In vitro assessment of various cytochromes P450 and glucuronosyltransferases using the antiarrhythmic propafenone as a probe drug

DDPH, a novel antihypertensive agent, is a potential dual inhibitor of hepatic CYP2D and CYP3A

DDPH (1-(2, 6-dimethylphenoxy)-2-(3, 4-dimethoxyphenylethylamino) propane hydrochloride) is a promising novel antihypertensive agent, with potent antihypertensive, neuroprotective and cardioprotective effects. This study aimed to investigate the effects of DDPH on the expression and activity of hepatic cytochrome P450 (CYP) isoforms and evaluate the metabolic drug-drug interactions of DDPH with propafenone. Our results showed that orally administered DDPH (12.5-50 mg/kg/d) for 7 days significantly inhibited CYP2D1 and CYP3A1 activity and mRNA and protein expression but weakly increased CYP1A2 activity and expression in rats. Enzyme kinetics studies showed that DDPH was a competitive inhibitor of CYP2D1 and mixed inhibitor of CYP3A1 in rat liver microsomes with Ki values of 3.70 ± 0.42 μM and 4.79 ± 1.10 μM respectively. With human liver microsomes, DDPH was a noncompetitive inhibitor of CYP2D6 (Ki = 0.85 ± 0.06 μM) and mixed inhibitor of CYP3A (Ki = 2.15 ± 0.41 μM). Further in vivo study showed that oral administration of DDPH (12.5-50 mg/kg/d) for 7 days in rats significantly increased the area under the plasma concentration-time curve (AUC) of propafenone by 25.4%-63.9%, with a concomitant decrease in the plasma clearance. In conclusion, the results indicated that DDPH inhibited CYP2D and CYP3A activities and down-regulated their protein expression and mRNA transcription. DDPH might cause metabolic drug-drug interactions through modulation of the activity and expression of CYP2D and CYP3A. This information could be important in the prediction of possible drug-drug interactions as well as for the effective therapy and the limitation of toxicity of DDPH in clinical practice.

Capillary electrophoretic investigation of the enantioselective metabolism of propafenone by human cytochrome P-450 SUPERSOMES: Evidence for atypical kinetics by CYP2D6 and CYP3A4

An enantioselective CE method was used to identify the ability of CYP450 enzymes and their stereoselectivity in catalyzing the transformation of propafenone (PPF) to 5-hydroxy-propafenone (5OH-PPF) and N-despropyl-propafenone (NOR-PPF). Using in vitro incubations with single CYP450 enzymes (SUPERSOMES), 5OH-PPF is shown to be selectively produced by CYP2D6 and N-dealkylation is demonstrated to be mediated by CYP2D6, CYP3A4, CYP1A2, and CYP1A1. For the elucidation of kinetic aspects of the metabolism with CYP2D6 and CYP3A4, incubations with individual PPF enantiomers and racemic PPF were investigated. With the exception of the dealkylation in presence of R-PPF only, which can be described by the Michaelis-Menten model, all CYP2D6-induced reactions were found to follow autoactivation kinetics. For CYP3A4, all NOR-PPF enantiomer formation rates as function of PPF enantiomer concentration were determined to follow substrate inhibition kinetics. The formation of NOR-PPF by the different enzymes is stereoselective and is reduced significantly when racemic PPF is incubated. Clearance values obtained for CYP3A4 dealkylation are stereoselective whereas those of CYP2D6 hydroxylation are not. This paper reports the first investigation of the PPF hydroxylation and dealkylation kinetics by the CYP2D6 enzyme and represents the first report in which enantioselective CE data provide the complete in vitro kinetics of metabolic steps of a drug.

Comparative molecular field analysis and QSAR on substrates binding to cytochrome P450 2D6

In this study, we utilized comparative molecular field analysis (CoMFA) to gain a better understanding of the steric and electrostatic features of the cytochrome p450 2D6 (CYP2D6) active site. The training set consists of 24 substrates with reported K(M) values from liver microsomal CYP2D6 spanning an activity range of almost three log units. The low energy conformers were fit by root mean square (RMS) to minaprine at the site of metabolism and to the protonated nitrogen. In this manner, we constructed two CoMFA models, one model with a distance constraint and another without. The model with the distance parameter (non-cross-validated R(2)=0.99) was approximately equal to the CoMFA without a distance parameter (non-cross-validated R(2)=0.98). Validation of our CoMFA was accomplished by predicting the K(M) values of 15 diverse CYP2D6 substrates not in the original training set resulting in a predictive R(2)=0.62. Finally, we also pursued correlations of pK(a) and log P with CYP2D6 substrate K(M) in an effort to investigate other physicochemical properties.

