Quantification of paclitaxel metabolites in human plasma by high-performance liquid chromatography

A novel ultra-performance liquid chromatography detection method development and validation for paclitaxel and its major metabolite in human plasma

BACKGROUND

Paclitaxel is a promising anticancer drug for patients with ovarian, breast, lung, gastrointestinal, genitourinary, prostate, and head-and-neck cancers. Paclitaxel follows nonlinear pharmacokinetics. The major metabolite of paclitaxel is 6-alpha-hydroxy paclitaxel, mediated by CYP2C8, while metabolism to two of its minor metabolites, 3'-p-hydroxypaclitaxel and 6a, 3'- p-dihydroxypaclitaxel, is catalyzed by CYP3A4. Therapeutic drug monitoring of paclitaxel could be a promising approach to improve the efficacy and safety of paclitaxel correct personalized doses and improve the overall benefit-risk ratio. A novel and highly sensitive chromatographic method for the detection of paclitaxel and its metabolite has been proposed that allows quantification in human plasma with 100% accuracy in terms of recovery without significant intraday or interday variations.

MATERIALS AND METHODS

The present study was planned following bioanalytical method validation guidance according to the U.S. Food and Drug Administration requirements. The validation of the analytical procedure was performed as per ICH Q2(R1) guidelines. It was done to assure the reliability of the results obtained for various parameters such as linearity, accuracy, precision, limit of detection (LOD), limit of quantification, robustness, stability, and system suitability.

RESULTS

The specificity of the method was established by ensuring no interference with peak obtained from paclitaxel and 6-alpha-hydroxy paclitaxel. LOD was found to be 0.05 and 0.033 while the limit of quantitation was 0.14 and 0.099 for paclitaxel and 6-alpha-hydroxy paclitaxel, respectively. Median (±interquartile range) accuracy for paclitaxel and 6-alpha-hydroxy paclitaxel was found to be 102.73 (±13.581) and 100.87 (±7.573), respectively.

CONCLUSION

This novel method of simultaneous detection of paclitaxel and its major metabolite 6-alpha-hydroxy paclitaxel demonstrated significant resolution and was sensitive enough for its quantification in human plasma.

In Vitro Evidence of Potential Interactions between CYP2C8 and Candesartan Acyl-β-D-glucuronide in the Liver

Growing evidence suggests that certain glucuronides function as potent inhibitors of CYP2C8. We previously reported the possibility of drug-drug interactions between candesartan cilexetil and paclitaxel. In this study, we evaluated the effects of candesartan N2-glucuronide and candesartan acyl-β-D-glucuronide on pathways associated with the elimination of paclitaxel, including those involving organic anion-transporting polypeptide (OATP) 1B1, OATP1B3, CYP2C8, and CYP3A4. UDP-glucuronosyltransferase (UGT) 1A10 and UGT2B7 were found to increase candesartan N2-glucuronide and candesartan acyl-β-D-glucuronide formation in a candesartan concentration-dependent manner. Additionally, the uptake of candesartan N2-glucuronide and candesartan acyl-β-D-glucuronide by cells stably expressing OATPs is a saturable process with K _m of 5.11 and 12.1 μM for OATP1B1 and 28.8 and 15.7 μM for OATP1B3, respectively; both glucuronides exhibit moderate inhibition of OATP1B1/1B3. Moreover, the hydroxylation of paclitaxel was evaluated using recombinant CYP3A4 and CYP3A5. Results show that candesartan, candesartan N2-glucuronide, and candesartan acyl-β-D-glucuronide inhibit the CYP2C8-mediated metabolism of paclitaxel, with candesartan acyl-β-D-glucuronide exhibiting the strongest inhibition (IC₅₀ is 18.9 µM for candesartan acyl-β-D-glucuronide, 150 µM for candesartan, and 166 µM for candesartan N2-glucuronide). However, time-dependent inhibition of CYP2C8 by candesartan acyl-β-D-glucuronide was not observed. Conversely, the IC₅₀ values of all the compounds are comparable for CYP3A4. Taken together, these data suggest that candesartan acyl-β-D-glucuronide is actively transported by OATPs into hepatocytes, and drug-drug interactions may occur with coadministration of candesartan and CYP2C8 substrates, including paclitaxel, as a result of the inhibition of CYP2C8 function. SIGNIFICANCE STATEMENT: This study demonstrates that the acyl glucuronidation of candesartan to form candesartan acyl-β-D-glucuronide enhances CYP2C8 inhibition while exerting minimal effects on CYP3A4, organic anion-transporting polypeptide (OATP) 1B1, and OATP1B3. Thus, candesartan acyl-β-D-glucuronide might represent a potential mediator of drug-drug interactions between candesartan and CYP2C8 substrates, such as paclitaxel, in clinical settings. This work adds to the growing knowledge regarding the inhibitory effects of glucuronides on CYP2C8.

The Role of CYP2C8 and CYP2C9 Genotypes in Losartan-Dependent Inhibition of Paclitaxel Metabolism in Human Liver Microsomes

The aim of the present study was to further investigate a previously identified metabolic interaction between losartan and paclitaxel, which is one of the marker substrates of CYP2C8, by using human liver microsomes (HLMs) from donors with different CYP2C8 and CYP2C9 genotypes. Although CYP2C8 and CYP2C9 exhibit genetic linkage, previous studies have yet to determine whether losartan or its active metabolite, EXP-3174 which is specifically generated by CYP2C9, is responsible for CYP2C8 inhibition. Concentrations of 6α-hydroxypaclitaxel and EXP-3174 were measured by high-performance liquid chromatography after incubations with paclitaxel, losartan or EXP-3174 in HLMs from seven donors with different CYP2C8 and CYP2C9 genotypes. The half maximal inhibitory concentration (IC50 ) values were not fully dependent on CYP2C8 genotypes. Although the degree of inhibition was small, losartan significantly inhibited the production of 6α-hydroxypaclitaxel at a concentration of 1 μmol/L in only HL20 with the CYP2C8*3/*3 genotype. HLMs with either CYP2C9*2/*2 or CYP2C9*1/*3 exhibited a lower losartan intrinsic clearance (Vmax /Km ) than other HLMs including those with CYP2C9*1/*1 and CYP2C9*1/*2. Significant inhibition of 6α-hydroxypaclitaxel formation by EXP-3174 could only be found at levels that were 50 times higher (100 μmol/L) than the maximum concentration generated in the inhibition study using losartan. These results suggest that the metabolic interaction between losartan and paclitaxel is dependent on losartan itself rather than its metabolite and that the CYP2C8 inhibition by losartan is not affected by the CYP2C9 genotype. Further study is needed to define the effect of CYP2C8 genotypes on losartan-paclitaxel interaction.

Quantification of taxanes in biological matrices: a review of bioanalytical assays and recommendations for development of new assays

Since the isolation of paclitaxel and its approval for the treatment of breast cancer, various taxanes and taxane formulations have been developed. To date, almost 100 bioanalytical assays have been published with the method development and optimization often extensively discussed by the authors. This Review presents an overview of assays published between January 1970 and September 2013 that described method development and validation of assays used to quantify taxanes in biological matrices such as plasma, urine, feces and tissue samples. For liquid chromatography assays, sample pretreatment, chromatographic separation and assay performance are compared. Since this Review discusses the limitations of previously developed liquid chromatography assays and gives recommendations for future assay development, it can be used as a reference for future development of liquid chromatography assays for the quantification of taxanes in various biological matrices to support preclinical and clinical studies.

A rapid analytical method for the quantification of paclitaxel in rat plasma and brain tissue by high-performance liquid chromatography and tandem mass spectrometry

RATIONALE

Paclitaxel, an antitumor agent for the treatment of several types of cancers, has recently been reported to cause impaired cognitive function and neuropathic pain in humans. To assess the effects of paclitaxel on the central nervous system, a sensitive and accurate method is required to quantify paclitaxel concentrations in plasma and brain tissue obtained from rodents receiving paclitaxel.

