Multimechanistic Single-Entity Combinations for Chronic Pain Control: A Narrative Review.

Atypical opioids such as tramadol, tapentadol, and cebranopadol combine two complementary mechanisms of action into a single molecule, creating novel analgesic agents. These are synthetic small molecules: cebranopadol is not yet market released; tramadol and tapentadol are commercially available and have immediate-release (IR) and extended-release (ER) formulations. Tramadol has been widely used in the United States in recent years and works as a prodrug in that its metabolites are active in inhibiting serotonin and norepinephrine reuptake. Tapentadol is a direct-acting agent with a faster onset of action and is a mu-opioid-receptor agonist and also inhibits noradrenaline reuptake. Cebranopadol is the newest of these drugs, a first-in-class atypical analgesic that combines mu-opioid receptor (MOR) agonism with activity at the nociception/orphanin (NOP) FQ petide receptors. Cebranopadol may be considered a partial kappa-opioid receptor agonist as well. The pharmacology of these unique single-entity agents allows them to offer analgesic benefit with fewer side effects and risks. Clinical studies have demonstrated the safety and efficacy of tramadol and tapentadol, and promising but limited studies for cebranopadol show good analgesic effect and safety. Serotonin toxicity or 'serotonin syndrome' may occur with accumulation of serotonin with tramadol. While the misuse of these agents is limited in the United States, tramadol misuse is prevalent in Iran and parts of Africa. Patients have been successfully rotated from one of these agents to another. All three agents show promise in the treatment of cancer and non-cancer pain and their unique formulation in a single molecule reduces the pill burden.