Title

 

Authors

P. Pedretti¹, C. Fusi¹, D. Preti², S. Materazzi¹, R. Nassini¹, P.G. Baraldi² and P. Geppetti¹

1. Department of Preclinical and Clinical Pharmacology, University of Florence, Italy
2. Department of Pharmaceutical Chemistry, University of Ferrara, Italy

 

Abstract

The transient receptor potential ankyrin 1 (TRPA1) is a recently identified receptor belonging to the TRP channels family, which exerts pleiotropic functions in a variety of cells and pathophysiological conditions (Nilius et al., 2007). TRPA1 localizes to primary sensory neurons of the trigeminal, vagal and dorsal root ganglia (DRG). More specifically, TRPA1 co-localizes with the capsaicin receptor channel, TRPV1, in a subset of somatosensory neurons with non-myelinated or thinly myelinated fibers with nociceptive and pain producing functions. Much interest has been directed to the study of the TRPV1 because, capsaicin, has been instrumental to discover the role of this subset of primary sensory neurons to cause nociceptive responses, to activate reflex pathways and to produce neurogenic inflammation (Szallasi et al., 2006). Most recently TRPA1 has attracted much attention. TRPA1 is targeted by a series of xenobiotics, including the ingredients of various spicy foods, as wasabi and mustard (allyl isotiocyanate), garlic (allicin), cinnamon (cinnamaldehyde) or irritant compounds, as chlorine/hypochlorite, cigarette smoke, formaldehyde and this action accounts for their irritant and pain producing action (Baraldi et al., 2010). In addition, recently it  has attracted much attention because it has been identified as a sensor for oxidative, nitrative and carbonylic stress (Bessac et al., 2008). In addition, we recently found that TRPA1 entirely and selectively mediates nociceptive behaviour and neurogenic inflammatory responses elicited by lipid peroxidation by-products generated by oxidative stress, including the highly reactive molecule, 4-hydroxy-2-nonenal (Trevisani et al., 2007) or cyclopentenone prostaglandins and isoprostane (Materazzi et al., 2008). From these pathophysiological observations we started a  drug discovery program aimed to identify new classes of selective and potent TRPA1 antagonists. The compound labelled as HC-030031 (2-(1,3-dimethyl-2,6-dioxo-1,2,3,6-tetrahydro -purin-7-yl)-N-(4-isopropyl-phenyl)-acetamide), developed by Hydra Biosciences, is the most cited TRPA1 antagonist so far reported (McNamare et al., 2007). This molecule, displaying micromolar potency in blocking TRPA1 channel, shows a central xanthine core functionalized at the 7-position with an acetamide chain. In order to improve HC-030031 potency, the effect of modifications of the central bicycle of the molecule has been explored and the purinone nucleus has been replaced by alternative nitrogen-containing bicycles functionalized with an appropriate acetamide chain. In particular, we synthesised a series of 7-substituted-1,3-dimethyl-1,5-dihydro-pyrrolo[3,2-d]pyrimidine-2,4-dione derivatives as 9-deaza-analog of the xanthine-based reference compound HC-030031 that showed antagonistic effect on TRPA1 channel. This new group of ligands showed a inhibitory action on the intracellular calcium elevation induced by selective TRPA1 activation. In particular, tested compounds block both human and rat TRPA1 with a potency in the range of micromolar concentration. These compounds have been tested in human fetal lung fibroblasts, in which TRPA1 channel has been originally cloned and in rat DRG neurons, important for future in vivo pharmacological studies. These data provide novel insights for the study and development of new classes of molecules able to gate the TRPA1 channel for the treatment of neuropathic pain.
 
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