Project Summary/Abstract CDC data show that between 2013 and 2020 fentanyl-linked deaths have increased 10-fold and are now twice those involving heroin. Fentanyl and fentanyl analogues (F/FA) are commonly found in seizure of stimulants and “club drugs.” Compounding this crisis is the concurrent use of F/FA with stimulants, particularly methamphetamine and cocaine, driving the so-called 4th wave of the opioid crisis. Illicitly manufactured fentanyl, heroin, cocaine, or methamphetamine (alone or in combination) were involved in nearly 85% of drug overdose deaths in 24 states and the District of Columbia during January-June 2019. Unfortunately, there is limited understanding of the drug-drug interactions and physiological consequences of F/FA co-intoxication with a stimulant, and there are no targeted therapies. Co-intoxication manifests in a complex continuum of clinical presentation due to different effect sites and drug-drug interactions, which can intensify or mask symptoms. Some symptoms include opioid-induced respiratory depression and muscle rigidity, and stimulant-induced increases in blood pressure and heart rate. The primary opioid reversal agent, naloxone, a µ-opioid receptor (MOR) antagonist, as a solo therapeutic, may provoke acute opioid withdrawal exacerbated by unopposed stimulant effects; naloxone's short duration of action may also result in re-narcotization. It is crucial to understand the drug-drug interactions in opioid-stimulant co-use. In addition, a single therapeutic that reverses the adverse effects of both co-intoxicants, simultaneously, through enhanced pharmacokinetic clearance would be highly beneficial. Our drug candidate, CS-1103, currently being studied as reversal agent for methamphetamine, also holds promise for treatment of opioid-stimulant co-intoxication. To better understand the in vivo physiologic and pharmacologic impact of F/FA-stimulant co-intoxication and to provide a foundation for CS-1103, and its use in conjunction with naloxone, we will undertake the following Aims in rodent studies of opioid intoxication: Aim 1 will determine the physiological and pharmacological effects of fentanyl- methamphetamine co-intoxication in awake, behaving rats. We will administer fentanyl and methamphetamine at various ratios, dose levels, and length of time between opioid and stimulant dosing. We will quantify effects on breathing, metabolism, hemodynamics, intoxicant pharmacokinetics, muscle rigidity, and sedation/immobility. These studies will provide a better understanding of the complex interaction between fentanyl and methamphetamine, to guide future therapies. Aim 2 will determine the physiological and pharmacological effects of fentanyl-methamphetamine co-intoxication reversal by CS-1103 in awake, behaving rats. We will identify doses of CS-1103 alone (and also in conjunction with naloxone) required to normalize intoxicant-induced effects identified in Aim 1. These studies will clarify the safety, efficacy, optimu...