Lipoic/Capsaicin-Related Amides: Synthesis and Biological Characterization of New TRPV1 Agonists Endowed with Protective Properties against Oxidative Stress

alpha-Lipoic acid is a sulfur-containing nutrient endowed with pleiotropic actions and a safe biological profile selected to replace the unsaturated alkyl acid of capsaicin with the aim of obtaining lipoic amides potentially active as a TRPV1 ligand and with significant antioxidant properties. Thus, nine compounds were obtained in good yields following a simple synthetic procedure and tested for their functional TRPV1 activity and radical-scavenger activity. The safe biological profile together with the protective effect against hypoxia damage as well as the in vitro antioxidant properties were also evaluated. Although less potent than capsaicin, almost all lipoic amides were found to be TRPV1 agonists and, specifically, compound 4, the lipoic analogue of capsaicin, proved to be the best ligand in terms of efficacy and potency. EPR experiments and in vitro biological assays suggested the potential protective role against oxidative stress of the tested compounds and their safe biological profile. Compounds 4, 5 and 9 significantly ameliorated the mitochondrial membrane potential caused by hypoxia condition and decreased F2-isoprostanes, known markers of oxidative stress. Thus, the experimental results encourage further investigation of the therapeutic potential of these lipoic amides.