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. 2016 Nov 29:12:1744806916677761.
doi: 10.1177/1744806916677761. Print 2016.

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

Sonya G Lehto  1 , Andy D Weyer  2 , Beth D Youngblood  3 , Maosheng Zhang  3 , Ruoyuan Yin  3 , Weiya Wang  3 , Yohannes Teffera  4 , Melanie Cooke  5 , Cheryl L Stucky  6 , Laurie Schenkel  7 , Stephanie Geuns-Meyer  7 , Bryan D Moyer  3 , Kenneth D Wild  3 , Narender R Gavva  3
Affiliations

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

Sonya G Lehto et al. Mol Pain. .

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.

Keywords: AMG0902; TRPA1; inflammatory; neuropathic; pain; rat.

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Figures

Figure 1.
Figure 1.
(a) Structure of AMG0902. (b) AMG0902 inhibits TRPA1 activation in stably expressing CHO cells by different chemical (80 µM AITC; 35 µM 4-ONE; 2.74 mM Methylglyoxol) and physical stimuli (100 mOsmol hypo-osmolarity) in a concentration-dependent manner. The maximum 45Ca2+uptake induced by each agonist alone is considered as 100%. AITC: allyl isothiocyanate.
Figure 2.
Figure 2.
Mechanically induced C-fiber action potentials are reduced in TRPA1 knockout mice and by AMG0902 in C57BL/6 mice. (a) Representative mechanically evoked action potentials in WT and KO mice. Upper trace shows response from a single fiber in a TRPA1 WT preparation; lower trace shows responses from a single fiber in a TRPA1 KO preparation to the indicated mechanical forces (mN). (b) Frequency of C-fiber spiking was significantly reduced in TRPA1 KO mice (circles; n = 22 fibers from 12 mice) compared to TRPA1 WT mice (squares; n = 24 fibers from 12 mice) at 120 mN and 160 mN mechanical stimulation forces (mean ± SEM, two-way ANOVA; p 
Figure 3.
Figure 3.
Open field analysis for the evaluation of the effect of AMG0902. (a) Sum of total distance traveled in centimeters during a 60-min observation. There was no significant effect of AMG0902. (b) Sum of total rearing behavior in seconds during a 60-min observation. There was a significant reduction of rearing following 200 mg/kg of gabapentin (p 
Figure 4.
Figure 4.
Behavioral models of mechanically activated hypersensitivity. (a) Sum of total rearing behavior in seconds during a 60-min observation that was 23 h post unilateral hind paw CFA injection. There was a significant reduction of rearing following CFA injection in animals orally dosed with vehicle (p  20-fold in excess of the in vivo IC90 of AITC flinching. (c) Gabapentin significantly reversed SNL-induced mechanical allodynia at 1 h (p < 0.05) and at 2 h (p < 0.001), but there was no significant effect with AMG0902. The mean unbound plasma concentration in this dosing group was 8.4 μM which is >4-fold in excess of the in vivo IC90 of AITC flinching. CFA: complete Freund’s adjuvant; SNL: spinal nerve ligation.
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