S(+)-ketamine effect on experimental pain and cardiac output: a population pharmacokinetic-pharmacodynamic modeling study in healthy volunteers.

BACKGROUND Low-dose ketamine behaves as an analgesic in the treatment of acute and chronic pain. To further understand ketamine's therapeutic profile, the authors performed a population pharmacokinetic-pharmacodynamic analysis of the S(+)-ketamine analgesic and nonanalgesic effects in healthy volunteers. METHODS Ten men and ten women received a 2-h S(+)-ketamine infusion. The infusion was increased at 40 ng/ml per 15 min to reach a maximum of 320 ng/ml. The following measurements were made: arterial plasma S(+)-ketamine and S(+)-norketamine concentrations, heat pain intensity, electrical pain tolerance, drug high, and cardiac output. The data were modeled by using sigmoid Emax models of S(+)-ketamine concentration versus effect and S(+)-ketamine + S(+)-norketamine concentrations versus effect. RESULTS Sex differences observed were restricted to pharmacokinetic model parameters, with a 20% greater elimination clearance of S(+)-ketamine and S(+)-norketamine in women resulting in higher drug plasma concentrations in men. S(+)-ketamine produced profound drug high and analgesia with six times greater potency in the heat pain than the electrical pain test. After ketamine-infusion, analgesia rapidly dissipated; in the heat pain test but not the electrical pain test, analgesia was followed by a period of hyperalgesia. Over the dose range tested, ketamine produced a 40-50% increase in cardiac output. A significant consistent contribution of S(+)-norketamine to overall effect was detected for none of the outcome parameters. CONCLUSIONS S(+)-ketamine displays clinically relevant sex differences in its pharmacokinetics. It is a potent analgesic at already low plasma concentrations, but it is associated with intense side effects.