Timolol Metabolism in Human Liver Microsomes Is Mediated Principally by CYP2D6

Timolol has mainly been used topically for the treatment of glaucoma. It has been suggested that the drug is metabolized by cytochrome P450 CYP2D6. The matter has not, however, been extensively studied. The aim here was to tentatively identify timolol metabolites and to determine the P450-associated metabolic and interaction properties of timolol in vitro. Four metabolites were identified, the most abundant being a hydroxy metabolite, M1. The Km value for the formation of M1 was 23.8 μM in human liver microsomes. Metabolism of timolol with recombinant P450s and correlation analysis have confirmed the conception that the drug is metabolized principally by CYP2D6, CYP2C19 being only a minor contributor (<10%) to the intrinsic microsomal clearance. The CYP2D6 inhibitor quinidine proved a potent competitive inhibitor of timolol metabolism, with an in vitro Ki value of 0.08 μM. Fluvoxamine, an inhibitor of CYP2C19, inhibited timolol metabolism to a lesser extent, confirming its minor contribution. Timolol itself did not inhibit CYP2D6-catalyzed dextromethorphan O-demethylation. Judging from the disappearance of timolol in human liver homogenate, the in vivo half-life was extrapolated to be about 3 h, an estimate close to the half-life of about 2 to 5 h observed in vivo. In conclusion, the inhibition of timolol metabolism by quinidine should be taken into account when patients are treated with timolol. However, when plasma timolol concentrations in patients remain low (≤0.2 μg/l), it is suggested that such interaction is of minor clinical relevance.