Walter J. Loos

10014 Background: Imatinib is mainly metabolized in the liver. We therefore studied the potential effects of metastatic liver involvement on exposure to imatinib in patients with GIST. Additionally, as clearance from a retrospective analysis has been suggested to decrease over time (Judson et al., Cancer Chemother Pharmacol, 2005), while 1 month steady state imatinib trough levels were suggested to be associated with clinical benefit (Demetri et al., J Clin Oncol, 2009), the secondary endpoint of our study was to assess the exposure to imatinib after multiple long-term time-points. Methods: Multicenter prospective population pharmacokinetic study in 50 GIST patients treated with imatinib. Systemic exposure to imatinib was assessed through 24-hour pharmacokinetic sampling on day 1, month 1, 6 and 12. Patients were categorized in 4 groups based on their proportional metastatic liver volumes on CT images which were assessed on day 1, month 6 and 12 of therapy using OsiriX Imaging Software (OsiriX Foundation, Geneva, Switzerland). Possible mean exposure differences between groups and over time were analyzed with one-way ANOVA and unpaired t-tests. Results: Mean imatinib trough levels and exposure showed no significant correlation with hepatic metastatic liver volumes. However, imatinib steady state trough levels and exposure at 6 months were significantly lower as compared to after 1 month, irrespective of metastatic liver involvement (1,204 ± 680 vs. 973 ± 573 ng/mL, P= .001 and 49.1 ± 23.4 vs. 39.3 ± 20.8 µg*h/mL, P=.008, respectively). Hence, trough levels at 6 months in our population dropped below the previously suggested clinically relevant threshold level of 1,100 ng/mL. Conclusions: This is the first prospective pharmacokinetic study in GIST, demonstrating a significant decrease of ~20% in imatinib exposure after long-term treatment. This means that trough level-clinical benefit analyses should be time-point specific. Liver involvement does not affect imatinib exposure.

2019 Background: Paclitaxel is mainly eliminated by CYP2C8- and CYP3A4/5-mediated oxidations at the C6α- and C3’-positions, respectively. Genetic polymorphisms in CYP2C8 and CYP3A4/5, causing reduced enzyme activity in vitro, have recently been described. The objective of this study was to evaluate the effects of known genetic polymorphisms in these genes on the pharmacokinetics of paclitaxel. Methods: A cohort of 107 Caucasian patients with cancer (median age, 57 years) received paclitaxel as an intravenous infusion (median dose, 232 mg; range, 88 to 468 mg). Concentrations of unbound paclitaxel in plasma, determined by equilibrium dialysis, were available in 97 patients. Pharmacokinetic parameter estimates, including clearance and volume of distribution, were obtained using NONMEM. Genomic DNA was isolated from whole blood or plasma and screened for several variants in the genes of interest [CYP2C8*2 (805A>T, I269F); CYP2C8*3 (416G>A, R139K); CYP2C8*4 (792C>G, I264M); CYP3A4*3 (1334T>C, M445T); CYP3A5*3C (6986A>G, splice variant); and ABCB1 3435C>T (I1143I)] using Pyrosequencing. Results: The population typical value of clearance of unbound paclitaxel was 301 L/h (individual clearance range, 83.7 to 1055 L/h). The allele frequencies for CYP2C8*2, CYP2C8*3, CYP2C8*4, CYP3A4*3, CYP3A5*3C, and ABCB1 3435C>T were 0.010, 0.090, 0.021, 0.005, 0.93, and 0.47, respectively, and all were in Hardy-Weinberg equilibrium. With the exception of CYP2C8*3 (n = 3), no patients were homozygous for any variant CYP2C8 allele. The CYP2C8 or CYP3A4/5 genotypes were not significantly associated with the clearance of unbound paclitaxel (P > .49, one-way ANOVA after logarithmic transformation). Likewise, no statistically significant association was observed between the ABCB1 3435C>T variant and any of the studied phenotypic measures (P > .27). Conclusions: This study indicates that the presently evaluated variant alleles in the CYP2C8, CYP3A4, CYP3A5, and ABCB1 genes are not associated with altered plasma concentrations of paclitaxel. Further investigation is needed to determine the relative role of genetic variation and environmental variables (e.g., concomitant drugs or herbs) on paclitaxel pharmacokinetics. No significant financial relationships to disclose.

