Bifunctional μ opioid and σ receptor ligands as novel analgesics with reduced side effects.

Opioid analgesics are highly effective painkillers for the treatment of moderate or severe pain, but they are associated with a number of undesirable adverse effects, including the development of tolerance, addiction, constipation and life-threatening respiratory depression. The development of new and safer analgesics with innovative mechanisms of action, which can enhance the efficacy in comparison to available treatments and reduce their side effects, is urgently needed. The sigma-1 receptor (σR), a unique Ca-sensing chaperone protein, is expressed throughout pain-modulating tissues and affects neurotransmission by interacting with different protein partners, including molecular targets that participate in nociceptive signalling, such as the μ-opioid receptor (MOR), N-methyl-d-aspartate receptor (NMDAR) and cannabinoid 1 receptor (CBR). Overwhelming pharmacological and genetic evidence indicates that σR antagonists induce anti-hypersensitive effects in sensitising pain conditions (e.g. chemically induced, inflammatory and neuropathic pain) and enhance opioid analgesia but not opioid-mediated detrimental effects. It has been suggested that balanced modulation of MORs and σRs may improve both the therapeutic efficacy and safety of opioids. This review summarises the functional profiles of ligands with mixed MOR agonist and σR antagonist activities and highlights their therapeutic potentials for pain management. Dual MOR agonism/σR antagonism represents a promising avenue for the development of potent and safer analgesics.