METHODS

The biological samples were prepared by liquid-liquid extraction and separated by a 3.5 min reversed-phase liquid chromatography (RPLC) method using a BDS Hypersil C8 column under isocratic conditions. Paclitaxel was quantified using multiple reaction monitoring (MRM) with a triple quadrupole tandem mass spectrometer working in the positive electrospray ionization (ESI+) mode. A stable isotope labeled analogue of paclitaxel was used as the internal standard (IS).

RESULTS

The method was validated to be precise and accurate within the dynamic range of 0.5-100 ng/mL based on 100 μL plasma and 1.5-300 ng/g based on 33 mg of brain tissue in homogenate. This method was applied to samples from 2 mg/kg intravenously dosed rats. The plasma concentrations were observed to be 26.62 ± 8.93 ng/mL and brain concentrations 11.08 ± 4.18 ng/g when measured 4 h post-dose.

CONCLUSIONS

This rapid LC/MS/MS method was validated to be sensitive, specific, precise and accurate for the quantification of paclitaxel in rat plasma and brain tissue homogenate. Application of the method to study samples provided sufficient proof of blood-brain barrier penetration of paclitaxel, allowing further investigation of its influence on the central nervous system.

Increased elimination of paclitaxel by magnesium isoglycyrrhizinate in epithelial ovarian cancer patients treated with paclitaxel plus cisplatin: a pilot clinical study

Magnesium isoglycyrrhizinate (MI) has been complementarily used for restoring the hepatic impairments caused by taxol plus platinum based chemotherapies in China. Due to the hepatic dependence of paclitaxel elimination, this pilot clinical study aimed to investigate the influence of MI on the pharmacokinetics of paclitaxel in epithelial ovarian cancer patients. During the standard chemotherapy of intravenous paclitaxel (125 mg/m(2) infused over a 3-h period) and intraperitoneal cisplatin (60 mg/m(2)) for patients with FIGO stage II epithelial ovarian cancer, 9 each of total 18 patients were respectively treated with intravenous MI (100 mg) or vehicle control for 4 days. Plasma paclitaxel was analyzed by HPLC and the pharmacokinetic parameters were calculated with non-compartmental analysis. The hematological, hepatic and renal status was monitored before and 3 days after paclitaxel administration. It was observed the terminal t 1/2 and MRT of paclitaxel were significantly (p = 0.002 and 0.001) reduced by MI, respectively, from 11.0 ± 2.2 and 5.6 ± 1.0 h to 7.7 ± 1.7 and 4.0 ± 0.3 h. Hematological toxicity indicated by platelet count and hepatic events marked with ALT, AST and γ-GT were significant in both groups. In spite of the insignificance of decreased system exposure of paclitaxel and recovered hepatic function by MI, they did correlate with each other. It was therefore deduced that the liver toxicities of paclitaxel plus cisplatin chemotherapy potentially decrease hepatic elimination and increase system exposure of paclitaxel, and the recovery of liver function by MI helps to restore hepatic clearance of paclitaxel. The clinical significance of this pharmacokinetic interaction requires further studies with larger population size.

Paclitaxel in self-micro emulsifying formulations: oral bioavailability study in mice

The anticancer drug paclitaxel is formulated for i.v. administration in a mixture of Cremophor EL and ethanol. Its oral bioavailability is very low due to the action of P-glycoprotein in the gut wall and CYP450 in gut wall and liver. However, proof-of-concept studies using the i.v. formulation diluted in drinking water have demonstrated the feasibility of the oral route as an alternative when given in combination with inhibitors of P-glycoprotein and CYP450. Because of the unacceptable pharmaceutical properties of the drinking solution, a better formulation for oral application is needed. We have evaluated the suitability of various self-micro emulsifying oily formulations (SMEOF’s) of paclitaxel for oral application using wild-type and P-glycoprotein knockout mice and cyclosporin A (CsA) as P-glycoprotein and CYP450 inhibitor. The oral bioavailability of paclitaxel in all SMEOF’s without concomitant CsA was low in wild-type mice, showing that this vehicle does not enhance intestinal uptake by itself. Paclitaxel (10 mg/kg) in SMEOF#3 given with CsA resulted in plasma levels that were comparable to the Cremophor EL-ethanol containing drinking solution plus CsA. Whereas the AUC increased linearly with the oral paclitaxel dose in P-glycoprotein knockout mice, it increased less than proportional in wild-type mice given with CsA. In both strains more unchanged paclitaxel was recovered in the feces at higher doses. This observation most likely reflects more profound precipitation of paclitaxel within the gastro-intestinal tract at higher doses. The resulting absolute reduction in absorption of paclitaxel from the gut was possibly concealed by partial saturation of first-pass metabolism when P-glycoprotein was absent. In conclusion, SMEOF’s maybe a useful vehicle for oral delivery of paclitaxel in combination with CsA, although the physical stability within the gastro-intestinal tract remains a critical issue, especially when applied at higher dose levels.

Safety and pharmacology of paclitaxel in patients with impaired liver function: a population pharmacokinetic-pharmacodynamic study

AIMS

To assess quantitatively the safety and pharmacology of paclitaxel in patients with moderate to severe hepatic impairment.

METHODS

Solid tumour patients were enrolled into five liver function cohorts as defined by liver transaminase and total bilirubin concentrations. Paclitaxel was administered as a 3-h intravenous infusion at doses ranging from 110 to 175 mg m(-2), depending on liver impairment. Covariate and semimechanistic pharmacokinetic-pharmacodynamic (PK-PD) population modelling was used to describe the impact of liver impairment on the pharmacology and safety of paclitaxel.

RESULTS

Thirty-five patients were included in the study, and PK data were assessed for 59 treatment courses. Most patients had advanced breast cancer (n = 22). Objective responses to paclitaxel were seen in four patients (11%). Patients in higher categories of liver impairment had a significantly lower paclitaxel elimination capacity (R2 = -0.38, P = 0.05), and total bilirubin was a significant covariate to predict decreased elimination capacity with population modelling (P = 0.002). Total bilirubin was also a significant predictor of increased haematological toxicity within the integrated population PK-PD model (P < 10(-4)). Data simulations were used to calculate safe initial paclitaxel doses, which were lower than the administered doses for liver impairment cohorts III-V.

CONCLUSIONS

Total bilirubin is a good predictor of paclitaxel elimination capacity and of individual susceptibility to paclitaxel-related myelosuppression in cancer patients with moderate to severe liver impairment. The proposed, adapted paclitaxel doses need validation in prospective trials.

Determination of paclitaxel in human and rat blood samples after administration of low dose paclitaxel by HPLC-UV detection

A simple and sensitive HPLC-UV method was developed for the determination of paclitaxel (TXL) in human and rat blood samples. 4-Hydroxybenzoic acid n-hexyl ester was used as an internal standard. TXL was extracted by a liquid-liquid extraction with tert-butylmethyl ether. The disturbing peaks in the case of serum sample were removed by pre-extraction with hexane. The separation of TXL was achieved within 25 min using an ODS column with 50% acetonitrile aqueous solution as a mobile phase at a flow rate of 1.0 mL/min. The eluent was monitored at 230 nm, and the resulted retention times of TXL and IS were 11.2 and 20.4 min. The detection limits of TXL for human plasma, serum and rat plasma samples at a signal-to-noise ratio of 3 were 10, 9.5 and 7.5 ng/mL, respectively. The proposed methods were applicable to the determination of TXL in human patients' plasma ranging from 15 to 27 ng/mL. Furthermore, monitoring of the time course of TXL after its single administration to rat could be demonstrated.

Acquired resistance to the anticancer drug paclitaxel is associated with induction of cytochrome P450 2C8

INTRODUCTION

We have previously shown that human colorectal cancer tissue is able to inactivate the anticancer drug paclitaxel through cytochrome P450 (CYP)2C8 and CYP3A4 metabolisms. The aim of this study was to evaluate whether changes in the expression levels of genes coding for such enzymes are related to anticancer drug resistance after long-term exposure to the drug.

