A Human TRPA1-Specific Pain Model

Abstract

The cation channel transient receptor potential ankyrin 1 (TRPA1) plays an important role in sensing potentially hazardous substances. However, TRPA1 species differences are substantial and limit translational research. TRPA1 agonists tested previously in humans also have other targets. Therefore, the sensation generated by isolated TRPA1 activation in humans is unknown. The availability of 2-chloro-N-(4-(4-methoxyphenyl)thiazol-2-yl)-N-(3-methoxypropyl)-acetamide (JT010), a potent and specific TRPA1 agonist, allowed us to explore this issue. To corroborate the specificity of JT010, it was investigated whether the TRPA1 antagonist (1E,3E)-1-(4-fluorophenyl)-2methyl-1-penten-3-one oxime (A-967079) abolishes JT010-elicited pain. Sixteen healthy volunteers of both sexes rated pain due to intraepidermal injections of different concentrations and combinations of the substances. The study design was a double-blind crossover study. All subjects received all types of injections, including a placebo without substances. Injections of the TRPA1 agonist dosedependently caused pain with a half-maximal effective concentration of 0.31 M. Coinjection of A-967079 dose-dependently reduced and at a high concentration abolished JT010-induced pain. Quantification of JT010 by HPLC showed that a substantial part is adsorbed when in contact with polypropylene surfaces, but that this was overcome by handling in glass vials and injection using glass syringes. Isolated TRPA1 activation in humans causes pain. Thus, intradermal JT010 injection can serve as a tool to validate new TRPA1 antagonists concerning target engagement. More importantly, TRPA1-specific tools allow quantification of the TRPA1-dependent component in physiology and pathophysiology.This study showed that activation of the ion channel transient receptor potential ankyrin 1 (TRPA1) alone indeed suffices to elicit pain in humans, independent of other receptors previously found to be involved in pain generation. The newly established TRPA1-specific pain model allows different applications. First, it can be tested whether diseases are associated with compromised or exaggerated TRPA1-dependent painful sensations in the skin. Second, it can be investigated whether a new, possibly systemically applied drug directed against TRPA1 engages its target in humans. Further, the general possibility of quantitative inhibition of TRPA1 allows identification of the TRPA1-dependent disease component, given that the substance reaches its target. This contributes to a better understanding of pathophysiology, can lay the basis for new therapeutic approaches, and can bridge the gap between preclinical research and clinical trials.

Selective activation of TRPA1 ion channel by skin sensitizer nitrobenzene DNFB

“…In contrast, covalent agents are highly reactive and are not stable in a cytosolic environment containing high concentrations of nucleophiles (Liu et al, 2020). Covalent TRPA1 agonists such as benzoquinone, JT010 and AITC, covalently binding to the cysteine residues of the electrophilic sensing domain of TRPA1, can make TRPA1 desensitization and deactivation, and only cause acute pain (Heber et al, 2019;Ibarra and Blair, 2013;Suo et al, 2020). We also made an effort in docking of DNCB, a derivative of DNFB, into the same electrophilic sensing pocket, which reveals a similar non-covalent binding through hydrogen bonds without the halogen bond to Y684 (data not shown) and DNCB also activates TRPA1, but the skin sensitization mediated by DNCB is much milder than that of DNFB (Hsieh et al, 1996;Tingle et al, 1990).…”

Novel Analgesics with Peripheral Targets

“…Alternative strategies, such as drug repurposing for TRPA1, might see the use of desvenlafaxine, paliperidone, and febuxostat as TRPA1 blockers [182]. With the advent of potent TRPA1 agonists, a new human TRPA1 model can be used to test TRPA1 antagonists in humans [183]. Aside from pain, other diseases could benefit from TRPA1 inhibition [184].…”

Non-Analgesic Symptomatic or Disease—Modifying Potential of TRPA1

“…With a specific TRPA1 stimulation, sensations were not different from those generated by capsaicin, with the subjects reporting a primarily burning pain. Importantly, subjects stated that they did not experience cold sensations [20].…”