Genetically determined variability in drug metabolism: dual slow acetylation and drug oxidation traits

Inhibitory Effects of Propafenone on the Pharmacokinetics of Caffeine in Humans

CYP1A2 is involved in the metabolism of both caffeine and propafenone, a class Ic antiarrhythmic agent. Despite the widespread consumption of caffeine, drug-drug interactions with this agent are often overlooked. This study investigated effects of propafenone on the pharmacokinetics of caffeine. Eight healthy volunteers were included in our study. A total of 300 mg of caffeine was given on 2 occasions, once alone and once during the coadministration of 300 mg propafenone. Serial blood samples were collected and pharmacokinetic parameters were estimated using a population pharmacokinetic approach. A one-compartment PK model with first-order absorption and elimination described plasma concentration profiles. Concomitant administration of propafenone decreased caffeine oral clearance from 8.3 +/- 0.9 L/h to 5.4 +/- 0.7 L/h (P < 0.05). Elimination half-life of caffeine was also increased 54% by propafenone. One of our volunteers was a poor metabolizer of CYP2D6. Concomitant administration of propafenone to this volunteer caused the greatest increase in caffeine plasma concentrations. These results support the concept of competitive inhibition between propafenone and caffeine. Our results suggest that propafenone causes significant inhibition of CYP1A2 activity leading to a decrease in the clearance of caffeine. Caffeine has intrinsic proarrhythmic effects; thus, its coadministration with an antiarrhythmic agent such as propafenone should be used with caution, especially in patients with poor CYP2D6 activity.

Altered distribution of debrisoquine oxidation phenotypes in patients with systemic lupus erythematosus

Oxidative metabolism in patients with systemic lupus erythematosus (SLE) was studied using the antihypertensive drug, debrisoquine. The metabolism of this drug to its principal metabolite, 4-hydroxydebrisoquine, is catalyzed by a discrete isozyme of cytochrome P-450. The extent of this reaction exhibits genetic polymorphism, with 2 phenotypes, "poor metabolizers" and "extensive metabolizers," discernible in the normal population. We observed the poor metabolizer debrisoquine phenotype in 9 of 42 patients with idiopathic SLE (21%), in contrast with 12 of 147 healthy volunteers (8%), which is a significant difference in frequency (P less than 0.04). These data provide further evidence for altered oxidative metabolism in SLE and support the concept that genetic differences in oxidative metabolism of endogenous compounds, such as sex steroid hormones, or of xenobiotics might influence susceptibility to SLE.

Debrisoquine oxidation in an Australian population

The standard laboratory method for determination of debrisoquine phenotype has been modified and shortened with no loss of sensitivity. Debrisoquine metabolic ratios (MR) at 4 and 8 h showed excellent correlation indicating that collection time can also be shortened. Same day phenotyping is therefore possible. One hundred normal, Caucasian Australian subjects were phenotyped (46 males, 54 females) and 6% were poor metabolisers (PM) of debrisoquine. Fifty of the original subjects were also acetylation phenotyped and 34% were fast and 66% slow acetylators. One PM of debrisoquine was a fast acetylator of sulphadimidine and four PM were slow acetylators. This was a non-significant association.