Flurbiprofen, a non-steroidal anti-inflammatory drug (NSAID), is metabolized by both oxidation via the cytochrome P450 system and by glucuronidation. The major oxidative pathway in flurbiprofen metabolism is to a 4'-hydroxy metabolite, and recently we demonstrated that cytochrome P450 2C9 and its R144C variant were involved in this process (Tracy et al., Biochem Pharmacol 49: 1269-1275, 1995). Using complementary DNA (cDNA)-expressed cell systems, it has been demonstrated that at physiological concentrations of flurbiprofen there is a lack of involvement of P450s 1A2, 2C8, 2E1, and 3A4. In evaluating flurbiprofen as a potential probe for cytochrome P450 2C9, it is important to assess the involvement of additional P450s in this process. To this end, further studies were undertaken using specific inhibitors of P450 2C9 and P450 cDNA-expressed microsomes for P450 1A1, 2A6, 2B6, 2C19, and 2D6 to assess their potential involvement. We observed the inhibition of (R)- and (S)-flurbiprofen 4'-hydroxylation by an inhibitor of P450 2C9, sulfaphenazole (Ki = 0.07 and 0.06 microM, respectively), and the NSAID piroxicam (Ki = 10 and 7 microM, respectively). Furthermore, using microsomes from a lymphoblastoid cell line, we found that P450s 1A1, 2A6, 2B6, 2C19, and 2D6 were not involved in flurbiprofen hydroxylation at physiological concentrations of flurbiprofen. This finding is particularly important due to the sequence homology and potential substrate overlap of P450 2C9 and 2C19. These studies then provide additional evidence to suggest that P450 2C9 may be the only isoform involved to any substantial degree in flurbiprofen 4'-hydroxylation, and thus this reaction is useful as an in vitro probe for this particularly cytochrome P450 isoform and may be useful as an in vivo probe.