This Article Abstract Full Text (PDF) eLetters: Submit a response to this article Purchase Article View Shopping Cart Alert me when this article is cited Alert me when eLetters are posted Alert me if a correction is posted Citation Map Services E-mail this article link to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Reprints/Permissions Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Elsayed, Y. A. Articles by Sausville, E. A. Search for Related Content PubMed PubMed Citation Articles by Elsayed, Y. A. Articles by Sausville, E. A.

Selected Novel Anticancer Treatments Targeting Cell Signaling Proteins

Developmental Therapeutics Program Clinical Trials Unit, Division of Cancer Treatment and Diagnosis, National Cancer Institute, NIH, Rockville, Maryland, USA

Correspondence: Edward A. Sausville, M.D., Ph.D., Developmental Therapeutic Program, Division of Cancer Treatment and Diagnosis, National Cancer Institute, NIH, Executive Plaza North, Room 8018, 6130 Executive Boulevard, Rockville, Maryland 20852, USA. Telephone: 301-496-8720; Fax: 301-402-0831; e-mail: sausville{at}nih.gov

	ABSTRACT

Top Abstract Introduction Cell Cycle Modulators in... Conclusions and Future... References

 
Empirical approaches to discovery of anticancer drugs and cancer treatment have made limited progress in the cure of cancer in the last several decades. Recent advances in technology and expanded knowledge of the molecular basis of tumorigenesis and metastasis have provided unique opportunities to design novel compounds that rationally target the abnormal molecular and biochemical signals leading to cancer. Several such novel agents have completed advanced stages in clinical development. The excellent clinical results achieved by some of these compounds are creating new paradigms in management of patients with neoplastic diseases. Clinical development of these agents also raises challenges to the traditional methods of drug evaluation and measurement of efficacy.

Key Words. Cell cycle • Cyclin-dependent kinase • Tyrosine kinase inhibitor • Chemotherapy • Histone deacetylase inhibitor • Proteasome inhibitor

	INTRODUCTION

Top Abstract Introduction Cell Cycle Modulators in... Conclusions and Future... References

 
Recent advances in molecular medicine have identified several molecular targets within cancer cell cycle regulation as a basis for anticancer treatments. Dysregulation of cell cycle control is a hallmark of human cancers, causing lack of differentiation and aberrant growth [1-3]. The cell cycle is a very complex and tightly regulated process that can result in cell division, differentiation, or growth, or contribute to programmed cell death through apoptosis. Proper regulation of this process involves environmental signals ultimately leading to the activation of cyclin-dependent serine/threonine kinases (CDK), regulated by activating cyclins and cyclin-dependent kinase inhibitors (CKI). Key steps regulated by CDKs are the DNA integrity control checkpoints, mediated by the retinoblastoma susceptibility tumor suppressor gene product (pRB), p53 tumor suppressor gene and E2F transcriptional factors family [4, 5]. To date, nine CDKs (cdk1-9) and at least 15 preferentially binding cyclins have been identified [6-8]. CDKs are typically small proteins of 300 amino acids in length and 33-40 kDa molecular weight. CDKs are activated through a 1:1 noncovalent binding with specific cyclins and trigger and coordinate the transition between the different phases of the cell cycle. Cyclin/CDK complex formation is usually transient and is affected by ubiquitin-mediated degradation of cyclins as a basis for regulating CDK activity, with rapid degradation of cyclins causing loss of CDK function. CDKs are also negatively regulated by small inhibitory molecules called endogenous CKIs. Two families of these inhibitors have been identified to date. The p21 gene family that includes p21^{WAF1/CIP1/SD11}, p27^KIP1, and p57, and the p16 gene family that includes p16^INK4A, p15^INK4B, p18^INK4C, and p19^INK4D [9, 10]. Members of p21 gene family interact with both cyclins and CDK subunits while members of p16 gene family interact only with CDKs [7].

Cell Cycle
Figure 1 is an overview of the cell cycle. The regulation of cell growth and division generally occurs in a precise and predictable manner dictated by tightly controlled successive waves of CDK/cyclin accumulation and rapid degradation of cyclins. The nondividing quiescent G₀ cell enters the G₁ phase in response to external mitogenic stimuli such as growth factor action or internal demands. Cyclin D is then expressed and binds to and activates CDK4 and/or CDK6 depending on the particular cell type. The cyclin-D/CDK4 or 6 complexes phosphorylate pRB (Fig. 2). Cyclin E is also expressed and binds to activate CDK2, resulting in phosphorylation of pRB on distinct sites. E2F transcriptional factors (E2F 1-5) then dissociate from the hyperphosphorylated pRB in order to activate S-phase promoting gene transcription including thymidylate synthase, dihydrofolate reductase, DNA polymerase, and others [11, 12]. By this point the cell has passed the "restriction point" beyond which the cell is committed to cell cycle progression and thus becomes independent of growth factors. Cyclins A and E bind and activate CDK2 and this allows the cell to traverse the S phase. Cyclin-A/CDK1 then facilitates the transition from S to G₂ phase. Cyclin B/CDK1 complex accumulates in late G₂ phase, and is required for progression of the cell through the M phase [13]. Following completion of anaphase, cyclin B is degraded, thus returning the cell to a G₁ state, which, in the presence of maintained growth factor stimulation, proceeds to successive rounds of cell division. The integrity of the synthesized DNA is examined and the repair of damaged DNA or apoptosis occurs at G₁ and G₂ checkpoints, an example of the checkpoint control operating to assure the fidelity of the replicated genome [14, 15]. CDKs play an important role in regulating these checkpoints. For example in response to various stress signals, p53, a transcriptional factor, is activated and causes transcriptional induction of p21 and establishment of the G₁ checkpoint [10]. The length of the individual phases of the cell cycle can vary depending on cell type and particular conditions. The CDK activities during the cell cycle are controlled at multiple levels including association with activating cyclins, transient expression and rapid degradation of these cyclins, post-translational modifications by kinases and phosphatases, interactions with CKIs, and intracellular translocations [9].

View larger version (17K): [in this window] [in a new window]   Figure 1. Schematic overview of cell cycle machinery. Following mitogenic signals that promote entry into early G1 phase, progression through the cell cycle is regulated by sequential activation of cell phase-specific cyclins and CDKs. Activation of CDK4 and CDK6 by cyclin D propels the cell through G1 phase. Activated CDK2 is required for progression through the S phase into G2 phase where CDK1/cyclin B complex then facilitates its passage into M phase. These steps are negatively regulated by endogenous cyclin-dependent kinase inhibitors. The p21 family of endogenous CDK inhibitors exerts a negative control on all cyclins and CDKs, while a member of the p16 family interacts specifically with CDK4 and CDK6.

View larger version (14K): [in this window] [in a new window]   Figure 2. The pRB/E2F pathway. The underphosphorylated and active retinoblastoma gene product (pRB) binds and actively represses the E2F family of transcriptional factors preventing cell cycle progression. Cyclin D/CDK4 and 6 complexes phosphorylate the RB early in the G1 phase resulting in its inactivation. Cyclin E/CDK2 adds further hyperphosphorylation of RB in late G1 phase. E2F family of transcriptional factors then dissociates from the inactive RB and promotes the transcription of target genes at the beginning of S phase.

	CELL CYCLE MODULATORS IN CLINICAL DEVELOPMENT

Top Abstract Introduction Cell Cycle Modulators in... Conclusions and Future... References

View larger version (29K): [in this window] [in a new window]   Figure 3. Chemical structures of selected protein kinase inhibitors currently in clinical development.

Pharmacology
The best antitumor activity in animal models was associated with frequent or prolonged exposure to the drug and as such several phase I and II clinical trials have employed a 72-hour continuous infusion schedule every 2 weeks [73-76]. The maximal tolerated dose (MTD) was identified as 50 mg/m²/day for 3 days on this schedule, and the dose-limiting toxicity (DLT) was secretory diarrhea [73]. Prophylactic use of antidiarrheal regimens, a combination of loperamide and cholestyramine, allowed escalation to a higher MTD of 78 mg/m²/day for 3 days where the DLT was reversible hypotension. The mean steady state concentration (Css) at the recommended phase II dose of 50 mg/m²/day for 3 days is 271 nm/l. The half-life for the terminal phase is 11.6 hours (range 1.3 to 29.1 hours), and the total clearance is 17.23 l/h/m² (range 11.5 to 27.3 l/h/m²). The pharmacology of flavopiridol is predictable in the majority of patients, however, a 30% interpatient variability was observed in this study. Polymorphic metabolism has been suggested for flavopiridol glucuronidation in the liver and poor glucuronidation has been associated with diarrhea [77]. Since activity in hematopoietic and squamous cell cancer preclinical in vivo models appeared to be superior with bolus administration of flavopiridol [60, 78], a trial administering flavopiridol as a 1-hour infusion daily for 5 days every 3 weeks has recently been completed [79]. The recommended phase II dose of 37.5 mg/m²/day for 5 consecutive days was identified. At this dose level an area under the concentration curve (AUC) of 25.8 ± 0.9 and a maximum concentration (C_max) of 1.622 ± 0.4 µM were achieved.

Clinical Response
The NCI phase I study of 72-hour infusional flavopiridol [73] treated 76 patients and documented one partial response (50% shrinkage) in renal cancer, three minor responses (<50% shrinkage) in lymphoma, colon, and renal cancers, and several episodes of stable disease for 6 months in prostate, adenocystic, and renal cancers. Five patients were able to receive flavopiridol for more than a year and one patient for more than 2 years. Tumor site inflammation and necrosis have been reported with flavopiridol treatment [73, 75]. One patient with gastric carcinoma achieved complete remission in a phase I study [80]. Tested outside the phase I trial context, however, 72-hour infusional flavopiridol as a single agent had no significant objective response in subsequent phase II studies in gastric [75], non-small cell lung (NSCLC) [76], colorectal [81], or renal cell cancers [74]. Stable disease for more than 6 months has been documented with bolus administration of flavopiridol in colon cancer, melanoma, NSCLC, and lymphoma [79]. The combination of flavopiridol with paclitaxel and cisplatin resulted in clinical responses in patients with esophageal and lung cancer [82]. Some of those esophageal cancer patients had failed treatment with paclitaxel or paclitaxel and cisplatin combination before responding to flavopiridol plus the same cytotoxic agent. This raises the possibility that modulation of standard agent activity may be an equally interesting strategy to pursue.

Toxicity
In several clinical trials the DLT of flavopiridol at 50 mg/m²/day was identified as secretory diarrhea with no evidence of mucosal damage [73, 74]. This was attributed to a local effect of flavopiridol and its glucuronidated metabolites in the intestine causing chloride ion secretion [77, 83, 84]. Diarrhea was treatable and preventable with loperamide and cholestyramine, however, in some patients this approach was unsuccessful and required dose de-escalation or stopping the drug. A unique constellation of dose-related pro-inflammatory toxicities comprised of fever, malaise, myalgia, arthralgias, and tumor pain accompanied by an increase in serum levels of acute phase reactants such as C-reactive protein, haptoglobin, and fibrinogen, and a decrease in albumin was experienced by almost all patients in most studies [73-75]. The near universal chronic fatigue was debilitating in some patients. Recent studies have shown a dose-dependent parallel increase in interleukin-6 levels following flavopiridol infusion [Senderowicz, unpublished data].

Flavopiridol treatment has also been associated with variable incidences of thrombosis in different clinical trials. This occurrence varied by tumor type from none in chronic lymphocytic leukemia (CLL) and lymphomas to 26% in renal cell cancer [74] and 33% in gastric cancer [75]. Although the majority of these thrombotic events were central venous catheter-related, several cases of myocardial ischemia, transient ischemic attacks, strokes, and pulmonary embolisms have been reported [73, 74]. Other toxicities observed with flavopiridol treatment included nausea, vomiting, reversible hypotension that constituted a DLT at the higher dose levels, reversible lymphocytopenia, rash, isolated hyperbilirubinemia, and hyperglycemia more pronounced in diabetic patients [73-75]. No significant myelosuppression was seen, however, mild to moderate decreases in hemoglobin and platelets were seen in some studies [74, 75]. A protocol employing administration of flavopiridol as a 1-hour bolus has a somewhat different toxicity profile such as dose-limiting nausea, vomiting, and neutropenia [79]. Grade 3 and 4 neutropenia have been reported in patients who received the combination of flavopiridol, paclitaxel, and cisplatin [82].

