Disposition of acetylmethadol in relation to pharmacologic action

The levels of acetylmethadol and its active biotransformation products were quantitated in the plasma and urine of subjects on acetylmethadol maintenance by the use of solvent extraction and gas-liquid chromatography. The time-course of plasma acetylmethadol, noracetylmethadol, and dinoracetylmethadol over 48 hr was determined concurrently with the time-action of pupillary constriction in 8 subjects receiving an average oral maintenance dose of 50 mg. The plasma level of acetylmethadol reached a peak at 4 hr and had nearly disappeared by 24 hr. The mean apparent half-life was 7 hr. The plasma level of noracetylmethadol peaked at 4 to 8 hr and slowly declined over the next 40 hr. The mean apparent half-life of noracetylmethadol was 48 hr. Dinoracetylmethadol plasma levels remained relatively constant throughout the dosing interval. The miotic effect reached a peak at 8 hr and decayed at a rate slower than that of plasma elimination of acetylmethadol but more rapidly than that of noracetylmethadol and dinoracetylmethadol. Substantial variation in the plasma levels of the compounds was observed in subjects after the same doses. The study indicated that the relatively long duration of opiate effects of acetylmethadol results from biotransformation to active and persistent metabolites.