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 microM) 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 microM) on 3-demethylation of caffeine (0 - 1,000 microM) was assessed. Metabolite concentrations were estimated by reversed-phase HPLC methods.


RESULTS
TA Km values without inhibitors were 60 and 142 microM, Vmax was 177 and 220 pmol/min/mg protein, respectively. Mean inhibitor induced changes of 4'-hydroxy-TA formation were as follows: Furafylline 25 microM (CYP1A2), complete inhibition (-100%); omeprazole 250 microM (CYP1A2 inhibitor/CYP2C 19 substrate), -30%; coumarin 25 microM (CYP2A6), -11%; quinidine 25 microM (CYP2D6), -9%; ketoconazole 25 microM (CYP3A), -18%; and erythromycin 250 microM (CYP3A), -8%. In the reverse inhibition studies, TA competitively inhibited caffeine 3-demethylation with Ki values of 65 and 111 microM, respectively.


CONCLUSIONS
4'-hydroxylation of TA in humans appears to be mediated exclusively by CYP1A2. Inhibition or induction of CYP1A2 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.
Fuhr
U
U
Institute for Pharmacology, Clinical Pharmacology, University of Cologne, Germany.
Kober
S
S
Zaigler
M
M
Mutschler
E
E
Spahn-Langguth
H
H
D002110:Caffeine / Q000378:metabolism / Q000494:pharmacology; D002851:Chromatography, High Pressure Liquid; D003374:Coumarins / Q000378:metabolism / Q000494:pharmacology; D019388:Cytochrome P-450 CYP1A2 / Q000378:metabolism*; D065609:Cytochrome P-450 CYP1A2 Inhibitors; D004305:Dose-Response Relationship, Drug; D004791:Enzyme Inhibitors / Q000494:pharmacology; D004917:Erythromycin / Q000378:metabolism / Q000494:pharmacology; D006801:Humans; D006900:Hydroxylation / Q000187:drug effects; D007654:Ketoconazole / Q000378:metabolism / Q000494:pharmacology; D007700:Kinetics; D008862:Microsomes, Liver / Q000187:drug effects / Q000201:enzymology / Q000378:metabolism*; D009853:Omeprazole / Q000494:pharmacology; D011802:Quinidine / Q000378:metabolism / Q000494:pharmacology; D013379:Substrate Specificity; D013463:Sulfuric Acid Esters / Q000378:metabolism; D013806:Theophylline / Q000031:analogs & derivatives / Q000494:pharmacology; D014223:Triamterene / Q000031:analogs & derivatives / Q000378:metabolism* / Q000494:pharmacology
D003374:Coumarins; D065609:Cytochrome P-450 CYP1A2 Inhibitors; D004791:Enzyme Inhibitors; D013463:Sulfuric Acid Esters; C029656:4-hydroxytriamterene; D002110:Caffeine; D004917:Erythromycin; C030123:coumarin; D013806:Theophylline; C050131:furafylline; D019388:Cytochrome P-450 CYP1A2; D011802:Quinidine; D009853:Omeprazole; D007654:Ketoconazole; D014223:Triamterene