ROLE OF HUMAN LIVER MICROSOMAL CYP2C9 IN THE BIOTRANSFORMATION OF LORNOXICAM

Objective: The nature of the enzyme(s) catalysing the biotransformatio n of lornoxicam to one of its major metabolites, 5'-hydroxy-lornoxicam , has been investigated in human liver microsomes. The reaction kineti cs were characterised, the affinity of lornoxicam for three major huma n drug metabolising cytochrome P-450 isozymes (CYP2C9, CYP2D6 and CYP3 A4) was determined, and inhibition of the reaction by known substrates (diclofenac, ibuprofen, mefenamic acid, phenytoin, tolbutamide and wa rfarin) and the prototype inhibitor (sulphaphenazole) of CYP2C9 was in vestigated. Results: Lornoxicam 5'-hydroxylation displayed single enzy me Michaelis-Menten kinetics, with a K-M of 3.6 mu mol . l(-1) and a V -max of 2.6 nmol . h(-1). mg(-1) microsomal protein. The apparent affi nity of lornoxicam was high for CYP2C9, but negligible for CYP3A4 and CYP2D6. Inhibition of lornoxicam 5'-hydroxylation by CYP2C9 substrates and sulphaphenazole was comparable in all livers preparations, values predicted from their K-M or K-i for CYP2C9 determined in separate stu dies assuming competitive inhibition. phenazole competitively and comp letely lornoxicam 5'-hydroxylation (K-i = 0.31 mu mol . l(-1)) as well as lornoxicam clearance (K-i = 0.33 mu mol . l(-1)), partial metaboli c clearance (f(m)) = 0.95). Conclusion: 5'-Hydroxylation appears to be the only cytochrome P-450 catalysed metabolic reaction of lornoxicam by human liver microsomes and this major in vivo biotransformation pat hway is catalysed virtually exclusively by CYP2C9.