2015 Background: The ATP-binding cassette transporter ABCG2 (breast cancer resistance protein; BCRP) functions as an efflux transporter for many drugs and is expressed at high levels in the human small intestine. It has been demonstrated that some individuals possess a nonsynonymous single-nucleotide polymorphism at nucleotide 421, substituting lysine for glutamine on position 141 at exon 5, and express low amounts of ABCG2. Methods: In this prospective exploratory study, duodenal biopsies were obtained before oral administration of topotecan on day 1 (dose, 2.3 mg/m²) and i.v. infusion on day 2 (dose, 1.5 mg/m²) in 18 adult Caucasian cancer patients (10 female/8 male; median age, 54 years). Pharmacokinetic data were obtained on days 1 and 2. Topotecan concentrations in plasma were determined by liquid chromatography with fluorescence detection, and analyzed by noncompartmental methods. Enterocyte (villin)-corrected mRNA expression levels of ABCG2 and ABCB1 (P-glycoprotein) were measured by real-time TaqMan analysis, and all patients were genotyped by cycle sequencing for ABCG2 421C>A. Results: Complete topotecan pharmacokinetic data were available for 13 patients. The mean (± SD) area under the curves after oral and i.v. administration were 109 ± 35.7 ng×h/mL and 218 ± 62.3 ng×h/mL, respectively. Substantial intra- and interindividual variation in mRNA expression of ABCG2 and ABCB1 was observed in the intestinal samples (N = 12). Two patients were heterozygous variant for ABCG2421C>A (14%; allele frequency, 7.14%), and 12 patients carried the wild-type sequence; 4 samples showed no amplification. The heterozygous CA allele was associated with a 1.3-fold increased oral bioavailability of topotecan [31.4% (wild-type, N = 10) versus 42.0% (heterozygous, N = 2); P = 0.037]. The lowest combined mRNA expression of ABCG2 and ABCB1 was observed in one of the patients that was heterozygous variant for ABCG2 421C>A. Conclusions: This pilot study indicates that ABCG2 genotype is correlated with altered topotecan pharmacokin-etics. The current observation warrants confirmation and further investigation in a larger patient population involving different ethnic groups. No significant financial relationships to disclose.

New solubilizers, including Sorporol 230, Sorporol 120Ex, Aceporol 345-T, Aceporol 460 and Riciporol 335, as potential new delivery vehicles for paclitaxel were investigated, since recent studies have shown that the paclitaxel delivery vehicle Cremophor EL significantly alters the pharmacokinetics of paclitaxel. Cremophor EL and Tween 80 were used as a reference. As in the case of Cremophor EL, alteration of blood distribution of paclitaxel occurred in the presence of all tested vehicles. Also, no differences in the affinity of paclitaxel for the tested solubilizers was found during equilibrium dialysis experiments. The different vehicles could be distinguished by a different rate of esterase-mediated breakdown, which was correlated with the fatty acid content of the solubilizers. The activation of the complement cascade was less pronounced for all solubilizers, except Riciporol 335, compared to Cremophor EL. The strategies presented here provide the possibility to rapidly screen future candidate delivery vehicles with optimal characteristics for use as a solubilizer in clinical formulations of paclitaxel or other poorly water-soluble drugs.

An additional chromatographic peak was observed in plasma samples of patients receiving NX 211, a liposomal formulation of the topoisomerase I inhibitor lurtotecan. We have isolated and purified this product by sequential solid-phase extractions, and we report its structure and cytotoxicity relative to lurtotecan and related agents. Nuclear magnetic resonance data indicate that cleavage of the piperazino moiety occurred at the N-C bond of the B-ring, yielding 7-methyl-10,11-ethylenedioxy-20(S)-camptothecin (MEC). Tests of the growth inhibition potential of MEC in seven human tumor cell lines showed that the compound was approximately 2-18-fold more cytotoxic than lurtotecan, topotecan, and 7-ethyl-10-hydroxy-20(S)-camptothecin (SN-38). Subsequently, we found that MEC was the product of rapid photolysis of lurtotecan, with the rate of degradation inversely proportional to NX 211 concentrations, and greatly depends on light intensity. Furthermore, MEC concentrations were found to increase significantly in plasma samples exposed to laboratory light but not in blood. MEC was not produced from NX 211 in the presence of human liver microsomes, suggesting that it is not a product of cytochrome P-450 metabolism. Using a validated analytical method, trace levels of MEC were quantitated in blood samples of two patients. These observations confirm that the precautions for protection from light currently specified for preparation and administration of NX 211 dose solutions are critical. Procedures to minimize formation of MEC, by the use of amber vials for NX 211 and by preparation of dilutions immediately before clinical use in a fashion completely protected from light, are now being routinely implemented.