Comparison of the effects of liposomal amphotericin B and conventional amphotericin B on propafenone metabolism and hepatic cytochrome P-450 in rats

The effects of conventional amphotericin B (AmB) dissolved in sodium deoxycholate on microsomal cytochrome P-450 concentrations and propafenone metabolism to 5-hydroxy-propafenone and N-desalkyl-propafenone were compared with those of liposomal AMB (Li-AMB) in rats. AmB (3 mg/kg/day, intravenously [i.v.]) given for 4 days caused a significant decrease in the concentration of hepatic microsomal cytochrome P-450 (0.43 +/- 0.06 nmol/mg versus 0.62 +/- 0. 05 nmol/mg for the control [P < 0.05]). Following the application of Li-AMB (15 mg/kg/day, i.v.), hepatic microsomal cytochrome P-450 concentrations were unchanged at 0.64 +/- 0.08 nmol/mg. AmB decreased ex vivo propafenone metabolism to 5-hydroxy-propafenone and N-desalkyl-propafenone significantly. Sodium deoxycholate (the vehicle of AmB) by itself induced a significant decline of 5-hydroxy-propafenone and N-desalkyl-propafenone production, while microsomal cytochrome P-450 concentrations remained unchanged. In contrast, Li-AMB did not change the levels of production of 5-hydroxy-propafenone or of N-desalkyl-propafenone at either substrate concentration tested (50 micromol and 200 micromol). Microsomal AmB concentrations were significantly higher following Li-AMB application (21.1 +/- 6.2 microg/g versus 3.7 +/- 1.4 microg/g for AmB [P < 0.05]). We conclude that Li-AMB, in contrast to AmB, decreases neither hepatic microsomal cytochrome P-450 nor hepatic propafenone metabolism in rats ex vivo. Sodium deoxycholate alone decreases propafenone metabolism in a similar way to AmB, suggesting that it participates in AmB-induced disturbance of hepatic metabolic function.

Electron shuttle between membrane-bound cytochrome P450 3A4 and b5 rules uncoupling mechanisms

Contradictory mechanisms involving conformational or redox effects have been proposed for the enhancement of cytochrome P450 activities by cytochrome b5 in reconstituted systems. These mechanisms were reinvestigated for human liver P450 3A4 bound to recombinant yeast membranes including human P450 reductase and various levels of human b5. Species conversions were calculated on the basis of substrate, oxygen, and electronic balances in six different substrate conditions. Electron flow from P450 reductase to ferric 3A4 was highly dependent on the nature of substrate but not on the presence of b5. P450 uncoupling by hydrogen peroxide formation was decreased by b5, leading to a corresponding increase in the rate of ferryl-oxo complex formation. Nevertheless, the major b5 effects mainly relied on an increased partition of ferryl-oxo complex to substrate oxidation compared to reduction to water, which could support a conformation change based mechanism. However, further steady-state investigations evidenced that electron carrier properties of b5 were strictly required for this modulation and that redox state of b5 was ruled by the nature and concentration of 3A4 substrates. Moreover, rapid kinetic analysis of b5 reduction following NADPH addition suggested that b5 was reduced by the 3A4 ferrous-dioxygen complex and reoxidized by subsequent P450 oxygenated intermediates. A kinetic model involving a 3A4-b5 electron shuttle within a single productive P450 cycle was designed and adjusted. This model semiquantitatively simulated all presented experimental data and can be made compatible with the effect of the redox-inactive b5 analogue previously reported in reconstituted systems. In this model, synchronization of the b5 and 3A4 redox cycles, binding site overlap between b5 and reductase, and dynamics of the b5-3A4 complex were critical features. This model opened the way for designing complementary experiments for unification of b5 action mechanisms on P450s.