Abstract

Studies were performed to determine the cytochromes P450 (P450) responsible for the biotransformation of (S)-13[(dimethylamino)methyl]-10,11,14,15-tetrahydro-4,9:16,21-dimetheno-1H, 13H-dibenzo[e,k]pyrrolo[3,4-h][1,4,13]oxadiazacyclohexadecene-1,3(2H)-dione (LY333531) to its equipotent metabolite, N-desmethyl LY333531, and to examine the ability of these two compounds to inhibit P450-mediated metabolism. Kinetic studies indicated that a single enzyme in human liver microsomes was able to formN-desmethyl LY333531 with an apparentK_M value of approximately 1 μM. The formation rate of N-desmethyl LY333531 was correlated with markers of nine P450s in a bank of 20 human liver microsomes. The only significant correlation observed was with the form-selective activity for CYP3A. Of the nine cDNA-expressed P450s examined, only CYP3A4 and CYP2D6 formed N-desmethyl LY333531. However, CYP3A4 formed N-desmethyl LY333531 at a rate 57-fold greater than that observed with CYP2D6. In incubations with human liver microsomes, quinidine, an inhibitor of CYP2D6, demonstrated little inhibition of metabolite formation while ketoconazole, an inhibitor of CYP3A, demonstrated almost complete inhibition. Thus, CYP3A is responsible for the formation of N-desmethyl LY333531. LY333531 and N-desmethyl LY333531 were also examined for their ability to inhibit metabolism mediated by CYP2D6, CYP2C9, CYP3A, and CYP1A2. LY333531 and N-desmethyl LY333531 were found to competitively inhibit CYP2D6 with calculatedK_i values of 0.17 and 1.0 μM, respectively. Less potent inhibition by these compounds of metabolism mediated by the other three P450s examined was observed. In conclusion, CYP3A is primarily responsible for forming N-desmethyl LY333531. Therefore, alterations in the activity of this enzyme have the potential to affect LY333531 clearance. In addition, LY333531 and its metabolite are predicted to be potential inhibitors of CYP2D6-mediated reactions in vivo.