Reston, VA (July 24, 2025)—Naloxone (also known as Narcan, the commonly used drug to treat narcotic overdoses) has greater binding to opioid receptors in women's brains than in men's brains, according to new research published in the July issue of The Journal of Nuclear Medicine. The first-in-human whole-body PET study suggests men and women may respond differently to opioid use disorder treatments and offers new insights to advance neuropharmacology.
Opioid misuse is a worldwide epidemic associated with high overdose and mortality rates. The μ-opioid receptor (MOR) is the target of opioid drugs including fentanyl and heroin, as well as well as naloxone, an opioid antagonist used to treat opioid use disorder. MORs are widely expressed in the central nervous system, peripheral organs, and immune system, making them especially useful for assessing functional changes in people with opioid use disorder.
"A better understanding of the interactions of opioid drugs with the MOR could help to elucidate the etiology, prevention, and treatment of opioid use disorder," said Jacob Dubroff, MD, PhD, associate professor of radiology in the Division of Nuclear Medicine Imaging & Therapy at Perelman School of Medicine at the University of Pennsylvania in Philadelphia. "Using whole-body PET, we investigated MOR physiology in men and women before and after pretreatment with naloxone."
Six female and seven male healthy subjects underwent two 11C-carfentanil whole-body PET imaging sessions—one at baseline and one immediately after pretreatment with the MOR antagonist naloxone. 11C-carfentanil brain distribution volume ratios were determined using the occipital cortex and the visual cortex within it as reference regions. Peripheral organ distribution volume ratios were also measured using the descending aorta and biceps/triceps as reference regions.
On the whole, naloxone reduced MOR availability by 40–50 percent in brain regions known to express high levels of MORs; a greater reduction was seen in women compared to men. For peripheral MOR distribution, the descending aorta reference region showed less variance than the extremity muscle, but both showed blocking effects of naloxone.
"This research is significant because it suggests that there may be sex-based differences in how men and women respond to opioid overdose treatments," noted Dubroff. "Additionally, we showed how regions outside the brain can be used to help measure radiotracer binding in the brain. This strategy could be applied to other radiotracers used for PET brain imaging and boost the development of new ones."
The authors of " [11C]Carfentanil PET Whole-Body Imaging of μ-Opioid Receptors: A First in-Human Study " include Jacob G. Dubroff, Chia-Ju Hsieh, Hsiaoju Lee, Alexander Schmitz, Elizabeth J. Li, Erin K. Schubert, and Robert Mach, Department of Radiology, Perelman School of Medicine, University of Pennsylvania, Philadelphia, Pennsylvania; Corinde E. Wiers, Department of Psychiatry, Perelman School of Medicine, University of Pennsylvania, Philadelphia, Pennsylvania; and Henry R. Kranzler, Department of Psychiatry, Perelman School of Medicine, University of Pennsylvania, Philadelphia, Pennsylvania, and Mental Illness Research, Education and Clinical Center, Crescenz VAMC, Philadelphia, Pennsylvania.
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