Effect of P-glycoprotein inhibitor, verapamil, on oral bioavailability and pharmacokinetics of irinotecan in rats
Introduction
Irinotecan (CPT-11), a water-soluble camptothecin, is the agent of choice for treatment of metastatic carcinomas of colon and rectum. It exerts anti-tumor activity by inhibiting the intranuclear enzyme topoisomerase I. Irinotecan itself is a prodrug and gets converted in vivo to 100–1000 times more potent active metabolite, 7-ethyl-10-hydroxy camptothecin (SN-38) by carboxylesterase enzyme. Majority of the studies performed with irinotecan are using intravenous administration (Chu et al., 1997, de Forni et al., 1994, Gupta et al., 1996, Iyer et al., 2002). However, it has been given orally in early clinical studies and its pharmacokinetic profile is characterized by relatively poor and highly variable oral bioavailability (Drengler et al., 1999, Schoemaker et al., 2005, Soepenberg et al., 2005, Zamboni et al., 1998). The main adverse effects of irinotecan in humans are gastrointestinal toxicity and myelosuppression which limits its usage and administration (Mathijssen et al., 2001, Yang et al., 2005). Since biliary excretion of irinotecan and its metabolites is a major elimination pathway, accounting for about 60% of the administered dose, several hypotheses for its toxicity directly involve the biliary excretion of these compounds (Chu et al., 1998).
The involvement of active transporters particularly P-glycoprotein (P-gp) in the transport of both irinotecan and SN-38 is demonstrated by various researchers (Itoh et al., 2005, Iyer et al., 2002, Luo et al., 2002, Takemoto et al., 2006, Yamamoto et al., 2001). P-gp is the major efflux transporter protein responsible for poor absorption of many drugs. Presence of P-gp transporter in intestinal epithelial membrane and biliary canalicular membrane makes its inhibition a logical strategy to enhance irinotecan oral bioavailability and ameliorating diarrhoeal toxicities. Verapamil is the most extensively characterized P-gp inhibitor and multi-drug resistance (MDR) reversal agent that has entered clinical trials (Perez-Tomas, 2006). It is also reported that verapamil has increased the AUC of doxorubicin and paclitaxel in rodents (Candussio et al., 2002, Choi and Li, 2005). Therefore, the effect of co-administration of verapamil on the oral bioavailability and pharmacokinetics of irinotecan is the subject of current investigation. Cyclosporin, PSC-833, ketoconazole, loperamide, probenecid and imatinib as the inhibitors of active transporters and cytochrome enzymes have modulated pharmacokinetics of irinotecan (Arimori et al., 2003, Gupta et al., 1996, Horikawa et al., 2002, Iyer et al., 2002, Kehrer et al., 2002, Liu et al., 1996, Stewart et al., 2004, Tobin et al., 2005). The main objective of the study is to evaluate the scope of improvement in oral delivery of irinotecan via co-treatment with oral P-gp inhibitor. P-gp inhibitors are generally administered intravenously and no suggestion is made regarding their oral intake. Parenteral administration of P-gp inhibitors in therapeutic doses into humans may cause severe clinical consequences. This is because in addition to intestine, P-gp is present in hepatocytes, bile canaliculi, brain and kidney suggesting its role in distribution, metabolism and excretion. Consequently, clinical pharmacokinetic interactions could be anticipated when P-gp inhibitors are co-administered. An ideal P-gp inhibitor should give nearly maximal inhibition of P-gp transport locally at intestine with a minimal systemic effect and whole body burden (Dantzig et al., 2003, Varma et al., 2003).
The design of present study includes an initial in vitro investigation on Caco-2 cells to evaluate the effect of verapamil on the function of intestinal P-gp. This was followed by determination of irinotecan pharmacokinetics (plasma concentrations and biliary excretion) following oral and intravenous (i.v.) administration with and without co-administration of verapamil in female Wistar rats. Based on these studies, it might be feasible to develop an oral irinotecan formulation which is much more convenient than the i.v. dosage forms.
Section snippets
Chemicals and apparatus
Irinotecan (>99%), SN-38 (>96%) and topotecan (>98%) originated from Dabur Pharma Limited (U.P., India). Verapamil, disodium ethylene diamine tetra acetic acid and non-essential amino acids were purchased from Sigma–Aldrich (St. Louis, MO, USA). Acetonitrile of HPLC grade was obtained from J.T. Baker (USA). O-phosphoric acid and DMSO were from Merck Ltd. (India). Ketamine (Aneket) and xylazine HCl (Xylaxin) were obtained from Neon Labs Ltd. (Mumbai, India) and Indian Immunologicals Ltd. (A.P.,
Bi-directional transport of irinotecan in Caco-2 cells
No significant change was observed in the TEER value (>400 Ω cm2) measured before (0 h) and after (2 h) completion of transport studies. The transport rate of lucifer yellow was also not changed in the presence of various P-gp modulators. The results obtained with rhodamine 123 indicated its polarized transport (PappB–A: 128 ± 12 nm s−1; PappA–B: 5 ± 2 nm s−1) similar to that reported earlier (Troutman and Thakker, 2003), enabling the complete standardisation of Caco-2 cell monolayer (Irvine et al., 1999).
Discussion
Irinotecan, a FDA approved anti-cancer agent, is widely indicated for the treatment of various malignancies. It is currently marketed for intravenous use although few reports of its oral drug administration exist (Drengler et al., 1999, Schoemaker et al., 2005, Soepenberg et al., 2005, Stewart et al., 1997). Data available on its absorption and disposition showed encouraging results with variable absorption, poor efficacy and toxicity profiles. As a result novel formulations of irinotecan
Conclusions
The simultaneous administration of verapamil significantly modulates the oral bioavailability and pharmacokinetics of irinotecan. Oral absorption and bioavailability of irinotecan are markedly increased (4.3-fold) suggesting that its low systemic exposure after oral administration is, at least in part, due to its high affinity for P-gp efflux pump. P-gp in the gastro-intestinal mucosa limits the absorption of orally administered xenobiotics and mediates its excretion into the bile. The effect
Acknowledgements
One of the authors (T.B.) is grateful to the Council of Scientific & Industrial Research (CSIR), New Delhi, India for providing senior research fellowship.
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