METHODS

Human colorectal cancer cells (Caco-2) that are sensitive to paclitaxel were exposed to increasing concentrations of the drug from 0-250 nM during one year, in order to select paclitaxel-resistant cells. Subsequently, we compared the sensitivity to paclitaxel and the extent of expression of the CYP2C8, CYP3A4 and CYP3A5 genes in original and resistant cells.

RESULTS

Resistant cancer cells displayed a 246-fold increased lethal dose (LD)50 to paclitaxel (p < 0.004) as compared with original cancer cells. A 4.4-fold (p = 0.005) enhancement of CYP2C8 expression and a 5.6-fold (p = 0.001) increase of multidrug resistance (MDR)1 expression was observed in resistant cells exposed to paclitaxel. When paclitaxel was removed from the culture medium, CYP2C8, but not MDR1 expression, reverted to basal levels and the resistance to paclitaxel decreased 3.2-fold (p = 0.005). No major changes in the expression levels of CYP3A4 and CYP3A5 were observed.

CONCLUSIONS

Caco-2 cells are capable of increasing the expression levels of CYP2C8 as a response to long-term exposure to paclitaxel. This study provides evidence for a mechanism of acquired resistance to anticancer therapy based on the induction of anticancer-metabolizing enzymes.

Effect of several compounds on biliary excretion of paclitaxel and its metabolites in guinea-pigs

The objective of this study was to evaluate the in vivo metabolic profile of paclitaxel and to examine the effect of potential co-administered drugs on the biliary secretion of paclitaxel and its metabolites in guinea-pigs. We first investigated in vitro paclitaxel metabolism using liver microsomes obtained from various species to identify the most suitable animal model with a similar metabolism to humans. Then, in vivo paclitaxel metabolism was investigated in male guinea-pigs. The levels of paclitaxel and its metabolites were measured by high-performance liquid chromatography in bile samples from guinea-pigs after paclitaxel i.v. injection (6 mg/kg). We further evaluated the effects of various drugs (quercetin, ketoconazole, dexamethasone, cotrimoxazole) on the biliary secretion of paclitaxel and its metabolites in guinea-pigs. This work demonstrated significant in vitro interspecies differences in paclitaxel metabolism. Our findings showed both in vitro and in vivo similarities between human and guinea-pig biotransformation of paclitaxel. 6alpha-Hydroxypaclitaxel, the main human metabolite of paclitaxel, was found in guinea-pig bile. After paclitaxel combination with ketoconazole or quercetin in guinea-pigs, the cumulative biliary excretion of paclitaxel and its metabolites up to 6 h was significantly decreased by 62 and 76%, respectively. The co-administration of cotrimoxazole or pretreatment with dexamethasone did not alter significantly cumulative biliary excretion. The guinea-pig is a suitable model to study metabolism and biliary excretion of paclitaxel, and to investigate in vivo drug interactions.

Quantitative effect of gender, age, liver function, and body size on the population pharmacokinetics of Paclitaxel in patients with solid tumors

BACKGROUND

The aim of this study was to quantitatively assess the effect of anthropometric and biochemical variables and third-space effusions on paclitaxel pharmacokinetics in solid tumor patients.

MATERIALS AND METHODS

Plasma concentration-time data of paclitaxel were collected in patients with non-small cell lung cancer (n = 84), ovarian cancer (n = 40), and various solid tumors (n = 44), totaling 168 patients. Paclitaxel was given as a 3-hour infusion (n = 163) at doses ranging from 100 to 250 mg/m(2), or as a 24-hour infusion (n = 5) at a dose of 135 or 175 mg/m(2). Data were analyzed using nonlinear mixed-effect modeling.

RESULTS

A three-compartment model with saturable elimination and distribution was used to describe concentration-time data. Male gender and body surface area were positively correlated with maximal elimination capacity of paclitaxel (VM(EL)); patient age and total bilirubin were negatively correlated with VM(EL) (P < 0.005 for all correlations). Typically, male patients had a 20% higher VM(EL); a 0.2 m(2) increase of body surface area led to a 9% increase of VM(EL); a 10-year increase of patient age led to a 5% decrease of VM(EL); and a 10-micromol increase of total bilirubin led to a 14% decrease of VM(EL). Third-space effusions were not correlated with paclitaxel pharmacokinetics.

CONCLUSIONS

This extended retrospective population analysis showed patient gender to significantly and independently affect paclitaxel distribution and elimination. Body surface area, total bilirubin, and patient age were confirmed to affect paclitaxel elimination. This pharmacokinetic model allowed quantification of the covariate effects on the elimination of paclitaxel and may be used for covariate-adapted paclitaxel dosing.

Measurement of paclitaxel and its metabolites in human plasma using liquid chromatography/ion trap mass spectrometry with a sonic spray ionization interface

A quantitative liquid chromatography/ion trap mass spectrometry method for the simultaneous determination of paclitaxel, 6alpha-hydroxypaclitaxel and p-3'-hydroxypaclitaxel in human plasma has been developed and validated. 6alpha-,p-3'-Dihydroxypaclitaxel was also quantified using paclitaxel as a reference and docetaxel as an internal standard. The substances were extracted from 0.500 mL plasma using solid-phase extraction. The elution was performed with acetonitrile and the samples were reconstituted in the mobile phase. Isocratic high-performance liquid chromatography analysis was performed by injecting 50 microL of reconstituted material onto a 100 x 3.00 mm C12 column with a methanol:1% trifluoroacetic acid/ammonium trifluoroacetate in H(2)O 70:30 mobile phase at 350 microL/min. The [M+H](+) ions generated in the sonic spray ionization interface were isolated and fragmented using two serial mass spectrometric methods: one for paclitaxel (transition 854 --> 569 & 551) and the dihydroxymetabolite (transition 886 --> 585 & 567) and one for the hydroxy metabolites (transition 870 --> 585 & 567; transition 870 --> 569 & 551) and docetaxel ([M+Na](+), transition 830 --> 550). Calibration curves were created ranging between 0.5 and 7500 ng/mL for paclitaxel, 0.5 and 750 ng/mL for 6alpha-hydroxypaclitaxel, and 0.5 and 400 ng/mL for p-3'-hydroxypaclitaxel. Adduct ion formation was noted and investigated during method development and controlled by mobile phase optimization. In conclusion, a sensitive method for simultaneous quantification of paclitaxel and its metabolites suitable for analysis in clinical studies was obtained.

Population pharmacokinetics of orally administered paclitaxel formulated in Cremophor EL

AIM

The vehicle Cremophor EL (CrEL) has been shown to impair the absorption of paclitaxel by micellar entrapment of the drug in the gastrointestinal tract. The goal of this study was to develop a semimechanistic population pharmacokinetic model to study the influence of CrEL on the oral absorption of paclitaxel.

METHOD

Paclitaxel plasma-concentration time profiles were available from 55 patients (M:F, 17 : 38; total 67 courses; 797 samples), receiving paclitaxel orally once or twice daily (dose range 60-360 mg m(-2)) together with 12-15 mg kg(-1) cyclosporin A. A population pharmacokinetic model was developed using the nonlinear mixed effect modelling program NONMEM.

RESULTS

After absorption, paclitaxel pharmacokinetics were best described using a two-compartment model with linear distribution from the central compartment into a peripheral compartment and first-order elimination. Paclitaxel in the gastrointestinal tract was modelled as free fraction or bound to CrEL, with only the free fraction available for absorption into the central compartment. The equilibrium between free and bound paclitaxel was influenced by the concentration of CrEL present in the gastrointestinal tract. The concentration of CrEL in the gastrointestinal tract decreased with time with a first order rate constant of 1.73 h(-1). The bioavailability of paclitaxel was independent of the dose and of CrEL. Estimated apparent paclitaxel clearance and volume of distribution were 127 l h(-1) and 409 l, respectively. Large interpatient variability was observed. Covariate analysis did not reveal significant relationships with any of the pharmacokinetic parameters.

