Enantioselective inhibition of the biotransformation and pharmacological actions of isoidide dinitrate by diphenyleneiodonium sulphate

Abstract

1 We have shown previously that the D- and L- enantiomers of isoidide dinitrate (D-IIDN and L-IIDN) exhibit a potency difference for relaxation and cyclic GMP accumulation in isolated rat aorta and that this is related to preferential biotransformation of the more potent enantiomer (D-IIDN). The objective of the current study was to examine the effect of the flavoprotein inhibitor, diphenyleneiodonium sulphate (DPI), on the enantioselectivity of IIDN action. 2 In isolated rat aortic strip preparations, exposure to 0.3 μM DPI resulted in a 3.6 fold increase in the EC50 value for D-IIDN-induced relaxation, but had no effect on L-IIDN-induced relaxation. 3 incubation of aortic strips with 2 μM D- or L-IIDN for 5 min resulted in significantly more D-isoidide mononitrate formed (5.0 ± 1.5 pmol mg protein-1) than L-isoidide mononitrate (2.1 ± 0.7 pmol mg protein-1) and this difference was abolished by pretreatment of tissues with 0.3 μM DPI. DPI had no effect on glutathione S-transferase (GST) activity or GSH-dependent biotransformation of D- or L-IIDN in the 105,000 x g supernatant fraction of rat aorta. 4 Consistent with both the relaxation and biotransformation data, treatment of tissues with 0.3 μM DPI significantly inhibited D-IIDN-induced cyclic GMP accumulation, but had no effect on L-IIDN-induced cyclic GMP accumulation. 5 In the intact animal, 2 mg kg-1 DPI significantly inhibited the pharmacokinetic and haemodynamic properties of D-IIDN, but had no effect L-IIDN. 6 These data suggest that the basis for the potency difference for relaxation by the two enantiomers is preferential biotransformation of D-IIDN to NO, by an enzyme that is inhibited by DPI. Given that DPI binds to and inhibits NADPH-cytochrome P450 reductase, the data are consistent with a role for the cytochromes P450-NADPH-cytochrome P450 reductase system in this enantioselective biotransformation process.