Comparative disposition of codeine and pholcodine in man after single oral doses

Four healthy male subjects received single oral doses of 15, 30 and 60 mg of codeine and pholcodine according to a balanced cross-over design with an interval of 7 days between the six treatments. Blood samples were collected for 8 h after each drug administration. In phase 2 of the study six different male volunteers received single oral doses of 60 mg of codeine and pholcodine with a 14 day interval between successive drug treatments. Blood was sampled for 12 h after codeine and 121 h after pholcodine administration. Plasma concentrations of free (unconjugated) and total (unconjugated plus conjugated) codeine, pholcodine and morphine were determined by radioimmunoassay and selected pharmacokinetic parameters were derived from these data. Pharmacokinetics of both drugs were independent of dose. Codeine was absorbed and eliminated relatively rapidly [elimination t1/2 = 2.3 +/- 0.4 h (mean +/- s.d.)]. While codeine kinetics were adequately described by a one-compartment open model with first-order absorption, a two-compartment model was required to describe pholcodine elimination from plasma (t1/2,z = 37.0 +/- 4.2 h). Plasma concentrations of conjugated codeine were much greater than those of the unconjugated alkaloid. By contrast, pholcodine appeared to undergo little conjugation. Biotransformation of codeine to morphine was evident in all subjects, although the extent of this metabolic conversion varied considerably between subjects. Morphine was not detectable in the plasma of any subject after pholcodine administration.