Stereoselective pharmacokinetics of ifosfamide in male and female rats

Abstract

The stereoselective pharmacokinetics of ifosfamide (IF) were investigated in male and female Sprague-Dawley rats. Following intravenous administration of IF deuterium-labeled pseudoracemates into rats at 40 mg/kg, IF enantiomers and their metabolites, 4-hydroxyIF (HOIF), N2-dechloroethylIF (N2D), N3-dechloroethylIF (N3D), and isophosphoramide mustard (IPM) were quantitated in plasma and urine using gas chromatographic-mass spectrometry techniques with appropriately deuterium-labeled analogs as the internal standards. In addition, the intrinsic clearances of IF isomers in rat liver microsomes were estimated by the in vitro metabolism study. Following drug administration in male rats, (R)-IF exhibited a lower area under the curve value and a shorter half-life of 34.2 minutes than (S)-IF, which gave a half-life of 41.8 minutes. In female rats, the half-lives of (R)- and (S)-IF were found to be 62.1 and 75.1 minutes, respectively, significantly longer than those in male rats. No change in volume of distribution or renal clearance for IF enantiomers in all rats was observed, and the protein binding value was low, with no enantioselectivity. Both in vitro and in vivo studies showed that metabolism of (R)-IF proceeded in favor of the 4-hydroxylation pathway, whereas (S)-IF preferentially underwent N2- and N3-dechloroethylation. The observed stereoselectivity and gender difference in pharmacokinetics of IF in the rat are mainly attributed to its stereoselective metabolism.

Stereoselectivity in metabolism of ifosfamide by CYP3A4 and CYP2B6

“…The hepatic microsomes from these rats as well as from untreated animals were prepared by a differential ultracentrifugation method and used to study the in vitro metabolism of IFA (Wang et al 2000). Protein concentrations were determined by the Lowry method using bovine serum albumin as a standard (Lowry et al 1951).…”

“…Despite the same chemical composition, IFA enantiomers differ in efficacy and toxicity (Farmer 1988;Wainer et al 1994a, b). Rodent model and clinical studies have demonstrated significant differences in the metabolism and disposition of the two enantiomers with (R)-IFA preferentially metabolized via 4-hydroxylation, whereas (S )-IFA is preferentially biotransformed by N-dechloroethylation Wainer et al 1996;Wang et al 2000). Enantioselective metabolism was shown to be the major contributing factor to the observed stereoselectivity and gender difference in pharmacokinetics of IFA in rats ( Wang et al 2000).…”

“…Rodent model and clinical studies have demonstrated significant differences in the metabolism and disposition of the two enantiomers with (R)-IFA preferentially metabolized via 4-hydroxylation, whereas (S )-IFA is preferentially biotransformed by N-dechloroethylation Wainer et al 1996;Wang et al 2000). Enantioselective metabolism was shown to be the major contributing factor to the observed stereoselectivity and gender difference in pharmacokinetics of IFA in rats ( Wang et al 2000). Phenobarbital pretreatment altered the stereoselectivity in pharmacokinetics and metabolism of IFA in rats, indicating that the hydroxylation and dechloroethylation of IFA enantiomers were catalysed by different cytochromes P450 (CYPs), which may have different stereochemical selectivities (Granvil et al 1994;Lu et al 1998).…”

“…It was noted that in some investigations (Roy et al 1999;Chen et al 2005) the formation of acrolein, a co-product of decomposition of the ring-open tautomer of HOIF, was used to measure of 4-hydroxylase activity; however, the assay might also detect secondary metabolites ( Wang and Chan 1995a), rendering potentially questionable results. We have developed a GC/MS-stable isotope method and directly measured individual primary metabolites of IFA ( Wang and Chan 1995b;Lu et al 1998;Wang et al 2000). Additionally, we employed a reconstituted pseudoracemate of IFA and this technique enables discrimination, by means of the labels, of enantioselective generation of all metabolites ( Wang and Chan 1995b;Lu et al 1998;Wang et al 2000).…”

“…We have developed a GC/MS-stable isotope method and directly measured individual primary metabolites of IFA ( Wang and Chan 1995b;Lu et al 1998;Wang et al 2000). Additionally, we employed a reconstituted pseudoracemate of IFA and this technique enables discrimination, by means of the labels, of enantioselective generation of all metabolites ( Wang and Chan 1995b;Lu et al 1998;Wang et al 2000). Furthermore, analysis of the products formed from enantiomeric precursors in the same metabolizing system reduces the problem of interindividual variability, which occurs frequently when experiments are performed separately.…”