The Plasma Membrane may Serve as a Drug Depot to Drive the Extreme Potency of Fentanyl

The drug overdose crisis in the United States is driven largely by the ultrapotent synthetic (UPS) opioid fentanyl; however, fentanyl’s extreme potency is poorly understood. Here we used state-of-the-art molecular dynamics simulations and experiments to test a hypothesis that fentanyl’s extreme potency is driven by its ability to partition into the plasma membrane, creating a drug reservoir near the receptor. The estimated effective permeability of fentanyl at pH 7.5 is on the order of 10⁻⁷ cm/s – at least two orders of magnitude faster than morphine. In contrast, isotonitazene (a newly emerged UPS opioid) and naloxone (an antagonist) effectively do not partition into the membrane under the same conditions. The simulations captured the proton-coupled permeation processes, challenging the long-standing pH-partition hypothesis. Subsequent reporter cell line experiments demonstrated that cells exposed to fentanyl, but not morphine, reactivated the receptor even after washout and addition of naloxone. Immobilized affinity membrane chromatography confirmed that fentanyl has significantly higher phospholipid affinity than morphine. Our findings strongly support the drug depot hypothesis and highlight the importance of membrane-dependent opioid pharmacology for understanding toxicity and guiding the design of more effective antagonists. The simulation methodology enables detailed analysis of membrane permeation by ionizable inhibitors, supporting ADME optimization in drug development.