A Randomized Trial of the N-methyl-d-aspartate Receptor Glycine Site Antagonist Prodrug 4-chlorokynurenine in Treatment-Resistant Depression.

BACKGROUND: Ketamine has rapid-acting antidepressant effects but is associated with psychotomimetic and other adverse effects. 7-chlorokynurenic acid (7-Cl-KYNA) is a potent and specific glycine site N-methyl-d-aspartate (NMDA) receptor antagonist but crosses the blood-brain barrier inefficiently. Its prodrug, L-4-chlorokynurenine (4-Cl-KYN) exerts acute and sustained antidepressant-like effects in rodents and has no reported psychotomimetic effects in either rodents or healthy volunteers. This study examined whether 4-Cl-KYN has rapid antidepressant effects in individuals with treatment-resistant depression (TRD). METHODS: After a two-week drug-free period, 19 participants with TRD were randomized to receive daily oral doses of 4-Cl-KYN monotherapy (1,080mg/day for seven days, then 1,440mg/day for seven days) or placebo for 14 days in a randomized, placebo-controlled, double-blind, crossover manner. The primary outcome measure was Hamilton Depression Rating Scale (HAM-D) score, assessed at several time points over a two-week period; secondary outcome measures included additional rating scale scores. Pharmacokinetic measures of 7-Cl-KYNA and 4-Cl-KYN and pharmacodynamic assessments were obtained longitudinally and included 1H-magnetic resonance spectroscopy (MRS) brain glutamate levels, resting-state functional magnetic resonance imaging (rsfMRI), and plasma and cerebrospinal fluid measures of kynurenine metabolites and neurotrophic factors. RESULTS: Linear mixed models detected no treatment effects, as assessed by primary and secondary outcome measures. No difference was observed for any of the peripheral or central biological indices or for adverse effects at any time between groups. 4-Cl-KYN was safe and well-tolerated, with generally minimal associated adverse events. CONCLUSIONS: In this small crossover trial, 4-Cl-KYN monotherapy exerted no antidepressant effects at the doses and treatment duration studied.