INTERACTION OF TERFENADINE AND ITS PRIMARY METABOLITES WITH CYTOCHROME-P450 2D6

The substrate structure-activity relationships described for the major human drug-metabolizing cytochrome P450 (P450 or CYP) enzymes suggest that the H-1 receptor antagonist terfenadine could interact with CYP2 D6 either as a substrate or as an inhibitor, in addition to its known ability to act as a substrate for CYP3A4. Based on this substrate stru cture-activity relationship, computer modeling studies were undertaken to explore the likely interactions of terfenadine with CYP2D6. An ove rlay of terfenadine and dextromethorphan, a known substrate of CYP2D6, showed that it was possible to superimpose the site of hydroxylation (t-butyl group) and the nitrogen atom of terfenadine with similar regi ons in dextromethorphan. These observations were substantiated by the ease of docking of terfenadine into a protein model of CYP2D6. Experim entally, terfenadine inhibited CYP2D6 activity in human liver microsom es with an IC50 of 14-27 mu M, depending on the CYP2D6 substrate used. The inhibition of CYP2D6 was further defined by determining the K-i f or terfenadine against bufuralol 1'-hydroxylase activity in four human livers. Terfenadine inhibited bufuralol 1'-hydroxylase activity with a K-i of approximately 3.6 mu M. The formation of the hydroxylated met abolite (hydroxyterfenadine) in microsomes prepared from human liver a nd specific P450 cDNA-transfected B lymphoblastoid cells indicated tha t only CYP2D6 and CYP3A4 were involved in this transformation. As expe cted, the rate of formation was greatest with CYP3A4 (V-max = 1257 pmo l/min/nmol of P450), with CYP2D6 forming the metabolite at a 6-fold lo wer rate (V-max = 206 pmol/min/nmol of P450). The two enzymes had simi lar K-M values (9 and 13 mu M, respectively). These data indicate that , as predicted from modeling studies, terfenadine has the structural f eatures necessary for interaction with CYP2D6, CYP2D6 substrate used. The inhibition of CYP2D6 was further defined by determining the K-i fo r terfenadine against bufuralol 1'-hydroxylase activity in four human livers. Terfenadine inhibited bufuralol 1'-hydroxylase activity with a Ki of approximately 3.6 mu M The formation of the hydroxylated metabo lite (hydroxyterfenadine) in microsomes prepared from human liver and specific P450 cDNA-transfected B lymphoblastoid cells indicated that o nly CYP2D6 and CYP3A4 were involved in this transformation. As expecte d, the rate of formation was greatest with CYP3A4 (V-max = 1257 pmol/m in/nmol of P450), with CYP2D6 forming the metabolite at a 6-fold lower rate (V-max = 206 pmol/min/nmol of P450). The two enzymes had similar K-M values (9 and 13 mu M, respectively). These data indicate that, a s predicted from modeling studies, terfenadine has the structural feat ures necessary for interaction with CYP2D6.