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Clinical Pharmacology

Prolonged Versus Standard Gemcitabine Infusion: Translation of Molecular Pharmacology to New Treatment Strategy

^aDivision of Experimental Therapy and ^bDepartment of Clinical Pharmacology, The Netherlands Cancer Institute/Antoni van Leeuwenhoek Hospital, Amsterdam, The Netherlands; ^cDepartment of Pharmacy & Pharmacology, Slotervaart Hospital, Amsterdam, The Netherlands; ^dFaculty of Science, Department of Pharmaceutical Sciences, Section of Biomedical Analysis, Division of Drug Toxicology, Utrecht University, Utrecht, The Netherlands

Key Words. Gemcitabine • Pharmacology • Fixed dose rate • Clinical activity

Correspondence: Stephan A. Veltkamp, Pharm.D., Division of Experimental Therapy, NKI-AvL, Postbus 90203, 1006 BE, Amsterdam, The Netherlands. Telephone: 31-20-512-2047; Fax: 31-20-512-2050; e-mail: s.veltkamp{at}nki.nl

Received October 31, 2007; accepted for publication January 28, 2008.

Disclosure: No potential conflicts of interest were reported by the authors, planners, reviewers, or staff managers of this article.

	Learning Objectives

Top Footnotes Learning Objectives Abstract Introduction Molecular Pharmacology of... Mechanisms of Action of... Preclinical Pharmacology of... Clinical Pharmacology of... Phase I/II Dose-Finding Studies... Phase II/III Randomized Trials... Discussion and Future... References

 
After completing this course, the reader will be able to:

1. Describe the molecular pharmacology of nucleoside analogues.
   
2. Explain transport, metabolism, and elimination in relation to the activity of gemcitabine.
   
3. Describe the clinical pharmacology of gemcitabine in relation to its rate of administration.
   

	ABSTRACT

Top Footnotes Learning Objectives Abstract Introduction Molecular Pharmacology of... Mechanisms of Action of... Preclinical Pharmacology of... Clinical Pharmacology of... Phase I/II Dose-Finding Studies... Phase II/III Randomized Trials... Discussion and Future... References

 
Gemcitabine is frequently used in the treatment of patients with solid tumors. Gemcitabine is taken up into the cell via human nucleoside transporters (hNTs) and is intracellularly phosphorylated by deoxycytidine kinase (dCK) to its monophosphate and subsequently into its main active triphosphate metabolite 2',2'-difluorodeoxycytidine triphosphate (dFdCTP), which is incorporated into DNA and inhibits DNA synthesis. In addition, gemcitabine is extensively deaminated to 2',2'-difluorodeoxyuridine, which is largely excreted into the urine. High expression levels of human equilibrative nucleoside transporter type 1 were associated with a significantly longer overall survival duration after gemcitabine treatment in patients with pancreatic cancer.

Clinical studies in blood mononuclear and leukemic cells demonstrated that a lower infusion rate of gemcitabine was associated with higher intracellular dFdCTP levels. Prolonged infusion of gemcitabine at a fixed dose rate (FDR) of 10 mg/m² per minute was associated with a higher intracellular accumulation of dFdCTP, greater toxicity, and a higher response rate than with the standard 30-minute infusion of gemcitabine in patients with pancreatic cancer.

In the current review, we discuss the molecular pharmacology of nucleoside analogues and the influence of hNTs and dCK on the activity and toxicity of gemcitabine, which is the basis for clinical studies on FDR administration, and the results of FDR gemcitabine administration in patients. These findings might aid optimal clinical application of gemcitabine in the future.

	INTRODUCTION

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Nucleoside anticancer drugs constitute an important class of antimetabolites that are used in the treatment of patients with cancer. The pyrimidine nucleoside analogues cytarabine (cytosine arabinoside [ara-C]) and gemcitabine (2',2'-difluorodeoxycytidine [dFdC]) are frequently used in the treatment of patients with solid tumors and hematological malignancies [1–4]. The chemical structures of gemcitabine and cytarabine are depicted in Figure 1.

View larger version (17K): [in this window] [in a new window]   Figure 1. Chemical structure of the pyrimidine nucleosides gemcitabine (A) and cytarabine (B).

In the current review, we discuss the molecular pharmacology of nucleoside analogues and the influence of human nucleoside transporters (hNTs) and dCK on the activity and toxicity of gemcitabine that is the basis for preclinical and clinical studies on FDR dosing compared with standard gemcitabine administration, of which trial results are summarized. These findings can be relevant for the optimal use of gemcitabine in patients in the future.

