A Novel Class of Potent, Allosteric TRPA1 Antagonists Reverse Hyperalgesia in Multiple Rat Models of Neuropathic Pain
Authors: Herz Jeffery M , Kesicki Edward , Tian Jinbin , Zhu Michael X , Thomsen William J 
Published: November 2023

There has been little recent success in identifying novel analgesic drug targets and discovering better therapeutics for treating neuropathic pain. Existing analgesic drugs have significant safety, efficacy, tolerability and addiction concerns limiting their use. Recent studies have identified a critical ligand‐gated ion channel on peripheral sensory nerve fiber endings, transient receptor potential ankyrin 1 (TRPA1), as a promising novel analgesic target for treating neuropathic pain. TRPA1 is a validated analgesic target based upon TRPA1 knockout studies in established pain models, pharmacological studies with TRPA1 antagonists and human medical genetic data. Peripherally acting TRPA1 antagonists may provide a novel, safer and more effective option for treating neuropathic pain than existing analgesics. Utilizing a rational drug discovery approach, we have discovered a novel structural class of small molecule, reversible TRPA1 antagonists with excellent drug‐like features using a human TRPA1 fluorescence‐based intracellular calcium assay. Five potent antagonists, ALGX‐2513, ALGX‐2541, ALGX‐2563, ALGX‐2561 and ALGX‐2542, inhibited FFA agonist‐mediated responses with IC50s ranging from 23–65nM and were found to be highly selective (>100X) in assays for numerous related TRP channels. ALGX‐2437 and ALGX‐2513 inhibited AITC‐mediated TRPA1 calcium responses in isolated mouse dorsal root ganglion (DRG) neurons, confirming target engagement. In rat PK studies, dosing of ALGX‐2437 and ALGX‐2460 (30 mg/kg) produced robust in vivo drug exposure, achieving high Cmax values (5–10X IC50) with tmax values of 2 hr and t1/2 values between 3–5 hrs. The analgesic effects of compounds in this series were evaluated in the spinal nerve ligation model (SNL) of neuropathic pain. A single dose (10–50mg/kg) of ALGX‐2437 showed dose‐dependent reversal (EC50 = 25mg/kg) of mechanical hyperalgesia in the SNL pain model with a maximal possible efficacy (%MPE) of 79% at 2 hr post‐dosing (p<0.0001). Analysis of brain and blood levels showed low brain penetrance of the drug at all dose levels. No adverse behavioral effects, including impairment of rotarod performance, were observed at all doses. In addition, ALGX‐2460, ALGX‐2465 (30mg/kg) and ALGX‐2542 (10mg/kg) also inhibited mechanical allodynia in the SNL model with %MPE values of 38 to 65%. The effect of ALGX‐2542 (10mg/kg) was also evaluated in the paclitaxel chemotherapeutic‐induced neuropathic pain model and displayed similar efficacy in inhibiting mechanical hyperalgesia as observed in the SNL pain model. These results demonstrate Algomedix TRPA1 antagonists possess the requisite high potency, specificity, and pharmacokinetic properties to achieve substantial efficacy in multiple neuropathic pain models and hold promise as a new class of analgesic drugs for effectively and safely treating neuropathic pain patients.

Support or Funding Information

Supported by NIH grants 1R43DA031516 and 1R43AR065330 (JMH)