Current Clinical Trials
Several problems have been identified with flavopiridol as an anticancer treatment including its relatively nonselective action (pan CDK inhibitor) and existence of metabolic pathways that contribute to its toxicity profile, and thus far low antitumor activity as a single agent. The significantly increased apoptosis and synergistic effect when flavopiridol is combined with other cytotoxic drugs is promising as a basis for combination therapy. Evidence of this was clear from phase I combination studies with paclitaxel and cisplatin [82] and also from the curative responses seen in colon cancer xenografts when flavopiridol was combined with CPT-11 [85]. At present several clinical trials using flavopiridol as a single agent or in combination are under way in CLL, breast cancer, NSCLC, non-Hodgkin's lymphoma, colon, prostate, and other solid tumors [86]. Different administration schedules in relation to other agents are being explored to identify the optimal antitumor combinations.

UCN-01
The parent compound of UCN-01, staurosporine, was originally isolated from Streptomyces species and found to be a nonspecific inhibitor of many kinases causing cell cycle arrest in G₁ and G₂ phases in different cell types. It has significant toxicities, which precluded its clinical development. UCN-01, 17-hydroxystaurosporine (Fig. 3B) is a naturally occurring derivative of staurosporine. UCN-01 is relatively more selective for PKC (IC₅₀ = 30 nM) [87], although many kinases are also affected. It possesses significant antiproliferative activity in several human tumor cell lines [88, 89] and a tolerable toxicity profile. UCN-01 has demonstrated significant cytotoxic effects against human xenografts in vivo [90].

The effects of UCN-01 on cell growth are complex. In Rb-competent cells, the drug causes arrest in G₁ with loss of CDK activity [89]. In cells released from nocadazole synchrony at M phase, arrest in G₁ and S phases can occur [91]. In lymphoblasts, an early effect of the drug was actual loss of G₂/M fraction, and induction of apoptosis [92]. UCN-01-induced apoptosis of lymphoblasts can be related to inappropriate activation of CDK1 and CDK2 resulting in inappropriate cell entry into M phase and induction of apoptosis [92]. A potential mechanism for this effect has been provided by the demonstration of its capacity to inhibit checkpoint kinase 1, an inhibitor of cdc25 phosphatase, at a low drug concentration [93]. This could lead to enhanced cell death after DNA damaging-agents including radiation [94], 5-fluorouracil (5-FU) [95], cisplatin [96], and mitomycin C. Increased cytotoxicity to UCN-01 is also seen in cells containing the mutated p53 gene [97]. UCN-01 was recently found to inhibit E2F expression by a ubiquitin-proteasome-dependent process [98].

Pharmacology
The cytotoxic effect of UCN-01 is dependent on exposure duration, and the 72-hour infusion was found to yield the maximal benefit both in vitro and in vivo [90, 99-101]. A surprising difference between UCN-01 pharmacology in humans and in rodents emerged when it was documented that UCN-01 binds avidly to -acidic glycoprotein (AAG) resulting in a prolonged half-life of 600-1,000 hours in humans [100-102]. Eighty-eight to 98% plasma protein binding was also seen with other staurosporine derivatives [103]. Owing to these properties a 4-weekly dosing schedule was found to be appropriate, with retreatments with 50% of the original dose.

In a recently published phase I study [101], 47 patients were treated and the recommended phase II dose was identified at 42.5 mg/m²/day for 3 days every 4 weeks. A mean total plasma concentration of 36.4 µM (400 nM free UCN-01) was achieved at this dose level with half-life ranging from 447 to 1,176 hours. A median salivary concentration of UCN-01 was 111 nM at the MTD, and the drug was detectable in saliva for several weeks following infusion.

No alteration in UCN-01 pharmacology was seen when it was combined with 5-FU [104]. One-hour bolus administration of UCN-01 had similar pharmacokinetics and low clearance [105].

Clinical Response
In the phase I study described above [101], one patient with metastatic melanoma achieved a partial response for more than 6 months, and a heavily treated patient with progressive anaplastic lymphoma kinase-positive large cell lymphoma had an initial response followed by stable disease for more than 3.5 years. Nineteen other patients had a stable disease for a median of 5 months duration. UCN-01 also may have enhanced the sensitivity of chemotherapy-resistant plasmacytoid lymphoma to EPOCH chemotherapy resulting in complete response in a 68-year-old man who had progressive lymphoma in spite of prior high-dose EPOCH therapy [106]. Partial response and stable disease were reported in two cases of progressive cholangiocarcinoma treated with staurosporine-derivative PKC412 [103]. Similar responses were reported in CLL patients treated with UCN-01 and fludarabine [107].

Toxicity
In the NCI phase I study, the DLTs were hyperglycemia, nausea, vomiting, and pulmonary dysfunction at 53 mg/m²/day. Hyperglycemia was a consistent toxicity at all dose levels tested, with its severity dose dependent. Some patients developed evidence of metabolic acidosis. Although hyperglycemia was more common in patients with existing glucose intolerance, it also occurred in patients with no prior abnormality in glucose metabolism. UCN-01-induced hyperglycemia was accompanied by increase in serum insulin and immunoreactive C-peptide levels [101] indicating a normal islet cell response to increase in glucose stress and is suggestive of increased peripheral tissue resistance to insulin, although still somewhat responsive to exogenous insulin administration. Staurosporine has been demonstrated to inhibit insulin-stimulated translocation of GLUT1 and GLUT4 glucose transporters [108]. Other adverse events observed with UCN-01 included pulmonary toxicity characterized by hypoxemia with small bilateral pleural effusions or no significant radiological findings, headache at the higher dose levels, asymptomatic hypotension, fever, fatigue, rash, and myalgia [101]. Hypotension emerged as a significant DLT when UCN-01 was administered as a 1-hour bolus infusion at 95 mg/m² [105]. Single agent UCN-01 treatment was not myelosuppressive and no significant adverse effect was noted on renal or hepatic functions. Myelosuppression became significant when UCN-01 was combined with other chemotherapeutic agents such as fludarabine [107].

Current Clinical Trials
Several clinical trials are ongoing to explore the utility of UCN-01 as a single agent or in combination with radiation and other chemotherapeutic agents. Approaches to decrease the binding of UCN-01 to AAG, such as the use of displacing agents or structural modification of UCN-01 to bind less avidly with AAG, are being investigated. Studies combining UCN-01 with fludarabine in CLL and indolent lymphoma [107] and with 5-FU [104] are under way.

STI571 (Gleevec, CGP 57148b)
STI571 (imatinib mesylate), a 2-phenylaminopyrimidine derivative (Fig. 3C), emerged from an effort by Novartis Pharmaceuticals to inhibit PDGF-R, and was also found to inhibit the abl-related tyrosine kinases including p210^bcr-abl, p185^bcr-abl, v-abl, and c-abl in addition to c-kit tyrosine kinase [109]. STI571 has been proposed to bind to a distinctive "inactive" conformation of the centrally located activation loop of abl kinase, preventing its catalytic utilization of ATP, and therefore activation [110]. Release of cytochrome c from mitochondria to cytosol, activation of caspase-9 and caspase-3, DNA fragmentation, and apoptosis follow inhibition of p210^bcr-abl kinase by STI571 [111]. Continuous exposure is required to maintain the inhibition of bcr-abl kinase activity [112]. In addition to its direct inhibitory effects, STI571 also plays a role in modulating resistance pathways in tumors expressing PDGF-R and c-kit kinases. STI571 has also been shown to inhibit the growth of glioblastoma (perhaps by PDGF-R effects) [113] and small cell lung cancer (SCLC) known to express c-kit [114]. In Philadelphia-chromosome positive (Ph⁺) leukemia cell lines, STI571 produced synergistic effects with -interferon and vincristine, and an additive effect in combination with cyclophosphamide, hydroxyurea, cytarabine, doxorubicin, and etoposide. Methotrexate exerted an antagonistic effect when combined with STI571 [115].

Several mechanisms of resistance to STI571 have been proposed including a bcr-abl amplification, reduction of STI571 uptake, and overexpression of multidrug resistant P-glycoproteins among others. Studies have documented the effect of STI571 in reversing chemotherapy resistance [116], and a synergistic effect has been seen with other chemotherapeutic agents including interferon- (IFN-), daunorubicin, cytarabine, and etoposide [116, 117].

Pharmacology
STI571 is well absorbed orally and once daily administration of 400 mg achieved a steady-state mean maximal concentration of 2.3 µg/ml and a 24-hour trough concentration of 0.72 µg/ml [118]. These values are well above that required for inhibition of bcr-abl kinase. The drug has a half-life of 13 to 16 hours. STI571 is metabolized in the liver primarily through CYP3A4 enzyme, and its plasma levels may be affected by other drugs that induce this enzyme [118].

Clinical Response
Recently published studies in patients with chronic phase CML [118], blast crisis, and Ph⁺ acute lymphocytic leukemia (ALL) [119] demonstrated significant antileukemic effects. Ninety-eight percent of patients with chronic phase CML who received a dose of 300 mg/day or more achieved complete hematologic response and 31% achieved complete or major cytogenetic response. The hematologic response was apparent as early as 3 weeks after initiation of treatment and was maintained for a median of 265 days in 96% of patients. Complete cytogenetic responses (28%) occurring within 3 to 9 months were reported in a phase II trial of chronic-phase CML [120]. Response rates of 44% and 62% were reported in phase II studies of accelerated phase and myeloid blast crisis, respectively. Fifty-five percent of patients with myeloid and 70% with lymphoid blast crisis achieved either a complete hematologic response or a marrow response with 15% blasts [119], however, the majority of patients with lymphoid phenotype later relapsed [120].

An excellent clinical response to 400 mg of daily STI571 was also noted in one patient with metastatic gastrointestinal stromal tumor (GIST) expressing c-kit who failed several prior chemotherapy regimens [121]. c-kit is implicated in the pathogenesis of several human tumors including mast cell leukemia, SCLC, GIST, germ cell tumors, ovarian cancer, melanoma, breast cancer, and neuroblastoma. A subsequent European Organization for the Research and Treatment of Cancer phase I study in GIST and other soft tissue sarcoma documented partial responses in four patients, and stable diseases and clinical improvement in 8 of 20 treated patients [122]. Positron-emission tomography (PET) proved to be a valuable method for evaluating metabolic response to this agent. Dramatic reduction in ¹⁸FDG uptake was seen as early as day 8 of treatment [122]. The reduction in the metabolic activity of tumors correlated well with decrease in tumor size on magnetic resonance imaging [121].

Toxicity
Treatment with STI571 was associated with a tolerable toxicity profile. In the phase I study [118], patients were treated with a dose range of 25-1,000 mg without identification of MTD. Toxicities were minimal at the lower dose levels, and no grade 3 or 4 toxicity was reported at the 300 mg daily dose. In both published studies [118, 119] the most common toxicity observed was nausea in 43%-55% of patients. Myalgias were reported in 21%-41% of patients, edema in 39%-41%, diarrhea in 17%-25%, fatigue in 10%-20%, rash in 17%-19%, vomiting in 18%-41%, anorexia in 10%, and arthralgias in 13%. Other infrequent side effects included grade I/II anemia, elevation of liver enzymes, exfoliative dermatitis, gastric hemorrhage, renal failure, and congestive heart failure with possible relation to STI571. Cases of solid tumor bleeding were also reported and related to rapid tumor lysis [122]. A more frequent grade 3 or 4 elevation in liver enzymes was reported in the second study [119] with no obvious relation to the doses given. Frequent, but not dose-limiting, myelosuppression was noted with STI571 treatment. Thirty to 69% of patients in both studies developed grade 3 or 4 thrombocytopenia, and 34%-66% developed grade 3 or 4 neutropenia. These toxicities were treated with dose reduction or temporary treatment interruption.

Current Clinical Trials
Currently several studies are under way to test the efficacy of STI571 in various tumors. A phase III trial is comparing STI571 with IFN- and cytarabine in newly diagnosed patients with CML. Although STI571 has shown significant response in all phases of CML and GIST, subpopulations can be defined that are resistant to its antitumor effect. Combination with other chemotherapeutic agents and other tyrosine kinase inhibitors is being considered. Clinical trials in glioblastoma and SCLC are being initiated.