	MOLECULAR PHARMACOLOGY OF NUCLEOSIDE ANTICANCER DRUGS

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Uptake by hNTs
The nucleoside anticancer drugs gemcitabine and cytarabine are hydrophilic compounds. Their uptake into cells is largely dependent on the activity of human equilibrative nucleoside transporters (hENTs) and human concentrative nucleoside transporters (hCNTs) [15–18]. The hENTs are capable of transporting pyrimidine and purine nucleosides from outside to inside the cell and vice versa, and are widely distributed in most human cells [19]. The hCNTs transport substrates, such as pyrimidine and purine nucleosides, only from outside to inside the cell, and are highly expressed in liver [20], kidney [21, 22], and intestine [23, 24]. hCNTs generally have a higher affinity for transport of nucleosides than hENTs (e.g., hCNT1 has a tenfold higher affinity for gemcitabine than hENT1).

Metabolism: Activation and Deactivation
Gemcitabine and cytarabine are phosphorylated by dCK into their corresponding monophosphate forms, which is the rate-limiting step in the formation of their active triphosphate metabolites, which are incorporated into DNA, thereby inhibiting DNA synthesis and DNA repair. Gemcitabine and cytarabine are extensively deaminated by cytidine deaminase (CDA) [7, 25] in liver, kidney, and plasma to their corresponding uracil metabolites 2',2'-difluorodeoxyuridine (dFdU) and arabinosyluracil, respectively. These metabolites have a much lower cytotoxicity than their parent compounds. Because of this rapid deamination, the elimination half-life (t_1/2) of gemcitabine and cytarabine is short, in the range of 10–30 minutes.

	MECHANISMS OF ACTION OF GEMCITABINE

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The chemical structures of dFdC, dFdU, and dFdCTP and the metabolic scheme and proposed pharmacological actions of gemcitabine and its metabolites are depicted in Figure 2. dFdCTP, the main active metabolite of gemcitabine, competes with deoxycytidine triphosphate (dCTP) for incorporation into DNA. Once dFdCTP is incorporated, two phosphate molecules are split off, leaving dFdC monophosphate (dFdCMP) in the DNA chain. Once dFdCTP is incorporated into DNA, only one more natural deoxynucleoside triphosphate can be incorporated, after which DNA replication terminates. dFdCMP is resistant to removal from the DNA strand by proofreading enzymes (i.e., polymerase ), thus impairing its ability to repair the DNA strand, a mechanism also known as "masked-chain termination." In addition, gemcitabine diphosphate (dFdCDP) inhibits ribonucleotide reductase [26], thereby depleting dCTP pools and facilitating incorporation of dFdCTP into DNA. As depicted in Figure 2, gemcitabine has various self-potentiating mechanisms of action contributing to the maintenance of dFdCDP and dFdCTP levels for prolonged periods of time.

View larger version (27K): [in this window] [in a new window]   Figure 2. Chemical structures of dFdC, dFdU, and dFdCTP and metabolic scheme and proposed pharmacological mechanisms of action of gemcitabine and its metabolites. dFdC is transported by hNTs into the cell. dFdC is intracellularly phosphorylated by dCK to its monophosphate dFdCMP, and subsequently into its active dFdCDP and dFdCTP metabolites. dFdCTP is incorporated into DNA, thereby competing with dCTP for incorporation. dFdCDP inhibits RR, which inhibits the conversion of CDP to dCDP and inhibits synthesis of dCTP, leading to an elevation in the intracellular dFdCTP/dCTP ratio and enhanced incorporation of dFdCTP into DNA. Since dCTP inhibits dCK, decreased dCTP pools can result in higher dCK activity and an increase in phosphorylation of dFdC to its active metabolites. dFdC is deaminated by CDA to dFdU. dFdCMP can be dephosphorylated to dFdC by 5'-NT and deaminated to dFdUMP by dCMPD. dFdCTP can inhibit dCMPD, which stimulates its own formation. Abbreviations: 5'-NT, 5'-nucleotidase; CDA, cytidine deaminase; CDP, cytidine diphosphate; dCDP, deoxycytidine diphosphate; dCK, deoxycytidine kinase; dCMPD, deoxycytidylate deaminase; dCTP, deoxycytidine triphosphate; dFdC, 2',2'-difluorodeoxycytidine; dFdCDP, dFdC diphosphate; dFdCMP, dFdC monophosphate; dFdCTP, dFdC triphosphate; dFdU, 2',2'-difluorodeoxyuridine; dFdUMP, 2',2'-difluorodeoxyuridine monophosphate; hNTs, human nucleoside transporters; RR, ribonucleotide reductase.

