Kinetics of metabolism and degradation of mometasone furoate in rat biological fluids and tissues

Mometasone furoate (MF) is a potent glucocorticoid developed for the treatment of glucocorticoid-responsive inflammatory disorders. The in-vitro and ex-vivo kinetics of the degradation and metabolism of MF were studied in selected biological fluids of rat and subcellular fractions of different rat tissues. In-vitro, MF was found to degrade slowly into four products in serum and urine, and metabolized rapidly and extensively in rat liver, minimally in extrahepatic tissues, including intestine, stomach, lung and kidney. Further investigation found that the microsomal fraction was the major intracellular site of MF 6beta-hydroxylation in rat liver. Using chemical inhibitors, CYP3A was found to be the major enzyme involved in the in-vitro MF 6beta-hydroxylation in rat liver microsomes. Enzyme kinetic studies in rat liver microsomes showed that the overall metabolic process of MF followed biphasic Michaelis-Menten kinetics, while 6beta-hydroxylation obeyed monophasic Michaelis-Menten kinetics. The kinetic parameters derived from the kinetic models along with the enzyme inhibition studies suggest that MF is mainly metabolized via 6beta-hydroxylation mediated by CYP3A primarily, and also biotransformed via other pathway(s) catalysed by other enzymes in rat liver in-vitro.