Dextromethorphan and Memantine After Ketamine Analgesia: A Randomized Control Trial

Purpose: Intravenous ketamine is often prescribed in severe neuropathic pain. Oral N-methyl-D-aspartate receptor (NMDAR) antagonists might prolong pain relief, reducing the frequency of ketamine infusions and hospital admissions. This clinical trial aimed at assessing whether oral dextromethorphan or memantine might prolong pain relief after intravenous ketamine.

Patients and methods: A multicenter randomized controlled clinical trial included 60 patients after ketamine infusion for refractory neuropathic pain. Dextromethorphan (90 mg/day), memantine (20 mg/day) or placebo was given for 12 weeks (n=20 each) after ketamine infusion. The primary endpoint was pain intensity at one month. Secondary endpoints included pain, sleep, anxiety, depression, cognitive function and quality of life evaluations up to 12 weeks.

Results: At 1 month, dextromethorphan maintained ketamine pain relief (Numeric Pain Scale: 4.01±1.87 to 4.05±2.61, p=0.53) and diminished pain paroxysms (p=0.03) while pain intensity increased significantly with memantine and placebo (p=0.04). At 3 months, pain remained lower than at inclusion (p=0.001) and was not significantly different in the three groups. Significant benefits were observed on cognitive-affective domains and quality of life for dextromethorphan and memantine (p<0.05).

Conclusions: Oral dextromethorphan given after ketamine infusion extends pain relief during one month and could help patients to better cope with pain. Future studies should include larger populations stratified on pharmacogenetics screening. Optimization of an oral drug that could extend ketamine antihyperalgesia, with fewer hospital admissions, remains a prime challenge in refractory neuropathic pain.

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Figure 4 Effect of dextromethorphan, memantine and placebo on Brief Pain Inventory (BPI) score and Short-Form 36 (SF-36) parameters assessed after ketamine infusion (postK), Month 1, 2 and 3 (M1, M2, M3). Notes: (A) Scores of “Worst pain” and “Walking inability” BPI sub-scores assessed at M1 were significantly decreased in the dextromethorphan group compared to memantine and placebo groups. (B) Scores of “General health” and “Vitality” SF-36 sub scores were significantly higher in the memantine group at M2 and M3 respectively compared to dextromethorphan and placebo groups. (C) Percentage change (%) represents the difference between postK and M3 of “Role emotional” SF-36 sub score. At M3 “Role emotional” sub score increases significantly in memantine group compared to dextromethorphan and placebo groups.