	PRECLINICAL PHARMACOLOGY OF GEMCITABINE

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The intracellular accumulation of active metabolites and cytotoxicity of gemcitabine are influenced by multiple factors, such as (a) the dosing schedule, (b) phosphorylation/activation (dCK), (c) cellular transport (hNTs), (d) degradation/inactivation (CDA), and (e) genetic factors (e.g., single nucleotide polymorphisms in dCK and CDA) [29–32]. In this review, we discuss the effect of the dosing schedule and the role of dCK and hNTs on the activity and toxicity of gemcitabine.

The Importance of dCK
Phosphorylation of gemcitabine to dFdCMP by dCK was shown to be the rate-limiting step in the formation of dFdCTP [33, 34]. The affinity of dCK is higher for gemcitabine than for cytarabine and other nucleoside anticancer drugs (e.g., cladribine and fludarabine) [35]. The activity of dCK is an important factor in the overall cytotoxicity of gemcitabine. A2780 human ovarian carcinoma cell lines deficient in dCK were resistant to gemcitabine [36]. Greater expression of dCK cDNA in HT-29 human colon carcinoma xenografts in nude mice was positively correlated with enhanced dFdCTP accumulation and with the antitumor activity of gemcitabine [37].

The Importance of hNTs
Transport of gemcitabine occurs particularly by hCNT1, hENT1, and hENT2 [18, 38, 39], and is saturable [40]. Sensitivity to gemcitabine positively correlated with the expression of hENT1 [41], and hENT1-deficient cells were highly resistant to cytotoxicity by gemcitabine and cytarabine [42–44]. Inhibitors of nucleoside transporters, such as nitrobenzylmercaptopurine riboside (NBMPR), a specific hENT1 inhibitor at low nanomolar concentrations, and dipyridamole, a hENT1/2 inhibitor, reduced sensitivity to gemcitabine by 39- and 1,800-fold [18]. hENT1 transports gemcitabine with a high affinity and low capacity, whereas hENT2 transports gemcitabine with a low affinity and high capacity [40]. Restoration of hCNT1 activity in human pancreatic cancer cells positively correlated with the cytotoxicity of gemcitabine [45].

	CLINICAL PHARMACOLOGY OF GEMCITABINE

Top Footnotes Learning Objectives Abstract Introduction Molecular Pharmacology of... Mechanisms of Action of... Preclinical Pharmacology of... Clinical Pharmacology of... Phase I/II Dose-Finding Studies... Phase II/III Randomized Trials... Discussion and Future... References

 
Gemcitabine is generally administered as a standard 30-minute infusion at a dose of 1,000 mg/m² once weekly for 3 weeks of a 4-week cycle. Following i.v. administration, gemcitabine has a short t_1/2 (8–20 minutes) because of rapid deamination to dFdU [46], which has a long t_1/2 (50 hours) and is largely excreted in the urine [47]. dFdCTP peak concentrations were obtained within 30 minutes of the end of the infusion, and dFdCTP elimination was linear at low concentrations, and became biphasic at high concentrations in leukemic and blood mononuclear cells [46, 48, 49]. The long retention time of dFdCTP likely contributes to the fact that gemcitabine has activity against both rapidly proliferating and slowly dividing tumors.

dCK and Intracellular Levels of Nucleoside Triphosphates
Gemcitabine was shown to be a good substrate for phosphorylation by dCK, having a Michaelis-Menten constant (Km) value of 5–10 µM [34, 50]. Inhibition of the reaction was shown at higher substrate concentrations [34]. Therefore, it was expected that the rate of phosphorylation became saturated at gemcitabine concentrations >20 µM, with a negative effect on phosphorylation at higher levels. Clinical pharmacokinetic studies strongly suggested that the rate of intracellular accumulation of dFdCTP was saturated at gemcitabine concentrations of 15–20 µM in plasma [46, 51], which were achieved with gemcitabine infusions at a dose rate of 8–10 mg/m² per minute [51]. A standard 30-minute infusion of gemcitabine at doses of 800–2,600 mg/m² generates gemcitabine plasma concentrations of 20–60 µM, which exceed levels at which the formation of dFdCTP becomes saturated [46, 49], and which may also inhibit the process of phosphorylation of gemcitabine [34]. Consequently, cells are not able to phosphorylate a substantial portion of the infused drug and the main portion of gemcitabine is deaminated to dFdU. These findings suggested that dose schedules resulting in gemcitabine plasma concentrations of 15–20 µM were optimal to achieve maximal intracellular dFdCTP levels. Additionally, the maximal accumulation of dFdCTP was likely achieved by more prolonged infusions at a dose rate that resulted in maintenance of a gemcitabine plasma concentration that saturated the formation of gemcitabine nucleotides.

