EN3427: a novel cationic aminoindane with long-acting local anesthetic properties.

Abstract

Currently approved local anesthetic drugs provide relatively brief local anesthesia that is appropriate and even desirable in some settings, but an extended duration of action beyond their capabilities would be a distinct benefit in other clinical situations. We implemented a drug discovery program that sought to identify novel local anesthetic molecules that specifically demonstrated a long-acting, preferential action on nociceptor sensory afferents that expressed transient receptor potential (TRP) channels. The hypothesis we tested was whether relatively membrane-impermeant local anesthetic molecules could confer long-lasting anesthesia if neuronal access was facilitated by TRP channel activation. The current work describes in vivo studies on a lead molecule that emerged from the discovery program, EN3427, in several rodent pain models. Studies were performed on male Sprague-Dawley rats using 2 models of acute mechanical paw-pinch-evoked and pinprick-evoked nociceptive pain. Behavioral responses to noxious stimuli were assessed at baseline, that is, before any pharmacologic intervention, and at various timepoints after a single perisciatic or subcutaneous administration of either EN3427 alone or in combination with lidocaine. Paw withdrawal thresholds or cutaneous trunci reflexes were quantified, and pre-post drug values were compared statistically with analysis of variance followed by post hoc Dunnett multiple range test. A single perisciatic injection of lidocaine (2%) produced relief of paw-pinch-evoked pain that was significantly different from baseline through to the 1-hour timepoint (Dunnett multiplicity-adjusted P = 0.0081), as assessed using paw withdrawal or vocalization end points. EN3427 (0.2%), in the same model, produced a long-lasting block, with pain thresholds being significantly above baseline through to the 18-hour timepoint (Dunnett multiplicity-adjusted P = 0.0002); the combination of EN3427 (0.2%) plus lidocaine (2%) produced even longer lasting analgesia, with pain thresholds being significantly above baseline through to the 24-hour timepoint (Dunnett multiplicity-adjusted P = 0.0073). Similar results were obtained with use of the pinprick approach. A single subcutaneous injection of lidocaine (2%) produced complete loss of sensation to cutaneous pinprick through 0.5 hours, but sensitivity thresholds were no different to baseline by the 1-hour timepoint, a similar injection of EN3427 alone (0.2%) produced a loss of sensation that was significantly different from baseline through the 8-hour timepoint (Dunnett multiplicity-adjusted P = 0.0045), and the combination of lidocaine (2%) plus EN3427 (0.2%) appeared to further enhance duration of analgesia, although this was significantly different from baseline only through the 10-hour timepoint (Dunnett multiplicity-adjusted P = 0.0048). Analgesic efficacy was dose related; using the combined injection approach, we found that increases in the dose of EN3427 with a fixed 2% lidocaine led to substantially extended analgesia and increasing doses of lidocaine combined with a fixed dose of EN3427 (0.2%) led to only modestly increased duration of action. The present studies demonstrate that a new molecular entity, EN3427, produces effective and long-lasting analgesia in 2 rodent pain models. The analgesic effects of EN3427 are significantly longer-lasting than lidocaine and are further extended when EN3427 is combined with lidocaine. The results are discussed with respect to a possible lidocaine-mediated TRP channel activation and facilitated neuronal access of EN3427, with subsequent entrapment conferring extended-duration efficacy.