Analgesic and Antidepressant Effects of the Clinical Glutamate Modulators Acetyl-L-Carnitine and Ketamine

Pain and depression are leading causes of disability and of profound social and economic burden. Their impact is aggravated by their chronicity and comorbidity and the insufficient efficacy of current treatments. Morphological and functional metabolism studies link chronic pain and depressive disorders to dysfunctional neuroplastic changes in fronto-limbic brain regions that control emotional responses to painful injuries and stressful events. Glutamate modulators are emerging new therapies targeting dysfunctional brain areas implicated in the generation and maintenance of chronic pain and depression. Here, we report the effects of two clinically approved glutamate modulators: acetyl-L-carnitine (ALCAR) and S, R(±)ketamine (KET). ALCAR is a natural neurotrophic compound currently marketed for the treatment of neuropathies. KET is the prototypical non-competitive antagonist at N-methyl-D-aspartate glutamate receptors and a clinically approved anesthetic. Although they differ in pharmacological profiles, ALCAR and KET both modulate aminergic and glutamatergic neurotransmissions and pain and mood. We assessed in rats the effects of ALCAR and KET on cerebral metabolic rates for glucose (rCMRglc) and assessed clinically the effects of ALCAR in chronic pain and of KET in post-operative pain. ALCAR and KET increased rCMRglc at similar degrees in prefrontal, somatosensory, and cingulate cortices, and KET increased rCMRglc at a different, much larger, degree in limbic and dopaminergic areas. While rCMRglc increases in prefrontal cortical areas have been associated with analgesic and antidepressant effects of ALCAR and KET, the marked metabolic increases KET induces in limbic and dopaminergic areas have been related to its psychotomimetic and abuse properties. In patients with chronic neuropathic pain, ALCAR (1,000 mg/day) yielded to a fast (2 weeks) improvement of mood and then of pain and quality of life. In day-surgery patients, KET improved dischargeability and satisfaction. In obese patients undergoing bariatric surgery, a single, low dose of KET (0.5 mg/kg) at induction of anesthesia determined a very fast (hours) amelioration of post-operative depression and pain and an opioid-sparing effect. These findings indicate that ALCAR and KET, two non-selective glutamate modulators, still offer viable therapeutic options in comorbid pain and depression.

Biomarkers of ketamine’s antidepressant effect: a clinical review of genetics, functional connectivity, and neurophysiology

Major depressive disorder (MDD) is one of the leading causes of morbidity and all-cause mortality (including suicide) worldwide, and, unfortunately, first-line monoaminergic antidepressants and evidence-based psychotherapies are not effective for all patients. Subanesthetic doses of the N-methyl-D-aspartate receptor antagonists and glutamate modulators ketamine and S-ketamine have rapid and robust antidepressant efficacy in such treatment-resistant depressed patients (TRD). Yet, as with all antidepressant treatments including electroconvulsive therapy (ECT), not all TRD patients adequately respond, and we are presently unable to a priori predict who will respond or not respond to ketamine. Therefore, antidepressant treatment response biomarkers to ketamine have been a major focus of research for over a decade. In this article, we review the evidence in support of treatment response biomarkers, with a particular focus on genetics, functional magnetic resonance imaging, and neurophysiological studies, i.e. electroencephalography and magnetoencephalography. The studies outlined here lay the groundwork for replication and dissemination.

Comparative efficacy between ketamine, memantine, riluzole and d-cycloserine in patients diagnosed with drug resistant depression: a meta-analysis

Background: Glutamate modulators are having immense potential and are newer entities for treating drug resistant depression. The objectives were to generate statistical evidence on basis of existing data of ketamine, memantine, riluzole and d-cycloserine in resistant depression.Methods: A total of 14 RCTs following PRISMA guidelines and matching inclusion and exclusion criteria were collected of ketamine (5), memantine (3), riluzole (2) and d-cycloserine (4) vs placebo in drug resistant depression. Only RCTs with primary diagnosis of drug resistant depression (Previously on two standard antidepressant therapy) were included. Studies with treatment response rate, 50% reduction in total score of the depression rating scale-Montgomery-Åsberg Depression Rating Scale or the Hamilton Depression Rating Scale or Beck Depression Inventory was chosen as clinical outcome measure. RevMan 5.3 software was used for the analysis.Results: In ketamine group using random effect model SMD was 2.122 (95% CI 0.659-3.584). P-value was statistically significant (random effect p <0.005 and in fixed effect <0.001). In memantine group, using random effect model -0.963 was SMD and (95% CI -1.958-0.0324). P-value was <0.001, significant in fixed effect. In riluzole group, SMD was -0.564 with (95% CI -3.927-2.799) in random effect. P-value was 0.741. In d-cycloserine group SMD was 0.316 with (95% CI -1.252-1.885) in random effect. P-value was 0.690.Conclusions: Ketamine showed best efficacy followed by memantine. Riluzole and DCS as such have no efficacy although its acts by same glutamate pathway. More molecular based research is required in use of glutamate modulators in resistant depression.