The formation of ara-CTP was also saturable during high-dose cytarabine therapy at cytarabine plasma concentrations >10 µM [52–54]. Exposure to ara-CTP in acute leukemic blasts positively correlated with response following administration of cytarabine either on an intermittent schedule [55] or by continuous infusion. Nonlinear elimination kinetics of ara-CTP were observed at high dose regimens of cytarabine corresponding to plasma concentrations >100 µM [56–58].

The Role of hNTs in the Cytotoxicity of Nucleosides
In 21 patients with pancreatic cancer treated with gemcitabine, hENT1 expression levels correlated with the median overall survival (OS) time. Patients with high expression levels of hENT1 had a significantly longer OS time of 25.7 months (95% confidence interval [CI], 17.6–33.7 months), compared with the OS time of 8.5 months (95% CI, 7.0–9.9 months) in patients with low levels of hENT1 [59]. Additionally, characterization of gene-expression patterns in tumor specimens of 102 patients with pancreatic cancer revealed that patients with high expression of hENT1 had a significantly longer OS time of 25.7 months, compared with the 12.4-month OS time in patients with low transcription levels (p < .001) [60].

Prolonged infusion of gemcitabine (1,200–2,800 mg/m²) at an FDR of 10 mg/m² per minute was safe and resulted in cumulative myelotoxicity with a lower maximum-tolerated dose (MTD) than with the standard 30-minute gemcitabine infusion [61]. Prolonged gemcitabine infusion at low dose levels (300 mg/m² in 6 hours) was associated with more pronounced nonhematological toxicity (e.g., alanine aminotransferase/aspartate aminotransferase elevations) [62, 63]. This might be a result of more pronounced accumulation of gemcitabine in the liver (high hCNT1 expression) than in other tissues. Because of the higher affinity of hCNT1 than hENT1 for gemcitabine, prolonged infusion of gemcitabine at a low dose level might allow selective uptake of gemcitabine into tumor cells with high expression of hCNT1, while sparing bone marrow cells that predominantly contain hENTs.

Intracellular accumulation of ara-CTP in leukemic cells of patients was limited and directly correlated with the number of membrane nucleoside transporters at low (<1 µM) cytarabine concentrations [17, 64, 65] after low-dose cytarabine therapy (100–200 mg/m² per day) [66, 67]. Concordant with these findings, patients with acute myelogenous leukemia (AML) who did not respond to cytarabine therapy (100–200 mg/m² per day) demonstrated low cytarabine uptake and low numbers of NBMPR binding sites in leukemic blasts [16]. Thus, at low cytarabine plasma concentrations, transport of cytarabine by hNTs is the rate-limiting step, whereas at high cytarabine concentrations, intracellular phosphorylation of cytarabine by dCK becomes the rate-limiting step in the formation of ara-CTP.

Coadministration of an inhibitor of hENT1 to cytarabine enhanced its cytotoxicity toward cells that coexpressed hENTs and hCNTs, which was explained by inhibition of efflux of cytarabine by hENT1, and enhanced intracellular uptake by hCNT1.

The Concept of FDR Dosing of Gemcitabine
Gemcitabine plasma concentrations of 20–60 µM are achieved following a standard 30-minute infusion of gemcitabine at a dose of 1,000–1,200 mg/m², which exceeds the levels of 15–20 µM at which dCK becomes saturated [46, 48–50]. Prolongation of the duration of infusion of gemcitabine at lower dose levels might increase the intracellular concentration of dFdC. In addition, it leads to the maintenance of plasma gemcitabine concentrations at levels at which dCK is saturated for prolonged periods of time, which increases intracellular accumulation of dFdCTP [48, 68]. This strategy was hypothesized to result in a higher antitumor activity of gemcitabine in patients. Clinical studies on the prolonged administration of gemcitabine at an FDR (e.g., 10 mg/m² per minute) compared with the standard 30-minute infusion (e.g., 30 mg/m² per minute) are discussed below.

