Animal experiment:

Mice: PZM21 is dissolved in 0.9% sodium chloride. Mice are injected with either vehicle, morphine (5 mg/kg, or 10 mg/kg), TRV130 (1.2 mg/kg) or PZM21 (10 mg/kg; 20 mg/kg; or 40 mg/kg). After injection of drug, the analgesic effect expressed as percentage maximum possible effect (%MPE) is measured at 15, 30, 60, 90 and 120 min after drug treatment[1].

PZM21 is a potent and selective μ opioid receptor agonist with an EC50 of 1.8 nM.

PZM21 has no detectable κOR or nociceptin receptor agonist activity-it is actually an 18 nM κOR antagonist-while it is a 500-fold weaker δOR agonist, making it a selective μOR agonist. At hERG, PZM21 has an IC50 of between 2 and 4 μM, 500- to 1,000-fold weaker than its potency as a μOR agonist. Signalling by PZM21 and other μOR agonists appears to be mediated primarily by the heterotrimeric G protein Gi/o, as its effect on cAMP levels is eliminated by pertussis toxin and no activity is observed in a calcium release assay [1].

PZM21 is 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 displays dose-dependent analgesia in a mouse hotplate assay, with a per cent maximal possible effect (% MPE) of 87% reached 15 min after administration of the highest dose of drug tested [1].PZM21 has a long-lasting analgesic effect on CNS mediated-pain responses, but does not cause respiratory depression and constipation, two key side effects of opioid agonists. PZM21 does not exhibit the type of biomarker responses, such as hyperlocomotion or conditioned place preference response, that are observed when morphine and other opioids are used and are associated with reinforcement and addiction[2].

[1]. Manglik A, et al. Structure-based discovery of opioid analgesics with reduced side effects. Nature. 2016 Sep 8;537(7619):185-190. [2]. Kostic M, et al. Biasing Opioid Receptors and Cholesterol as a Player in Developmental Biology. [3]. Araldi D, et al. Mu-opioid Receptor (MOR) Biased Agonists Induce Biphasic Dose-dependent Hyperalgesia and Analgesia, and Hyperalgesic Priming in the Rat. Neuroscience. 2018 Oct 17;394:60-71.