Inhibitors of EGFR-TK
EGFR (erbB-1) is a transmembrane glycoprotein with an external ligand binding domain and an intracellular tyrosine kinase domain. It is a part of the erb-B family of receptors that form homodimers or heterodimers on ligand binding (transforming growth factor- [TGF-] or EGF). This leads to autophosphorylation of the tyrosine residues within the intracellular domain and activation of downstream pathways such as ras/MAP kinase and STAT-3 transcription factors. These signal transduction events are critical for the growth of many tumors. EGFR is overexpressed in an extensive range of human cancers including NSCLC, colorectal, head and neck, bladder, brain, pancreas, breast, ovary, prostate, and gastric cancers [123, 124]. There is a high level of expression of EGFR in squamous cell carcinomas, and 90% of head and neck tumors overexpress this protein. Overexpression of EGFR is associated with poor outcome in several tumor types [125, 126]. Several approaches to blocking the EGFR-TK in human diseases are being explored including monoclonal antibodies and small molecule inhibitors. Cetuximab (C-225) is a human-chimeric monoclonal antibody that has shown promising results in phase I/II studies and is now entering phase III clinical trials [127]. Of the small molecule compounds that act intracellularly to inhibit EGFR-TK, ZD 1839 and OSI-74 have reached advanced clinical development.

ZD 1839 (IRESSA)
ZD 1839 (Fig. 3D) is an anilinoquinazoline compound developed as a specific potent inhibitor of EGFR-TK. ZD 1839 inhibits EGFR-TK through competitive binding to the ATP-binding site. Specificity for EGFR-TK was demonstrated in cell cultures of KB human tumors with an IC₅₀ of 0.08-0.09 µM and 3.64 µM for EGF-stimulated and non-EGF-stimulated growth respectively, and no significant inhibitory effect on other related PTKs such as erb-B2 kinase, KDR, c-flt kinase, PKC, and MAPK. In vivo animal models revealed significant inhibitory effect on xenograft of several tumor types including A431, A549, HT29, DU145, and MCF-7. Although ZD 1839 has cytostatic effects, cytotoxicity was seen at higher dose levels. Continuous administration is required for maintaining this inhibition.

ZD 1839 produced supra-additive and enhanced antitumor effects of cisplatin, carboplatin, oxaliplatin, paclitaxel, docetaxel, etoposide, topotecan, ralitrexed, and doxorubicin in several tumor types resulting in complete regression in some xenograft tumors [128, 129]. Similar responses were seen in combination with radiation [130]. No additive effects were seen with gemcitabine [128]. Schedule dependency seems to exist between ZD 1839 and some of these drugs, such as cisplatin. The cytotoxicity of cisplatin is greater with pre-exposure to ZD 1839 for 48 hours rather than 1 hour [131]. Other studies have demonstrated that pretreatment with low doses of cisplatin enhanced the sensitivity of NSCL carcinoma cells to ZD 1839 [132]. An important finding in several studies is the apparent effectiveness of ZD 1839 regardless of the levels of EGFR protein [128] or gene expression [133]. Thus, additional targets for the drug action may exist and would be of interest to define.

Pharmacology
Preclinical studies documented a 50% bioavailability of orally administered ZD 1839 and several phase I studies evaluated its pharmacology in humans [134-136]. Daily oral administration with doses ranging from 50 to 700 mg in an intermittent schedule of 14 of 28 consecutive days or continuous administration for 28 days was examined. With intermittent repeated daily dosing, a median half-life was estimated at 46-49 hours [134, 135]. Mean C_max and AUC_0-24hr were 113-2,255 ng/ml and 1.8-38.5 µg/ml, respectively [134].

Continuously administered ZD 1839 reaches a steady-state plasma concentration by day 7 of treatment. Lower doses of ZD 1839 were required when it was combined with other chemotherapeutic agents, and this was independent of the levels of EGFR expression [128]. Five hundred mg per day of ZD 1839 was found to be safe when administered in combination with paclitaxel and carboplatin [137]. No changes in the exposure to paclitaxel or carboplatin were observed during this combination.

Clinical Response
ZD 1839, given either intermittently or continuously, resulted in partial responses in NSCLC and prostate cancer, and stable disease lasting more than 4 months in several patients with various tumor types [134-136]. Clinical trials conducted by Baselga et al. [136] documented an improvement in quality of life in patients treated with ZD 1839. Quality of life was determined by scores on the functional assessment of cancer therapy-lung (FACT-L) patient's questionnaire and disease-related symptoms. Of 23 Japanese patients with NSCLC treated with an intermittent schedule ZD 1839, five patients with adenocarcinoma achieved partial response [135]. In an ongoing phase II combination study of ZD 1839, carboplatin, and paclitaxel in chemonaïve advanced NSCLC patients, 28% achieved partial response and 40% had stable disease at day 56 of treatment [137].

Toxicity
On the schedule of daily administration for 14 days, diarrhea and elevated transaminases were identified as the DLTs at 700 mg [134, 135]. Grade 3 diarrhea was reported in three patients on the continuous daily schedule [136]. Interestingly, no DLT was identified in the 28-consecutive-day schedule up to an 800 mg/day dose level [136]. Another frequent toxicity was an acneiform rash that has been documented in more than 50% of treated patients. Other toxicities included nausea, vomiting, and anorexia. These toxicities were reversible, and the rash resolved in some patients even with continued treatment.

Current Clinical Trials
Combining ZD 1839 with cytotoxic drugs produced greater antiproliferative effects even in tumors (e.g., LX-1) with minimally detectable EGFR expression [128], and the mechanism for this phenomenon is not clear. For these reasons, ZD 1839 has a promising role in enhancing the therapeutic benefit of existing standard agents, and several studies evaluating these combinations are ongoing. Phase III studies in lung cancer are evaluating ZD 1839 in combination with gemcitabine/cisplatin or paclitaxel/cisplatin and also its combination with paclitaxel and carboplatin. Another study is evaluating ZD 1839 in head and neck cancer.

Recent preclinical studies demonstrated efficacy of ZD 1839 in Herceptin-resistant cell lines with enhanced induction of apoptosis when ZD 1839 was combined with Herceptin [138]. A role for ZD 1839 is also anticipated in breast cancer patients who become resistant to tamoxifen [139]. The impact of EGFR overexpression on the efficacy of treatment is being correlated with clinical outcome in several studies, and this may provide important information on optimal candidates for such therapy.

OSI-774 (CP-358, 774)
OSI-774 (Fig. 3E) is another orally active and potent inhibitor of EGFR-TK with an IC₅₀ of 2 nM in biochemical assays. It reversibly inhibits EGFR-TK through competitive binding to the ATP site. Inhibition of EGFR-TK and its downstream P13/MAPK signal transduction pathways by OSI-774 results in accumulation of p27^KIP1, cell cycle arrest at G₁ phase, and induction of apoptosis [137]. EGFR-TK was shown to have more than a 1,000-fold sensitivity to OSI-774 inhibition compared with other tyrosine kinases [140]. A substantial inhibitory activity against EGFR-overexpressing tumor cells and inhibition of tumor growth were demonstrated in human tumor xenograft models (50% effective dose = 10 mg/kg daily for 20 days) [141]. Evaluation of pre- and 3-day-post-treatment biopsy specimens from patients with squamous cell carcinoma of the head and neck treated with OSI-774 revealed a 60% reduction in phosphorylated EGFR and complete abolition of phosphorylated Akt [142]. Significantly enhanced cytotoxicity was also demonstrated when combined with cisplatin [141], doxorubicin, gemcitabine, and other agents.

Pharmacology
Several phase I studies have evaluated the dose and scheduling of OSI-774 as a once-weekly treatment for 3 of 4 weeks, or continuous daily dosing for consecutive 21 of 28 days [143, 144]. Patients who received 100 mg or greater daily doses of OSI-774 achieved the projected target average plasma concentration for clinical efficacy (Cavg) of 500 ng/ml [143, 145]. Large inter- and intrapatient variability and evidence of dose-related accumulation in exposure were observed with oral OSI-774 treatment [144, 145]. In another phase I study evaluating several administration schedules, an average half-life of 24.4 ± 14.6 hours, V_dss of 136.4 ± 93.1 l, and Css of 1.20 ± 0.62 µg/ml at the 150 mg/day dose level were documented [143]. Liver metabolism of OSI-571 is cytochrome P-450-dependent and involves the CYP-3A4 enzyme system.

Clinical Response
Partial responses were observed in patients with renal and colon cancers, and stable disease for more than 5 months was documented in patients with head and neck, NSCL, prostate, and cervical cancers [143]. Partial and minimal responses in head and neck cancers in addition to significant reduction in metabolic activity assessed by PET scan were reported also in other phase I studies [146]. In a phase II study of OSI-774 in patients with EGFR-positive, platinum-refractory NSCLC, 11% of patients achieved partial response and 34% had stable disease [147]. These responses did not correlate with higher percentages or intense EGFR staining. A similar 13% partial response and 29% rate of stable disease were also reported in patients with advanced squamous cell carcinoma of the head and neck [148]. Advanced refractory ovarian carcinoma had a similar pattern of response [149].

Toxicity
On continuous daily dosing, diarrhea became a DLT at 200 mg/day. No DLT was identified in the weekly schedule up to 1,600 mg/day dose level. Other frequent toxicities were an acneiform rash, fatigue, headache, mucositis, nausea, and transient rise in bilirubin and transaminases [144]. The maculopapular rash was similar to that seen with ZD 1839 and occurred in up to 78% of patients affecting mainly the face and upper part of the body [147]. Biopsies of the rash revealed subepidermal neutrophilic infiltration and epidermal hyperproliferation.

Current Clinical Trials
In addition to its single-agent activity, OSI-571 is also promising in combination with chemotherapy, and its evaluation is under way or planned in several human tumors.

Proteasome Inhibitor PS-341
The proteasome is an exciting new target for cancer treatment. The ubiquitin-proteasome pathway plays an important role in cell cycle regulation through precisely programmed degradation of intracellular proteins resulting in either activation or blockage of certain signal transduction pathways. The 26S proteasome degrades proteins that have been marked for elimination and conjugated to multiple units of polypeptide ubiquitin [150] (Fig. 4). Examples of proteins that are degraded through the ubiquitin-proteasome are the transcription regulator IB. Degradation of IB results in activation of NF-B, which plays a significant role in cell survival, adhesion, metastasis, and angiogenesis [151]. Cell cycle arrest can result from increased 20S proteasome-mediated cyclin degradation and loss of CDK activity. On the other hand, inhibition of 20S proteasome activity can lead to accumulation of CKIs and cell arrest with or without apoptosis [152]. Several inhibitors of the proteasome pathway have been identified including lactacystin, peptide aldehydes, and dipeptide boronate derivatives.

View larger version (24K): [in this window] [in a new window]   Figure 4. The ubiquitin-proteasome pathway. Ubiquitin is activated by ubiquitin-activating enzymes (E1) and then transferred via transacylation to ubiquitin-carrier/conjugating enzymes (E2). The ubiquitin ligase (E3) helps the activated ubiquitin bind to a lysine residue of a substrate/protein and generates a polyubiquitin chain. The polyubiquitinated substrates then bind to the ubiquitin receptor subunit of 19S complex and are then degraded to small peptides. This process is ATP-dependent. The ubiquitin is recycled. The multi-subunit 26S proteasome is assembled from two 19S regulatory complexes attached at each side of the cylinder-shaped 20S core catalytic unit.

Pharmacology
PS-341 is rapidly removed from the vascular compartment and is widely distributed with a half-life of 10 minutes [152]. The degree of 20S proteasome inhibition in whole blood was adopted as a surrogate marker for the drug activity [160]. Several phase I studies evaluated various schedules of PS-341 administration, such as twice weekly injections every 2 weeks followed by 1 week off treatment, and twice or once weekly for 4 of 6 weeks. At the MTD recommended for phase II studies (1.25-1.3 mg/m²), a 65%-72% inhibition of 20S proteasome was achieved [156, 161]. An average 54% inhibition of proteasome was achieved in patients' tumors [162].

Clinical Response
Phase I studies documented a partial response in an extensively treated patient with a bronchoalveolar NSCLC [161], and 50% reduction in lung metastatic lesions and stable cutaneous lesions in two patients with melanoma [162]. Stable disease was seen in other patients with sarcoma, lung adenocarcinoma, and malignant fibrous histiocytoma [162].

