Asian Journal of Pharmacodynamics and Pharmacokinetics ...

Gao J et al. Asian Journal of Pharmacodynamics and Pharmacokinetics 2010(4):287-299hydroxylation and glucuronidation reactions,were performed, and the metabolic pathway oflicofelone was elucidated. After glucuronidation,predominantly catalyzed by UDPglucuronosyltransferase (UGT) isoformsUGT2B7, UGT1A9, and UGT1A3, M1 isconverted into the hydroxy-glucuronide M3 in aCYP2C8-dependent reaction. The enzymespecificities were investigated using recombinanthuman cytochrome P450 and UGT isoforms astest systems. In vitro drug-interaction studiesusing the 6α-hydroxylation of paclitaxel ascontrol reaction confirmed that neither licofelonenor M1 is a relevant inhibitor of CYP2C8. Theformation of M3 was also observed with livermicrosomes from cynomolgus monkeys, but inincubations with mouse and rat liver microsomes,M1 remained unchanged. [9]Biotransformation pathwayIn conventional in vitro assays using livermicrosomes and NADPH as cosubstrate, a highmetabolic stability of licofelone was observed. Incontrast, M1 remained at the level of a tracemetabolite. In that respect, the disposition oflicofelone in humans is different from all thestandard animal species (mouse, rat, dog, monkey)in which systemic concentrations of M2 werenegligible even on chronic dosing. A furtherhydroxy-metabolite of licofelone is M4. Thiscompound was initially identified in microsomalexperiments but not in plasma samples fromhumans after single and repeated administration oftherapeutic doses (i.e., 200 or 400 mg b.i.d.).Relevant concentrations were determined in plasmasamples from subjects who were treated withincreasing doses to determine the maximumtolerated dose. The chemical structures oflicofelone and its metabolites are shown in Fig.1.Fig 1. Chemical structure of licofelone and its proposed biotransformation pathway.The results from in vitro metabolism studiesshow that in humans hydroxylation of theglucuronide M1 represents the pivotal step in thebiosynthesis of M2. [9] Although the cytochromeP450 (P450)-dependent hydroxylation ofglucuronides has been described in theliterature. [34,35] The formation of M2 represents aunique example as the systemic exposure ofhumans to this major metabolite is based on theglucuronidation of the parent drug followed byhydroxylation of the glucuronide.In the presence of UDPglucuronic acid,licofelone is rapidly converted into thecorresponding acyl glucuronide, M1. These resultsare in conflict with data from clinical studies. Afteradministration of licofelone to humans, M1 plasmaconcentrations were negligibly low, whereas theexposure of the hydroxy-metabolite M2 achieved293

Gao J et al. Asian Journal of Pharmacodynamics and Pharmacokinetics 2010(4):287-299The whole-body autoradiograms (Fig 6)metabolic capacity of hepatocytes does not allowurine(Fig 5). [36] characteristics with a rapid initial decrease ofthe elucidation of metabolic pathways. Furthermore,this test system is less suitable to identify theenzymes responsible for biotransformations ofinterest. Finally, hepatocytes from dogs, mice, ormonkeys are not ubiquitously available, whichlimits their use for interspecies comparison of drugmetabolism. [9]In Fig. 3, the biotransformation rates, relative tothe initial substrate concentrations, and theshow that an a increased radioactivity is left inthe stomach, the highest tissue level is deterctedin the lung, liver, kidneys, heart, large intestineand small intestine(Fig 6); the tissue levelscumulation of radioactivity is also seen in thespleen, levels in the lung, liver and kidneys arelower relative to the intestine levels (Fig 6b); andat 48 h, the tissue distribution is similar to theresult obtained affer 24 h (Fig 6c).concentrations are given. With 10 and 30 µmollicofelone, the sum of glucuronides (M1 < M3) was3.41 and 9.92µmol, which correspond tobiotransformation rates of 34 and 33%. With 100µl of licofelone, the relative glucuronide contentwas 25%. The biotransformation rate of M2 andM3 decreased with ascending substrateconcentrations. The highest M3 concentration (5.02± 0.11 µmol) was observed at 30 µmol, whereasonly 2.41 ± 0.56µmol was determined afterincubation of 100 µmol licofelone. These dataindicate that the glucuronidation capacity of the testsystem was limited at concentrations greater thanFig 4. Plasma level of rasdioactivity of 14C-ML3000after oral administration of a single dose 28.6 mg.kg -1to rats.30 µM. In contrast, hydroxylation of M1 was partlyinhibited in incubations with 100 µmol. M4concentrations increased with ascending substrateconcentrations in an almost proportional manner.The biotransformation rate to M4 was essentiallyconstant. [9]Pharmacokinetics in animalsPlasma levels and distribution ofradioactivity were examined using whole-bodyautoradiography after oral administration of14 C-labeled licofelone (13.7 to 28.6 mg·kg -1 ) tofemale rats (Fig 4). Plasma levels of licofeloneFig 5. Excretion of radioactivuy with feces(-) and urine(---) after oral administration of 28.6 mg.kg -1 dose.peaked at 3 to 4 h after administration, with aplasma t 1/2 of about 11 h. The highest tissuelevels of licofelone were detected in the lung,liver, kidney, heart and intestine. Almost nopenetration of the blood-brain barrier was noted;however, after 48 h there was a minoraccumulation in fat. Of the total radioactivity,Pharmacokinetics in humansIn humans, after p.o. administration ofimmediate-release tablets, licofelone is rapidlyabsorbed from the gastrointestinal tract, andmaximum plasma concentrations are achievedapproximately 2 to 3 h after administration.58.3% was found in the feces and 7.9% in the Systemic elimination follows biphasicplasma concentration [T ½α = 1 h] and a slow296