Łucja Szumiec, MSc - 2022

DOCTORAL DISERTATION

Badanie działania nowych funkcjonalnie selektywnych agonistów receptorów opioidowych

Łucja Szumiec

Department of Molecular Neuropharmacology Maj Institute of Pharmacology Polish Academy of Science

Opioids are powerful painkillers, commonly used in medicine. Those drugs also possess psychoactive effects, contributing to their recreational use. Popular opioids, such as morphine, fentanyl or oxycodone, exert a biological effect by the activation of the µopioid receptors. The therapeutic potential of opioids is wide, but their clinical utility is severely limited by side effects - potentially fatal respiratory depression, constipation and the risk of addiction development. Therefore, substantial efforts aimed at developing new opioid drugs with a safer action profile and a lower addiction potential are particularly important. An agonist binding to the µ-opioid receptor results in the activation of two intracellular signaling pathways, G protein- and β-arrestin-2-dependent, respectively. Studies in transgenic mice lacking the gene encoding β-arrestin-2 have shown that morphine administration in these animals is associated with potentiation and prolongation of antinociception as well as with the reduction of some side effects. The aim of designing new µ-opioid receptor agonists has therefore become the synthesis of compounds characterized by the functional selectivity/biased agonism, which enables the preferential activation of a selected cell signaling pathway. µ-opioid receptor agonists, activating the G protein-dependent pathway and minimally involving β-arrestin-2, have been proposed as a potentially safer and less addictive alternative to conventional agonists of this receptor. To date, several compounds presenting such profile of action have been discovered, and one of them, TRV130 (oliceridine), has been approved for medical use in the United States. This dissertation aimed to investigate the effects of novel functionally selective μopioid receptor agonists. The compounds with high G protein bias factors - PZM21, SR14968 and SR-17018 – were used. Using a battery of behavioral tests supplemented with biochemical studies, the effects of these agonists were assessed, with particular emphasis on their addictive potential in laboratory rodents. All the compounds induced dose-dependent and long-lasting antinociception. Unlike SR-14968 and SR-17018, PZM21 did not elicit a maximal antinociceptive response. The effects of the tested agonists on various aspects of opioid-related addictive behavior were then investigated. Repeated administration of the tested compounds led to antinociceptive tolerance, but in the case of SR-14968 and SR-17018, the development of tolerance was slightly delayed. The tested agonists also induced physical dependence, as measured by the severity of withdrawal symptoms. In mice treated with PZM21, this effect occurred at the highest dose evaluated, while SR-14968 and SR-17018 caused physical dependence even at low doses. The compounds differed in terms of their influence on locomotor activity. PZM21 and SR-17018 did not significantly change the activity of animals, while SR-14968-treated mice presented enhanced locomotor activity after a single drug administration, followed by tolerance to this effect during repeated treatment and high expression of activity after the incubation period. The rewarding properties of 14968 and SR-17018 have been demonstrated in the conditioned place preference test. PZM21 was not rewarding in this paradigm. Moreover, at the tested doses, it was not self-administered intravenously by rats and did not induce drug-seeking behavior during the abstinence. Therefore, in this part of the study, it was shown that the tested agonists differently influence the selected types of addiction-related behaviors. In the next stage of the research, the influence of PZM21, SR-14968 and SR-17018 on the behavioral response to morphine was assessed. The tested agonists intensified morphine-induced antinociception. Additionally, SR-14968 and SR-17018 slightly delayed the development of morphine antinociceptive tolerance. Interestingly, the compounds attenuated some symptoms of the withdrawal syndrome in mice physically dependent on morphine. Moreover, PZM21 abolished the rewarding effects of morphine in the conditioned place preference test. As PZM21 had an interesting profile of action (lack of rewarding/reinforcing effects and low physical dependence), additional experiments aimed to assess its effects in various stress-related behavioral paradigms were performed. Even though there is evidence showing protective effects of µ receptor agonists under stress conditions, there were no effects of PZM21 on acute response to stress or trauma-related memory in the fear conditioning model. This compound did not also affect cognitive functioning under neutral conditions. Therefore, in the studied area of behaviors, PZM21 had no considerable influence on the phenotype of mice. In conclusion, this doctoral dissertation has provided a wide characterization of selected μ-opioid receptor agonists, reported to present a high G protein bias. The obtained data indicate that such compounds are a heterogenous group with varying effects. The most crucial differences between the tested agonists are that SR-4968 and SR- 17018 induce stronger antinociception than PZM21. On the other hand, unlike SR agonists, PZM21 is not rewarding and does not cause profound physical dependence. The therapeutic potential of the tested agonists can also be extended to control selected aspects of opioid addiction, i.e. alleviating some morphine withdrawal symptoms, and, in the case of PZM21, weakening its rewarding properties. Due to this, the tested agonists may be potentially used in the pharmacotherapy of opioid use disorder.