INVOLVEMENT OF HUMAN CYP1A ISOENZYMES IN THE METABOLISM AND DRUG-INTERACTIONS OF RILUZOLE IN-VITRO

Cytochrome P450 (CYP) and uridine diphosphate glucuronosyltransferase (UGT) isoenzymes involved in riluzole oxidation and glucuronidation we re characterized in (1) kinetic studies with human hepatic microsomes and isoenzyme-selective probes and (2) metabolic studies with genetica lly expressed human CYP isoenzymes from transfected B-lymphoblastoid a nd yeast cells. In vitro incubation of [C-14]riluzole (15 mu M) with h uman hepatic microsomes and NADPH or UDPGA cofactors resulted in forma tion of N-hydroxyriluzole (K-m = 30 mu M) or an unidentified glucuroco njugate (K-m = 118 mu M). Human microsomal riluzole N-hydroxylation wa s most strongly inhibited by the CYP1A2 inhibitor alpha-naphthoflavone (IC50 = 0.42 mu M). Human CYP1A2-expressing yeast microsomes generate d N-hydroxyriluzole, whereas human CYP1A1-expressing yeast microsomes generated N-hydroxyriluzole, two additional hydroxylated derivatives a nd an O-dealkylated derivative. CYP1A2 was the only genetically expres sed human P450 isoenzyme in B-lymphoblastoid microsomes to metabolize riluzole. Riluzole glucuronidation was inhibited most potently by prop ofol, a substrate for the human hepatic UGT HP4 (UGT1.8/9) isoenzyme. In vitro, human hepatic microsomal hydroxylation of riluzole (15 mu M) was weakly inhibited by amitriptyline, diclofenac, diazepam, nicergol ine, clomipramine, imipramine, quinine and enoxacin (IC50 approximate to 200-500 mu M) and cimetidine (IC50 = 940 mu M). Riluzole (1 and 10 mu M) produced a weak, concentration-dependent inhibition of CYP1A2 ac tivity and showed competitive inhibition of methoxyresorufin O-demethy lase. Thus, riluzole is predominantly metabolized by CYP1A2 in human h epatic microsomes to N-hydroxyriluzole; extrahepatic CYP1A1 can also b e responsible for the formation of several other metabolites. Direct g lucuronidation is a relatively minor metabolic route. In vivo, riluzol e is unlikely to exhibit significant pharmacokinetic drug interaction with coadministered drugs that undergo phase I metabolism.