CONCLUSION

A pharmacokinetic model was developed that described the pharmacokinetics of orally administered paclitaxel. CrEL strongly influenced paclitaxel absorption from the gastrointestinal tract resulting in time-dependent but no significant dose-dependent absorption over the examined dose range studied.

Utilization of human liver microsomes to explain individual differences in paclitaxel metabolism by CYP2C8 and CYP3A4

Paclitaxel is widely used for treatment of malignant tumors. Paclitaxel is metabolized by CYP2C8 and CYP3A4, and these enzymes are known to differ between individuals, although the details have not been clarified. Recent progress in pharmacogenetics has shown that genetic polymorphisms of metabolic enzymes are related to these individual differences. We investigated the effect of the polymorphisms on paclitaxel metabolism by analyzing metabolic activities of CYP2C8 and CYP3A4 and expressions of mRNA and protein. Production of 6alpha-hydroxypaclitaxel, a metabolite of CYP2C8, was 2.3-fold larger than 3'-p-hydroxypaclitaxel, a metabolite of CYP3A4. Significant inter-individual differences between these two enzyme activities were shown. The expressions of mRNA and protein levels correlated well with the enzyme activities, especially with CYP3A4. Although it was previously reported that CYP2C8*3 showed lower activity than the wild type, two subjects that had the CYP2C8*3 allele did not show lower activities in our study. Inter-individual differences in paclitaxel metabolism may be related to CYP2C8 and CYP3A4 mRNA expression. CYP2C8 is the primary metabolic pathway of paclitaxel, but there is a "shifting phenomenon" in the metabolic pathway of paclitaxel in the liver of some human subjects.

Normal phase high performance liquid chromatography for determination of paclitaxel incorporated in a lipophilic polymer matrix

A normal phase (NP) high performance liquid chromatography (HPLC) method was developed for analysis of paclitaxel incorporated in poly(sebacic-co-ricinoleic acid), a lipophilic polymer matrix utilized for preparation of an injectable formulation for the localized delivery of paclitaxel. Thin layer chromatography experiments revealed that separation of paclitaxel from the polymer is dependent on the eluting strength (solvent strength) of the mobile phase. The HPLC system consists of a Purospher STRAR Si analytical HPLC column (5 microm, 250mm x 4mm, Merck), and 1-2.5% (v/v) methanol in dichloromethane as the mobile phase. Detection was by UV absorbance at 240 and 254 nm. The effect of the mobile phase composition on paclitaxel retention, peak shape and column efficiency, and the influence of the sample loading on the shape of the paclitaxel peak were studied. The mobile phases used for the chromatography consisted of 1.5% (v/v) methanol in dichloromethane. Paclitaxel was determined in the formulation and in the samples from degradation studies using UV detection at a wavelength of 254 nm. UV detection at 240 nm has advantages for following polymer matrix degradation products due to higher detector response at this wavelength. The utility of the proposed NP HPLC approach was demonstrated by assessment of intra- and inter-batch content uniformity, and by the determination of paclitaxel content after 7 and 60 days exposure of the paclitaxel-loaded polymer matrix to in vitro and in vivo degradation.

Bayesian Pharmacokinetically Guided Dosing of Paclitaxel in Patients with Non-Small Cell Lung Cancer

PURPOSE

Paclitaxel is a taxane derivative with a profound antitumor activity against a variety of solid tumors. In a previous clinical study in patients with non-small cell lung cancer (NSCLC) treated with paclitaxel, it was shown that paclitaxel plasma concentrations of 0.1 micro mol/liter for > or = 15 h were associated with prolonged survival. The purpose of this study was to evaluate the feasibility of Bayesian dose individualization to attain paclitaxel plasma concentrations >0.1 micromol/liter for > or = 15 h.

EXPERIMENTAL DESIGN

Patients with stage IIIb-IV NSCLC were treated with paclitaxel and carboplatin once every 3 weeks for a maximum of six courses. During the first course, a standard paclitaxel dose of 175 mg/m(2) was administered i.v. in 3 h. In subsequent courses, the paclitaxel dose was individualized based on observed paclitaxel concentrations in plasma during the previous course(s) using a Bayesian algorithm. The paclitaxel dose of a subsequent course was increased to the lowest dose for which the predicted time period during which the paclitaxel plasma concentration exceeds 0.1 micromol/liter was >15 h.

RESULTS

A total of 25 patients have been included in the study (92 evaluable courses). During the first course, the median time period above the threshold concentration was 16.3 h (range, 7.6-31.6 h), and was <15 h for 9 patients (36%). During subsequent individualized courses, the time period above the threshold concentration was <15 h in 23% (5 of 22), 14% (2 of 14), 23% (3 of 13), 11% (1 of 9), and 11% (1 of 9) of the patients in the second, third, fourth, fifth, and sixth course, respectively. Dose increments, ranging from 5 to 65 mg/m(2), were performed in 29 of the 67 individualized courses. Patients with increased individualized doses had similar regimen related toxicities compared with those remaining at a dose of 175 mg/m(2). Toxicity was reversible and manageable, and was mainly hematological (granulocytopenia CTC grade 3/4 in 80% of the patients). The objective response rate was 20%.

CONCLUSIONS

The results indicate that the applied pharmacokinetically guided dosing strategy for paclitaxel is safe and technically feasible. A randomized study is necessary to demonstrate whether dose individualization may result in improved activity and efficacy in patients with NSCLC.

Development and validation of a method to determine the unbound paclitaxel fraction in human plasma

Paclitaxel is pharmaceutically formulated in a mixture of Cremophor EL and ethanol (1:1, v/v). The unbound fraction of the anticancer drug paclitaxel in plasma is dependent on both plasma protein binding and entrapment in Cremophor EL micelles. We have developed a simple and reproducible method for the quantification of the unbound paclitaxel fraction in human plasma. Human plasma was spiked with [3H]paclitaxel and [14C]glucose (unbound reference) and incubated at 37 degrees C for 30 min. Plasma ultrafiltrate was prepared by a micropartition system (MPS-1) and collected in a sample cup containing 100 microl of plasma to prevent the loss of paclitaxel due to adsorption. The radionuclides were separated after combustion of the biological samples using a sample oxidizer and the radioactivity was determined by liquid scintillation counting. The unbound fraction of paclitaxel was calculated by dividing the ratios of 3H and 14C in plasma ultrafiltrate and in plasma. The method was thoroughly validated using human plasma spiked with pharmacologically relevant concentrations of paclitaxel (10-1000 ng/ml) and Cremophor EL (0.25-2.0%). The method was precise, with a within-day precision ranging from 3.9 to 11.0% and a between-day precision ranging from 5.8 to 13.1%. In patient plasma with low serum albumin values containing 1% of Cremophor EL, the unbound fraction appeared to be significantly higher than that in plasma with normal albumin values. The determination of the unbound fraction of paclitaxel proved to be stable during a 10-week storage at -20 degrees C. Furthermore, the assay was applicable in patient samples. This assay can be used to determine the unbound fraction of paclitaxel in plasma. Moreover, its design should allow the determination of the unbound concentrations of other hydrophobic drugs.

Quantitative determination of paclitaxel in human plasma using semi-automated liquid-liquid extraction in conjunction with liquid chromatography/tandem mass spectrometry

This paper describes a high-throughput sample preparation procedure combined with LC-MS/MS analysis to measure paclitaxel in human plasma. Paclitaxel and an internal standard were extracted from plasma by a semi-automated robotic method using liquid-liquid extraction. Thereafter compounds were separated on a RP C18 column. Detection was by a PE Sciex API 3000 mass spectrometer equipped with a TurboIonSpray interface. The compounds were detected in positive ion mode using the mass transition m/z 854.6-->286.2 and m/z 831.6-->263.2 for paclitaxel and the internal standard, respectively. The limit of quantitation for paclitaxel was 1 ng/ml with an imprecision of 5.2% following extraction of 0.1 ml of plasma. Linearity was confirmed over the whole calibration range (1-1000 ng/ml) with correlation coefficients higher than 0.99 indicating good fits of the regression models. The inter and intra-day precision was better than 9.5% and the accuracy ranged from 90.3 to 104.4%. The assay was simple, fast, specific and exhibited excellent ruggedness.

