Rate-limiting biotransformation of triamterene is mediated by CYP1A2.

Objective: Triamterene (TA), a potassium-sparing diuretic, is extensively metabolized by hydroxylation in 4'-position and subsequent conjugation by cytosolic sulfotransferases. To identify the cytochrome P450 enzyme(s) catalyzing hydroxylation of triamterene (the rate-limiting step in the formation of the sulfate ester (STA)), in vitro incubation studies were performed with human liver microsomes. Methods: Initial rates of TA hydroxylation (0 - 300 μM) were determined during a ten-minute-incubation period with liver microsomes of two donors. The role of individual CYP enzymes was determined by pre-incubation with selective inhibitors/alternative substrates. Vice versa, the effect of TA (0 - 500 μM) on 3-demethylation of caffeine (0 - 1,000 μM) was assessed. Metabolite concentrations were estimated by reversed-phase HPLC methods. Results: TA K m values without inhibitors were 60 and 142 μM, V m a x was 177 and 220 pmol/min/mg protein, respectively. Mean inhibitor induced changes of 4'-hydroxy-TA formation were as follows: Furafylline 25 μM (CYP1A2), complete inhibition (-100%); omeprazole 250μM (CYP1A2 inhibitor/CYP2C19 substrate), -30%; coumarin 25 μM (CYP2A6), -11%; quinidine 25μM (CYP2D6), -9%; ketoconazole 25 μM (CYP3A), -18%; and erythromycin 250 μM (CYP3A), -8%. In the reverse inhibition studies, TA competitively inhibited caffeine 3-demethylation with K i values of 65 and 111 μM, respectively. Conclusion: 4'-hydroxylation of TA in humans appears to be mediated exclusively by CYP1A2. Inhibition or induction of CYP 1 A2 will change the time course of both TA and its active phase-II metabolite. The net pharmacodynamic effect of such changes is difficult to predict and needs to be evaluated in clinical studies.