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-1-hydroxycyclohexyl)-1,2,3,4-tetrahydroquinazoline (D HTQ). The formation of DHTQ proceeds non-enzymatically, whereas chat of DBA A requires NADPH. Studies have been performed to identify the CYP isozyme(s ) involved in the formation of DBAA using human liver microsomes and micros omes expressing recombinant human CYP isozymes (1A1, 1A2, 2A6, 2B6, 2C8, 2C 9, 2C19, 2D6, 2E1, 3A4 and 4A11). 2. The apparent V-max and K-m for the formation of DBAA were 472 +/- 192 pm ol/ min/mg protein and 248 +/- 40.6 mu M respectively (mean +/- S.D., n = 3 ). 3. Of the recombinant CYP examined, only CYP3A4 metabolized ambroxol to DBA A. The apparent V-max and K-m were 1.42 pmol/min/pmol P-450 and 287 mu M re spectively. 4. Among the CYP inhibitors examined (furafylline, sulphaphenazole, quinidi ne, diethyldithiocarbamic acid, ketoconazole), only ketoconazole inhibited the production of DBAA (> 80%) at 1 mu M and anti-CYP3A antiserum almost co mpletely inhibited the formation of DBAA. 5. These results suggest that CYP3A4 is predominantly involved in the metab olism of ambroxol to DBAA in humans.