Identification of CYP3A4 as the predominant isoform responsible for the metabolism of ambroxol in human liver microsomes

1. In humans, ambroxol is metabolized to dibromoanthranilic acid (DBAA) and 6,8- dibromo-3-(trans-4-hydroxycyclohexyl)-1,2,3,4-tetrahydroquinazoline (DHTQ). The formation of DHTQ proceeds non-enzymatically, whereas that of DBAA requires NADPH. Studies have been performed to identify the CYP isozyme(s) involved in the formation of DBAA using human liver microsomes and microsomes expressing recombinant human CYP isozymes (1A1, 1A2, 2A6, 2B6, 2C8, 2C9, 2C19, 2D6, 2E1, 3A4 and 4A11). 2. The apparent V_max and K_m for the formation of DBAA were 472 +/- 192 pmol/min/mg protein and 248 +/- 40.6 muM respectively (mean +/- S.D., n = 3). 3. Of the recombinant CYP examined, only CYP3A4 metabolized ambroxol to DBAA. The apparent V_max and K_m were 1.42 pmol/min/pmol P450 and 287 muM respectively. 4. Among the CYP inhibitors examined (furafylline, sulphaphenazole, quinidine, diethyldithiocarbamic acid, ketoconazole), only ketoconazole inhibited the production of DBAA (>80%) at 1 muM and anti-CYP3A antiserum almost completely inhibited the formation of DBAA. 5. These results suggest that CYP3A4 is predominantly involved in the metabolism of ambroxol to DBAA in humans.