Clinical pharmacology and pharmacotherapy of opioid switching in cancer patients.

Pain is one of the most common and often most feared symptoms in patients with cancer. Ongoing or progressive pain is physically debilitating and has a marked impact on quality of life. Since a third of the population will die from cancer, and of these, 80% will experience severe pain in their final year of life, effective treatment of cancer-related pain remains both a high priority and an ongoing challenge in clinical practice. Individuals with moderate to severe cancer-related pain require treatment with strong analgesics, namely opioids. There is evidence to support the therapeutic maneuver of opioid switching in clinical practice, but further evidence is needed to elucidate the underlying mechanisms for interindividual differences in response to different opioids. Large, robust clinical trials will be needed if clinical differences among side-effect profiles of different opioids are to be clearly demonstrated. This review discusses candidate genes, which contribute to opioid response; many other genes have also been implicated in "pain" from animal or human studies. In order to continue to evaluate the genetic contributions to both pain susceptibility and analgesic response, further candidate genes need to be considered. Good pain control remains a high priority for clinicians and patients, and there is much work to be done to further individualize analgesic therapy for patients with cancer.