Papers of the Week

Post-herpetic neuralgia (PHN) is a type of neuropathic (nerve) pain that persists for more than 3 months after crusting of the last shingles lesion. It is difficult to relieve with analgesic/adjuvant medications, and so novel analgesics are needed. Our aim was to use a rat model of varicella zoster virus (VZV)-induced neuropathic pain to assess the pain relief efficacy of several small molecule angiotensin II type 2 (AT) receptor antagonists (PD123,319, EMA300, and EMA401) relative to clinically used analgesic/adjuvant agents from four different pharmacological classes. Male Wistar rats received a unilateral intraplantar injection of VZV-infected MRC-5 cells (2 × 10 infected cells) and paw withdrawal thresholds (PWTs) in the ipsilateral hindpaws were assessed using von Frey filaments. Animals with PWTs ≤ 8 g received single doses of PD123,319 (0.03-3 mg/kg), EMA300 (0.3-5 mg/kg), EMA401 (0.03-1 mg/kg), gabapentin (10-60 mg/kg), amitriptyline (5-30 mg/kg), morphine (0.1-3 mg/kg), meloxicam (5-20 mg/kg) or vehicle and PWT versus time curves were generated. Single doses of PD123,319, EMA300, EMA401, gabapentin and morphine-evoked dose-dependent anti-allodynia in the hindpaws of VZV-rats. The mean (95% confidence intervals) EDs were 0.57 (0.04-1.7), 2.5 (1.0-3.7) and 0.41 (0.12-0.87) mg/kg for PD123,319, EMA300, and EMA401, respectively. The EDs for gabapentin and morphine were 39.9 (25.1-64.8) and 0.04 (0.16-2.09) mg/kg, respectively. In conclusion, the anti-allodynic efficacy of EMA401 in a VZV-rat model of neuropathic pain is aligned with its analgesic efficacy in a Phase 2a clinical trial in patients with PHN. This model has utility for anti-allodynic efficacy assessment of novel AT receptor antagonists from drug discovery.