Novel Approaches, Drug Candidates, and Targets in Pain Drug Discovery.

Because of the problems associated with opioids, drug discovery efforts have been employed to develop opioids with reduced side effects using approaches such as biased opioid agonism, multifunctional opioids, and allosteric modulation of opioid receptors. Receptor targets such as adrenergic, cannabinoid, P2X3 and P2X7, NMDA, serotonin, and sigma, as well as ion channels like the voltage-gated sodium channels Nav1.7 and Nav1.8 have been targeted to develop novel analgesics. Several enzymes, such as soluble epoxide hydrolase, sepiapterin reductase, and MAGL/FAAH, have also been targeted to develop novel analgesics. In this review, old and recent targets involved in pain signaling and compounds acting at these targets are summarized. In addition, strategies employed to reduce side effects, increase potency, and efficacy of opioids are also elaborated. This review should aid in propelling drug discovery efforts to discover novel analgesics.