Biological and environmental monitoring of hospital personnel exposed to antineoplastic agents: a review of analytical methods

In order to assess occupational exposure of hospital personnel involved in the preparation and administration of antineoplastic drugs, biological and environmental monitoring are essential to identify the main exposure routes and to quantify potential health risks. If workplace contamination cannot be completely avoided, it is of utmost importance to reduce exposure to the lowest possible levels. To this aim, not only do education and training of the exposed subjects play an important role, but accurate standardized sampling techniques and analytical methods are also required. A critical overview of the most significant methods available in the literature is presented and their value is discussed, especially with respect to their sensitivity and specificity. In addition, attention is given to validation procedures and, consequently, to their reliability. The results from the most important surveys carried out at hospital departments are also discussed, with a view to improving both monitoring strategies and moreover working conditions.

Drug interactions of paclitaxel metabolism in human liver microsomes

The human liver metabolism of paclitaxel (Taxol), an anticancer drug, leads to three metabolites: 6alpha-hydroxypaclitaxel, 3'-p-hydroxypaclitaxel and 6alpha,3'-p-dihydroxypaclitaxel. The inter-individual variability of paclitaxel metabolism was investigated first in vitro using 22 human liver microsomes. Three metabolites have been detected by HPLC. This preliminary work revealed marked inter-individual differences in paclitaxel metabolism. The amount of major metabolite 6alpha-hydroxypaclitaxel formed varied 16-fold (0.7 to 11.5 nmol/mg/h). We next studied the effect of 29 compounds (antineoplastics, antiemetics, histamine-2 receptor antagonist, antalgics, antifungals, antivirals, psychotropics, antibiotic, corticoid, antiarrhythmic, calcium channel blocker) on paclitaxel metabolism in human liver microsomes. Among the compounds studied, quercetin, antifungal drugs such as ketoconazole and miconazole, and the antineoplastic drug doxorubicin inhibited formation of 6alpha-hydroxypaclitaxel. Dixon plots indicated that quercetin and doxorubicin inhibited 6alpha-hydroxypaclitaxel formation through a competitive mechanism with a Ki of 10.1 microM and 64.8 microM, respectively. The inhibition of this metabolite by ketoconazole was through a noncompetitive mechanism with a Ki of 11.8 microM. Our data thus suggest that special attention should be paid when these drugs are combined in clinical practice.

Weekly oral paclitaxel as first-line treatment in patients with advanced gastric cancer

BACKGROUND

Pharmacokinetic study has shown that co-administration of cyclosporin A (CsA), which acts as a P-glycoprotein (P-gp) and CYP-3A blocker, resulted in an 8-fold increase in the systemic exposure of oral paclitaxel. Two doses of oral paclitaxel on 1 day in combination with CsA resulted in higher systemic exposure than single dose administration.

PATIENTS AND METHODS

In this phase II study, chemonaïve patients with advanced gastric cancer received oral paclitaxel weekly in two doses of 90 mg/m(2) on the same day; CsA (10 mg/kg) was given 30 min before each dose of oral paclitaxel.

RESULTS

In 25 patients, the main toxicities were: nausea CTC grade 2/3, 10 patients (40%); vomiting grade 2/3, 4 patients (20%); diarrhea grade 2/3, 6 patients (24%); neutropenia grade 3/4, 5 patients (20%). In the 24 evaluable patients, eight partial responses were observed, resulting in an overall response rate (ORR) of 33% [95% confidence interval (CI) 18% to 52%]. Eleven patients had stable disease (46%) and 5 patients showed progressive disease (21%). The ORR in the total population was 32% (95% CI 17% to 50%). The median time to progression was 16 weeks (95% CI 9-22). Pharmacokinetic analyses revealed that the mean area under the plasma concentration-time curve (AUC) of orally administered paclitaxel (+/- standard deviation) was 3757.6 +/- 939.4 ng.h/ml in week 1 and 3928.4 +/- 1281 ng.h/ml in week 2. The intrapatient variability in the AUC was 12%.

CONCLUSIONS

Oral paclitaxel in combination with CsA is both active and safe in chemonaïve patients with advanced gastric cancer. Toxicities were mainly gastrointestinal.

Sensitive liquid chromatography-mass spectrometry assay for quantitation of docetaxel and paclitaxel in human plasma

We have developed a high-performance liquid chromatography-electrospray ionization mass spectrometry (LC-MS) method for quantifying docetaxel and paclitaxel in human plasma. The assay fulfills the need for defining the lower plasma concentrations of these antineoplastic agents that result from a number of changes in how these agents are used clinically. The assay uses paclitaxel as the internal standard for docetaxel, and vice versa; solid-phase extraction; a Phenomenex Hypersil ODS (5 micrometer, 100x2 mm) reversed-phase analytical column; an isocratic mobile phase of 0.1% formic acid in methanol-water (70:30, v/v); and mass spectrometric detection using electrospray positive mode electron ionization. The assay has a lower limit of quantitation (LLOQ) of 0.3 nM and is linear between 0.3 nM and 1 microM for docetaxel. For paclitaxel, the LLOQ was 1 nM, and the assay is linear between 1 nM and 1 microM. We demonstrated the suitability of this assay for docetaxel by using it to quantify the docetaxel concentrations in plasma of a patient given 40 mg/m(2) of docetaxel and comparing those results to results produced when the same samples were assayed with an HPLC assay using absorbance detection. In a similar manner, the suitability of the assay for paclitaxel was demonstrated by using it to quantify the concentrations of paclitaxel in the plasma of a patient given 15 mg/m(2) of paclitaxel and comparing those results to results produced when the same samples were assayed with an HPLC assay using absorbance detection. The LC-MS assay, which proved superior because of its greater sensitivity and relatively short (7 min) run time, should be an important tool for future pharmacokinetic analyses of docetaxel and paclitaxel.

Phase II and pharmacologic study of weekly oral paclitaxel plus cyclosporine in patients with advanced non-small-cell lung cancer

PURPOSE

A phase II study was performed to assess the efficacy and toxicity of oral cyclosporine (CsA) plus paclitaxel in advanced non-small-cell lung cancer (NSCLC).

PATIENTS AND METHODS

Chemotherapy-naive or previously treated patients (one regimen) with measurable disease and World Health Organization performance status <or= 2 were eligible. Oral paclitaxel was given weekly in a dose of 90 mg/m(2) bid. CsA (10 mg/kg) was given 30 minutes before each dose of oral paclitaxel.

RESULTS

Twenty-six patients with a median age of 54 years (range, 32 to 77 years) were entered onto this study. Eighteen patients (69%) had received one prior chemotherapy regimen. The most frequently recorded toxicities were as follows: National Cancer Institute common toxicity criteria grade 3 neutropenia, eight patients (31%); grade 4, six patients (23%); grade 4 febrile neutropenia, three patients (12%); grade 2/3 neurotoxicity, three patients (12%); and grade 2 nail changes, four patients (15%). The overall response rate (ORR) of the 23 assessable patients was 26% (95% confidence interval [CI], 10% to 48%). In the intention-to-treat population, the ORR was 23% (95% CI, 9% to 44%). The median time to progression was 3.5 months (95% CI, 1.2 to 3.9 months), and median overall survival was 6.0 months (95% CI, 2.3 months to not available). Pharmacokinetics revealed that the mean area under the concentration-time curve (AUC) of oral paclitaxel was 5.0 +/- 2.3 micro mol/L/h in week 1 and 4.6 +/- 2.0 micro mol/L/h in week 2, with interpatient variabilities (coefficient of variation [%CV]) of 45% and 42%, respectively. The intrapatient variability (%CV) of the AUC was 14.5%.

CONCLUSION

Oral paclitaxel plus CsA is active and safe in advanced NSCLC, including in patients previously treated with chemotherapy.

