Acetylmethadol metabolites influence opiate receptors and adenylate cyclase in amygdala

The activity of acetylmethadol and two major metabolites of this drug, noracetylmethadol and dinoracetyl-methadol were studied in monkey brain amygdaloid tissue. This tissue contains opiate receptors which may be assessed by direct binding studies; also these receptors are coupled to and capable of modulating a dopamine-stimulated adenylate cyclase system. Etrophine and D-ALa2-Met-enkephalin exhibited similar potencies when assessed for inhibition of dopamine-stimulated adenylate cyclase or competition for [3H] D-Ala2-Met-enkephalin binding sites. While acetylmethadol displaced [3H] D-Ala2-Met-enkephalin binding sites with a Ki of 7.4 X 10(-7) M, it had no detectable activity in the opiate receptor coupled adenylate cyclase system. Noracetylmethadol and dinoracetylmethodol, however, were capable of both binding (Ki values, 5.6 and 1140 nM respectively) as well as inhibiting the dopamine stimulated adenylate cyclase system (IC50 values, 1.2 and 800 nM respectively). Thus, it appears that metabolism of acetylmethadol to its mono-demethylated form results in a compound more active in both assay systems. The further demethylation of noracetylmethadol results in a second, but less potent, active metabolite. This process of biological activation can be assumed to account for the slow onset and long duration of action of acetylmethadol.