Selective antagonism of TRPA1 produces limited efficacy in models of inflammatory- and neuropathic-induced mechanical hypersensitivity in rats

Abstract

The transient receptor potential ankyrin 1 (TRPA1) channel has been implicated in pathophysiological processes that include asthma, cough, and inflammatory pain. Agonists of TRPA1 such as mustard oil and its key component allyl isothiocyanate (AITC) cause pain and neurogenic inflammation in humans and rodents, and TRPA1 antagonists have been reported to be effective in rodent models of pain. In our pursuit of TRPA1 antagonists as potential therapeutics, we generated AMG0902, a potent (IC90 of 300 nM against rat TRPA1), selective, brain penetrant (brain to plasma ratio of 0.2), and orally bioavailable small molecule TRPA1 antagonist. AMG0902 reduced mechanically evoked C-fiber action potential firing in a skin-nerve preparation from mice previously injected with complete Freund’s adjuvant, supporting the role of TRPA1 in inflammatory mechanosensation. In vivo target coverage of TRPA1 by AMG0902 was demonstrated by the prevention of AITC-induced flinching/licking in rats. However, oral administration of AMG0902 to rats resulted in little to no efficacy in models of inflammatory, mechanically evoked hypersensitivity; and no efficacy was observed in a neuropathic pain model. Unbound plasma concentrations achieved in pain models were about 4-fold higher than the IC90 concentration in the AITC target coverage model, suggesting that either greater target coverage is required for efficacy in the pain models studied or TRPA1 may not contribute significantly to the underlying mechanisms.

Targeting nociceptive transient receptor potential channels to treat chronic pain: current state of the field

“…In contrast, HC‐030031 had no effect on nerve firing following capsaicin stimulation, indicating these effects were not due to non‐specific inhibition of action potential generation (Kerstein et al, ). Similar effects were noted with the chemically distinct AMG0902 (Lehto et al, ). Consistent with this, A‐967079, a potent analogue of AP‐18, also reduced firing of wide‐dynamic range neurons in naïve rats after noxious pinch (McGaraughty et al, ).…”

“…Consistent with this, A‐967079, a potent analogue of AP‐18, also reduced firing of wide‐dynamic range neurons in naïve rats after noxious pinch (McGaraughty et al, ). In addition, TRPA1 KO mice showed attenuated mechanical nociception and reduced firing in response to force in a skin‐nerve preparation (Kerstein et al, ; Lehto et al, ), though the in vivo effect is subtle and varies somewhat among reports (Kwan et al, ; Petrus et al, ). A large data set collected from many studies also supports the role of TRPA1 channels in mechanical hyperalgesia induced by inflammation.…”

“…However, not all compounds seem to significantly reduce mechanical hypersensitivity in this model; for example, AMG0902 exerted only a mild effect in a CFA mechanical model at a dose that showed robust inhibition of AITC‐induced pain behaviours. The reason for this discrepancy remains unclear, though it may be related to the reduced ability of AMG0902 to inhibit activity induced by endogenously produced activators such as 4‐ONE and methylglyoxal, compared to AITC (Lehto et al, ).…”

“…Consistent results have been observed in models of peripheral neuropathic pain and more ambiguous data with central neuropathic pain. Amgen reports that AMG0902 does not show efficacy in a Chung model of neuropathic pain due to nerve ligation (Lehto et al, ) whereas Merck reported that the Hydra compound HC‐030031 reduced mechanical hypersensitivity (Eid et al, ). These compounds may exert different effects due to intrinsic properties, or the discrepant results may be due to the divergent protocols employed: Merck tested 6 weeks after L5/L6 ligation (Eid et al, ) and Amgen tested 2 weeks post‐surgery (Lehto et al, ).…”

Behavioral characterization of a CRISPR-generated TRPA1 knockout rat in models of pain, itch, and asthma

“…However, these experiments used low potency first-generation TRPA1 inhibitors such as HC030031 and AP18, increasing the likelihood that some of the effects of these compounds could be off-target. Indeed, when high potency TRPA1 inhibitors have been tested in the CFA model (and neuropathic pain models), only limited, if any, efficacy has been observed 35 . It was surprising that TRPA1 KO rats display normal scratching behavior in the calcipotriol-induced chronic itch model, since several high-quality studies showed a robust requirement for TRPA1 in itch using KO mice 12,15,36 .…”