Fondaparinux sodium is not metabolised in mammalian liver fractions and does not inhibit cytochrome P450-mediated metabolism of concomitant drugs

Objective: To investigate the in vitro metabolism of the antithrombotic agent fondaparinux sodium in mammalian liver fractions and to evaluate its potential inhibitory effect on human cytochrome P450 (CYP)-mediated metabolism of other drugs. Methods: Metabolism was evaluated by incubating radioisotope-labelled fordaparinux sodium with postmitochondrial liver fractions of rat, rabbit, monkey or human origin (three subjects). Human liver microsomal preparations and an NADPH-generating system were incubated with phenacetin, coumarin, tolbutamide, S-mephenytoin, bufuralol, chlorzoxazone or nifedipine. These are selectively metabolised by CYP isoforms: CYP1A2, CYP2A6, CYP2C9, CYP2C9, CYP2D6, CYP2E1 or CYP3A4, respectively. Experiments were designed to determine apparent K-i (inhibitory constant) values for fondaparinux sodium against each CYP isoform, by varying concentrations of fondaparinux sodium and the selective substrate. Each experiment included control reaction mixtures containing an isoform- selective inhibitor. After incubation, the mixtures were analysed by LC-MS/MS or with fluorometric detection. Results: All liver fractions were enzymatically active, as demonstrated by degradation of [C-14]testosterone. No metabolism of fordaparinux sodium was detectable in postmitochondrial liver fractions. Apparent Ki values for fordaparinux sodium against the CYP isoforms could not be determined because the oxidative metabolism of the isoform-selective CYP substrates was not significantly inhibited in pooled microsomal reaction mixtures. In the presence of selective CYP inhibitors, metabolism of each substrate was significantly reduced, confirming that inhibition could be observed in these assays. Conclusion: The demonstrated lack of mammalian hepatic metabolism of fondaparinux sodium is consistent with animal and human studies. The absence of inhibition of the human CYP isoforms commonly involved in the metabolism of drugs suggests that clinical treatment with fordaparinux sodium is unlikely to interfere with the pharmacokinetics and metabolism of a wide range of other drugs which are associated with CYP inhibition.