Received for publication January 2, 2008.
Revised February 7, 2008.
Accepted for publication February 8, 2008.

In vitro metabolism of Licofelone (ML3000), an Inhibitor of Cyclooxygenases-1 and -2 and 5-Lipoxygenase

Abstract

Licofelone (2-[6-(4-chlorophenyl)-2,2-dimethyl-7-phenyl-2,3-dihydro-1H-pyrrolizin-5-yl] acetic acid) is a dual inhibitor of both cyclooxygenase isoforms (COX-1/-2) and 5-lipoxygenase (5-LOX) and under development for treatment of osteoarthritis. In conventional in vitro assays using liver microsomes and NADPH as cosubstrate, a high metabolic stability of licofelone was observed. In the presence of UDP-glucuronid acid (UDPGA), licofelone is rapidly converted into the corresponding acyl glucuronide, M1. These results are in conflict with data from clinical studies. After administration of licofelone to humans, M1-plasma concentrations were negligibly low while the exposure of the hydroxy-metabolite M2 achieved values of approx. 20 % compared to that of the parent drug. Metabolism studies with human hepatocytes and dual-activity assays with microsomes, which allowed the simultaneous monitoring of hydroxylation and glucuronidation reactions, were performed and the metabolic pathway of licofelone was elucidated. After glucuronidation, predominantly catalyzed by UGT-isoforms 2B7, 1A9 and 1A3, M1 is converted into the hydroxy-glucuronide M3 in a CYP2C8 dependent reaction. The enzyme specificities were investigated using recombinant human CYP and UGT-isoforms as test systems. In vitro drug-interaction studies using the 6-hydroxylation of paclitaxel as control reaction confirmed that neither licofelone nor M1 are relevant inhibitors of CYP2C8. The formation of M3 was also observed with liver microsomes from Cynomolgus monkeys but in incubations with mouse and rat liver microsomes, M1 remained unchanged. The clinical relevance of these findings is discussed.

Key words: anti-inflammatory drugs, cytochrome P450 catalyzed oxidations, drug disposition, UDP glucuronyltransferases