The Oncologist, Vol. 9, Suppl 2, 3–8,
June 2, 2004
© 2004 AlphaMed Press
Preclinical Pharmacology of the Taxanes: Implications of the Differences
Joseph Gligorov,
Jean Pierre Lotz
CancerEst, APHP Tenon, Medical Oncology, Paris, France
Correspondence: Joseph Gligorov, M.D., CancerEst, APHP Tenon, Medical Oncology, 4 rue de la Chine, 75970 Paris cedex 20, France. Telephone: 33-156-016024; Fax: 33-156-016452; e-mail: joseph.gligorov{at}tnn.ap-hop-paris.fr
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LEARNING OBJECTIVES
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After completing this course, the reader will be able to:
- Identify the differences in molecular pharmacology of the taxanes.
- Describe the impact of the pharmacokinetic and pharmacodynamic profiles of the taxanes on their administration and toxicity.
- Outline the toxicity profiles of paclitaxel and docetaxel.
Access and take the CME test online and receive 1 hour of AMA PRA category 1 credit at CME.TheOncologist.com
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ABSTRACT
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Taxanes are one of the most powerful classes of compounds among all chemotherapeutic drugs. Only 30 years separate the isolation of the first taxane from the results of direct clinical comparisons in metastatic breast, ovarian, and lung cancer between the two taxanes available in routine clinical practice. These results suggest a more favorable benefit-to-risk ratio for docetaxel compared to paclitaxel when these drugs are used as single agents or in combination with other chemotherapeutic agents in an every-3-week dosing regimen. Pharmacological data support the difference between the taxanes, likely explaining the clinical results. Considering the molecular pharmacology of the two drugs, docetaxel appears to bind to ß-tubulin with greater affinity and has a wider cell cycle activity than paclitaxel. Docetaxel also appears to have direct antitumoral activity via an apoptotic effect mediated by bcl-2 phosphorylation. In addition, docetaxel has a longer retention time in tumor cells than paclitaxel because of greater uptake and slower efflux. Pharmacokinetics and pharmacodynamics of the taxanes show both agents to be extensively metabolized in the liver, and paclitaxel has a nonlinear pharmacokinetic behavior while docetaxel has linear pharmacokinetics. These differences explain the more simple treatment schedule and favorable results for docetaxel as a single agent and in combination therapy. Last, but not least, there is a pharmacokinetic interaction between paclitaxel and the anthracyclines, an active class of compounds commonly used in the treatment of breast cancer. This pharmacokinetic interaction is associated with greater cardio- and myelotoxicities, which are sequence dependent. These pharmacological data likely explain the different clinical development strategies for the two molecules as well as the different clinical results from individual trials and direct comparisons.
Key Words. Taxane • Docetaxel • Paclitaxel • Pharmacology • Pharmacokinetics
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INTRODUCTION
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Among new chemotherapeutic agents, the taxanes have emerged as one of the most powerful classes of compounds, exhibiting a wide range of activity. Randomized clinical trials evaluating docetaxel and paclitaxel in the first-line treatment setting for metastatic breast, lung, ovarian, and digestive cancers, as well as in the adjuvant setting for breast cancer, have confirmed that taxanes are leading contributors in the armamentarium of cancer treatments.
Paclitaxel was isolated in 1971 from the Pacific yew (Taxus brevifolia), and docetaxel, a semi-synthetic taxane analogue from the European yew (Taxus baccata), was identified in the 1980s. Though the taxanes share similar mechanisms of action, differences are apparent in their molecular pharmacology, pharmacokinetics, and pharmacodynamic profiles. Indeed, these differences may account for the differences observed between the taxanes in their clinical activity and toxicity.
