Opiate analgesics continue to be the most effective drugs for the management of cancer-related pain. These drugs mainly work by binding to the mu opioid receptor in multiple areas along the central nervous system. For many years, the clinical management of pain using opiate analgesics involved opioid dose escalation. This was primarily due to the fact that all opiates were perceived as sharing the same mechanism of action and binding site. It was therefore considered—unnecessary to change from one opiate to another. Our team initially observed that in patients with developed opiate-induced neurotoxicity, changing the type of opioid was associated with a rapid and dramatic reduction in neurotoxicity. It also became clear that, paradoxically, patients who underwent rotation required a lower opiate dose to achieve the same or better analgesia. These initial clinical findings were confirmed by other groups and later followed by research showing that there is considerable heterogeneity in mu receptors, opioid-induced hyperalgesia, and neurotoxic opioid metabolites.
Breakthrough pain has been described as the presence of brief periods of uncontrolled pain in patients who otherwise have good pain control on a regular opiate analgesic. The initial management of cancer-related pain included the use of opiate analgesics only as needed. The emergence of extended-release opiates changed that practice to regular use in order to maintain a constant blood level. Findings from our team in clinical studies led to a better understanding that combination of a regular opioid plus an intermittent immediate-release opioid was necessary, since the majority of patients experience breakthrough pain. Further research by our team has found that 90% of patients are able to achieve excellent breakthrough pain control with oral opiate analgesics, making the need for rapid opioids such as transmucosal, buccal, or intranasal fentanyl unnecessary in the vast majority of patients.
Methadone is an old synthetic opioid agonist that combines mu binding with NMDA antagonist properties. Initial attempts to use methadone as an analgesic during the 1960s and the 1970s failed, mainly due to severe toxicity when other opioids were replaced by methadone at excessively high equal analgesic dosages. Our team started to use methadone in carefully selected cases of refractory pain in an inpatient palliative care unit setting. These daily titrations allowed us to learn that while the opiate dose ratio for all other opiate agonists was linear and flat, in the case of methadone the opiate dose ratio became progressively more potent as patients received a higher MEDD of the previous opioid. This knowledge allowed for safer titration and rotation to methadone, resulting in better management of refractory pain.
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