CPT-11 belongs to the class of topoisomerase I inhibitors, and it acts as a prodrug of SN-38, which is approximately 100-1000-fold more cytotoxic than the parent drug. CPT-11 has shown a broad spectrum of antitumor activity in preclinical models as well as clinically, with responses observed in various disease types including colorectal, lung, cervical, and ovarian cancer. The pharmacokinetics and metabolism of CPT-11 are extremely complex and have been the subject of intensive investigation in recent years. Both CPT-11 and SN-38 are known in an active lactone form and an inactive carboxylate form, between which an equilibrium exists that depends on the pH and the presence of binding proteins. CPT-11 is subject to extensive metabolic conversion by various enzyme systems, including esterases to form SN-38, UGT1A1 mediating glucuronidation of SN-38, as well as CYP3A4, which forms several pharmacologically inactive oxidation products. Elimination routes of CPT-11 also depend on the presence of drug-transporting proteins, notably P-glycoprotein and canalicular multispecific organic anion transporter, present on the bile canalicular membrane. The various processes mediating drug elimination, either through metabolic breakdown or excretion, likely impact substantially on interindividual variability in drug handling. Strategies to individualize CPT-11 administration schedules based on patient differences in enzyme or protein expression or by coadministration of specific agents modulating side effects are under way and may ultimately lead to more selective chemotherapeutic use of this agent.

Gender-dependent differences in the clinical pharmacokinetic behavior of various drugs have been documented previously. Most commonly, these differences are associated with differences in body composition, renal elimination, drug absorption or hepatic metabolism. Gender-dependent differences in the pharmacokinetics of topotecan (Hycamtin®) have not yet been described. In this report, pharmacokinetic data of the lactone and carboxylate forms of topotecan were derived from clinical studies in which topotecan was administered either orally or i.v. to a total of 55 males and 37 females. A significant difference (p=0.0082) of 38% was found between the apparent clearance of topotecan lactone after oral administration in males (237±105 l/h) and females (163±62.5 l/h). When adjusted for body surface area, this difference remained significant (p=0.031). Similarly, differences were noted in the percentage of topotecan in the lactone form (37.1±5.32 versus 41.7±6.51%, p=0.0076). Statistical analysis revealed that individual hematocrit values, which were consistently lower in females (p<0.023), were a significant predictor of the apparent topotecan lactone clearance. This was confirmed experimentally in in vitro incubation studies in whole blood using artificially altered hematocrit values and in blood samples from both male and female volunteers. Topotecan is thus subject to significant gender-dependent differences in pharmacokinetics that arise as a result of a physiological difference in hematocrit values between males and females. This finding may have significant implications for the interpretation of the relationships between pharmacokinetics and pharmacodynamic outcome of topotecan treatment, and may provide a basis for the development and refinement of future clinical protocols.

Anticancer drugs still are dosed based on the body-surface area (BSA) of the individual patient, although the BSA is not the main predictor of the clearance for the majority of drugs. The relevance of BSA-based dosing has not been evaluated for topotecan yet. A retrospective pharmacological analysis was performed of kinetic data from four clinical Phase I studies in which topotecan was administered p.o. as a single agent combined with data from a combination study of topotecan and cisplatin. A strong correlation (r = 0.91) was found between the area under the plasma concentration time curve of the lactone and carboxylate forms of topotecan by plotting 326 data sets obtained from 112 patients receiving oral topotecan at dose levels ranging from 0.15-2.70 mg/m2. The intrapatient variability, studied in 47 patients sampled for 3 or more days, for the apparent lactone clearance, ranged from 7.4-69% (mean, 24 +/- 13%; median, 20%). The interpatient variabilities in the lactone clearance, calculated with the data of all studied patients, expressed in liter/h/m2 and in liter/h were 38% and 42%, respectively. In view of the relatively high inter- and intrapatient variabilities in topotecan clearance, in contrast to a variability of only 12% in the BSA of the studied patients, no advantage of BSA-based dosing was found over fixed dose regimens.

Several analogs of the topoisomerase I inhibitor camptothecin (CPT) have been introduced in clinical practice in the last decade. All CPT analogs are sensitive to a pH-dependent reversible conversion between a pharmacologically active lactone form and its inactive, lactone ring-opened, carboxylate form. The reversible conversion is also dependent on the, sometimes species-dependent, protein binding properties of the two forms, resulting in different lactone to carboxylate plasma ratios for the various analogs. Pharmacokinetic analysis of the CPT analogs is helpful in understanding the pharmacodynamic outcome of drug treatment, in clinical as well preclinical studies. Measurement of these analogs is habitually complicated by the chemical instability of the lactone moiety and necessitates a rapid centrifugation of the blood sample, preferably at the bedside of the patient, to collect the plasma supernatant. Since the lactone forms of these drugs are able to diffuse across cell membranes, including those of the red blood cells, rapid collection and processing is even necessary in the case where only the total concentrations of the CPT analogs are to be measured. Sample pretreatment procedures of the CPT analogs topotecan, irinotecan, 9-aminocamptothecin and lurtotecan are summarized and discussed in this review.