Valnoctamide, a non-teratogenic amide derivative of valproic acid, inhibits arachidonic acid activation in vitro by recombinant acyl-CoA synthetase-4.

Valnoctamide [valmethamide or 2-ethyl-3-methyl pentanamide (VCD)] (Fig. 1) is an over-the-counter drug available in many European countries as a sedative-hypnotic (1, 2). VCD is a chiral constitutional isomer of valpromide (VPD), the corresponding amide of valproic acid (VPA), a mood stabilizer approved by the FDA for treating bipolar disorder (BD) (2–5). Compared to VPA, however, VCD is significantly less teratogenic in mice (2, 6). It is not transformed into its homologous acid, valnoctic acid, or converted to VPA in vivo (4, 7, 8). It distributes better than VPA in brain (8), and is three times more potent as an anticonvulsant (3, 9). Fig. 1 Structure of valproic acid (VPA) and valnoctamide (VCD). VPA is effective in treating bipolar mania, but has a significant risk for birth defects and developmental delay through its epigenetic effects, particularly from inhibition of histone deacetylase (HDAC) (10–12). Having a similarly acting antimanic drug that is not teratogenic would be of clinical relevance. In this regard, in a Phase III trial, VCD was more effective than placebo as an add-on to risperidone for treating bipolar mania (13). On the basis of its clinical antimanic efficacy, metabolic stability, and lack of teratogenicity, VCD has a potential to become a new BD drug. VPA and the other FDA-approved mood stabilizers, carbamazepine, lithium, and lamotrigine, when given chronically to unanesthetized rats to produce therapeutically relevant plasma concentrations, downregulate markers of the brain arachidonic acid [(AA) 20:4n-6] cascade (14, 15): AA turnover in brain phospholipids or AA influx from plasma, expression of cyclooxygenase (COX)-2, and prostaglandin E (PGE2) concentration. Since markers of the AA cascade are upregulated in the postmortem BD brain, in association with excitotoxicity, neuroinflammation, apoptosis and synaptic loss (16–18), dampening the cascade by these drugs may contribute to their efficacy in BD (14). AA undergoes rapid deacylation-reacylation recycling within brain phospholipids (19–21), and it and its products (e.g., prostaglandins, thromboxanes, leukotrienes) have multiple biological effects and participate in neurotransmission and neuroinflammation (14, 15). As part of the deacylation-reacylation cycle, AA is hydrolyzed from membrane phospholipid by AA-selective calcium-dependent cytosolic phospholipase A2 (cPLA2) IVA, which is transcriptionally downregulated in rat brain following treatment with the mood stabilizers, carbamazepine and lithium. On the other hand, VPA’s downregulation of AA turnover in rat brain has been ascribed to its ability to uncompetitively inhibit activation of AA to AA-CoA by AA-selective acyl-CoA synthetase (Acsl, E.C.6.2.1.3)-4 (22–24). In uncompetitive inhibition, the inhibitor binds to the enzyme-substrate complex [ES] only and not to the free enzyme [E], while in noncompetitive inhibition the inhibitor binds to [E] or [ES]. VPA uncompetitively inhibits recombinant Acsl4 in vitro at a Ki of 25 mM (23). In view of VPA’s ability to inhibit recombinant Acsl4 in vitro, we thought it of interest to test whether other agents that are not as teratogenic would do so well. As a first approach, we showed that a non-teratogenic constitutional isomer of VPA, propylisopropylacetic acid (PIA), also inhibited recombinant Acsl4 in vitro, but with a higher inhibition efficiency than VPA (25) and at a lower Ki, 11.4 mM. In the present study, we used similar in vitro Michaelis–Menten kinetics to test whether VCD also would inhibit recombinant Acsl4 activity. Briefly, we found that VCD inhibited Acsl4-mediated activation of AA to AA-CoA by recombinant Acsl4 in vitro, with a lower Ki than reported for VPA and PIA (23, 25). Butyrate, an analog of VPA and also a histone deacetylase inhibitor (26), had no inhibitory action.