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MOLECULAR PHARMACOLOGY
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Tubulin is the "building block" of microtubules, and agents that bind tubulin are believed to block cell division by interfering with the function of the mitotic spindle, blocking the cells at the metaphase-anaphase junction of mitosis. Microtubules are complex structures involved in numerous cellular functions, including the maintenance of cell shape, intracellular transport, secretion, and neurotransmission. Moreover, microtubules are highly dynamic and unstable structures that are constantly incorporating free dimers and releasing dimers into the soluble tubulin pool. Taxane antitumor activity is the result of the binding of drug to the beta subunits of tubulin, which causes the stabilization of tubulin polymerization. This stabilization results in cell cycle arrest at the G2/M phase, thus inhibiting mitosis [1]. Docetaxel and paclitaxel are different in their molecular pharmacology, potentially explaining their different activity and toxicity profiles (Table 1
). Docetaxel exhibits greater affinity to ß-tubulin, targeting centrosome organization and acting on cells in three phases of the cell cycle (S/G2/M), whereas paclitaxel causes cell damage by affecting the mitotic spindle in the G2 and M phases of the cell cycle [2]. Docetaxel was proven to be almost totally lethal against S-phase cells, while the maximum resistance to paclitaxel is early in the S phase [3]. The bcl-2 gene encodes the oncoprotein that prevents apoptosis, and this gene is overexpressed in several solid tumors, including breast, lung, prostate, and nasopharyngeal cancers. Docetaxel induces Bcl-2 phosphorylation and apoptotic cell death at concentrations 100-fold less than those required by paclitaxel [2]. Presently, the best described mechanism of resistance to antitubulin agents is the multidrug resistant (MDR) phenotype, which is mediated by Pgp, the 170-kD ATP-binding transport protein encoded by the MDR1 gene [1]. Additional differences between the taxanes include greater uptake of docetaxel into tumor cells and slower efflux of docetaxel from tumor cells, thus leading to longer retention times, providing a possible explanation for the incomplete cross-resistance between the drugs [4]. In vitro, the antiproliferative actions of docetaxel and paclitaxel were studied against a variety of freshly explanted human tumor specimens, including breast, lung, ovarian, and colorectal cancers. At concentrations of 10 mg/ml, 41% of docetaxel-treated specimens achieved significant inhibition, compared with 33% of paclitaxel-treated specimens, demonstrating high cytotoxicity with docetaxel as well as incomplete cross-resistance between the taxanes [5].
The incomplete cross-resistance between the taxanes found preclinically, specifically the activity of docetaxel in paclitaxel-resistant disease, has been confirmed in several tumor types, specifically, breast, ovarian, and lung cancers (Table 2) [6–8]. Valero and colleagues [6] evaluated the efficacy and safety of docetaxel in breast cancer patients who had previously received at least two chemotherapy regimens for metastatic disease and were progressing during paclitaxel therapy. The objective response rates observed were not significantly different when stratified by previously received paclitaxel dose. Interestingly, there were no responses observed in 12 patients who had received paclitaxel by a continuous 24-hour infusion; however, the response rate in 32 patients who received paclitaxel by either a 1-hour or 3-hour infusion was 25%. A second phase II trial evaluated two dose levels of docetaxel in patients with paclitaxel-resistant Müllerian carcinoma [7]. Resistance to paclitaxel was defined as disease progression during treatment, failure to achieve tumor regression after a minimum of four courses of therapy, or recurrence of disease within 6 months of completion of paclitaxel therapy. Of the 19 patients that progressed while receiving paclitaxel, 11% responded to docetaxel, while 45% of those in other paclitaxel-resistant categories responded to docetaxel. The docetaxel responders had a median taxane-free interval of 73 weeks while nonresponders had a taxane-free interval of only 19 weeks, indicating that docetaxel is active in paclitaxel-resistant Müllerian carcinoma, especially in those patients with long taxane-free intervals.
The results of a phase III trial reported by Fossella and colleagues [8] that compared two dose levels of docetaxel with standard non-small cell lung cancer (NSCLC) treatments further demonstrate the incomplete cross-resistance exhibited with taxane therapy. Of the 250 patients randomized to treatment with docetaxel, 73% had received prior paclitaxel therapy. A subset analysis of this group determined that the likelihood of response to docetaxel was not impacted by previous treatment with paclitaxel because the partial response rates were similar for those with prior exposure (20%) and those without prior exposure (15%) [8].
As previously stated, taxanes arrest cells in the G2/M phase of mitosis, which is also the phase of the cell cycle that is most radiosensitive. Preclinical data evaluating the ability of docetaxel to enhance tumor response and influence radiation injury to normal tissues have shown that, if docetaxel is administered within 2 days before radiation therapy, it results in greater therapeutic gains from radiation [9, 10]. Mice were treated with 33 mg/kg of docetaxel with or without 9–21 Gy of radiation, with delay of tumor growth being the primary end point [9]. To determine the effect on normal tissue, jejunal cells were evaluated. Docetaxel enhanced tumor radioresponse by a factor of 2.33 when administered 48 hours prior to radiation; it only slightly enhanced radiation-induced damage to the jejunal cells and only when given 3–9 hours before radiation. A second murine trial using the same dose of docetaxel with or without 5–65 Gy of radiation also resulted in delayed tumor growth and enhanced response to radiation [11]. Furthermore, docetaxel induced infiltration of the tumor with lymphocytes; percentages were <2% in the control and 27% with docetaxel [11]. This was secondary to the influx of helper T cells and natural killer cells, indicating that docetaxel also possesses immunomodulating properties. The radiosensitizing effects of docetaxel relative to paclitaxel were evaluated in an in vitro comparative analysis using three human cancer cell lines (cervical cancer, mesothelioma, and lung cancer) [12]. Results showed that all three cell lines were more sensitive to docetaxel than to paclitaxel and that, although mesothelioma cells were intrinsically resistant to both radiation and taxanes, the resistance was partially overcome with administration of docetaxel before radiation. These preclinical data suggest that docetaxel is a potent radiosensitizer [9–12].