Toxicity
PS-341 has a dose- and schedule-dependent toxicity profile with severe toxicity expected when the proteasome inhibition exceeds 80%. In phase I dose-escalation trials, DLTs were reported as painful neuropathy, diarrhea, fatigue, and orthostatic hypotension [161]. Other frequent toxicities noted with PS-341 treatment were nausea, vomiting, fever, and thrombocytopenia. Patients usually had more toxicity with the second cycle of treatment.

Current Clinical Trials
Several phase II clinical trials evaluating PS-341 as a single agent in hematologic malignancies, breast, brain, and several other solid tumors, as well as in pediatric patients are ongoing. In addition, PS-341 in combination with 5-FU, doxorubicin, CPT-11, gemcitabine, and etoposide is also being evaluated in a phase II trial [163]. The interaction of PS-341 with paclitaxel and docetaxel is being evaluated in a phase I study. Since PS-341 has a radiosensitizer effect probably mediated through NF-B inhibition, its combination with concurrent radiation in squamous cell carcinoma of head and neck is also being evaluated in a phase I study [163].

Histone Deacetylase Inhibitors
Histone acetylation by histone acetyltransferases is important for promoting the action of several transcription factors. Acetylation facilitates binding of transcription factors to specific target DNA sequences by destabilizing nucleosomes bound to the promoter region of the target genes [164]. On the other hand, histone deacetylases (HDAC) facilitate suppression of the transcriptional activity [165]. The transcriptional repression activity of HDACs was documented for pRB/E2F [166]. Disruption of histone acetylation is associated with cancer development in several human neoplasms. Inhibition of HDAC activity was found to be cytostatic and to arrest cells in G₁ and G₂/M phases. Therefore, inhibition of HDAC activity is being pursued as a novel approach to cancer treatment, and several classes of HDAC inhibitors (HDACI) have recently been identified including butyrate, trichostatin A, oxamflatin, depsipeptide, and benzamide compounds (MS-275). Depsipeptide and MS-275 have exhibited significant preclinical antitumor activities and are being evaluated in phase I and II clinical trials.

Depsipeptide (FR901228)
Depsipeptide (Fig. 3G) is a fermentation product from Chromobacterium violaceum that demonstrated potent cytotoxic activity against several human cell lines and xenograft models of human tumors in nanomolar concentration [167]. Depsipeptide causes downregulation of cyclin D1, upregulation of cyclin E, and p21-dependent cell cycle arrest at G₁ and G₂ [168]. Exposure to depsipeptide in vitro induced proapoptotic changes in B-cell CLL including a decrease in bcl-2:bax ratio and p27 expression [169].

In a phase I study conducted at the NCI [170], depsipeptide was administered intravenously over a 4-hour period on days 1 and 5 of a 21-day cycle, to 27 patients in eight dose levels. MTD was established at 17.8 mg/m². Several toxicities were experienced with depsipeptide treatment including neutropenia, thrombocytopenia, hypocalcemia, fatigue, nausea, vomiting, and T-wave inversions on electrocardiogram not associated with elevation in cardiac enzymes. This study documented an elimination half-life of 12 hours and volume of distribution of 14 L. Partial responses occurred in three patients with cutaneous T-cell lymphoma and a complete response in a patient with peripheral T-cell lymphoma [171]. Hyperacetylation of histones was demonstrated in Sézary cells after treatment with depsipeptide in this study. A phase II study of depsipeptide in T-cell lymphoma is now open for accrual at the NCI.

MS-275
MS-275 (Fig. 3H) is a benzamide-derivative that induces p53-independent accumulation of p21^WAF/CIP1 and cell cycle arrest [172]. MS-275 also induced TGF-ß type II receptor expression in human breast cancer [173], and this may contribute to its induction of p21^WAFI/CIP1 expression. MS-275 has significant growth inhibitory effects on several human tumor cell lines including myeloma, pro-myelocytic leukemia, colon, lung, and ovarian tumors. This effect is time dependent. Compared with 5-FU, MS-275 has a comparable or superior antitumor effect in xenograft models of several human tumors including St-4 gastric, KB-3-1 epidermoid, Ca-pan-1 pancreatic, HT-29 colon, A2780 ovarian, and Calu-3 lung tumors [172]. MS-275 is absorbed orally with 28%-55% bioavailability. Its antiproliferative effect requires prolonged exposure. In animal models the main toxicities were gastrointestinal and bone marrow suppression. A phase I study evaluating orally administered MS-275 in lymphomas and solid tumors has just started at the NCI.

Geldanamycin Derivatives and Immunoconjugates
Geldanamycin is a highly cytotoxic benzoquinoid ansamycin antibiotic produced by the actinomycete Streptomyces hygroscopicus. Ansamycin antibiotics were found to inhibit several PTKs, reverse the morphology of cells transformed by PTK oncogenes such as src, yes, fps, c-myc, enhance expression of hyperphosphorylated pRB, and induce G₂/M cell cycle arrest [174, 175]. Geldanamycin and its derivatives bind with high affinity to and interfere with the chaperone function of the cytosolic heat-shock-protein-90 (Hsp90), and enhance the proteasomal degradation of several cell cycle regulatory proteins including receptor tyrosine kinases, steroid receptors, RAF, and CDKs [176-178]. Preclinical data demonstrated in vitro and in vivo anti-tumor activities in several tumor types including melanoma, prostate, ovary, and breast cancers.

Since the parent compound geldanamycin is not suitable for clinical use due to significant hepatic toxicity, other approaches have been identified to utilize its cytotoxic properties. A structurally modified derivative, 17-(Allylamino-)-17-Demethoxygeldanamycin (17-AAG) has a better toxicity profile and exerts similar antitumor effects. Two phase I clinical studies [179, 180] presented this year documented the tolerability and pharmacology of 17-AAG in humans. 17-AAG was administered as a 1-hour daily infusion for 5 consecutive days and repeated every 3 weeks. An MTD of 40 mg/m² was identified in the NCI study [179]. DLT in this study was reversible hepatotoxicity. Other adverse events observed with 17-AAG treatment were fever, emesis, fatigue, thrombocytopenia and diarrhea [179, 180]. Terminal half-life of 2.5 ± 0.5 hour and C_max of 1,860 ± 660 nM at the 40 mg/m² dose were identified [179]. Plasma drug concentrations exceeded the levels (10-500 nM) required for cell death in vitro and in xenograft models. Both studies suggested evidence of disease stabilization.

Another promising approach is utilization of geldanamycin in immunoconjugates. Recent studies have demonstrated the feasibility of conjugating derivatives of geldanamycin to internalizing anti-HER2 monoclonal antibodies [181]. These immunoconjugates exhibited enhanced antiproliferative activity compared with the native monoclonal antibody. Conjugation to monoclonal antibodies directed at other growth hormones and oncogenic receptors is being explored. This promising approach will likely continue to develop and reach clinical trials in the near future.

Rapamycin Analog (CCI-779)
Rapamycin (Sirolimus, Rapamune) (Fig. 3I) is a macrolide fungicide that binds intracellularly to the immunophilin FKBP12, and the resultant complex inhibits the activity of a 290-kDa kinase known as mammalian target of rapamycin (mTOR). mTOR, also known as FRAP, RAFT1, and RAP1, is a kinase member of phosphoinositide 3 kinase related kinases family that is activated in response to growth signaling through the PI3K/Akt pathway. Activation of mTOR results in increased translation of several critical cell cycle regulatory mRNAs through two downstream effector kinases, p70^s6k and 4E-BP1/PHAS [182, 183]. Rapamycin causes G₁ cell cycle arrest by increasing the turnover of cyclin D1 [184], upregulating p27^KIP1, and inhibiting cyclin A-dependent kinase activity [185]. Blockage of mTOR function results in inhibition of PI3K/Akt-mediated proliferative signals and cell arrest. In addition to its antimicrobial and immunosuppressive properties, rapamycin was found to have significant antiproliferative effects in human tumors [186]. CCI-779, an ester of rapamycin, has a significant antiproliferative effect and favorable toxicology profile and is being studied in several phase I clinical trials in human cancer [187, 188]. In these Phase I studies CCI-779 was administered on a weekly or daily basis for 5 days every 2 weeks schedule. Toxicities observed with CCI-779 treatment included hypocalcaemia, neutropenia, thrombocytopenia, mucositis, hypertriglyceridemia, rash, reversible decrease in testosterone levels in men, and allergic reactions. A median half-life of 17.3 hours was documented [188]. Several partial responses have been documented in renal cell carcinoma, NSCLC, neuroendocrine tumors, and breast cancer in addition to minor responses and stable disease in several tumor types [187, 188]. Several studies evaluating combination of CCI-779 with other chemotherapeutic agents are under way.

	CONCLUSIONS AND FUTURE DIRECTIONS

Top Abstract Introduction Cell Cycle Modulators in... Conclusions and Future... References

 
Development of therapeutic anticancer drugs has entered a new era of targeted treatment that promises rational modulation of malignant cell growth with acceptable toxicity. The development of these drugs depends on the identification of suitable targets within the pathophysiologic pathway of carcinogenesis and on designing agents that modulate these targets. Targets are being discovered continuously and this has been made possible by recent advances in technology such as DNA microarray and proteomics. Although many of the agents described in this article are rationally targeted to specific effectors in the neoplastic process, several issues have emerged regarding the development of such agents, their range of toxicities, and how to measure their clinical effect.

Several of the new agents achieve antitumor effects through a wide range of mechanisms including cell cycle arrest, antiangiogenesis, induction of apoptosis, and cytotoxicity. These processes are linked and result from modulation of several survival and death pathways downstream of the drugs' targets. These sequential and time-dependent changes may result in apoptosis and cell death. A unique feature of most of these new compounds is the reversibility of action and the need for continuous administration to attain significant growth inhibition and allow for the apoptotic process to take place. While newer generations of these agents such as the EGFR-TK inhibitor CI-1033 (PD 183805) have irreversible effects, which may result in a sustained blockage of carcinogenic signals, there is also a potential for greater toxicity.

All inhibitors of signal transduction under clinical development at present exhibit a concentration-dependent inhibitory effect on a wide range of signaling events. It is anticipated that more active and selective chemical inhibitors of CDK and tyrosine kinases will be available in the near future. The clinically successful compounds are anticipated to have very defined and limited targets and also a reasonable selectivity to avoid side effects. It is estimated that there are 2,000 kinases in the human body and therefore the required selectivity of these compounds may be difficult to achieve given that all kinases have an ATP-binding pocket with some high degree of homology. The recently described inhibitory mechanism of STI571 by binding to an inactive form of Abl tyrosine kinase [110] and preventing its activation provides an exciting approach for designing new highly selective inhibitors. Inactive kinases may have more distinctive conformations that allow for better selectivity.

Expression levels of cell surface antigens and receptors associated with a specific PTK may be of theoretical importance to the maximal inhibitory effect of these drugs. However, recent studies have not found a correlation between the degree of EGFR positivity and the clinical response [128, 133]. Unlike monoclonal antibodies such as Herceptin, where the degree of expression may be essential for better response, "small" molecules acting intracellularly may not require a higher expression of targets. This may allow administration of these drugs to specific tumor types without the need for assessing the degree of receptor expression. They also question, however, the specific relevance of the target against which the drugs were designed.

These new agents raise a challenge to the traditional dose-finding approach in cancer drug discovery and the concept that more is better. A new approach that determines the biologically optimal dose may be more appropriate than the traditional MTD. Biological endpoints should determine the recommended dose for certain conditions if that dose correlates with inhibition of target kinase and clinical outcome. The majority of the newly designed drugs achieved a biologically active plasma concentration that is sufficient to inhibit the target kinase at doses well below the MTD.

Assessing the clinical response to these novel agents is an area of extensive discussion. Many of the novel targeted antitumor agents that affect signal transduction may have a cytostatic endpoint rather than the traditional cytotoxic one. Therefore, the traditional methods of assessing disease response may not be valid, and new approaches to examine the changes in rate of tumor growth and perhaps other biological surrogate markers may be useful in determining which of these drugs are most suitable for advanced phase development. Such issues are important if drugs with greater utility in earlier stage disease (i.e., adjuvant application) are not discarded owing to need-to-document cytotoxic responses in late-stage disease patients. PET using ¹⁸[F]-fluoro-deoxyglucose imaging has shown promising potential as a marker of tumor metabolic activity treated with several novel agents including STI571 and OSI-774 [121, 147, 189]. Other endpoints that may be useful include time to progression, stable disease as best response, symptomatic benefit, and other quality-of-life measures. Another important issue is that these agents may require a longer treatment period before a sensible reduction or stabilization in the tumor size could be appreciated. The current 2-3 months interval of assessment may not be suitable for these agents.

