Comparative Toxicological Profiling Reveals Divergent Redox, Mitochondrial, and Cellular Toxicity to Structurally Diverse Opioids

Abstract

Opioid neurotoxicity is increasingly associated with oxidative stress, mitochondrial dysfunction, and DNA damage; however, direct mechanistic comparisons among structurally diverse opioids remain scarce. We comparatively evaluated morphine, codeine, heroin, buprenorphine, and methadone in human SH-SY5Y neuroblastoma cells using a multiparametric approach encompassing cytotoxicity; redox homeostasis; mitochondrial membrane potential; LDH release; caspase-3, -8, and -9 activation; PI3K/Akt signaling; DNA strand breaks; and in silico pharmacokinetic profiling. The opioids exhibited clearly divergent toxicity patterns. Buprenorphine showed the highest cytotoxicity, mitochondrial depolarization, and activation of caspase-9 and caspase-3, consistent with mitochondrial caspase-dependent apoptotic signaling. Morphine also reduced mitochondrial membrane potential and activated caspase-9, whereas heroin and codeine increased caspase-9 activity without significant mitochondrial depolarization. Methadone produced the most pronounced oxidative phenotype, with increased ROS, glutathione depletion, mitochondrial depolarization, and the only significant increase in LDH release but without significant caspase activation, indicating a distinct caspase-independent injury pattern. Akt activation increased significantly after buprenorphine and morphine exposure. Morphine and codeine induced greater DNA damage than methadone at equimolar exposure, demonstrating that oxidative burden did not parallel genotoxicity. Overall, these findings reveal compound-specific redox, mitochondrial, signaling, genotoxic, and cell-death responses and show that opioid toxicity cannot be adequately explained by oxidative stress alone.