The P-Glycoprotein Antagonist PSC 833 Increases the Plasma Concentrations of 6a-Hydroxypaclitaxel, a Major Metabolite of Paclitaxel

Purpose: Overexpression of P-glycoprotein (Pgp) is one mechanism of drug resistance in cancer chemotherapy. A Phase I trial was conducted using PSC 833, a Pgp antagonist, in combination with paclitaxel in patients with refractory cancer. The objective of this study was to assess the effect of PSC 833 on the metabolism of paclitaxel and characterize the differences in 6a-hydroxypaclitaxel pharmacokinetics. In addition, we examined the possibility of enhanced cytotoxicity of paclitaxel by the coexistence of 6a-hydroxypaclitaxel. Experimental Design: Patients received paclitaxel 35 mg/m/day by continuous intravenous infusion (CIVI) 3 4 days without PSC 833 in cycle 1 and escalating doses of paclitaxel (13.1, 17.5, or 21.3 mg/m/day CIVI 3 4 days) with 5 mg/kg PSC 833 by mouth every 6 h 3 7 days in cycle 2. Plasma samples were analyzed for both paclitaxel and its major metabolite with high-performance liquid chromatography methods. Using human liver microsomes, we studied the effect of PSC 833 on the metabolism of paclitaxel. In addition, the in vitro cytotoxicity of 6a-hydroxypaclitaxel alone and in combination with paclitaxel was evaluated. Results: Twenty-one of 22 patients had a metabolite peak (6a-hydroxypaclitaxel) observed in the chromatogram of plasma samples from cycle 2 when they received paclitaxel in combination with PSC 833. This metabolite was not detectable in plasma obtained during the first cycle when they received paclitaxel without PSC 833. During cycle 2, the mean concentrations of 6a-hydroxypaclitaxel and paclitaxel were 0.10 6 0.074 and 0.079 6 0.041 mg/ml, respectively. A moderate association was observed between total bilirubin and 6a-hydroxypaclitaxel concentrations (P 5 0.015, r 5 0.52; n 5 21). Human liver microsome experiments showed that a PSC 833 concentration as high as 10 mM did not affect the production of 6a-hydroxypaclitaxel. Paclitaxel cytotoxicity in HL60 and K562 human leukemia cells was increased in the presence of noncytotoxic concentrations of 6ahydroxypaclitaxel. Conclusions: PSC 833 increases the plasma concentration of 6a-hydroxypaclitaxel during paclitaxel therapy. Inhibition of cytochrome P-450 3A4 by PSC 833 may explain this in part, although other mechanisms cannot be excluded. INTRODUCTION Pgp is a multidrug transporter that is expressed in normal tissues, including the apical surface of intestinal epithelia, endometrial glands, the blood-brain barrier, and the proximal tubule of the kidney (1, 2). Although the physiological function of Pgp is not known, it has been suggested that Pgp may play a role in defense against xenobiotics in normal tissues (3). Overexpression of Pgp, which is encoded by the MDR1 gene, confers multidrug resistance in cancer cells (4). In patients, overexpression can correlate with a poor prognosis (5–10). A number of chemotherapeutic agents including the Vinca alkaloids, the anthracyclines, the taxanes, and other natural products have been identified as substrates for Pgp (11). In addition, numerous antagonists have been identified including verapamil, cyclosporine A, quinidine, and several potent second-generation antagonists, currently under clinical investigation. PSC 833, a cyclosporine D analogue, is a nonimmunosuppressive, nonnephrotoxic Pgp antagonist (12). Biotransformation of PSC 833 is CYP3A dependent (13). Paclitaxel was originally isolated from the bark of the Pacific yew tree and was subsequently identified as an antineoplastic agent (14). Paclitaxel stabilizes microtubules resulting in cell cycle arrest. Clinical trials have documented paclitaxel activity against ovarian, breast, lung, and other types of cancer (15–19). Paclitaxel metabolism has been characterized in several studies with three major metabolites identified: 6a-hydroxypaclitaxel, 39-p-hydroxypaclitaxel, and 6a,39-pdihydroxypaclitaxel. CYP2C8 is responsible for the transformation of paclitaxel to 6a-hydroxypaclitaxel and the formation of 6a,39-p-dihydroxypaclitaxel from 39-p-hydroxypaclitaxel (20– Received 11/24/00; revised 3/8/01; accepted 3/13/01. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact. 1 Supported by the United States Government Office of Research on Minority Health. 2 To whom requests for reprints should be addressed, at Medicine Branch, National Cancer Institute, NIH, Building 10, Room 5A01, 9000 Rockville Pike, Bethesda, MD 20892. Phone: (301) 402-3622; Fax: (301) 402-8606; E-mail: [email protected]. 3 The abbreviations used are: Pgp, P-glycoprotein; CIVI, continuous intravenous infusion; CYP, cytochrome P450; HPLC, high-performance liquid chromatography; ANC, absolute neutrophil count; CsA, cyclosporine A; MTT, 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide. 1610 Vol. 7, 1610–1617, June 2001 Clinical Cancer Research Research. on September 12, 2017. © 2001 American Association for Cancer clincancerres.aacrjournals.org Downloaded from 23). CYP3A4 converts paclitaxel to 39-p-hydroxypaclitaxel and also converts 6a-hydroxypaclitaxel to 6a,39-p-dihydroxypaclitaxel (Fig. 1; Ref. 24). A Phase I trial combining oral PSC 833 and infusional paclitaxel in patients with advanced cancer was conducted at the National Cancer Institute (25). The objectives of the study were to determine the maximum tolerated dose of PSC 833 in combination with paclitaxel and to compare the pharmacokinetics of paclitaxel administered alone with the pharmacokinetics of paclitaxel with concomitant administration of PSC 833. Four different dose levels of PSC 833 ranging from 1.25 to 5 mg/kg p.o. every 6 h 3 7 days were explored. Fifty patients received paclitaxel and PSC 833 separately in the first cycle and a combination of paclitaxel and PSC 833 in subsequent cycles. The maximum tolerated doses identified were: 5 mg/kg PSC 833 administered every 6 h 3 7 days in combination with a paclitaxel dose of either 13.1 mg/m/day by CIVI 3 4 days without filgrastim or 17.5 mg/m/day CIVI 3 4 days with filgrastim. Paclitaxel doses of 13.1 or 17.5 mg/m/days CIVI 3 4 days with PSC 833 resulted in mean plasma concentrations that were equivalent to those achieved with a paclitaxel dose of 35 mg/m/days CIVI 3 4 days without PSC 833. However, considerable interpatient variation was observed in the magnitude of paclitaxel concentration changes with PSC 833. In the course of analyzing patient plasma samples, we observed, in a majority of patients who received 5 mg/kg PSC 833 administered p.o. every 6 h, unusually high concentrations of a metabolite subsequently identified as 6a-hydroxypaclitaxel. A peak corresponding to 6a-hydroxypaclitaxel was not detected in the first cycle when paclitaxel was administered alone. This study describes the identification of this metabolite as 6ahydroxypaclitaxel and its quantification. Attempts to understand the mechanisms responsible for the appearance of the metabolite and to find a correlation between the metabolite and the occurrence of toxicities are also discussed. MATERIALS AND METHODS Clinical Protocol. Eligible patients were .18 years of age and had a Karnofsky performance status of 70% or better, a life expectancy of .3 months, aspartate aminotransferase and alanine aminotransferase #2 3 upper limit of normal, creatinine clearance $50 ml/min, WBC $3,000/mm, ANC $1,000/mm, and platelet count $100,000/mm. Patients who had received chemotherapy, immunotherapy, or radiation therapy in the 4 weeks before study entry, or had undergone a prior bone marrow transplantation or extensive radiation resulting in compromised bone marrow reserve, were not eligible. Agents proven to interact with CsA or suspected to interact with P-glycoprotein were restricted during the study. The protocol was approved by the Institutional Review Board of the National Cancer Institute, and all patients signed an informed consent document