Morphine is an alkaloid from the opium poppy used to treat pain. The potentially lethal side effects of morphine and related opioids—which include fatal respiratory depression—are thought to be mediated by μ-opioid-receptor (μOR) signalling through the β-arrestin pathway or by actions at other receptors. Conversely, G-protein μOR signalling is thought to confer analgesia. Here we computationally dock over 3 million molecules against the μOR structure and identify new scaffolds unrelated to known opioids. Structure-based optimization yields PZM21—a potent Gi activator with exceptional selectivity for μOR and minimal β-arrestin-2 recruitment. Unlike morphine, PZM21 is more efficacious for the affective component of analgesia versus the reflexive component and is devoid of both respiratory depression and morphine-like reinforcing activity in mice at equi-analgesic doses. PZM21 thus serves as both a probe to disentangle μOR signalling and a therapeutic lead that is devoid of many of the side effects of current opioids. Computational docking to the the μ-opioid-receptor identifies PZM21, a novel selective biased agonist that generates substantial affective analgesia in mice without altering respiration or inducing drug reinforcement. Morphine and other alkaloids from the opium poppy are μ-opioid receptor agonists that have been used to treat pain for many centuries. These authors used a computational approach to dock over three million small molecules to the μ-opioid receptor. Structure-based optimization of the most promising structures led to the identification of a potent agonist, PZM21, with exceptional subtype selectivity for the μ-opioid receptor. In mice, PZM21 generates substantial analgesia, which is fully ablated in μ-opioid receptor knockout animals. This small molecule seems to reduce the affective component of pain, without detectably altering reflexive behaviours, and has little effect on respiration.

Structure-based discovery of opioid analgesics with reduced side effects