Ketamine for Depression, 3: Does Chirality Matter?

Ketamine is a racemic mixture of the enantiomers R-ketamine and S-ketamine (esketamine). S-ketamine has greater analgesic and anesthetic effects than R-ketamine and is less likely to cause psychotomimetic and other adverse effects. There is therefore an emerging interest favoring the use of S-ketamine over racemic ketamine when the drug is used for analgesia or anesthesia. This article examines preclinical and clinical literature on the antidepressant properties of S-ketamine. Animal data suggest potential advantages for R-ketamine over S-ketamine. Case reports, case series, and some small randomized controlled trials suggest that single or repeated intravenous infusions (0.2-0.4 mg/kg) or intranasal administrations (28-84 mg) of S-ketamine have antidepressant action in patients with medication-refractory depression and that the observed benefits are similar in magnitude to the antidepressant benefits reported with racemic ketamine. However, there are no direct comparisons between S-ketamine and either R-ketamine or racemic ketamine in depressed patients; therefore, it is not possible to make an informed choice when considering the enantiomers and the racemate for the indication of depression.