CYP3A4-MEDIATOD OXIDATION OF LISOFYLLINE TO LISOFYLLINE 4,5-DIOL IN HUMAN LIVER-MICROSOMES

The cytochrome P450s responsible for the conversion of lisofylline, a drug being developed to prevent the complications of high-dose chemoth erapy, to lisofylline 4,5-diol, one of two principal metabolites in hu man liver microsomes, were evaluated. Lisofylline diol formation in mi crosomes prepared from five adult human livers was biphasic, with resp ective K-m values of 0.0230 +/- 0.015 and 4.23 +/- 2.8 mM (mean +/- SD ) and respective V-max values of 0.0565 +/- 0.052 and 0.429 +/- 0.15 n mol/min/mg of protein. Through studies with isoform selective chemical inhibitors, CYP3A4 was implicated as the low K-m enzyme from 89.0 +/- 11.2% inhibition of lisofylline 4,5-diol formation by troleandomycin at 50 mu M substrate and CYP2A6 was implicated as the high K-m enzyme. The formation of lisofylline 4,5-diol by these enzymes was confirmed with cDNA-expressed human CYP3A4 and CYP2A6.