• Terhi J Lohela, Satu Poikola, Daniel Backmansson, Outi Lapatto-Reiniluoto, Janne T Backman, Klaus T Olkkola, and Tuomas O Lilius.
• From the Department of Clinical Pharmacology, Faculty of Medicine, University of Helsinki and Helsinki University Hospital, Helsinki, Finland.
• Anesth. Analg. 2024 Mar 1; 138 (3): 598606598-606.

BackgroundSubanesthetic ketamine may reduce perioperative consumption of opioids. We studied whether intravenous S-ketamine alters the pharmacokinetics of oral morphine in healthy volunteers.MethodsIn this paired, randomized, double-blind, crossover trial, 12 participants under a 2-hour intravenous S-ketamine (0.57 mg/kg/h) or placebo infusion received oral morphine (0.2 mg/kg) at 30 minutes. Plasma concentrations of ketamine, morphine, and their major metabolites were quantified for 24 hours. The primary end point was area under the curve (AUC) 0-24 of morphine. Other pharmacokinetic variables for morphine and its metabolites were studied as secondary end points. The data were analyzed as between-phase comparisons for each participant using Wilcoxon matched-pairs signed-rank tests ( tmax ) or paired t -tests on log-transformed variables (other variables).ResultsWhile the AUC 0-24 was similar between the 2 phases, S-ketamine reduced the AUC 0-1.5 of oral morphine by 69% (ratio to control, 0.31; 90% confidence interval [CI], 0.15-0.65; P = .0171) and increased its tmax from 0.5 (range, 0.50-1.5) to 1.0 hour (range, 0.50-4.0; P = .010). The AUC 0-1.5 of morphine-6-glucuronide (M6G) was reduced by 84% (0.16; 90% CI, 0.07-0.37; P = .0025) and maximum plasma concentration ( Cmax ) by 43% (0.57; 90% CI, 0.40-0.81; P = .0155), while its tmax was increased from 1.5 (range, 1.0-2.0) to 4.0 (range, 1.0-8.0; P = .0094) hours by S-ketamine. Similarly, the AUC 0-1.5 of morphine-3-glucuronide (M3G) was reduced by 85% (0.15; 90% CI, 0.05-0.43; P = .0083), and tmax increased from 1.0 (range, 0.5-1.5) to 4.0 hours (range, 1.0-8.0; P = .0063). In addition, the M6G-to-morphine and M3G-to-morphine metabolic AUC ratios were decreased by 47% (0.53; 90% CI, 0.39-0.71; P = .0033) and 52% (0.48; 90% CI, 0.27-0.85; P = .0043) during 0 to 1.5 hours and by 15% (0.85; 90% CI, 0.78-0.92; P = .0057) and 10% (0.90; 90% CI, 0.83-0.98; P = .0468) during 0 to 24 hours, respectively. One participant was excluded from the analyses due to vomiting in the S-ketamine phase.ConclusionsIntravenous S-ketamine inhibited the metabolism of oral morphine and delayed its absorption, resulting in a net reduction in the exposure to morphine during the first 1.5 hours. Intravenous S-ketamine may delay the absorption and impair the efficacy of orally administered analgesics and other drugs.Copyright © 2023 The Author(s). Published by Wolters Kluwer Health, Inc. on behalf of the International Anesthesia Research Society.

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