Biophysical Society Thematic Meeting | Bucharest 2026

Biophysics of Membrane Reactions in Brian

Thursday Speaker Abstracts

MEMBRANE CROSSING OF OPIOIDS REVEALED BY CONTINUOUS CONSTANT PH MOLECULAR DYNAMICS SIMULATIONS AND CELL EXPERIMENTS Jana Shen University of Maryland School of Pharmacy, Pharmaceutical Sciences, Baltimore, MD, USA Fentanyl is a leading cause of drug overdose deaths in the United States, yet mechanism driving its extreme in vivo potency remains poorly understood. Here we used novel molecular dynamics (MD) and experimental approaches to test whether membrane plays a role in driving the potency of fentanyl. The weighted-ensemble continuous constant pH MD (CpHMD) simulations estimated fentanyl's effective permeability to be on the order of 10 -7 cm/s, approximately 100 times that of morphine. Interestingly, the structurally similar naloxone did not cross the membrane on the simulation timescale, while isotonitazene, a newly emerged opioid even more potent than fentanyl, permeates orders of magnitude more slowly than morphine. Simulations captured the opioid permeation processes at atomic resolution with concurrent pH titration. Remarkably, BRET reporter cell lines demonstrated that cells exposed to fentanyl, but not morphine, reactivated the receptor after multiple washout and in competition with naloxone. Further experiments confirmed that fentanyl was retained by the cell and subsequently repartitioned into the extracellular solution. Taken together, the computational and experimental findings suggest that fentanyl's exceptional in vivo potency stems at least in part from its remarkable lipophilicity, which facilitates receptor rebinding through membrane-mediated pathways. Our work establishes membrane-mediated receptor access as a fundamental mechanism underlying opioid toxicity, with implication for developing more effective countermeasures. Weighted-ensemble CpHMD provides mechanistic elucidation of membrane permeation of ionizable molecules, both challenging and refining the long-standing pH partition hypothesis.

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