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Potency, dissociation kinetics and reversibility of fentanyls and nitazenes by naloxone at the μ opioid receptor
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  • Norah Alhosan,
  • Damiana Cavallo,
  • Marina Santiago,
  • Eamonn Kelly,
  • Graeme Henderson
Norah Alhosan
University of Bristol
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Damiana Cavallo
University of Bristol
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Marina Santiago
Macquarie University
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Eamonn Kelly
University of Bristol
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Graeme Henderson
University of Bristol

Corresponding Author:graeme.henderson@bristol.ac.uk

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Abstract

Background and Purpose: Fentanyls and nitazenes are μ opioid receptor agonists responsible for a large number of opioid overdose deaths. Here, we compared the potency, dissociation kinetics and antagonism by naloxone at the μ receptor of several fentanyl and nitazene analogues and compared them to morphine and DAMGO. Experimental Approach: In vitro assays of G protein activation and signalling and arrestin recruitment were performed. AtT20 cells expressing μ receptors were loaded with a membrane potential dye and changes in fluorescence used to determine agonist potency, dissociation kinetics and susceptibility to antagonism by naloxone. BRET experiments were undertaken in HEK293T cells expressing μ opioid receptors, to assess Gi protein activation and β-arrestin 2 recruitment. Key Results: The rate of agonist dissociation from the μ receptor varied, with morphine, DAMGO, alfentanil and fentanyl dissociating rapidly whereas isotonitazene, etonitazene, ohmefentanyl and carfentanil dissociated slowly. Slowly dissociating agonists were more resistant to antagonism by naloxone. For carfentanil, the slow rate of dissociation was not due to G protein receptor kinase-mediated arrestin recruitment as its rate of dissociation was not affected by inhibition of GRKs with Compound 101. The in vitro relative potencies of fentanyls and nitazenes compared to morphine were much lower than that previously observed in in vivo experiments. Conclusions and Implications: With fentanyls and nitazenes, that slowly dissociate from the opioid receptor, antagonism by naloxone is pseudo competitive. In overdoses involving fentanyls and nitazenes higher doses of naloxone may be required for reversal than those normally used to reverse heroin overdose.
08 May 2024Submitted to British Journal of Pharmacology
08 May 2024Submission Checks Completed
08 May 2024Assigned to Editor
08 May 2024Review(s) Completed, Editorial Evaluation Pending
13 Jun 2024Editorial Decision: Revise Minor
12 Aug 20241st Revision Received
13 Aug 2024Assigned to Editor
13 Aug 2024Submission Checks Completed
13 Aug 2024Review(s) Completed, Editorial Evaluation Pending
19 Aug 2024Reviewer(s) Assigned
10 Sep 2024Editorial Decision: Revise Minor
10 Sep 20242nd Revision Received
11 Sep 2024Submission Checks Completed
11 Sep 2024Assigned to Editor
11 Sep 2024Review(s) Completed, Editorial Evaluation Pending
20 Sep 2024Editorial Decision: Accept