Expression of paclitaxel-inactivating CYP3A activity in human colorectal cancer: implications for drug therapy

Cytochrome P450 3A is a drug-metabolising enzyme activity due to CYP3A4 and CYP3A5 gene products, that is involved in the inactivation of anticancer drugs. This study analyses the potential of cytochrome P450 3A enzyme in human colorectal cancer to impact anticancer therapy with drugs that are cytochrome P450 3A substrates. Enzyme activity, variability and properties, and the ability to inactivate paclitaxel (taxol) were analysed in human colorectal cancer and healthy colorectal epithelium. Cytochrome P450 3A enzyme activity is present in healthy and tumoral samples, with a nearly 10-fold interindividual variability. Nifedipine oxidation activity+/-s.d. for colorectal cancer microsomes was 67.8+/-36.6 pmol min(-1) mg(-1). The K(m) of the tumoral enzyme (42+/-8 microM) is similar to that in healthy colorectal epithelium (36+/-8 microM) and the human liver enzyme. Colorectal cancer microsomes metabolised the anticancer drug paclitaxel with a mean activity was 3.1+/-1.2 pmol min(-1) mg(-1). The main metabolic pathway is carried out by cytochrome P450 3A, and it is inhibited by the cytochrome P450 3A-specific inhibitor ketoconazole with a K(I) value of 31 nM. This study demonstrates the occurrence of cytochrome P450 3A-dependent metabolism in colorectal cancer tissue. The metabolic activity confers to cancer cells the ability to inactivate cytochrome P450 3A substrates and may modulate tumour sensitivity to anticancer drugs.

On-line solid-phase extraction and determination of paclitaxel in human plasma

The application of coupled-column liquid chromatographic analysis to pharmacokinetic studies eliminates the need for sample clean-up from plasma. Considering lipophilic antineoplastic agents, we tested this approach to analyze paclitaxel under unfavourable circumstances (i.e., weekly low-dose regimen, plasma protein binding >90%, UV detection at 229 nm). The excellent quality control data (recovery: 95.6-100.7%, inter-assay relative standard deviation on 5 days: 1.3-3.2%, accuracy: 0.9-2.7%) and the detection limit of 19 nM indicates the usefulness of this method for the analysis of paclitaxel in plasma using on-line solid-phase extraction.

The antineoplastic drug Paclitaxel has immunosuppressive properties that can effectively promote allograft survival in a rat heart transplant model

INTRODUCTION

Recurrent and de novo neoplasms are ominous risk factors for transplant patients. In particular, when organ transplantation is attempted to cure isolated cancers, conventional immunosuppression likely promotes cancer reestablishment. Therefore, drugs with both immunosuppressive and antineoplastic activity are needed. We show that the anticancer agent paclitaxel may fulfill these diverse expectations.

METHODS

Heterotopic heart transplantation was performed in the ACI-to-Lewis or Lewis-to-ACI rat-strain combination and paclitaxel was injected i.p. daily (days 0-14) at doses from 0.75-1.5 mg/kg. Serum cytotoxic antidonor antibody levels were measured using a complement-mediated cell cytotoxicity assay. In vitro, the effect of paclitaxel on Lewis lymphocyte viability and apoptosis was determined. Also, Lewis lymphocytes preconditioned with irradiated ACI cells+/-paclitaxel, were restimulated with ACI cells and tested for cytotoxic T cell (CTL) activity and interleukin-2 (IL-2) production.

RESULTS

Paclitaxel promoted heart allograft survival in a dose-dependent manner in both high- and low-responder transplant combinations. Furthermore, low-doses of paclitaxel (0.75-1.0 mg/kg) and cyclosporine (1 mg/kg) in combination synergistically increased transplant survival. Immunologically, paclitaxel markedly reduced the antidonor cytotoxic antibody response. In vitro, nearly 90% of prestimulated lymphocytes were killed by paclitaxel and cells became positive for the apoptosis marker, annexin-V. Furthermore, paclitaxel reduced CTL activity and IL-2 production after alloantigen rechallenge.

CONCLUSION

Paclitaxel, a clinically proven antineoplastic agent, also has potent immunosuppressive properties in rodent organ transplantation. This drug could be extremely valuable in transplant situations where de novo cancer develops, or when organ transplantation is performed to treat isolated, but typically recurrent, neoplasms.

Analytical approaches for traditional chinese medicines exhibiting antineoplastic activity

Traditional Chinese medicines have attracted great interest in recent researchers as alternative antineoplastic therapies. This review focuses on analytical approaches to various aspects of the antineoplastic ingredients of traditional Chinese medicines. Emphasis will be put on the processes of biological sample extraction, separation, clean-up steps and the detection. The problems of the extraction solvent selection and different types of column chromatography are also discussed. The instruments considered are gas chromatography, capillary electrophoresis (CE) and high-performance liquid chromatography (HPLC) connected with various detectors (ultraviolet, fluorescence, electrochemistry, mass, etc.). In addition, determinations of antineoplastic herbal ingredients, including camptothecin, taxol (paclitaxel), vinblastine. vincristine, podophyllotoxin, colchicine, and their related compounds, such as irinotecan, SN-38, topotecan, 9-aminocamptothecin, docetaxel (taxotere) and etoposide, are briefly summarized. These drugs are structurally based on the herbal ingredients, and some of them are in trials for clinical use. Evaluation of potential antineoplastic herbal ingredients, such as harringtonine, berberine, emodin, genistein, berbamine, daphnoretin, and irisquinone, are currently investigated in laboratories. Other folk medicines are excluded from this paper because their antineoplastic ingredients are unknown.

The co-solvent Cremophor EL limits absorption of orally administered paclitaxel in cancer patients

The purpose of this study was to investigate the effect of the co-solvents Cremophor EL and polysorbate 80 on the absorption of orally administered paclitaxel. 6 patients received in a randomized setting, one week apart oral paclitaxel 60 mg m(-2) dissolved in polysorbate 80 or Cremophor EL. For 3 patients the amount of Cremophor EL was 5 ml m(-2), for the other three 15 ml m(-2). Prior to paclitaxel administration patients received 15 mg kg(-1) oral cyclosporin A to enhance the oral absorption of the drug. Paclitaxel formulated in polysorbate 80 resulted in a significant increase in the maximal concentration (C(max)) and area under the concentration-time curve (AUC) of paclitaxel in comparison with the Cremophor EL formulations (P = 0.046 for both parameters). When formulated in Cremophor EL 15 ml m(-2), paclitaxel C(max) and AUC values were 0.10 +/- 0.06 microM and 1.29 +/- 0.99 microM h(-1), respectively, whereas these values were 0.31 +/- 0.06 microM and 2.61 +/- 1.54 microM h(-1), respectively, when formulated in polysorbate 80. Faecal data revealed a decrease in excretion of unchanged paclitaxel for the polysorbate 80 formulation compared to the Cremophor EL formulations. The amount of paclitaxel excreted in faeces was significantly correlated with the amount of Cremophor EL excreted in faeces (P = 0.019). When formulated in Cremophor EL 15 ml m(-2), paclitaxel excretion in faeces was 38.8 +/- 13.0% of the administered dose, whereas this value was 18.3 +/-15.5% for the polysorbate 80 formulation. The results show that the co-solvent Cremophor EL is an important factor limiting the absorption of orally administered paclitaxel from the intestinal lumen. They highlight the need for designing a better drug formulation in order to increase the usefulness of the oral route of paclitaxel

Rapid high-performance liquid chromatographic determination of docetaxel (Taxotere) in plasma using liquid-liquid extraction

A new rapid and sensitive high-performance liquid chromatographic method for analysis of docetaxel (Taxotere) in human plasma was developed and validated. After adding an internal standard (paclitaxel, Taxol), plasma was extracted following a simple liquid-liquid extraction with diethyl ether. Extraction efficiency averaged 95% for docetaxel. Separation was performed using a Nucleosil (C18) 5 microm column, monitored at 227 nm. The isocratic mobile phase consisted of acetonitrile-acetate buffer, pH 5-tetrahydrofuran (45:50:5, v/v) pumped at a flow-rate of 1.8 ml/min. The limit of quantification for docetaxel in plasma was 12.5 ng/ml. Retention times for docetaxel and paclitaxel were 7.7 and 9 min, respectively. Standard curves were linear over a range of 25-1,000 ng/ml. This new method is rapid since it does not require time-consuming extraction procedures, or complex chromatographic conditions. This rapidity, along with the lack of chromatographic interferences with various other drugs likely to be administered to the cancer patients (pain killers, corticoids, antiemetics drugs) make this method suitable for daily routine analysis of Taxotere, a major anticancer drug extensively used in clinical oncology.

