Xylazine Exacerbates Fentanyl-Induced Respiratory Depression and Bradycardia.

Fatal opioid overdoses in the United States have nearly tripled during the past decade, with greater than 92% involving a synthetic opioid like fentanyl. Fentanyl potently activates the µ-opioid receptor to induce both analgesia and respiratory depression. The danger of illicit fentanyl has recently been exacerbated by adulteration with xylazine, an α2-adrenergic receptor agonist typically used as a veterinary anesthetic. In 2023, over a 1,000% increase in xylazine-positive overdoses was reported in some regions of the U.S. Xylazine has been shown to potentiate the lethality of fentanyl in mice, yet a mechanistic underpinning for this effect has not been defined. Herein, we evaluate fentanyl, xylazine, and their combination in whole-body plethysmography (to measure respiration) and pulse oximetry (to measure blood oxygen saturation and heart rate) in male and female CD-1 mice. We show that xylazine decreases breathing rate more than fentanyl by increasing the expiration time. In contrast, fentanyl primarily reduces breathing by inhibiting inspiration, and xylazine exacerbates these effects. Fentanyl but not xylazine decreased blood oxygen saturation, and when combined, xylazine did not change the maximum level of fentanyl-induced hypoxia. Xylazine also reduced heart rate more than fentanyl. Finally, loss in blood oxygen saturation correlated with the frequency of fentanyl-induced apneas, but not breathing rate. Together, these findings provide insight into how the addition of xylazine to illicit fentanyl may increase the risk of overdose.

Structure-Activity Relationships of the Fentanyl Scaffold: Identification of Antagonists as Potential Opioid Overdose Reversal Agents.

Opioid-related overdoses account for almost half of all drug overdose deaths in the United States and cause more preventable deaths every year than car crashes. Fentanyl, a highly potent mu opioid receptor (MOR) agonist and its analogues (fentalogues) are increasingly found in illicit drug samples, both where the primary drug of abuse is an opioid and where it is not. The prevalence of fentalogues in the illicit drug market is thought to be the primary driver of the increased number of opioid-related overdose deaths since 2016. In fact, fentanyl and its analogues are involved in more than 70% of opioid-related overdoses. The standard opioid overdose rescue therapy naloxone is often insufficient to reverse opioid overdoses caused by fentalogue agonists under current treatment paradigms. However, the pharmacology of many fentalogues is unknown. Moreover, within the fentalogue series of compounds, it is possible that antagonists could be identified that might be superior to naloxone as opioid overdose reversal agents. In this report, we explore the pharmacology of 70 fentalogues and identify compounds that behave as MOR antagonists in vitro and demonstrate with one of these reversals of fentanyl-induced respiratory depression in the mouse. Such compounds could provide leads for the development of effective agents for the reversal of opioid overdose.

Opioid Antagonists from the Orvinol Series as Potential Reversal Agents for Opioid Overdose.

The opioid crisis continues to claim many lives, with a particular issue being the ready availability and use (whether intentional or accidental) of fentanyl and fentanyl analogues. Fentanyl is both potent and longer-acting than naloxone, the standard of care for overdose reversal, making it especially deadly. Consequently, there is interest in opioid reversal agents that are better able to counter its effects. The orvinol series of ligands are known for their high-affinity binding to opioid receptors and often extended duration of action; generally, compounds on this scaffold show agonist activity at the kappa and the mu-opioid receptor. Diprenorphine is an unusual member of this series being an antagonist at mu and only a partial agonist at kappa-opioid receptors. In this study, an orvinol antagonist, 14, was designed and synthesized that shows no agonist activity in vitro and is at least as good as naloxone at reversing the effects of mu-opioid receptor agonists in vivo.