A unique constellation of toxicities has emerged with these agents. The occurrence of diarrhea, rash, hyperglycemia, hepatic dysfunction, and inflammatory syndromes was common with these agents. This syndrome is most likely related to the blockage of ATP binding and interference with downstream cellular signaling pathways that regulate common features in human physiology. The exact affected pathways resulting in these adverse events have not been identified yet, but the majority of these side effects were easily treatable.

As mentioned above, the combination of various cell signaling inhibitors with conventional agents and radiation is being pursued in several studies based on the promising preclinical data. Schedule dependency has emerged as an important factor to achieve the optimal antiproliferative activity desired and also to minimize toxicity. This raises the question of how to combine different agents to achieve maximal blockage of multiple intracellular pathways leading to cancer development. These targeted therapies are likely to have a wide range of applications in cancer treatment, however, greater roles are envisioned as adjuvant, chronic stabilizing, and preventive modalities.

	ACKNOWLEDGMENT

Top Abstract Introduction Cell Cycle Modulators in... Conclusions and Future... References

 
This is a U.S. government work.

	REFERENCES

Top Abstract Introduction Cell Cycle Modulators in... Conclusions and Future... References

 

1. Sherr CJ. Cancer cell cycles. Science 1996;274:1672–1677.[Abstract/Free Full Text]
2. Hartwell LH, Kastan MB. Cell cycle control and cancer. Science 1994;266:1821–1828.[Abstract/Free Full Text]
3. Dictor M, Ehinger M, Mertens F et al. Abnormal cell cycle regulation in malignancy. Am J Clin Pathol 1999;112(suppl 1):S40–S52.[Medline]
4. van den Heuvel S, Harlow E. Distinct roles for cyclin-dependent kinases in cell cycle control. Science 1993;262:2050–2054.[Abstract/Free Full Text]
5. Weinberg RA. The retinoblastoma protein and cell cycle control. Cell 1995;81:323–330.[CrossRef][Medline]
6. Draetta G. Cell cycle control in eukaryotes: molecular mechanisms of cdc2 activation. Trends Biochem Sci 1990;15:378–383.[CrossRef][Medline]
7. Sherr CJ, Roberts JM. CDK inhibitors: positive and negative regulators of G₁-phase progression. Genes Dev 1999;13:1501–1512.[Free Full Text]
8. Sausville EA, Johnson J, Alley M et al. Inhibition of CDKs as a therapeutic modality. Ann N Y Acad Sci 2000;910:207–222.[Medline]
9. Morgan DO. Principles of CDK regulation. Nature 1995;374:131–134.[CrossRef][Medline]
10. Xiong Y, Hannon GJ, Zhang H et al. p21 is a universal inhibitor of cyclin kinases. Nature 1993;366:701–704.[CrossRef][Medline]
11. Dyson N. The regulation of E2F by pRB-family proteins. Genes Dev 1998;12:2245–2262.[Free Full Text]
12. Hatakeyama M, Weinberg RA. The role of RB in cell cycle control. Prog Cell Cycle Res 1995;1:9–19.[Medline]
13. Sherr CJ. G₁ phase progression: cycling on cue. Cell 1994;79:551–555.[CrossRef][Medline]
14. Hartwell LH, Weinert TA. Checkpoints: controls that ensure the order of cell cycle events. Science 1989;246:629–634.[Abstract/Free Full Text]
15. Buolamwini JK. Cell cycle molecular targets in novel anticancer drug discovery. Curr Pharm Des 2000;6:379–392.[CrossRef][Medline]
16. Hunter T, Pines J. Cyclins and cancer. II: Cyclin D and CDK inhibitors come of age. Cell 1994;79:573–582.[CrossRef][Medline]
17. Karp JE, Broder S. Molecular foundations of cancer: new targets for intervention. Nat Med 1995;1:309–320.[CrossRef][Medline]
18. Draetta GF. Mammalian G₁ cyclins. Curr Opin Cell Biol 1994;6:842–846.[CrossRef][Medline]
19. Dreyling MH, Bullinger L, Ott G et al. Alterations of the cyclin D1/p16-pRB pathway in mantle cell lymphoma. Cancer Res 1997;57:4608–4614.[Abstract/Free Full Text]
20. Hall M, Peters G. Genetic alterations of cyclins, cyclin-dependent kinases, and Cdk inhibitors in human cancer. Adv Cancer Res 1996;68:67–108.[Medline]
21. Leach FS, Elledge SJ, Sherr CJ et al. Amplification of cyclin genes in colorectal carcinomas. Cancer Res 1993;53:1986–1989.[Abstract/Free Full Text]
22. Motokura T, Arnold A. Cyclin D and oncogenesis. Curr Opin Genet Dev 1993;3:5–10.[CrossRef][Medline]
23. Davis MA, Sturzl MA, Blasig C et al. Expression of human herpesvirus 8-encoded cyclin D in Kaposi's sarcoma spindle cells. J Natl Cancer Inst 1997;89:1868–1874.[Abstract/Free Full Text]
24. Keyomarsi K, Herliczek TW. The role of cyclin E in cell proliferation, development and cancer. Prog Cell Cycle Res 1997;3:171–191.[Medline]
25. Gudas JM, Payton M, Thukral S et al. Cyclin E2, a novel G₁ cyclin that binds Cdk2 and is aberrantly expressed in human cancers. Mol Cell Biol 1999;19:612–622.[Abstract/Free Full Text]
26. Buckley MF, Sweeney KJ, Hamilton JA et al. Expression and amplification of cyclin genes in human breast cancer. Oncogene 1993;8:2127–2133.[Medline]
27. Wang J, Zindy F, Chenivesse X et al. Modification of cyclin A expression by hepatitis B virus DNA integration in a hepatocellular carcinoma. Oncogene 1992;7:1653–1656.[Medline]
28. Kim JH, Kang MJ, Park CU et al. Amplified CDK2 and cdc2 activities in primary colorectal carcinoma. Cancer 1999;85:546–553.[CrossRef][Medline]
29. Ikeda K, Monden T, Tsujie M et al. Cyclin D, CDK4 and p16 expression in colorectal cancer [article in Japanese]. Nippon Rinsho 1996;54:1054–1059.[Medline]
30. Costello JF, Plass C, Arap W et al. Cyclin-dependent kinase 6 (CDK6) amplification in human gliomas identified using two-dimensional separation of genomic DNA. Cancer Res 1997;57:1250–1254.[Abstract/Free Full Text]
31. Kamb A, Gruis NA, Weaver-Feldhaus J et al. A cell cycle regulator potentially involved in genesis of many tumor types. Science 1994;264:436–440.[Abstract/Free Full Text]
32. Hayashi N, Sugimoto Y, Tsuchiya E et al. Somatic mutations of the MTS (multiple tumor suppressor) 1/CDK4l (cyclin-dependent kinase-4 inhibitor) gene in human primary non-small cell lung carcinomas. Biochem Biophys Res Commun 1994;202:1426–1430.[CrossRef][Medline]
33. Cairns P, Mao L, Merlo A et al. Rates of p16 (MTS1) mutations in primary tumors with 9p loss. Science 1994;265:415–417.[Free Full Text]
34. Drexler HG. Review of alterations of the cyclin-dependent kinase inhibitor INK4 family genes p15, p16, p18 and p19 in human leukemia-lymphoma cells. Leukemia 1998;12:845–859.[CrossRef][Medline]
35. Meltzer SJ. The molecular biology of esophageal carcinoma. Recent Results Cancer Res 1996;142:1–8.[Medline]
36. Takamura H, Fushida S, Hashimoto T et al. Analysis of the p16INK4, p15INK4B genes abnormality and the amplification of cyclin D1 gene in esophageal cancer [article in Japanese]. Nippon Rinsho 1996;54:1043–1048.[Medline]
37. Cordon-Cardo C. Mutations of cell cycle regulators. Biological and clinical implications for human neoplasia. Am J Pathol 1995;147:545–560.[Abstract]
38. Porter PL, Malone KE, Heagerty PJ et al. Expression of cell-cycle regulators p27Kip1 and cyclin E, alone and in combination, correlate with survival in young breast cancer patients. Nat Med 1997;3:222–225.[CrossRef][Medline]
39. Catzavelos C, Bhattacharya N, Ung YC et al. Decreased levels of the cell-cycle inhibitor p27Kip1 protein: prognostic implications in primary breast cancer. Nat Med 1997;3:227–230.[CrossRef][Medline]
40. Catzavelos C, Tsao MS, DeBoer G et al. Reduced expression of the cell cycle inhibitor p27Kip1 in non-small cell lung carcinoma: a prognostic factor independent of Ras. Cancer Res 1999;59:684–688.[Abstract/Free Full Text]
41. Loda M, Cukor B, Tam SW et al. Increased proteasome-dependent degradation of the cyclin-dependent kinase inhibitor p27 in aggressive colorectal carcinomas. Nat Med 1997;3:231–234.[CrossRef][Medline]
42. Lee RJ, Albanese C, Stenger RJ et al. pp60(v-src) induction of cyclin D1 requires collaborative interactions between the extracellular signal-regulated kinase, p38, and Jun kinase pathways. A role for cAMP response element-binding protein and activating transcription factor-2 in pp60(v-src) signaling in breast cancer cells. J Biol Chem 1999;274:7341–7350.[Abstract/Free Full Text]
43. Albanese C, Johnson J, Watanabe G et al. Transforming p21ras mutants and c-Ets-2 activate the cyclin D1 promoter through distinguishable regions. J Biol Chem 1995;270:23589–23597.[Abstract/Free Full Text]
44. Lovec H, Grzeschiczek A, Kowalski MB et al. Cyclin D1/bcl-1 cooperates with myc genes in the generation of B-cell lymphoma in transgenic mice. EMBO J 1994;13:3487–3495.[Medline]
45. Daksis JI, Lu RY, Facchini LM et al. Myc induces cyclin D1 expression in the absence of de novo protein synthesis and links mitogen-stimulated signal transduction to the cell cycle. Oncogene 1994;9:3635–3645.[Medline]
46. Wang TC, Cardiff RD, Zukerberg L et al. Mammary hyperplasia and carcinoma in MMTV-cyclin D1 transgenic mice. Nature 1994;369:669–671.[CrossRef][Medline]
47. Bodrug SE, Warner BJ, Bath ML et al. Cyclin D1 transgene impedes lymphocyte maturation and collaborates in lymphomagenesis with the myc gene. EMBO J 1994;13:2124–2130.[Medline]
248. Michalides R, van Veelen N, Hart A et al. Overexpression of cyclin D1 correlates with recurrence in a group of forty-seven operable squamous cell carcinomas of the head and neck. Cancer Res 1995;55:975–978.[Abstract/Free Full Text]
49. Gansauge S, Gansauge F, Ramadani M et al. Overexpression of cyclin D1 in human pancreatic carcinoma is associated with poor prognosis. Cancer Res 1997;57:1634–1637.[Abstract/Free Full Text]
50. Kim HK, Park IA, Heo DS et al. Cyclin E overexpression as an independent risk factor of visceral relapse in breast cancer. Proc Am Soc Clin Oncol 2000;19:609a.
51. Tsihlias J, Kapusta L, Slingerland J. The prognostic significance of altered cyclin-dependent kinase inhibitors in human cancer. Annu Rev Med 1999;50:401–423.[CrossRef][Medline]
52. Sausville EA, Zaharevitz D, Gussio R et al. Cyclin-dependent kinases: initial approaches to exploit a novel therapeutic target. Pharmacol Ther 1999;82:285–292.[CrossRef][Medline]
53. Walker DH. Small-molecule inhibitors of cyclin-dependent kinases: molecular tools and potential therapeutics. Curr Top Microbiol Immunol 1998;227:149–165.[Medline]
54. Webster KR. Therapeutic potential of targeting the cell cycle. Chem Res Toxicol 2000;13:940–943.[CrossRef][Medline]
55. Meijer L, Kim SH. Chemical inhibitors of cyclin-dependent kinases. Methods Enzymol 1997;283:113–128.[Medline]
56. Garrett MD, Fattaey A. CDK inhibition and cancer therapy. Curr Opin Genet Dev 1999;9:104–111.[CrossRef][Medline]
57. Gray N, Detivaud L, Doerig C et al. ATP-site directed inhibitors of cyclin-dependent kinases. Curr Med Chem 1999;6:859–875.[Medline]
58. Sedlacek HH, Czech J, Naik R et al. Flavopiridol (L86-8275, NSC -649890), a new kinase inhibitor for tumor therapy. Int J Oncol 1996;9:1143–1168.
59. Kaur G, Stetler-Stevenson M, Sebers S et al. Growth inhibition with reversible cell cycle arrest of carcinoma cells by flavone L86-8275. J Natl Cancer Inst 1992;84:1736–1740.[Abstract/Free Full Text]
60. Arguello F, Alexander M, Sterry JA et al. Flavopiridol induces apoptosis of normal lymphoid cells, causes immunosuppression, and has potent antitumor activity in vivo against human leukemia and lymphoma xenografts. Blood 1998;91:2482–2490.[Abstract/Free Full Text]
61. Konig A, Schwartz GK, Mohammad RM et al. The novel cyclin-dependent kinase inhibitor flavopiridol downregulates Bcl-2 and induces growth arrest and apoptosis in chronic B-cell leukemia lines. Blood 1997;90:4307–4312.[Abstract/Free Full Text]
62. Parker BW, Kaur G, Nieves-Neira W et al. Early induction of apoptosis in hematopoietic cell lines after exposure to flavopiridol. Blood 1998;91:458–465.[Abstract/Free Full Text]
63. Bible KC, Kaufmann SH. Flavopiridol: a cytotoxic flavone that induces cell death in noncycling A549 human lung carcinoma cells. Cancer Res 1996;56:4856–4861.[Abstract/Free Full Text]
64. Carlson BA, Dubay MM, Sausville EA et al. Flavopiridol induces G₁ arrest with inhibition of cyclin-dependent kinase (CDK) 2 and CDK4 in human breast carcinoma cells. Cancer Res 1996;56:2973–2978.[Abstract/Free Full Text]
65. Losiewicz MD, Carlson BA, Kaur G et al. Potent inhibition of CDC2 kinase activity by the flavonoid L86-8275. Biochem Biophys Res Commun 1994;201:589–595.[CrossRef][Medline]
66. Worland PJ, Kaur G, Stetler-Stevenson M et al. Alteration of the phosphorylation state of p34cdc2 kinase by the flavone L86-8275 in breast carcinoma cells. Correlation with decreased H1 kinase activity. Biochem Pharmacol 1993;46:1831–1840.[CrossRef][Medline]
67. Carlson B, Lahusen T, Singh S et al. Down-regulation of cyclin D1 by transcriptional repression in MCF-7 human breast carcinoma cells induced by flavopiridol. Cancer Res 1999;59:4634–4641.[Abstract/Free Full Text]
