Low Intrinsic Efficacy Alone Cannot Explain the Improved Side Effect Profiles of New Opioid Agonists

Biased agonism at G protein–coupled receptors describes the phenomenon whereby some drugs can activate some downstream signaling activities to the relative exclusion of others. Descriptions of biased agonism focusing on the differential engagement of G proteins versus β-arrestins are commonly limited by the small response windows obtained in pathways that are not amplified or are less effectively coupled to receptor engagement, such as β-arrestin recruitment. At the μ-opioid receptor (MOR), G protein–biased ligands have been proposed to induce less constipation and respiratory depressant side effects than opioids commonly used to treat pain. However, it is unclear whether these improved safety profiles are due to a reduction in β-arrestin–mediated signaling or, alternatively, to their low intrinsic efficacy in all signaling pathways. Here, we systematically evaluated the most recent and promising MOR-biased ligands and assessed their pharmacological profile against existing opioid analgesics in assays not confounded by limited signal windows. We found that oliceridine, PZM21, and SR-17018 had low intrinsic efficacy. We also demonstrated a strong correlation between measures of efficacy for receptor activation, G protein coupling, and β-arrestin recruitment for all tested ligands. By measuring the antinociceptive and respiratory depressant effects of these ligands, we showed that the low intrinsic efficacy of opioid ligands can explain an improved side effect profile. Our results suggest a possible alternative mechanism underlying the improved therapeutic windows described for new opioid ligands, which should be taken into account for future descriptions of ligand action at this important therapeutic target.

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Evaluation of G protein bias and β-arrestin 2 signaling in opioid-induced respiratory depression.

AJP Cell Physiology ◽

10.1152/ajpcell.00259.2021 ◽

2021 ◽

Author(s):

Jordan T. Bateman ◽

Erica S. Levitt

Keyword(s):

Recent Work ◽

G Protein ◽

Respiratory Depression ◽

G Proteins ◽

Side Effect ◽

Opioid Analgesics ◽

Substantial Evidence ◽

Intrinsic Efficacy ◽

Protein Versus

Respiratory depression is a potentially fatal side effect of opioid analgesics and major limitation to their use. G-protein-biased opioid agonists have been proposed as "safer" analgesics with less respiratory depression. These agonists are biased to activate G proteins rather than β-arrestin signaling. Respiratory depression has been shown to correlate with both G-protein bias and intrinsic efficacy, and recent work has refuted the role of β-arrestin signaling in opioid-induced respiratory depression. In addition, there is substantial evidence that G-proteins do, in fact, mediate respiratory depression by actions in respiratory-controlling brainstem neurons. Based on these studies, we provide the perspective that protection from respiratory depression displayed by newly developed G-protein biased agonists is due to factors other than G-protein versus arrestin bias.

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The Effect of Intrinsic Efficacy on Opioid Tolerance

Anesthesiology ◽

10.1097/00000542-199505000-00018 ◽

1995 ◽

Vol 82 (5) ◽

pp. 1226-1236 ◽

Cited By ~ 150

Author(s):

Alokesh Duttaroy ◽

Byron C. Yoburn

Keyword(s):

Dose Response ◽

Opioid Analgesics ◽

Tail Flick ◽

Subcutaneous Injections ◽

Intermittent Administration ◽

Intrinsic Efficacy ◽

Opioid Agonists ◽

End Of Treatment ◽

Effective Doses ◽

Development Of Tolerance

Background The intrinsic efficacy of opioid analgesics has been suggested to play a role in the development of tolerance to these agents. However, the effect of differences in dosing protocol on tolerance to opioid analgesics of high or low efficacy has not been addressed. Therefore, the effect of opioid intrinsic efficacy on tolerance in mice was determined in protocols of continuous and intermittent administration of equieffective doses of opioid agonists. Methods Initial antinociceptive median effective doses (ED50s) for five opioid agonists that vary in intrinsic efficacy were estimated in untreated mice. Groups of mice received continuous infusions of morphine, fentanyl, or etorphine for 72 h or 7 days from osmotic minipumps implanted subcutaneously. The infusion doses were calculated as multiples of the initial antinociceptive ED50. An inert placebo was implanted subcutaneously in controls. At the end of treatment, the pumps and placebos were removed, and 4-24 h later, mice were tested in dose-response studies (tail flick) using the same drug that had been chronically administered. In another study using intermittent dosing, mice received subcutaneous injections every 24 h for 3 days of saline or morphine, etorphine, fentanyl, oxycodone, or meperidine, or received subcutaneous injections every 24 h for 7 days of saline or morphine, etorphine, or fentanyl. Daily doses were calculated as multiples of the initial antinociceptive ED50. Twenty-four hours after the last injection, mice were tested in dose-response studies. Results High-intrinsic-efficacy compounds (e.g., etorphine and fentanyl) produced less tolerance than a lower-intrinsic-efficacy drug (morphine) in 72-h and 7-day infusion studies. Tolerance for all compounds after intermittent treatment with equieffective doses was similar, and intrinsic efficacy had no effect on the magnitude of tolerance after intermittent dosing. Conclusions These results indicate that the intrinsic efficacy of opioid analgesics is inversely related to the degree of tolerance after continuous infusion, but that intrinsic efficacy does not significantly affect tolerance after once-daily intermittent administration of these agents. These findings may be of clinical utility in understanding the development of tolerance to the antinociceptive effects of opioids.

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