CODEINE AND MORPHINE IN EXTENSIVE AND POOR METABOLIZERS OF SPARTEINE - PHARMACOKINETICS, ANALGESIC EFFECT AND SIDE-EFFECTS

Objective: Codeine O-demethylation to morphine is catalysed by the gen etic polymorphic sparteine oxygenase (CYP2D6). The objective of the pr esent study was to assess the analgesic effect of codeine on different types of experimental pain in relation to sparteine phenotype. Method s: Fourteen extensive (EMs) and 14 poor metabolizers (PMs) of spartein e completed a randomized, double-blind, three-way, cross-over study wi th a single oral dose of codeine (75 or 100 mg) against morphine (20 o r 30 mg) and placebo. Pain tests performed before and 1, 2, 3, and 4 h after medication included the cold presser test and gain thresholds f or heat and pressure stimulation. Adverse effects were rated by a stru ctured interview. Results: After morphine, morphine and morphine-6-glu curonide were present in equal amounts in plasma of PMs and EMs. After codeine, neither morphine nor morphine-6-glucuronide could be detecte d in 13 of the 14 PMs, whereas at least one of the compounds could be detected in all EMs. Peak pain and discomfort rated on a VAS scale dur ing the cold presser test were significantly reduced by morphine in bo th EMs and PMs, with a median peak change of 8.5 and 7.0 mm, respectiv ely, for peak pain, and 11.5 and 15.5 mm, respectively, for discomfort . Codeine only reduced these pain measures significantly in EMs, with a median peak change of 5.5 mm for peak pain and 10.5 mm for discomfor t. Pain detection and tolerance thresholds to heat and pressure were n ot consistently altered by either morphine or codeine. In PMs, adverse effects were significantly more pronounced on morphine than on codein e and only showed a slight difference between codeine and placebo. In EMs, there was no difference between codeine and morphine and more pro nounced adverse effects on both drugs as compared to placebo. Conclusi on. This study confirms that codeine O-demethylation depends on CYP2D6 ; it shows that the 6-glucuronidation of morphine is independent of CY P2D6; it supports the theory that the analgesic effect of codeine depe nds on its O-demethylation; and it indicates that this is probably als o the case for the adverse effects. The resuls lend no support to the suggestion of a non-opioid analgesic effect of codeine.