Pharmacokinetics and biotransformations of oxaliplatin in comparison with ormaplatin following a single bolus intravenous injection in rats.

PURPOSE: Traditionally ultrafilterable Pt has been used to estimate the body exposure to platinum drugs. However, previous studies have shown that ultrafilterable Pt consists of both cytotoxic and inert biotransformation products of platinum drugs. Therefore, it has been proposed that pharmacokinetic parameters of the parent drug and its cytotoxic biotransformation products are more likely to be correlated with the drug toxicity and efficacy than those of ultrafilterable Pt. Oxaliplatin and ormaplatin are likely to form very similar biotransformation products in vivo based on previous studies. However, ormaplatin causes severe and irreversible neurotoxicity while oxaliplatin causes moderate and reversible neurotoxicity. To evaluate the hypothesis that the neurotoxicity is associated with the pharmacokinetics of active biotransformation products, we investigated the biotransformations and pharmacokinetics of oxaliplatin and ormaplatin in rats at equimolar doses.
METHODS: 3H-oxaliplatin and 3H-ormaplatin were administered to Wistar male rats through single bolus i.v. injections (20 micromol/kg). Blood was sampled from 3.5 min to 360 min and centrifuged at 2000 g to separate the plasma from red blood cells (RBCs). The RBCs were sonicated and centrifuged at 13000 g to separate the cytosol from the membrane fraction. Both plasma and RBC cytosol were filtered through YMT30 membranes (Mr = 30000 kDa), and the ultrafiltrates were analyzed using a single column HPLC technique to identify and quantitate the biotransformation products. The pharmacokinetics of oxaliplatin, ormaplatin, and their biotransformation products were characterized utilizing the curve stripping and nonlinear least-squares fitting program RSTRIP.
RESULTS: The decays of total, plasma, plasma ultrafilterable (PUF), RBC-bound, and plasma protein-bound Pt-dach (only Pt species with an intact dach carrier ligand were quantitated in this study) were described by biphasic curves. No significant kinetic differences between oxaliplatin and ormaplatin were observed for total, plasma, and PUF Pt-dach in the initial alpha decay phase. However, Pt-dach bound to plasma proteins fourfold more quickly for ormaplatin than for oxaliplatin, and the AUC for Pt-dach bound to plasma proteins was twofold higher for ormaplatin than for oxaliplatin. The concentration of RBC-bound Pt-dach was highest at the initial time-point of 3.5 min for both drugs, which suggested a very rapid RBC uptake. The binding of Pt-dach to RBCs was slightly greater initially for ormaplatin than for oxaliplatin. However, the RBC-bound Pt-dach decayed more rapidly for ormaplatin (t(1/2alphaRBC) = 5.1 min) than for oxaliplatin (t(1,2alphaRBC) = 15.3 min). Thus the AUC(RBC) was slightly greater for oxaliplatin than for ormaplatin. The AUC was also slightly greater for oxaliplatin than for ormaplatin for the Pt-dach associated with the RBC membrane and RBC cytosolic proteins. However, there was no significant difference between oxaliplatin and ormaplatin for Pt-dach in the RBC cytosolic ultrafiltrate. There was also no significant difference in the AUCpuf between oxaliplatin and ormaplatin. Both oxaliplatin and ormaplatin produced the same types of major plasma biotransformation products including Pt(dach)Cl2, Pt(dach)(Cys)2, Pt(dach)(GSH)2, Pt(dach)(GSH), Pt(dach)(Met), and free dach. The decays of oxaliplatin, ormaplatin, and their biotransformation products were described by biphasic curves. (ABSTRACT TRUNCATED)