	PHASE I/II DOSE-FINDING STUDIES INVESTIGATING PROLONGED (FDR) AND STANDARD INFUSION OF GEMCITABINE

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Grunewald and coworkers demonstrated, in a small subset of patients, that prolongation of the infusion duration of gemcitabine at a dose of 800 mg/m² increased the AUC of dFdCTP in leukemic cells (Table 1) [48]. In a dose-escalation study (10–1,000 mg/m² gemcitabine), they showed that a prolonged 60-minute dosing schedule of 800 mg/m² (400 mg/m² in 30 minutes), resulting in plasma gemcitabine concentrations of 20 µM, led to a fourfold higher AUC of dFdCTP compared with a 30-minute infusion of 790 mg/m², after which gemcitabine concentrations of 60 µM were obtained [49]. No significant difference in the AUC for dFdCTP was observed at gemcitabine plasma concentrations >20 µM [49]. In a different dose-escalation study with gemcitabine (10–1,000 mg/m²), Abbruzzese and colleagues demonstrated that, after the end of a 30-minute infusion, plasma levels of gemcitabine decreased rapidly (t_1/2 8 minutes) because of deamination to dFdU [46]. Both the rate of accumulation (µM/hour) and the maximum concentration (µM) of dFdCTP in mononuclear cells increased with the gemcitabine dose up to 350 mg/m², corresponding to gemcitabine plasma concentrations of 15–20 µM, without a further increase at higher dose levels, suggesting saturation of the intracellular formation of dFdCTP [46]. Administration of gemcitabine (1,200–6,400 mg/m²) at an FDR of 10 mg/m² per minute resulted in a linear increase in the AUC of dFdCTP with the infusion time and with the dose [51]. Additionally, inhibition of DNA synthesis in circulating blasts increased proportionately with the AUC of dFdCTP. A study by Touroutoglou and colleagues showed that FDR gemcitabine (1,200–2,800 mg/m²) was safe and well tolerated [61]. Once-weekly administration of gemcitabine for 7 weeks of an 8-week cycle and FDR dosing of 1,000 mg/m² in 150 minutes demonstrated that intracellular levels of dFdCTP in peripheral blood mononuclear cells (PBMCs) increased linearly with infusion duration [69]. The maximum concentration (C_max) of dFdCTP was 1.4-fold higher after FDR than after standard gemcitabine. Similarly, Gandhi and colleagues demonstrated that levels of dFdCTP in AML blasts increased linearly with infusion time following FDR gemcitabine dosing [70]. An abrupt decrease in DNA synthesis to values of 5%–20% of the pretreatment value at 1 hour after the infusion was observed, and DNA synthesis remained inhibited until 24 hours after the start of the infusion. Moreover, dATP pools decreased during gemcitabine infusion up to 24 hours after the start of the infusion. Bengala and coworkers found that FDR gemcitabine infusion in patients with advanced pancreatic adenocarcinoma resulted in a median time to progression (TTP) of 4.8 months, a median OS time of 7 months, and a 1-year survival rate (SR) of 21.9% [71]. The AUC of dFdU, the main deaminated metabolite of gemcitabine, and OS time significantly correlated with the expression and activity of cytidine deaminase (p < .05). Studies on the prolonged infusion of gemcitabine during 3, 4, 6, and 24 hours at low dose levels in patients with advanced solid tumors found low MTD values between 180 and 450 mg/m² [72–75]. Besides treatment with single-agent gemcitabine, FDR infusion of gemcitabine has been investigated in combination with other chemotherapeutic agents, such as carboplatin and cisplatin. Soo and colleagues found that combination therapy with carboplatin (AUC, 5 mg/ml·minute) and gemcitabine administered as a 75-minute infusion at a dose of 750 mg/m² was active and tolerable in patients with non-small cell lung cancer (NSCLC) and resulted in plasma gemcitabine concentrations >10 µM [76]. Others found that the combination of carboplatin (AUC, 5 mg/ml·minute) with gemcitabine administered as a 120-minute infusion at a dose of 1,200 mg/m² resulted in a median TTP of 7.0 months and an OS time of 12.0 months [77]. The gemcitabine concentration at the end of the infusion was positively correlated with the percentage reduction in leukocytes and platelets. The prolonged infusion of gemcitabine (120–250 mg/m²) over 6 hours with cisplatin (75 mg/m²) had an acceptable toxicity profile [78]. The median TTP was 6 months and the 1-year SR was 40%. Prolonged gemcitabine infusion over 96 hours at doses of 1–25 mg/m² per day resulted mainly in nonhematological toxicities (e.g., mucositis, fever, rash), in contrast to studies with shorter infusion durations (<2 hours), which predominantly resulted in myelotoxicity [79].

