Enantioselective pharmacokinetics of (R)- and (S)-ketamine after a 5-day infusion in patients with complex regional pain syndrome.

INTRODUCTION: This study determined the pharmacokinetics and pharmacodynamics of (R)- and (S)-ketamine and (R)- and (S)-norketamine following a 5-day moderate dose, as a continuous (R,S)-ketamine infusion in complex regional pain syndrome (CRPS) patients.
MATERIALS AND METHODS: Ketamine was titrated to 10-40 mg/h and maintained for 5 days. (R)- and (S)-Ketamine and (R)- and (S)-norketamine pharmacokinetic and pharmacodynamic studies were performed. Blood samples were obtained on Day 1 preinfusion, and at 60-90, 120-150, 180-210, and 240-300 min after the start of the infusion, on Days 2, 3, 4, 5, and on Day 5 at 60 min after the end of infusion. The plasma concentrations of (R)- and (S)-ketamine and (R)- and (S)-norketamine were determined using enantioselective liquid chromatography-mass spectrometry.
RESULTS: Ketamine and norketamine levels stabilized 5 h after the start of the infusion. (R)-Ketamine clearance was significantly lower resulting in higher steady-state plasma concentrations than (S)-ketamine. The first-order elimination for (S)-norketamine was significantly greater than that of (R)-enantiomer. When comparing the pharmacokinetic parameters of the patients who responded to ketamine treatment with those who did not, no differences were observed in ketamine clearance and the first-order elimination of norketamine.
CONCLUSION: The results indicate that (R)- and (S)-ketamine and (R)- and (S)-norketamine plasma concentrations do not explain the antinociceptive activity of the drug in patients suffering from CRPS.