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Fentanyl-Rewired: A 2‑Azaspiro[3.3]heptane Core Preserves μ‑Opioid Function.

Arran W Stewart1, Lisa M Eubanks1, Mingliang Lin1

  • 1Department of Chemistry and Immunology, The Skaggs Institute for Chemical Biology, Worm Institute of Research and Medicine (WIRM), The Scripps Research Institute, La Jolla, California 92037, United States.

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Summary
This summary is machine-generated.

Researchers explored novel fentanyl analogues by replacing a key chemical group, aiming to reduce respiratory risks. The new spiro analogue maintained pain relief but showed significantly less respiratory depression, offering a promising blueprint for safer opioids.

Keywords:
Biased agonismBioisostereFentanylPortabilityμ-opioid receptor

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Area of Science:

  • Medicinal Chemistry
  • Pharmacology
  • Neuroscience

Background:

  • Fentanyl is a potent μ-opioid receptor (MOR) agonist used for pain relief.
  • A major limitation of fentanyl is its significant respiratory depressant effects.
  • Developing novel analgesics with reduced respiratory liability is a critical unmet medical need.

Purpose of the Study:

  • To investigate the impact of replacing the piperidine moiety in fentanyl with a 2-azaspiro[3.3]-heptane group.
  • To assess the pharmacological profile, including receptor binding, signaling, and antinociceptive effects, of the novel spiro analogue.
  • To evaluate the respiratory effects and pharmacokinetic properties of the spiro analogue in vivo.

Main Methods:

  • Synthesis of a novel spiro analogue by substituting the piperidine ring of fentanyl with 2-azaspiro[3.3]-heptane.
  • In vitro receptor binding assays to determine affinity for MOR, KOR, and DOR.
  • In vitro assays to assess β-arrestin-2 recruitment.
  • In vivo antinociception studies using hot-plate and tail-flick tests in mice.
  • Pharmacokinetic studies measuring serum half-life after intravenous administration in mice.
  • In vivo respiratory function assessment using whole-body plethysmography in mice.

Main Results:

  • The spiro analogue exhibited MOR-preferred binding (MOR > KOR ≫ DOR) and did not recruit β-arrestin-2.
  • Full antinociception was observed in hot-plate and tail-flick tests, albeit with a ~100-fold lower potency compared to fentanyl.
  • The compound demonstrated favorable pharmacokinetics with a serum half-life of approximately 27 minutes in mice.
  • Dose-dependent respiratory depression was observed, but only at high doses, indicating a wider therapeutic window.

Conclusions:

  • Replacing the piperidine moiety with 2-azaspiro[3.3]-heptane preserves fentanyl-class antinociceptive activity while reducing respiratory liability.
  • This structural modification decouples analgesic potency from respiratory depression, mapping important boundary conditions of the opioid pharmacophore.
  • The spiro analogue represents a promising lead for developing next-generation opioid analgesics with an improved safety profile.