68. Melillo G, Sausville EA, Cloud K et al. Flavopiridol, a protein kinase inhibitor, down-regulates hypoxic induction of vascular endothelial growth factor expression in human monocytes. Cancer Res 1999;59:5433–5437.[Abstract/Free Full Text]
69. Kerr JS, Wexler RS, Mousa SA et al. Novel small molecule alpha v integrin antagonists: comparative anti-cancer efficacy with known angiogenesis inhibitors. Anticancer Res 1999;19:959–968.[Medline]
70. Bible KC, Bible RH, Kottke TJ et al. Flavopiridol binds to duplex DNA. Cancer Res 2000;60:2419–2428.[Abstract/Free Full Text]
71. Bible KC, Kaufmann SH. Cytotoxic synergy between flavopiridol (NSC 649890, L86-8275) and various antineoplastic agents: the importance of sequence of administration. Cancer Res 1997;57:3375–3380.[Abstract/Free Full Text]
72. Chien M, Astumian M, Liebowitz D et al. In vitro evaluation of flavopiridol, a novel cell cycle inhibitor, in bladder cancer. Cancer Chemother Pharmacol 1999;44:81–87.[CrossRef][Medline]
73. Senderowicz AM, Headlee D, Stinson SF et al. Phase I trial of continuous infusion flavopiridol, a novel cyclin-dependent kinase inhibitor, in patients with refractory neoplasms. J Clin Oncol 1998;16:2986–2999.[Abstract/Free Full Text]
74. Stadler WM, Vogelzang NJ, Amato R et al. Flavopiridol, a novel cyclin-dependent kinase inhibitor, in metastatic renal cancer: a University of Chicago Phase II Consortium study. J Clin Oncol 2000;18:371–375.[Abstract/Free Full Text]
75. Schwartz GK, Ilson D, Saltz L et al. Phase II study of the cyclin-dependent kinase inhibitor flavopiridol administered to patients with advanced gastric carcinoma. J Clin Oncol 2001;19:1985–1992.[Abstract/Free Full Text]
76. Shapiro GI, Supko JG, Patterson A et al. A phase II trial of the cyclin-dependent kinase inhibitor flavopiridol in patients with previously untreated stage IV non-small cell lung cancer. Clin Cancer Res 2001;7:1590–1599.[Abstract/Free Full Text]
77. Innocenti F, Stadler WM, Iyer L et al. Flavopiridol metabolism in cancer patients is associated with the occurrence of diarrhea. Clin Cancer Res 2000;6:3400–3405.[Abstract/Free Full Text]
78. Patel V, Senderowicz AM, Pinto Jr D et al. Flavopiridol, a novel cyclin-dependent kinase inhibitor, suppresses the growth of head and neck squamous cell carcinomas by inducing apoptosis. J Clin Invest 1998;102:1674–1681.[Medline]
79. Senderowicz AM, Messmann R, Arbuck S et al. A phase I trial of 1 hour infusion of Flavopiridol (FLA), a novel cyclin-dependent kinase inhibitor, in patients with advanced neoplasms. Proc Am Soc Clin Oncol 2000;19:796a.
80. Thomas J, Tutsch K, Arzoomanian R et al. Phase I clinical and pharmacokinetic trial of the cyclin-dependent kinase (CDK) inhibitor Flavopiridol. Proc Am Soc Clin Oncol 1998;17:804a.
81. Bennett P, Mani S, O'Reilly S et al. Phase II trial of Flavopiridol in metastatic colorectal cancer: Preliminary results. Proc Am Soc Clin Oncol 1999;19:1065a.
82. Shah MA, Kaubisch A, O'Reilly E et al. A phase IB clinical trial of the sequence dependent combination of paclitaxel (P) and cisplatin (C) with cyclin dependent kinase (CDK) inhibitor flavopiridol in patients with advanced solid tumors. Proc Am Assoc Cancer Res 2001;42:2917a.
83. Jager W, Zembsch B, Wolschann P et al. Metabolism of the anticancer drug flavopiridol, a new inhibitor of cyclin dependent kinases, in rat liver. Life Sci 1998;62:1861–1873.[CrossRef][Medline]
84. Kahn ME, Senderowicz A, Sausville EA et al. Possible mechanisms of diarrheal side effects associated with the use of a novel chemotherapeutic agent, flavopiridol. Clin Cancer Res 2001;7:343–349.[Abstract/Free Full Text]
85. Schwartz GK, Jung C, Sirotnak F et al. The cyclin dependent kinase inhibitor flavopiridol improves the response to CPT-11 and demonstrates cures in colon cancer Xenografts. Proc Am Soc Clin Oncol 2001;20:348a.
86. Wright J, Blatner GL, Cheson BD. Clinical trials referral resource. Clinical trials of flavopiridol. Oncology (Huntingt) 1998;12:1018, 1023-1024.
87. Takahashi I, Asano K, Kawamoto I et al. UCN-01 and UCN-02, new selective inhibitors of protein kinase C. I. Screening, producing organism and fermentation. J Antibiot (Tokyo) 1989;42:564–570.[Medline]
88. Akinaga S, Gomi K, Morimoto M et al. Antitumor activity of UCN-01, a selective inhibitor of protein kinase C, in murine and human tumor models. Cancer Res 1991;51:4888–4892.[Abstract/Free Full Text]
89. Akiyama T, Yoshida T, Tsujita T et al. G₁ phase accumulation induced by UCN-01 is associated with dephosphorylation of Rb and CDK2 proteins as well as induction of CDK inhibitor p21/Cip1/WAF1/Sdi1 in p53-mutated human epidermoid carcinoma A431 cells. Cancer Res 1997;57:1495–1501.[Abstract/Free Full Text]
90. Senderowicz AM, Sausville EA. Preclinical and clinical development of cyclin-dependent kinase modulators. J Natl Cancer Inst 2000;92:376–387.[Abstract/Free Full Text]
91. Akiyama T, Shimizu M, Okabe M et al. Differential effects of UCN-01, staurosporine and CGP 41 251 on cell cycle progression and CDC2/cyclin B1 regulation in A431 cells synchronized at M phase by nocodazole. Anticancer Drugs 1999;10:67–78.[Medline]
92. Wang Q, Worland PJ, Clark JL et al. Apoptosis in 7-hydroxystaurosporine-treated T lymphoblasts correlates with activation of cyclin-dependent kinases 1 and 2. Cell Growth Differ 1995;6:927–936.[Abstract]
93. Graves PR, Yu L, Schwarz JK et al. The Chk1 protein kinase and the Cdc25C regulatory pathways are targets of the anticancer agent UCN-01. J Biol Chem 2000;275:5600–5605.[Abstract/Free Full Text]
94. Busby EC, Leistritz DF, Abraham RT et al. The radiosensitizing agent 7-hydroxystaurosporine (UCN-01) inhibits the DNA damage checkpoint kinase hChk1. Cancer Res 2000;60:2108–2112.[Abstract/Free Full Text]
95. Hsueh CT, Kelsen D, Schwartz GK. UCN-01 suppresses thymidylate synthase gene expression and enhances 5-fluorouracil-induced apoptosis in a sequence-dependent manner. Clin Cancer Res 1998;4:2201–2206.[Abstract]
96. Bunch RT, Eastman A. Enhancement of cisplatin-induced cytotoxicity by 7-hydroxystaurosporine (UCN-01), a new G₂-checkpoint inhibitor. Clin Cancer Res 1996;2:791–797.[Abstract]
97. Wang Q, Fan S, Eastman A et al. UCN-01: a potent abrogator of G₂ checkpoint function in cancer cells with disrupted p53. J Natl Cancer Inst 1996;88:956–965.[Abstract/Free Full Text]
98. Hsueh CT, Chiu CF, Schwartz GK. UCN-01 suppresses E2F-1 mediated by ubiquitin-proteasome-dependent degradation. Proc Am Soc Clin Oncol 2000;19:798a.
99. Seynaeve CM, Stetler-Stevenson M, Sebers S et al. Cell cycle arrest and growth inhibition by the protein kinase antagonist UCN-01 in human breast carcinoma cells. Cancer Res 1993;53:2081–2086.[Abstract/Free Full Text]
100. Sausville EA, Lush RD, Headlee D et al. Clinical pharmacology of UCN-01: initial observations and comparison to preclinical models. Cancer Chemother Pharmacol 1998;42(suppl):S54–S59.
101. Sausville EA, Arbuck SG, Messmann R et al. Phase I trial of 72-hour continuous infusion UCN-01 in patients with refractory neoplasms. J Clin Oncol 2001;19:2319–2333.[Abstract/Free Full Text]
102. Fuse E, Tanii H, Kurata N et al. Unpredicted clinical pharmacology of UCN-01 caused by specific binding to human alpha1-acid glycoprotein. Cancer Res 1998;58:3248–3253.[Abstract/Free Full Text]
103. Propper DJ, McDonald AC, Man A et al. Phase I and pharmacokinetic study of PKC412, an inhibitor of protein kinase C. J Clin Oncol 2001;19:1485–1492.[Abstract/Free Full Text]
104. Shah MA, Kemeny N, Kelsen D et al. A phase I/pharmacologic study of UCN-01 (UCN) in combination with 5-fluorouracil (F) in patients with advanced solid tumors. Proc Am Soc Clin Oncol 2001;20:3135a.
105. Dees EC, O'Reilly S, Figg WD et al. A phase I and pharmacologic study of UCN-01. A protein kinase C inhibitor. Proc Am Soc Clin Oncol 2000;19:797a.
106. Wilson WH, Sorbara L, Figg WD et al. Modulation of clinical drug resistance in a B cell lymphoma patient by the protein kinase inhibitor 7-hydroxystaurosporine: presentation of a novel therapeutic paradigm. Clin Cancer Res 2000;6:415–421.[Abstract/Free Full Text]
107. Wilson WH, Gutierrez M, Stetler-Stevenson M et al. Phase I trial of 7-hydroxystaurosporine (UCN-01) and Fludarabine phosphate (FAMP); In vivo evidence of UCN-01 induced apoptosis in CLL. Proc Am Soc Hematol 2000;96:3268a.
108. Nishimura H, Simpson IA. Staurosporine inhibits phorbol 12-myristate 13-acetate- and insulin-stimulated translocation of GLUT1 and GLUT4 glucose transporters in rat adipose cells. Biochem J 1994;302:271–277.
109. Druker BJ, Lydon NB. Lessons learned from the development of an abl tyrosine kinase inhibitor for chronic myelogenous leukemia. J Clin Invest 2000;105:3–7.[Medline]
110. Schindler T, Bornmann W, Pellicena P et al. Structural mechanism for STI-571 inhibition of abelson tyrosine kinase. Science 2000;289:1938–1942.[Abstract/Free Full Text]
111. Svingen PA, Tefferi A, Kottke TJ et al. Effects of the bcr/abl kinase inhibitors AG957 and NSC 680410 on chronic myelogenous leukemia cells in vitro. Clin Cancer Res 2000;6:237–249.[Abstract/Free Full Text]
112. le Coutre P, Mologni L, Cleris L et al. In vivo eradication of human BCR/ABL-positive leukemia cells with an ABL kinase inhibitor. J Natl Cancer Inst 1999;91:163–168.[Abstract/Free Full Text]
113. Kilic T, Alberta JA, Zdunek PR et al. Intracranial inhibition of platelet-derived growth factor-mediated glioblastoma cell growth by an orally active kinase inhibitor of the 2-phenylaminopyrimidine class. Cancer Res 2000;60:5143–5150.[Abstract/Free Full Text]
114. Krystal GW, Honsawek S, Litz J et al. The selective tyrosine kinase inhibitor STI571 inhibits small cell lung cancer growth. Clin Cancer Res 2000;6:3319–3326.[Abstract/Free Full Text]
115. Kano Y, Akutsu M, Tsunoda S et al. In vitro cytotoxic effects of a tyrosine kinase inhibitor STI571 in combination with commonly used antileukemic agents. Blood 2001;97:1999–2007.[Abstract/Free Full Text]
116. Thiesing JT, Ohno-Jones S, Kolibaba KS et al. Efficacy of STI571, an abl tyrosine kinase inhibitor, in conjunction with other antileukemic agents against bcr-abl-positive cells. Blood 2000;96:3195–3199.[Abstract/Free Full Text]
117. Fang G, Kim CN, Perkins CL et al. CGP57148B (STI-571) induces differentiation and apoptosis and sensitizes Bcr-Abl-positive human leukemia cells to apoptosis due to antileukemic drugs. Blood 2000;96:2246–2253.[Abstract/Free Full Text]
118. Druker BJ, Talpaz M, Resta DJ et al. Efficacy and safety of a specific inhibitor of the BCR-ABL tyrosine kinase in chronic myeloid leukemia. N Engl J Med 2001;344:1031–1037.[Abstract/Free Full Text]
119. Druker BJ, Sawyers CL, Kantarjian H et al. Activity of a specific inhibitor of the BCR-ABL tyrosine kinase in the blast crisis of chronic myeloid leukemia and acute lymphoblastic leukemia with the Philadelphia chromosome. N Engl J Med 2001;344:1038–1042.[Abstract/Free Full Text]
120. Mauro MJ, Druker BJ. STI571: targeting BCR-ABL as therapy for CML. The Oncologist 2001;6:233–238.[Abstract/Free Full Text]
121. Joensuu H, Roberts PJ, Sarlomo-Rikala M et al. Effect of the tyrosine kinase inhibitor STI571 in a patient with a metastatic gastrointestinal stromal tumor. N Engl J Med 2001;344:1052–1056.[Free Full Text]
122. Van Oosterom AT, Judson I, Verweij J et al. STI 571, an active drug in metastatic gastro intestinal stromal tumors (GIST), an EORTC phase I study. Proc Am Soc Clin Oncol 2001;20:2a.
123. Salomon DS, Brandt R, Ciardiello F et al. Epidermal growth factor-related peptides and their receptors in human malignancies. Crit Rev Oncol Hematol 1995;19:183–232.[Medline]
124. Gullick WJ. Prevalence of aberrant expression of the epidermal growth factor receptor in human cancers. Br Med Bull 1991;47:87–98.[Abstract/Free Full Text]
125. Neal DE, Marsh C, Bennett MK et al. Epidermal-growth-factor receptors in human bladder cancer: comparison of invasive and superficial tumours. Lancet 1985;1:366–368.[CrossRef][Medline]
126. Ke LD, Adler-Storthz K, Clayman GL et al. Differential expression of epidermal growth factor receptor in human head and neck cancers. Head Neck 1998;20:320–327.[CrossRef][Medline]
127. Baselga J, Pfister D, Cooper MR et al. Phase I studies of anti-epidermal growth factor receptor chimeric antibody C225 alone and in combination with cisplatin. J Clin Oncol 2000;18:904–914.[Abstract/Free Full Text]
128. Sirotnak FM, Zakowski MF, Miller VA et al. Efficacy of cytotoxic agents against human tumor xenografts is markedly enhanced by coadministration of ZD1839 (Iressa), an inhibitor of EGFR tyrosine kinase. Clin Cancer Res 2000;6:4885–4892.[Abstract/Free Full Text]
129. Ciardiello F, Caputo R, Bianco R et al. Antitumor effect and potentiation of cytotoxic drugs activity in human cancer cells by ZD-1839 (Iressa), an epidermal growth factor receptor-selective tyrosine kinase inhibitor. Clin Cancer Res 2000;6:2053–2063.[Abstract/Free Full Text]
