pK a of opioid ligands as a discriminating factor for side effects

Sci Rep. 2019 Dec 18;9(1):19344. doi: 10.1038/s41598-019-55886-1.


The non-selective activation of central and peripheral opioid receptors is a major shortcoming of currently available opioids. Targeting peripheral opioid receptors is a promising strategy to preclude side effects. Recently, we showed that fentanyl-derived μ-opioid receptor (MOR) agonists with reduced acid dissociation constants (pKa) due to introducing single fluorine atoms produced injury-restricted antinociception in rat models of inflammatory, postoperative and neuropathic pain. Here, we report that a new double-fluorinated compound (FF6) and fentanyl show similar pKa, MOR affinity and [35S]-GTPγS binding at low and physiological pH values. In vivo, FF6 produced antinociception in injured and non-injured tissue, and induced sedation and constipation. The comparison of several fentanyl derivatives revealed a correlation between pKa values and pH-dependent MOR activation, antinociception and side effects. An opioid ligand's pKa value may be used as discriminating factor to design safer analgesics.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Analgesics, Opioid / adverse effects*
  • Analgesics, Opioid / chemical synthesis
  • Analgesics, Opioid / chemistry
  • Animals
  • Drug Design
  • HEK293 Cells
  • Humans
  • Hydrogen-Ion Concentration
  • Ligands
  • Male
  • Nociception / drug effects
  • Piperidines / adverse effects
  • Piperidines / chemical synthesis
  • Piperidines / chemistry
  • Rats, Wistar
  • Receptors, Opioid, mu / metabolism


  • Analgesics, Opioid
  • Ligands
  • Piperidines
  • Receptors, Opioid, mu