The pharmacogenetics of analgesia: toward a genetically-based approach to pain management

Interindividual differences in the experience of pain have been appreciated clinically for over a century. More recently, there has been a growing body of evidence demonstrating differences in analgesic response to various pharmacotherapies, although the source of this variability largely remains to be explained. To this end, basic science research is beginning to identify the allelic variants that underlie such antinociceptive variability using a multiplicity of animal models, and powerful genetic approaches are being exploited to accelerate this process. Although the vast majority of these studies have focused on the pharmacogenetics of opioids, owing to their prominent status as analgesics, the number of pharmacotherapies evincing genetically-based variability is rapidly expanding. In addition, analogous studies have been undertaken in humans, as a small but growing number of clinical trials have begun to evaluate prospectively the existence, if oftentimes not the origin, of interindividual differences in analgesic drug response. Importantly, with a few notable exceptions, such efforts have primarily identified differences in analgesic efficacy and/or potency between male and female human subjects. Looking toward the future development of one or more widely utilised, pharmacogenetic screens that would lead to modifications in treatment planning, at least with respect to the pharmacologic management of pain, this review will document the breadth of genetically-based variability in drug-mediated antinociception in animals. Specific examples in which the gene or genes underlying such variability have been postulated or identified will be given, while highlighting the effect of sex and its interactions with other genetic backgrounds. Finally, we will summarise and evaluate the literature on pharmacogenetic differences in human analgesic drug response, for which the influence of sex has served as one of the better studied and heuristically insightful examples.