Abstract

The pharmacokinetics and pharmacodynamics of the two main metabolites of tramadol, (+)-O-desmethyltramadol and (−)-O-desmethyltramadol, were studied in rats. Pharmacodynamic endpoints evaluated were respiratory depression, measured as the change in arterial blood pCO₂, pO₂, and pH levels; and antinociception, measured by the tail-flick technique. The administration of 10 mg/kg (+)-O-desmethyltramadol in a 10-min i.v. infusion significantly altered pCO₂, pO₂, and pH values in comparison with baseline and lower-dose groups (P < .05). However, 2 mg/kg administered in a 10-min i.v. infusion was enough to achieve 100% antinociception without respiratory depression. Moreover, the β-funaltrexamine pretreatment completely eliminated the antinociception of the 2-mg/kg dose, suggesting that such an effect is due to μ-opioid receptor activation. To describe and adequately characterize the in vivo antinociceptive effect of the drug, (+)-O-desmethyltramadol was given at different infusion rates of varying lengths (10–300 min). Pharmacokinetics was best described by a two-compartmental model. The time course of response was described using an effect compartment associated with a linear pharmacodynamic model. The estimates of the slope of the effect versus concentration relationship were significantly decreased (P < .05) as the length of infusion was increased, suggesting the development of tolerance. Doses of up to 8 mg/kg (−)-O-desmethyltramadol given in 10-min i.v. infusion did not elicit either antinociception in the tail-flick test or respiratory effects. These in vivo results are in accordance with the opiate and nonopiate properties reported for these compounds in several in vitro studies.