130. Williams KJ, Telfer BA, Stratford IA et al. Combination of ZD1839 ("Iressa"), an EGFR-TKI, and radiotherapy increases antitumor efficacy in a human colon cancer xenograft model. Proc Am Assoc Cancer Res 2001;42:3840a.
131. Al Hazaa A, Birchall MA, Bowen ID. ZD 1839 (Iressa), an EGFR-TK1, and cisplatin have an additive effect on programmed cell death in human head and neck squamous carcinoma cells in vitro. Clin Cancer Res 2000;6:4542a.
132. Ohmori T, Ao Y, Nishio K et al. Low dose cisplatin can modulate the sensitivity of human non-small cell lung carcinoma cells to EGFR tyrosine kinase inhibitor (ZD839; Iressa) in vivo. Proc Am Assoc Cancer Res 2000;41:482a.
133. Ozawa S, Ueda M, Ando N et al. Antiproliferative effects of ZD1839 (‘Iressa’) on esophageal squamous carcinoma cells. Proc Am Assoc Cancer Res 2001;42:8a.
134. Ferry D, Hammond L, Ranson M et al. Intermittent oral ZD1839 (Iressa), a novel epidermal growth factor receptor tyrosine kinase inhibitor (EGFR-TKI), shows evidence of good tolerability and activity: final results from phase I study. Proc Am Soc Clin Oncol 2000;19:5Ea.
135. Negoro S, Nakagawa K, Fukuoka M et al. Final results of a phase I intermittent dose-escalation trial of ZD1839 (‘Iressa’) in Japanese patients with various solid tumors. Proc Am Soc Clin Oncol 2001;20:1292a.
136. Baselga J, Herbst R, LoRusso P et al. Continuous administration of ZD1839 (Iressa), a novel oral epidermal growth factor receptor tyrosine kinase inhibitor (EGFR-TKI) in patients with five selected tumor types: evidence of activity and good tolerability. Proc Am Soc Clin Oncol 2000;19:686a.
137. Miller VA, Johnson D, Heelan RT et al. A pilot trial demonstrates the safety of ZD1839 (‘Iressa’), an oral epidermal growth factor receptor tyrosine kinase inhibitor (EGFR-TKI), in combination with carboplatin (C) and paclitaxel (P) in previously untreated advanced non-small cell lung cancer (NSCLC). Proc Am Soc Clin Oncol 2001;20:1301a.
138. Moulder SL, Yakes M, Bianco R et al. Small molecule EGF receptor tyrosine kinase inhibitor ZD1839 (IRESSA) inhibits HER2/Neu (erb-2) overexpressing breast tumor cells. Proc Am Soc Clin Oncol 2001;20:8a.
139. Gee JM, Hutcheson IR, Knowlden JM et al. The EGFR-selective tyrosine kinase inhibitor ZD1839 (‘Iressa’) is an effective inhibitor of tamoxifen-resistant breast cancer growth. Proc Am Soc Clin Oncol 2001;20:282a.
140. Moyer JD, Barbacci EG, Iwata KK et al. Induction of apoptosis and cell cycle arrest by CP-358,774, an inhibitor of epidermal growth factor receptor tyrosine kinase. Cancer Res 1997;57:4838–4848.[Abstract/Free Full Text]
141. Pollack VA, Savage DM, Baker DA et al. Inhibition of epidermal growth factor receptor-associated tyrosine phosphorylation in human carcinomas with CP-358,774: dynamics of receptor inhibition in situ and antitumor effects in athymic mice. J Pharmacol Exp Ther 1999;291:739–748.[Abstract/Free Full Text]
142. Hidalgo M, Malik S, Rowinsky E et al. Inhibition of epidermal growth factor receptor (EGFR) by OSI-774, a specific EGFR inhibitor in malignant and normal tissues of cancer patients. Proc Am Soc Clin Oncol 2001;20:281a.
143. Hidalgo M, Siu LL, Nemunaitis J et al. Phase I and pharmacologic study of OSI-774, an epidermal growth factor receptor tyrosine kinase inhibitor, in patients with advanced solid malignancies. J Clin Oncol 2001;19:3267–3279.[Abstract/Free Full Text]
144. Karp DD, Silberman SL, Csudae R et al. Phase I dose escalation study of epidermal growth factor receptor (EGFR) tyrosine kinase (TK) inhibitor CP-358,774 in patients with advanced solid tumors. Proc Am Soc Clin Oncol 1999;18:1499a.
145. Siu LL, Hidalgo M, Nemunaitis J et al. Dose and schedule-duration escalation of the epidermal growth factor receptor (EGFR) tyrosine kinase (TK) inhibitor CP-358,774: a phase I and pharmacokinetic (PK) study. Proc Am Soc Clin Oncol 1999;18:1498a.
146. Rowinsky EK, Hammond L, Siu L et al. Dose-schedule-finding, pharmacokinetic (PK), biologic and functional imaging studies of OSI-774, a selective epidermal growth factor receptor (EGFR) tyrosine kinase (TK) inhibitor. Proc Am Soc Clin Oncol 2001;20:5a.
147. Perez-Soler R, Chachoua A, Huberman M et al. A phase II trial of the epidermal growth factor receptor (EGFR) tyrosine kinase inhibitor OSI-774, following platinum-based chemotherapy, in patients (pts) with advanced, EGFR-expressing, non-small cell lung cancer (NSCLC). Proc Am Soc Clin Oncol 2001;20:1235a.
148. Senzer NN, Soulieres D, Siu L et al. Phase 2 evaluation of OSI-774, a potent oral antagonist of EGFR-TK in patients with advanced squamous cell carcinoma of the head and neck. Proc Am Soc Clin Oncol 2001;20:6a.
149. Finkler N, Gordon A, Crozier M et al. Phase 2 evaluation of OSI-774, a potent oral antagonist of EGFR-TK in patients with advanced ovarian carcinoma. Proc Am Soc Clin Oncol 2001;20:831a.
150. Hilt W, Wolf DH. Proteasomes: destruction as a programme. Trends Biochem Sci 1996;21:96–102.[CrossRef][Medline]
151. Palombella VJ, Rando OJ, Goldberg AL et al. The ubiquitin-proteasome pathway is required for processing the NF- kappa B1 precursor protein and the activation of NF-kappa B. Cell 1994;78:773–785.[CrossRef][Medline]
152. Adams J, Palombella VJ, Sausville EA et al. Proteasome inhibitors: a novel class of potent and effective antitumor agents. Cancer Res 1999;59:2615–2622.[Abstract/Free Full Text]
153. Teicher BA, Ara G, Herbst R et al. The proteasome inhibitor PS-341 in cancer therapy. Clin Cancer Res 1999;5:2638–2645.[Abstract/Free Full Text]
154. Perez-Soler R, Ling Y-H, Mendoza S et al. Effect of proteasome inhibitor PS341 on cell cycle progression and cell cycle-related events: implication for combination therapy with cell cycle dependent agents. Proc Am Soc Clin Oncol 2000;19:740a.
155. An WG, Hwang SG, Trepel JB et al. Protease inhibitor-induced apoptosis: accumulation of wt p53, p21WAF1/CIP1, and induction of apoptosis are independent markers of proteasome inhibition. Leukemia 2000;14:1276–1283.[CrossRef][Medline]
156. Erlichman C, Adjei AA, Thomas JP et al. A phase I trial of the proteasome inhibitor PS-341 in patients with advanced cancer. Proc Am Soc Clin Oncol 2001;20:337a.
157. Sunwoo JB, Chen Z, Dong G et al. Novel proteasome inhibitor PS-341 inhibits activation of nuclear factor-kappaB, cell survival, tumor growth, and angiogenesis in squamous cell carcinoma. Clin Cancer Res 2001;7:1419–1428.[Abstract/Free Full Text]
158. Logothetis CJ, Yang H, Daliani D et al. Dose-dependent inhibition of 20S proteasome results in serum Il-6 and PSA decline in patients (PTS) with androgen-independent prostate cancer (AIPCa) treated with the proteasome inhibitor PS-341. Proc Am Soc Clin Oncol 2001;20:740a.
159. Williams SA, Papandreou C, McConkey D. Preclinical effects of proteasome inhibitor PS-341 in combination chemotherapy for prostate cancer. Proc Am Soc Clin Oncol 2001;20:2427a.
160. Lightcap ES, McCormack TA, Pien CS et al. Proteasome inhibition measurements: clinical application. Clin Chem 2000;46:673–683.[Abstract/Free Full Text]
161. Aghajanian C, Soignet S, Dizon DS et al. A phase I trial of the novel proteasome inhibitor PS341 in advanced solid tumor malignancies. Proc Am Soc Clin Oncol 2001;20:338a.
162. Hamilton AL, Eder JP, Pavlick AC et al. PS-341: phase I study of a novel proteasome inhibitor with pharmacodynamic endpoints. Proc Am Soc Clin Oncol 2001;20:336a.
163. Wright J, Hillsamer VL, Gore-Langton RE et al. Clinical trials referral resource. Current clinical trials for the proteasome inhibitor PS-341. Oncology (Huntingt) 2000;14:1589-1590, 1593-1594, 1597.[Medline]
164. Struhl K. Histone acetylation and transcriptional regulatory mechanisms. Genes Dev 1998;12:599–606.[Free Full Text]
165. Wolffe AP. Histone deacetylase: a regulator of transcription. Science 1996;272:371–372.[CrossRef][Medline]
166. Luo RX, Postigo AA, Dean DC. Rb interacts with histone deacetylase to repress transcription. Cell 1998;92:463–473.[CrossRef][Medline]
167. Ueda H, Manda T, Matsumoto S et al. FR901228, a novel antitumor bicyclic depsipeptide produced by Chromobacterium violaceum No. 968. III. Antitumor activities on experimental tumors in mice. J Antibiot (Tokyo) 1994;47:315–323.[Medline]
168. Sandor V, Senderowicz A, Mertins S et al. P21-dependent g(1)arrest with downregulation of cyclin D1 and upregulation of cyclin E by the histone deacetylase inhibitor FR901228. Br J Cancer 2000;83:817–825.[CrossRef][Medline]
169. Byrd JC, Shinn C, Ravi R et al. Depsipeptide (FR901228): a novel therapeutic agent with selective, in vitro activity against human B-cell chronic lymphocytic leukemia cells. Blood 1999;94:1401–1408.[Abstract/Free Full Text]
170. Bates SE, Sandor V, Bakke S et al. A phase I study of FR901228 (depsipeptide), a histone deacetylase inhibitor. Proc Am Soc Clin Oncol 1999;18:693a.
171. Piekarz R, Robey R, Bakke S et al. Histone deacetylase inhibitor for the treatment of peripheral or cutaneous T-cell lymphoma. Proc Am Soc Clin Oncol 2001;20:2680a.
172. Saito A, Yamashita T, Mariko Y et al. A synthetic inhibitor of histone deacetylase, MS-27-275, with marked in vivo antitumor activity against human tumors. Proc Natl Acad Sci USA 1999;96:4592–4597.[Abstract/Free Full Text]
173. Lee BI, Park SH, Kim JW et al. MS-275, a histone deacetylase inhibitor, selectively induces transforming growth factor beta type II receptor expression in human breast cancer cells. Cancer Res 2001;61:931–934.[Abstract/Free Full Text]
174. Yamaki H, Nakajima M, Seimiya H et al. Inhibition of the association with nuclear matrix of pRB, p70 and p40 proteins along with the specific suppression of c-MYC expression by geldanamycin, an inhibitor of Src tyrosine kinase. J Antibiot (Tokyo) 1995;48:1021–1026.[Medline]
175. Murakami Y, Mizuno S, Hori M et al. Reversal of transformed phenotypes by herbimycin A in src oncogene expressed rat fibroblasts. Cancer Res 1988;48:1587–1590.[Abstract/Free Full Text]
176. Stebbins CE, Russo AA, Schneider C et al. Crystal structure of an Hsp90-geldanamycin complex: targeting of a protein chaperone by an antitumor agent. Cell 1997;89:239–250.[CrossRef][Medline]
177. Zheng FF, Kuduk SD, Chiosis G et al. Identification of a geldanamycin dimer that induces the selective degradation of HER-family tyrosine kinases. Cancer Res 2000;60:2090–2094.[Abstract/Free Full Text]
178. Neckers L, Schulte TW, Mimnaugh E. Geldanamycin as a potential anti-cancer agent: its molecular target and biochemical activity. Invest New Drugs 1999;17:361–373.[CrossRef][Medline]
179. Wilson RH, Takimoto C, Agnew EB et al. Phase I pharmacologic study of 17-(Allylamino)-17-Demethoxygeldanamycin (AAG) in adult patients with advanced solid tumors. Proc Am Soc Clin Oncol 2001;20:325a.
180. Munster PN, Tong W, Schwartz L et al. Phase trial of 17-(allylamino)-17-Demethoxygeldanamycin (17-AAG) in patients (Pts) with advanced solid malignancies. Proc Am Soc Clin Oncol 2001;20:327a.
181. Mandler R, Wu C, Sausville EA et al. Immunoconjugates of geldanamycin and anti-HER2 monoclonal antibodies: antiproliferative activity on human breast carcinoma cell lines. J Natl Cancer Inst 2000;92:1573–1581.[Abstract/Free Full Text]
182. Sekulic A, Hudson CC, Homme JL et al. A direct linkage between the phosphoinositide 3-kinase-AKT signaling pathway and the mammalian target of rapamycin in mitogen-stimulated and transformed cells. Cancer Res 2000;60:3504–3513.[Abstract/Free Full Text]
183. Gingras AC, Kennedy SG, O'Leary MA et al. 4E-BP1, a repressor of mRNA translation, is phosphorylated and inactivated by the Akt(PKB) signaling pathway. Genes Dev 1998;12:502–513.[Abstract/Free Full Text]
184. Hashemolhosseini S, Nagamine Y, Morley SJ et al. Rapamycin inhibition of the G₁ to S transition is mediated by effects on cyclin D1 mRNA and protein stability. J Biol Chem 1998;273:14424–14429.[Abstract/Free Full Text]
185. Kawamata S, Sakaida H, Hori T et al. The upregulation of p27Kip1 by rapamycin results in G₁ arrest in exponentially growing T-cell lines. Blood 1998;91:561–569.[Abstract/Free Full Text]
186. Eng CP, Sehgal SN, Vezina C. Activity of rapamycin (AY-22,989) against transplanted tumors. J Antibiot (Tokyo) 1984;37:1231–1237.[Medline]
187. Hidalgo M, Rowinsky E, Erlichman C et al. CCI-779, a rapamycin analog and multifaceted inhibitor of signal transduction: a phase I study. Proc Am Soc Clin Oncol 2000;19:726a.
188. Raymond E, Alexandre J, Depenbrock H et al. CCI-779, a rapamycin analog with antitumor activity: a phase I study utilizing a weekly schedule. Proc Am Soc Clin Oncol 2000;19:728a.
189. Price P. Changes in 18F-FDG uptake measured by PET as a pharmacodynamic end-point in anticancer therapy. How far have we got? Br J Cancer 2000;83:281–283.[CrossRef][Medline]