Simultaneous determination of taxol and its metabolites in microsomal samples by a simple thin-layer chromatography radioactivity assay--inhibitory effect of NK-104, a new inhibitor of HMG-CoA reductase

The inhibitory effect of NK-104, a potent inhibitor of HMG-CoA reductase, on taxol metabolism was examined using radio-TLC. This method is described for in vitro measurement of taxol metabolites as an alternative to the commonly used HPLC assay. After incubation of 14C-taxol with human liver microsomes, the supernatants were developed using a solvent system consisting of toluene-acetone-formic acid (60:39:1, v/v) and quantified with a bioimaging analyzer. The described method provides a valuable tool for the simultaneous determination of unchanged taxol and its major metabolites. There was no inhibitory effect of NK-104 on CYP-mediated metabolism of taxol in human liver microsomes.

The effect of different doses of cyclosporin A on the systemic exposure of orally administered paclitaxel

The objective of this study was to define the minimally effective dose of cyclosporin A (CsA) that would result in a maximal increase of the systemic exposure to oral paclitaxel. Six evaluable patients participated in this randomized cross-over study in which they received at two occasions two doses of 90 mg/m(2) oral paclitaxel 7 h apart in combination with 10 or 5 mg/kg CsA. Dose reduction of CsA from 10 to 5 mg/kg resulted in a statistically significant decrease in the area under the plasma concentration-time curve (AUC) and time above the threshold concentrations of 0.1 microM (T>0.1 microM) of oral paclitaxel. The mean (+/-SD) AUC and T>0.1 microM values of oral paclitaxel with CsA 10 mg/kg were 4.29+/-0.88 microM x h and 12.0+/-2.1 h, respectively. With CsA 5 mg/kg these values were 2.75+/-0.63 microM x h and 7.0+/-2.1 h, respectively (p=0.028 for both parameters). In conclusion, dose reduction of CsA from 10 to 5 mg/kg resulted in a significant decrease in the AUC and T>0.1 microM values of oral paclitaxel. Because CsA 10 mg/kg resulted in similar paclitaxel AUC and T>0.1 microM values compared to CsA 15 mg/kg (data which we have published previously), the minimally effective dose of CsA is determined at 10 mg/kg.

Co-administration of GF120918 significantly increases the systemic exposure to oral paclitaxel in cancer patients

Oral bioavailability of paclitaxel is very low, which is due to efficient transport of the drug by the intestinal drug efflux pump P-glycoprotein (P-gp). We have recently demonstrated that the oral bioavailability of paclitaxel can be increased at least 7-fold by co-administration of the P-gp blocker cyclosporin A (CsA). Now we tested the potent alternative orally applicable non-immunosuppressive P-gp blocker GF120918. Six patients received one course of oral paclitaxel of 120 mg/m(2)in combination with 1000 mg oral GF120918 (GG918, GW0918). Patients received intravenous (i.v.) paclitaxel 175 mg/m(2)as a 3-hour infusion during subsequent courses. The mean area under the plasma concentration-time curve (AUC) of paclitaxel after oral drug administration in combination with GF120918 was 3.27 +/- 1.67 microM x h. In our previously performed study of 120 mg/m(2)oral paclitaxel in combination with CsA the mean AUC of paclitaxel was 2.55 +/- 2.29 microM x h. After i.v. administration of paclitaxel the mean AUC was 15.92( )+/- 2.46 microM x h. The oral combination of paclitaxel with GF120918 was well tolerated. The increase in systemic exposure to paclitaxel in combination with GF120918 is of the same magnitude as in combination with CsA. GF120918 is a good and safe alternative for CsA and may enable chronic oral therapy with paclitaxel.

Metabolism and excretion of paclitaxel after oral administration in combination with cyclosporin A and after i.v. administration

The objective of this study was to compare the quantitative excretion of paclitaxel and metabolites after i.v. and oral drug administration. Four patients received 300 mg/m2 paclitaxel orally 30 min after 15 mg/kg oral cyclosporin A, co-administered to enhance the uptake of paclitaxel. Three weeks later these and three other patients received 175 mg/m2 paclitaxel by i.v. infusion. Blood samples, urine and feces were collected up to 48-96 h after administration, and analyzed for paclitaxel and metabolites. The area under the plasma concentration-time curve of paclitaxel after i.v. administration (175 mg/m2) was 16.2 +/- 1.7 microM x h and after oral administration (300 mg/m2) 3.8 +/- 1.5 microM x h. Following i.v. infusion of paclitaxel, total fecal excretion was 56 +/- 25%, with the metabolite 6alpha-hydroxypaclitaxel being the main excretory product (37 +/- 18%). After oral administration of paclitaxel, total fecal excretion was 76 +/- 21%, of which paclitaxel accounted for 61 +/- 14%. In conclusion, after i.v. administration of paclitaxel, excretion occurs mainly in the feces with the metabolites as the major excretory products. Orally administered paclitaxel is also mainly excreted in feces but with the parent drug in highest amounts. We assume that this high amount of parent drug is due to incomplete absorption of orally administered paclitaxel from the gastrointestinal tract.

Phase I and pharmacokinetic study of oral paclitaxel

PURPOSE

To investigate dose escalation of oral paclitaxel in combination with dose increment and scheduling of cyclosporine (CsA) to improve the systemic exposure to paclitaxel and to explore the maximum-tolerated dose (MTD) and dose-limiting toxicity (DLT).

PATIENTS AND METHODS

A total of 53 patients received, on one occasion, oral paclitaxel in combination with CsA, coadministered to enhance the absorption of paclitaxel, and, on another occasion, intravenous paclitaxel at a dose of 175 mg/m(2) as a 3-hour infusion.

RESULTS

The main toxicities observed after oral intake of paclitaxel were acute nausea and vomiting, which reached DLT at the dose level of 360 mg/m(2). Dose escalation of oral paclitaxel from 60 to 300 mg/m(2) resulted in significant but less than proportional increases in the plasma area under the concentration-time curve (AUC) of paclitaxel. The mean AUC values +/- SD after 60, 180, and 300 mg/m(2) of oral paclitaxel were 1.65 +/- 0.93, 3.33 +/- 2.39, and 3.46 +/- 1.37 micromol/L.h, respectively. Dose increment and scheduling of CsA did not result in a further increase in the AUC of paclitaxel. The AUC of intravenous paclitaxel was 15.39 +/- 3.26 micromol/L.h.

CONCLUSION

The MTD of oral paclitaxel was 300 mg/m(2). However, because the pharmacokinetic data of oral paclitaxel, in particular at the highest doses applied, revealed nonlinear pharmacokinetics with only a moderate further increase of the AUC with doses up to 300 mg/m(2), the oral paclitaxel dose of 180 mg/m(2) in combination with 15 mg/kg oral CsA is considered most appropriate for further investigation. The safety of the oral combination at this dose level was good.