	PHASE II/III RANDOMIZED TRIALS COMPARING FDR WITH STANDARD DOSING OF GEMCITABINE

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	DISCUSSION AND FUTURE PERSPECTIVES

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In summary, various clinical studies have shown that prolonged i.v. gemcitabine administration over 1–24 hours at lower rates of infusion (<30 mg/m² per minute) can be safely administered and has remarkable antitumor activity in patients with advanced solid tumors. Overall, there was a trend that prolonged infusion of gemcitabine resulted in a somewhat higher degree of nonhematological toxicity (e.g., elevations in transaminases) than with the standard 30-minute infusion, after which myelosuppression was the most common toxicity. The FDR strategy (e.g., 10 mg/m² per minute) resulted in gemcitabine plasma concentrations at levels at which dCK becomes saturated for prolonged periods of time and led to higher AUC values of dFdCTP in mononuclear and leukemic cells than with the standard gemcitabine infusion (e.g., 33 mg/m² per minute). Concordant with these findings, one would expect FDR gemcitabine to result in higher concentrations of dFdCTP in solid tumors of patients. However, in vivo metabolism and/or transport of gemcitabine might be somewhat different among blood mononuclear, leukemic, and solid tumor cells; for example, as a result of differences in expression of metabolic enzymes and hNTs. Thus far, not many studies have determined dFdCTP concentrations in tumor tissue following gemcitabine therapy. A study in 30 patients with head and neck cancer treated with i.v. gemcitabine (50–300 mg/m²) over 30 minutes resulted in dFdCTP concentrations of 733–3,817 pmol/g tumor biopsy, which was suggested to produce potent radiosensitization based on in vitro studies [88]. In a phase I trial in 52 patients with refractory solid cancers, a 30-minute infusion of gemcitabine (1500 mg/m²) resulted in 70 pmol of dFdCTP per gram wet weight tumor biopsy of patients with head and neck cancer (n = 2) [89].

Overall, clinical studies provide pharmacological evidence for an advantage of prolonged FDR dosing above the standard 30-minute infusion of gemcitabine, based on the increase in dFdCTP exposure in blood mononuclear cells, which could be used to further optimize gemcitabine treatment. Thus far, only the study by Tempero and coworkers demonstrated an advantage for FDR infusion over standard gemcitabine in terms of survival of patients. Recently, a phase III study compared the OS time seen with standard gemcitabine (1,000 mg/m² over 30 minutes weekly for 7 weeks over 56 days then weekly for 3 weeks every 28 days, A) with that seen with FDR gemcitabine (1,500 mg/m² over 150 minutes weekly for 3 weeks for 28 days, B) and with FDR gemcitabine (1,000 mg/m² over 100 minutes on day 1) plus oxaliplatin (100 mg/m² on day 2 every 14 days), C, in 750 patients with advanced pancreatic cancer [90]. An interim analysis showed median OS times of 4.96, 6.01, and 6.47 months for arm A, B, and C, respectively, without significant differences among the three treatment arms. In general, pancreatic cancer is an unresponsive disease, and anticancer drugs other than gemcitabine are needed for the treatment of this type of cancer. New well-powered, randomized clinical studies are warranted to establish the optimal dose and infusion duration of gemcitabine providing the greatest antitumor activity with acceptable toxicity in types of cancer other than pancreatic cancer. Furthermore, future studies should assess relationships between the exposure to dFdCTP in PBMCs and solid tumors following prolonged FDR and following standard gemcitabine therapy. These studies should prove whether prolonged FDR gemcitabine dosing is a better treatment option than standard gemcitabine.

	FOOTNOTES

 
Conception/design: Stephan A. Veltkamp, Jos H. Beijnen, Jan H.M. Schellens

Collection/assembly of data: Stephan A. Veltkamp

Data analysis and interpretation: Stephan A. Veltkamp, Jos H. Beijnen, Jan H.M. Schellens

Manuscript writing: Stephan A. Veltkamp, Jan H.M. Schellens

Final approval of manuscript: Stephan A. Veltkamp, Jos H. Beijnen, Jan H.M. Schellens

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