This article has been cited by other articles:

				J. P. Jani, R. S. Finn, M. Campbell, K. G. Coleman, R. D. Connell, N. Currier, E. O. Emerson, E. Floyd, S. Harriman, J. C. Kath, et al. Discovery and Pharmacologic Characterization of CP-724,714, a Selective ErbB2 Tyrosine Kinase Inhibitor Cancer Res., October 15, 2007; 67(20): 9887 - 9893. [Abstract] [Full Text] [PDF]

				S. Roy, M. Kaur, C. Agarwal, M. Tecklenburg, R. A. Sclafani, and R. Agarwal p21 and p27 induction by silibinin is essential for its cell cycle arrest effect in prostate carcinoma cells Mol. Cancer Ther., October 1, 2007; 6(10): 2696 - 2707. [Abstract] [Full Text] [PDF]

				C. J. Zheng, L. Y. Han, C. W. Yap, Z. L. Ji, Z. W. Cao, and Y. Z. Chen Therapeutic targets: progress of their exploration and investigation of their characteristics. Pharmacol. Rev., June 1, 2006; 58(2): 259 - 279. [Abstract] [Full Text] [PDF]

				C. Richard, D. Matthews, W. Duivenvoorden, J. Yau, P. S. Wright, and J. P.H. Th'ng Flavopiridol Sensitivity of Cancer Cells Isolated from Ascites and Pleural Fluids Clin. Cancer Res., May 1, 2005; 11(9): 3523 - 3529. [Abstract] [Full Text] [PDF]

				M. Michael and M.M. Doherty Tumoral Drug Metabolism: Overview and Its Implications for Cancer Therapy J. Clin. Oncol., January 1, 2005; 23(1): 205 - 229. [Abstract] [Full Text] [PDF]

				M. L. Janmaat and G. Giaccone Small-Molecule Epidermal Growth Factor Receptor Tyrosine Kinase Inhibitors Oncologist, December 1, 2003; 8(6): 576 - 586. [Abstract] [Full Text] [PDF]

				E. K. Rowinsky Challenges of Developing Therapeutics That Target Signal Transduction in Patients With Gynecologic and Other Malignancies J. Clin. Oncol., May 15, 2003; 21(90100): 175s - 186. [Abstract] [Full Text] [PDF]

				R. Nahta, G. N. Hortobagyi, and F. J. Esteva Growth Factor Receptors in Breast Cancer: Potential for Therapeutic Intervention Oncologist, February 1, 2003; 8(1): 5 - 17. [Abstract] [Full Text] [PDF]

				D. B. Mendel, A. D. Laird, X. Xin, S. G. Louie, J. G. Christensen, G. Li, R. E. Schreck, T. J. Abrams, T. J. Ngai, L. B. Lee, et al. In Vivo Antitumor Activity of SU11248, a Novel Tyrosine Kinase Inhibitor Targeting Vascular Endothelial Growth Factor and Platelet-derived Growth Factor Receptors: Determination of a Pharmacokinetic/Pharmacodynamic Relationship Clin. Cancer Res., January 1, 2003; 9(1): 327 - 337. [Abstract] [Full Text] [PDF]

				F. Ciardiello and G. Tortora Inhibition of bcl-2 as cancer therapy Ann. Onc., April 1, 2002; 13(4): 501 - 502. [Full Text] [PDF]

This Article Abstract Full Text (PDF) eLetters: Submit a response to this article Purchase Article View Shopping Cart Alert me when this article is cited Alert me when eLetters are posted Alert me if a correction is posted Citation Map Services E-mail this article link to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Reprints/Permissions Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Elsayed, Y. A. Articles by Sausville, E. A. Search for Related Content PubMed PubMed Citation Articles by Elsayed, Y. A. Articles by Sausville, E. A.