Opioids for cancer pain: the challenge of optimizing treatment.

During 2007, 11.7 million US men and women of all ages suffered from some form of invasive cancer. During their illness, at least 70% (8.2 million) will experience pain sufficiently severe to require chronic opioid treatment. Cancer-induced pain is usually described under 3 headings: acute pain, chronic pain, and breakthrough pain. Among patients with chronic, persistent cancer pain controlled by around-the-clock analgesics, there is a high prevalence of breakthrough pain-often precipitated by some form of physical activity. Breakthrough pain seems best treated by a powerful, fast-acting opioid such as intravenous morphine or transmucosal fentanyl. At present, opioids are virtually the only analgesics capable of controlling moderate and severe cancer pain. In recent years, a veritable arsenal of opioids with a wide range of pharmacologic properties has become available for use in different pain situations. The World Health Organization has developed a 3-step "analgesic ladder" to guide management of cancer pain, based on the pain's severity, estimated by means of a 1 to 10 numeric rating scale. As the severity of the pain escalates, more potent (World Health Organization Step III) opioids are used. When faced with a difficult case of cancer pain, the physician must choose-from an array of options-the safest and most effective opioid analgesic and the most appropriate delivery system. Such decisions require an adequate understanding of the available opioids and experience with their use. The pharmacodynamic response to a given opioid depends on the nature of the receptor to which the opioid binds and its affinity for the receptor. Morphine activates the μ-opioid receptors, resulting in not only analgesia and sedation, but also euphoria, respiratory depression, constipation, and pruritus. The existence of a number of opioid receptor subtypes, each with its own repertoire of responses, has given rise to the hope (as yet unrealized) that an opioid can be found (or engineered) that will selectively produce adequate analgesia and sedation without, at the same time, causing unwanted adverse effects. Furthermore, suitable neurostimulatory or neuroinhibitive methods involving the central nervous system are being sought that can amplify the analgesic action of opioids. In the search for antinociceptive agents as efficacious as currently available opioids, but without their troublesome adverse effects, the endogenous opioids, such as the endomorphins, are being examined as offering possible solutions to the adverse effect problem.