Abstract

The N-demethylation of LAAM, norLAAM, and methadone has been investigated in human liver microsomes and microsomes containing cDNA-expressed human P450s. Gas chromatography/mass spectrometry methods allowed detection of norLAAM and dinorLAAM formation from LAAM, dinorLAAM formation from norLAAM, and EDDP and EMDP formation from methadone. The rates of N-demethylation varied 4- to 7-fold in microsomes from four different donors with activities for LAAM and norLAAM consistently greater (5- to 14-fold) than for methadone. TheN-demethylation of LAAM, norLAAM, and methadone were significantly inhibited by ketoconazole. IC₅₀s could be determined for ketoconazole inhibition of LAAM and norLAAMN-demethylation of 1.6 and 1.1 μM, respectively. The ability of ketoconazole to reduce methadone N-demethylation below 40% varied in regard to liver donor. No other P450-selective inhibitors reduced the average activities more than 43%. cDNA-expressed P450 3A4 N-demethylated LAAM, norLAAM, and methadone at greater rates than the other cDNA-expressed P450s studied (1A2, 2C9, 2D6, or 2E1). P450 3A N-demethylation of LAAM, norLAAM, and methadone exceeded the next most active P450, respectively, by at least 2.5, 9.6, and 13.4 times when expressed per milligram protein and by 18.2, 6.0, and 6.1 times when expressed per nanomole P450. These results suggest that P450 3A4 is the primary site of N-demethylation of LAAM, norLAAM, and methadone in human liver. Although other enzymes may also be capable ofN-demethylating these compounds, identification of specific enzymes, except P450 3A4, has yet to be established. Knowledge of these enzymatic pathways is essential for assessment of the impact of metabolic drug-drug interactions on therapeutic success and/or adverse events.