Cremophor EL causes (pseudo-) non-linear pharmacokinetics of paclitaxel in patients

The non-linear plasma pharmacokinetics of paclitaxel in patients has been well established, however, the exact underlying mechanism remains to be elucidated. We have previously shown that the non-linear plasma pharmacokinetics of paclitaxel in mice results from Cremophor EL. To investigate whether Cremophor EL also plays a role in the non-linear pharmacokinetics of paclitaxel in patients, we have established its pharmacokinetics in patients receiving paclitaxel by 3-, 24- or 96-h intravenous infusion. The pharmacokinetics of Cremophor EL itself was non-linear as the clearance (Cl) in the 3-h schedules was significantly lower than when using the longer 24- or 96-h infusions (Cl175-3 h = 42.8+/-24.9 ml h(-1) m(-2); CI175-24 h = 79.7+/-24.3; P = 0.035 and Cl135-3 h = 44.1+/-21.8 ml h(-1) m(-1); Cl140-96 h = 211.8+/-32.0; P < 0.001). Consequently, the maximum plasma levels were much higher (0.62%) in the 3-h infusions than when using longer infusion durations. By using an in vitro equilibrium assay and determination in plasma ultrafiltrate we have established that the fraction of unbound paclitaxel in plasma is inversely related with the Cremophor EL level. Despite its relatively low molecular weight, no Cremophor EL was found in the ultrafiltrate fraction. Our results strongly suggest that entrapment of paclitaxel in plasma by Cremophor EL, probably by inclusion in micelles, is the cause of the apparent nonlinear plasma pharmacokinetics of paclitaxel. This mechanism of a (pseudo-)non-linearity contrasts previous postulations about saturable distribution and elimination kinetics and means that we must re-evaluate previous assumptions on pharmacokinetics-pharmacodynamics relationships.

Rapid and sensitive determination of paclitaxel in mouse plasma by high-performance liquid chromatography

This report describes a rapid, simple and sensitive isocratic high-performance liquid chromatography with diode array UV detection for micro-sample analysis of paclitaxel in mouse plasma. The analysis utilized a Capcell-pak octadecyl analytical column and a mobile phase consisting of acetonitrile--0.1% phosphoric acid in deionized water (55:45, v/v). Paclitaxel and n-hexyl p-hydroxybenzoic acid (internal standard) were extracted from plasma by one-step extraction with tert.-butyl methyl ether. Peak purity was determined over a UV wavelength range of 200 to 400 nm. Paclitaxel and the internal standard were eluted at 3.4 min and 5.4 min, respectively, at a mobile phase flow-rate of 1.3 ml/min. No interfering peaks were observed and the total run time was 10 min. The standard curve was linear (r = 0.9999) over the concentration range of 0.010-500 micrograms/ml. The extraction recovery was > 90% for both paclitaxel and n-hexyl p-hydroxybenzoic acid. The intra- and inter-day assay variabilities of paclitaxel ranged from 0.4 to 2.2% and 0.6 to 7.8%, respectively. The LOD and LOQ were 5 and 10 ng/ml, respectively, for paclitaxel using a plasma sample volume of 100 microliters. This highly sensitive and simple assay method was successfully applied to a pharmacokinetic study after i.v. administration of paclitaxel 20 mg/kg to mice.

European drug development and its impact on national activities: the Dutch example

The development of new anticancer agents is becoming an increasingly complex task which is often beyond the capabilities of a single institution or company. To ensure fast, efficient, high-quality drug development, good coordination and collaboration are essential prerequisites. In this paper, the Dutch national drug development program is described as an example and placed in the perspective of Europe-wide efforts. Since knowledge of new molecular targets and biological approaches by which the malignant growth of cells can be stopped or prevented is increasing, new guidelines for the development of noncytotoxic drugs are required. To avoid duplication and to make the results obtained using these new agents comparable so that patients will gain the maximum benefit of efforts in this field, further extension of international coordination will be of utmost importance in the coming years.

Human liver microsomal metabolism of paclitaxel and drug interactions

The aim of this study was to investigate the influence of several anticancer drugs and investigational multidrug resistance (MDR) reversing agents on the hepatic metabolism of paclitaxel (Taxol) to its primary metabolites, 6alpha-hydroxypaclitaxel (metabolite, MA) and 3'-p-hydroxypaclitaxel (metabolite, MB). There is significant inter-individual variability associated with the levels of these two metabolites. In many cases, 6alpha-hydroxypaclitaxel has been observed to be the predominant metabolite, in others, 3'-p-hydroxypaclitaxel has been the principal metabolite. The formation of 6alpha-hydroxypaclitaxel and 3'-p-hydroxypaclitaxel is catalyzed by cytochrome P450 isozymes CYP2C8 and CYP3A4, respectively. A number of factors, including co-administration of drugs and adjuvants, are known to influence the activity of these isozymes. Therefore, the influence of MDR reversing agents, R-verapamil, cyclosporin A (CsA) and tamoxifen and anti-cancer drugs doxorubicin, etoposide (VP-16) and cisplatin on paclitaxel metabolism was assessed employing human liver microsomes in vitro. Paclitaxel (10 microM) was incubated with human liver microsomes (1 mg protein, -0.34 nmol CYP) in the presence of a NADPH generating system at 37 degrees C for 1 h, with and without the presence of interacting drug. Controls included incubations with quercetin and ketoconazole, known inhibitors of 6alpha-hydroxypaclitaxel and 3'-p-hydroxypaclitaxel formation, respectively. At the end of the incubation period, paclitaxel and the metabolites were extracted in ethyl acetate and analyzed employing an HPLC method. Significant inhibition of paclitaxel conversion to 6alpha-hydroxypaclitaxel and 3'-p-hydroxypaclitaxel was observed in the presence of R-verapamil, tamoxifen and VP-16 (P 0.005). Doxorubicin significantly inhibited the formation of 3'-p-hydroxypaclitaxel and CsA inhibited the formation of 6alpha-hydroxypaclitaxel (P 0.005). This study demonstrates that co-administration of several of the above listed compounds could lead to significant changes in the pharmacokinetics of paclitaxel.

Influence of Cremophor EL on the quantification of paclitaxel in plasma using high-performance liquid chromatography with solid-phase extraction as sample pretreatment

For the quantitative determination of paclitaxel in human plasma reversed-phase high-performance liquid chromatographic (HPLC) methods with solid-phase extraction (SPE) as sample pretreatment procedure are frequently used. Recovery problems arose during the quantification of paclitaxel in plasma samples of patients. The major problems were a large batch-to-batch difference in performance of the SPE columns and the effects of the pharmaceutical vehicle Cremophor EL on the performance of the SPE. Cremophor EL concentrations exceeding 1.0% (v/v) had a great impact on the absolute recovery of paclitaxel from human plasma with the SPE procedure. The recoveries decreased approximately 10 to 40% depending on the quality of the batch SPE columns. The problems are avoided by using 2'-methylpaclitaxel as the internal standard. This study points out the importance of including the effects of a pharmaceutical vehicle, like Cremophor EL, in the validation programme of a bioanalytical assay and the use of an internal standard in HPLC paclitaxel assays preceded by SPE as sample pretreatment procedure.

Determination of paclitaxel in human plasma using single solvent extraction prior to isocratic reversed-phase high-performance liquid chromatography with ultraviolet detection

An isocratic reversed-phase high-performance liquid chromatographic method with ultraviolet detection at 230 nm has been developed for the determination of paclitaxel in human plasma. Plasma samples were prepared by a selective one-step liquid-liquid extraction involving a mixture of acetonitrile-n-butyl chloride (1:4, v/v). Paclitaxel and the internal standard docetaxel were separated using a column packed with ODS-80A material, and a mobile phase consisting of water-methanol-tetrahydrofuran-ammonium hydroxide (37.5:60:2.5:0.1, v/v). The calibration graph for paclitaxel was linear in the range 10-500 ng/ml, with a lower limit of quantitation of 10 ng/ml, using 1 ml plasma samples. The extraction recoveries of spiked paclitaxel and docetaxel to drug-free human plasma were 89.6+/-8.52 and 93.7+/-5.0%, respectively. Validation data showed that the assay for paclitaxel is sensitive, selective, accurate and reproducible. The assay has been used in a single pharmacokinetic experiment in a patient to investigate the applicability of the method in vivo.