Acute Biodistribution Comparison of Fentanyl and Morphine

Synthetic opioids such as fentanyl are key drivers of the opioid crisis, contributing to approximately 68% of the nearly 108,000 deaths linked to drug overdose in 2022 (CDC). Though fentanyl is a μ opioid receptor agonist, it demonstrates enhanced lipophilicity, heightened potency to induce respiratory depression, and more rapid central nervous system entry compared to certain other opioids, i.e., morphine. However, there are relatively few biodistribution comparison studies of fentanyl and classical opioids like morphine in mice, despite the use of mice as preclinical models of opioid effects, i.e., respiratory depression. Therefore, the current study compared acute fentanyl (0.3 mg/kg) and morphine (30 mg/kg) biodistribution in blood and 12 tissues at doses causing respiratory depression in male Swiss Webster mice. Whole-body plethysmography was used to select fentanyl and morphine doses producing comparable respiratory depression, and an LC/MS-MS protocol was developed to quantify fentanyl, morphine, and metabolites in diverse tissue samples. Drug distribution time courses varied by tissue, with fentanyl and morphine displaying similar time courses in the lung, stomach, and small intestine, but differing in the brain and spleen. Fentanyl exhibited greater distribution out of the blood and into the brain, liver, lung, and heart than morphine early after administration and out of the blood into fat at later time points after administration. The ratios of total drug distribution (area under the curve) in tissue–blood over time suggest that fentanyl accumulation in tissue relative to blood in several areas, such as lung, heart, kidney, spleen, fat, and small intestine, is greater than morphine. These findings indicate that fentanyl administration may affect several organs to a larger degree than morphine.