The TRPA1 channel mediates the analgesic action of dipyrone and pyrazolone derivatives.

BACKGROUND AND
PURPOSE: Although still used by hundreds of millions of people worldwide, the mechanism of the analgesic action of the pyrazolone derivatives (PDs), dipyrone, propyphenazone and antipyrine remains unknown. The transient receptor potential ankyrin 1 (TRPA1) channel, expressed by nociceptors, is emerging as a major pain transduction pathway. We hypothesized that PDs target the TRPA1 channel and by this mechanism produce their analgesic effect. EXPERIMENTAL APPROACH: Calcium responses and currents were studied in cultured TRPA1-expressing rodent dorsal root ganglion neurons and human cells. Acute nociception and mechanical hypersensitivity were investigated in naïve and genetically manipulated mice. KEY
RESULTS: Pyrazolone and PDs selectively inhibited calcium responses and currents in TRPA1-expressing cells and acute nocifensor responses in mice evoked by reactive channel agonists (allyl isothiocyanate, acrolein and H2 O2 ). In line with recent results obtained with TRPA1 antagonists and TRPA1 gene deletion, the two most largely used PDs, dipyrone and propyphenazone, attenuated TRPA1-mediated nociception and mechanical allodynia in models of inflammatory and neuropathic pain (formalin, carrageenan, partial sciatic nerve ligation and the chemotherapeutic drug, bortezomib). Notably, dipyrone and propyphenazone attenuated carrageenan-evoked mechanical allodynia, without affecting PGE2 levels. The main metabolites of PDs did not target TRPA1 and did not affect TRPA1-dependent nociception and allodynia. CONCLUSIONS AND IMPLICATIONS: Evidence that in rodents the nociceptive/hyperalgesic effect produced by TRPA1 activation is blocked by PDs suggests that a similar pathway is attenuated by PDs in humans and that TRPA1 antagonists could be novel analgesics, devoid of the adverse haematological effects of PDs.