Influence of renal function on the steady-state pharmacokinetics of the antiarrhythmic propafenone and its phase I and phase II metabolites.

The aim of this study was to investigate the disposition of propafenone and its Phase I and II metabolites in relation to kidney function under steady-state conditions. The mechanism of the renal handling of propafenone glucuronides (filtration, secretion) was also examined. Racemic (R/S) propafenone was administered to 7 young volunteers, to 5 older patients with a normal glomerular filtration rate and to 4 patients with chronic renal failure. No difference was found in the plasma concentrations of propafenone and 5-hydroxypropafenone between the three groups. The propafenone glucuronide (PPFG) concentration was elevated in the older compared to the younger subjects (S-PPFG: 544 vs. 222 nmol.ml-1.mol-1; R-PPFG: 576 vs. 304 nmol.ml-1.mol-1). Although Glomerular filtration rate did not differ, the renal clearance of propafenone glucuronides was reduced in the former group, which could be attributed to their impaired renal secretion. A dramatic increase in propafenone glucuronide concentration was observed in the patients with renal failure (S-PPFG: 2783 nmol.ml-1.mol-1; R-PPFG: 7340 nmol.ml-1.mol-1). In summary, the disposition of propafenone and of its active metabolite 5-hydroxypropafenone was not affected by kidney dysfunction, indicating that no dose adjustment is necessary in patients with renal failure. The accumulation of drug glucuronides in older patients with apparently normal kidney function should be taken into account as a possible factor modifying drug therapy.

Rapid determination of CYP2D6 phenotype during propafenone therapy by analysing urinary excretion of propafenone glucuronides.

Metabolism of the antiarrhythmic, propafenone, cosegregates with the sparteine/debrisoquine polymorphism. Patients devoid of CYP2D6 activity have a higher incidence of adverse effects than those with normal enzyme function. In this paper we present a method for rapid assignment of CYP2D6 phenotype using urinary excretion of intact glucuronides of propafenone (PPFG). After establishing an HPLC assay, urinary excretion of PPFG was quantified during one dosage interval and related to individual CYP2D6 activity as determined by phenotyping. We observed a close correlation of urinary excretion of PPFG with individual CYP2D6 activity (r = 0.84, P < 0.01) and conclude that this method is suitable for rapid assignment of phenotype during propafenone therapy.

Identification and characterization of the cytochrome P450 enzymes involved in N-dealkylation of propafenone: molecular base for interaction potential and variable disposition of active metabolites.

The activity of metabolizing enzymes determines plasma concentrations and hence effects of drugs. Identification of these enzymes may allow the prediction of both the interaction potential of drugs and the variability deriving from certain pathways. The antiarrhythmic propafenone is extensively biotransformed to the active metabolites 5-hydroxypropafenone and N-desalkylpropafenone. Whereas 5-hydroxylation is catalyzed by CYP2D6, the enzyme involved in N-dealkylation has not been identified. We, therefore, characterized the enzyme involved in the formation of N-desalkylpropafenone by using both in vitro [human liver microsomes, specific antibodies or inhibitors, and stably expressed cytochrome P450 (P450) enzymes] and in vivo (formation of N-desalkylpropafenone in patients under conditions of chronic therapy) approaches. Formation of N-desalkylpropafenone can be described by Michaelis-Menten kinetics. A strong correlation was observed between maximum rate of formation (Vmax) of N-desalkylpropafenone and the amount of CYP1A2 (r = 0.83, p < 0.001) and CYP3A (r = 0.54, p < 0.05) in the microsomal fraction of 20 human livers. In vitro intrinsic clearances (derived from Vmax/Km) indicated a wide interindividual variability in seven human livers (from 0.01 to 0.1 ml/hr/mg of protein). Antibodies directed against CYP3A and CYP1A2 inhibited formation of N-desalkylpropafenone by 54 +/- 10% and 24 +/- 16%, respectively. The CYP2D6-mediated formation of 5-hydroxypropafenone was unaffected by these antibodies. Verapamil (substrate of CYP3A4 and CYP1A2) and midazolam (substrate of CYP3A4) were competitive inhibitors of N-desalkylpropafenone formation (Ki = 70 microM and 25 microM for verapamil and midazolam, respectively). Coding sequences for CYP1A2 and CYP3A4 were inserted in a yeast expression vector and introduced into Saccharomyces cerevisiae strain W(R). Both CYP1A2 and CYP3A4 catalyzed N-dealkylation of propafenone, with specific activities of 0.32 pmol/min/pmol of P450 and 0.16 pmol/min/pmol of P450, respectively. Our data indicate that N-dealkylation of propafenone is mediated via CYP3A4 and CYP1A2. From experiments on the molecular level interactions of propafenone with other drugs that are metabolized by CYP3A4 and CYP1A2 can be predicted. Such interactions have been reported for cyclosporin, rifampicin, warfarin, and theophylline. Moreover, in vitro intrinsic clearances showed a wide interindividual variability. Therefore, variable plasma concentrations of the active metabolite N-desalkylpropafenone are expected in vivo. We tested this hypothesis in 14 patients (dose of 150 mg of propafenone three times per day) during chronic oral therapy and observed steady state plasma concentrations of N-desalkylpropafenone ranging from 4 to 293 ng/ml.(ABSTRACT TRUNCATED AT 400 WORDS)