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PHARMACOKINETICS AND PHARMACODYNAMICS
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There are substantial differences in the pharmacokinetic and pharmacodynamic profiles of docetaxel and paclitaxel, an issue which may contribute to the difficulties in defining the optimal delivery schedule for paclitaxel monotherapy and combination regimens, particularly with anthracyclines. Both taxanes are extensively metabolized in the liver by the cytochrome P-450 enzymes and undergo biliary excretion as their main route of elimination, thus resulting in the need for dose reductions in patients with elevated liver enzymes. A major fraction of the taxane dose is excreted in feces as parent drug or hydroxylated metabolites; the known metabolites of both taxanes are either inactive or less potent than the parent compounds. Both agents have wide tissue distribution, are highly protein bound, and approximately 6% of either drug is renally eliminated.
Studies in mice and humans have determined that paclitaxel exhibits nonlinear pharmacokinetics, a characteristic further compounded by the addition of Cremophor EL, a vehicle in the formulation to improve its water solubility [13]. Pharmacokinetic modeling of paclitaxel indicates that saturable distribution and elimination processes potentially explain its nonlinear pharmacokinetic behavior [14]. Clinical implications of nonlinear pharmacokinetics emerge as dose escalations or reductions of paclitaxel result in disproportionate increases or decreases in both the area under the time-concentration curve (AUC) and the peak plasma concentration (Cmax). In 30 ovarian and breast cancer patients receiving paclitaxel by 3- and 24-hour infusions, Gianni and colleagues [15] observed that neutropenia was not related to the AUC of paclitaxel or its metabolite, 6 alpha-hydroxypaclitaxel, or to Cmax, but was significantly correlated with the duration that the paclitaxel plasma concentration was 
0.05 µM. Thus, shorter infusions should be associated with fewer adverse clinical outcomes.
Conversely, docetaxel exhibits linear pharmacokinetics and is customarily administered over 1 hour, thus, any dose adjustments result in proportional modifications in the AUC and Cmax. Population pharmacokinetic studies have confirmed that clearance of docetaxel is significantly lower with increasing age, less body surface area, and higher liver function values [16]. Moreover, in patients with advanced cancer, the first-course docetaxel AUC is a significant predictor of hematologic toxicity (p < 0.001) [17]. It has been suggested that low-dose, weekly administration of docetaxel may potentially be better tolerated than the every-3-week schedule, thereby enabling the treatment of elderly and poor performance status patients—populations who generally have decreased drug clearance and, subsequently, more toxicity. A phase II study of 29 women with metastatic breast cancer receiving 40 mg/m2 weekly of docetaxel supports the concept that this administration tactic is effective, with a superior toxicity profile compared with every 3-week dosing [18]. Each cycle of therapy consisted of weekly docetaxel for 6 weeks followed by a 2-week rest period; treatment was continued until disease progression or removal from the study (due to either toxicity or patient preference). The median patient age was 57 years, and all patients but one had Eastern Cooperative Oncology Group (ECOG) performance status scores of 0 or 1. The overall response rate was 41%, and therapy was well tolerated, with no reports of grade 4 toxicity (only 28% of patients exhibited grade 3 toxicity). The most frequently reported grade 3 toxicities with the weekly regimen were myelosuppression (14%) and fatigue/asthenia (14%), which differs from the typical toxicities of neutropenia, sensory neuropathy, and nail changes associated with the every-3-week administration schedule.
Slaviero and colleagues [19] recently reported study results evaluating the relationship between clearance and toxicity of a weekly docetaxel regimen in patients with advanced cancer. Fifty-four patients (median age of 63 years) with a median ECOG performance status of 1 were treated with docetaxel at a dose of 40 mg/m2 weekly. The relationships among docetaxel clearance, patient characteristics (e.g., age, liver enzymes), and toxicities were estimated using linear regression and nonparametric statistics. Acute grade 3/4 toxicity after the first dose of therapy was found to be associated with decreased docetaxel clearance (p = 0.004) but was not associated with age or performance status (p > 0.05). It was determined, however, that mean docetaxel clearance was significantly correlated with liver function values, suggesting that reduced hepatic drug clearance clearly plays a role in predicting which patients will experience acute toxicity on a weekly schedule.
