Papers of the Week

The worsening opioid epidemic has highlighted the need for the development of new, safe, and effective analgesic therapeutics. Opioid therapy currently is associated with the risk of conversion to addiction, diversion from patients for whom use is intended, and the development of analgesic tolerance, or the loss of efficacy with continued treatment. To this end, we have developed a line of non-opioid, agmatine-based compounds and assessed them for their efficacy in reversing behavioral expressions of pain in animal models, as well as evaluated their safety following chronic exposure. We have previously shown that agmatine, decarboxylated L-arginine, is an N-methyl-D-aspartate (NMDA) receptor antagonist that preferentially antagonizes receptors that express NR2B subunits. This preferential antagonism is desirable for NMDA-based therapeutics as it can lead to a widening of the therapeutic window and avoidance of the side effects commonly associated with NMDA antagonism including motor ataxia and psychoactive effects. However, agmatine has shown limited penetration through the blood brain barrier (BBB) and a short systemic half-life, limiting its clinical utility. We have designed strategically-substituted agmatine (SSA) compounds with the goal of improving its penetration through the BBB by increasing the lipophilicity of agmatine, potentially improving distribution across the BBB and increasing its half-life following systemic delivery. To this end, we have evaluated this series of SSAs for safety and efficacy in multiple animal models of pain. Mice (21-30 g, M/F) were assessed for their baseline mechanical sensitivity, and then one of several models of pain was induced: inflammatory pain (Complete Freund's Adjuvant, (CFA) injected into a hindpaw), neuropathic pain (spared nerve injury surgery), or post-surgical pain (hindpaw muscle incision). Mechanical sensitivity was again assessed, then an SSA compound, agmatine (the parent compound), or vehicle control was administered, and mechanical sensitivity was recorded for up to three hours following administration. The SSA compounds effectively reversed pain behaviors in mice following administration in the various pain models. Additionally, side effects characteristic of NMDA receptor antagonists were assessed and not found at the range of doses that produced analgesia, indicating a wide therapeutic window. These data indicate that the strategically-substituted agmatines are anti-hyperalgesic compounds with a wide therapeutic window, avoiding the motor impairment typical of drugs of this class.