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DRUG INTERACTIONS
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The majority of drug interactions associated with the taxanes have primarily involved paclitaxel, the most significant being between paclitaxel and the anthracyclines. Paclitaxel-doxorubicin combinations have proven to be highly effective in patients with breast cancer, with response rates ranging from 58%-94% [20–22]. However, because of the associated cardiac toxicity with this combination, issues regarding the optimal schedule and sequence of administration remain in question. Clinically, it appears that this interaction is schedule dependent. Gianni and colleagues [23] conducted a trial evaluating paclitaxel (as a 3-hour infusion) plus doxorubicin (as a 15-minute bolus infusion) in 36 women with chemotherapy-naïve metastatic breast cancer. The objectives of that study were to assess the role of administration sequence, the time interval between drugs, and the duration of doxorubicin infusion on both paclitaxel and anthracycline disposition. The effect of paclitaxel on increasing doxorubicin-associated cardiotoxicity appeared greater when the paclitaxel was administered prior to the doxorubicin, if the interval between drug administrations was <1 hour, and if the paclitaxel infusion was <3 hours. Conversely, cardiotoxicity was less when doxorubicin administration preceded paclitaxel.
Because of the cardiotoxicity observed with the doxorubicin-paclitaxel combination and the fact that epirubicin is also an anthracycline that is classically associated with less cardiotoxicity, the combination of epirubicin-paclitaxel is logical. Therefore, the sequence effect of epirubicin and paclitaxel was studied in 39 patients with stage II breast cancer [24]. Patients received bolus epirubicin followed by a 3-hour paclitaxel (ET) infusion or the opposite sequence (TE) every 3 weeks for four cycles. Clinically, there was no significant difference in nonhematologic toxicities. However, a sequence-dependent effect on the myelotoxicity profile was observed, as evidenced by lower neutrophil and platelet nadirs and a significantly slower neutrophil recovery in the TE group. Furthermore, when paclitaxel preceded epirubicin, the AUC of epirubicin was increased by 37% (2,346 ± 1,162 ng/ml/hour versus 1,717 ± 542 ng/ml/hour; p = 0.002) and the total clearance was decreased, on average, by 25%. Overall, these findings confirm that there is a sequence-dependent pharmacokinetic interaction between paclitaxel and epirubicin, and clinical toxicity is minimized by maintaining the classic administration schedule of epirubicin followed by paclitaxel.
Docetaxel-anthracycline combinations have resulted in no clinically significant effect on the anthracycline pharmacokinetics. In a trial of breast cancer patients, epirubicin (90 mg/m2 i.v. bolus) was immediately followed either by docetaxel (70 mg/m2 as a 1-hour infusion) or paclitaxel (175 mg/m2 over 3 hours). While no change in the pharmacokinetic profile of epirubicin was observed when followed by either paclitaxel or docetaxel, there were clear differences between the effects that each taxane had on the epirubicin metabolites. Statistically significant increases in Cmax (78.5 ng/ml versus 61.9 ng/ml, p < 0.05) and AUC (1,521 ng/ml/hour versus 848 ng/ml/hour, p < 0.01) of epirubicinol (EOL) were observed when epirubicin was followed by paclitaxel but not when it was followed by docetaxel [25]. These data indicate that there is no apparent pharmacokinetic interaction between the taxanes and the parent compound, epirubicin, whereas the epirubicin metabolite EOL is clearly more affected by paclitaxel than docetaxel. Whether these effects on the epirubicin metabolites have any relevance to toxicity and efficacy remains to be determined.
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TOXICITIES OF THE TAXANES
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The differences in the taxanes are also apparent in their adverse effect profiles. Paclitaxel is associated with anaphylaxis and severe hypersensitivity reactions characterized by hypotension, dyspnea, angioedema, and generalized urticaria, likely attributable to the Cremophor EL solubilizer. Other reactions typically more related to paclitaxel than docetaxel include myalgias and neuropathy. The side-effect profile of docetaxel appears to be schedule dependent. When administered every 3 weeks, docetaxel is more frequently associated with reversible, noncumulative neutropenia, fluid retention, cutaneous reactions, and hyperlacrimation (Table 3). However, when administered via a weekly dosing schedule, the toxicity profile of docetaxel is different and includes fewer hematologic toxicities, less stomatitis, fewer cutaneous events, and fewer neurologic toxicities, but a greater amount of grade 3/4 fatigue/asthenia [18]. Side effects common to both methods of administration include hyperlacrimation and fluid retention. Less acute toxicity appears to be associated with weekly docetaxel therapy than with every-3-week docetaxel therapy. Results of more clinical trials will further define the schedule-dependent differences between the taxanes.
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SUMMARY
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Clearly, there are differences between the taxanes, ranging from their binding affinity and cell cycle specificity to their toxicity profiles (Table 4). As is often the case with similar compounds, a comparison between agents is rare and difficult to accomplish. Docetaxel and paclitaxel have been compared with one another (as single agents and/or in combination regimens in every-3-week dosing schedules) in patients with metastatic breast, ovarian, and lung cancers, with preliminary or final results now available. Results of these trials and of more clinical trials in various malignancies will further elucidate the clinical differences between the taxanes.
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Received February 27, 2004;
accepted for publication April 5, 2004.
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