Abstract This R21 proposal, “Repurposing L-NAC to prevent fentanyl-induced respiratory depression” seeks to expand on our evidence that a bolus intravenous injection of the clinically-approved drug, N-acetyl-L-cysteine (L-NAC), reverses the profound respiratory depression elicited by infusion of fentanyl in rats. The clinical effectiveness of opioid analgesics such as fentanyl are compromised by their adverse actions on breathing and arterial blood- gas (ABG) chemistry. Opioid-induced respiratory depression (OIRD) can be reversed by opioid receptor (OR) antagonists but these antagonists also reverse opioid-induced analgesia We are reporting on the efficacies of L- and D-thiolesters such as D-cysteine ethyl ester (D-CYSee) to reverse OIRD while preserving analgesia and our current NIDA funding is allowing us to examine the efficacy of D-CYSee as a reversal agent against fentanyl and analogues in rats (PI: Stephen Lewis, NIH/NIDA U01DA051373: Optimization of Novel Thiolesters as a Therapeutic Strategy for Combating Opioid Overdoses and Abuse) and goats (PI: Matt Hodges, NIH/NIDA 1RF1DA050571: Reversing opioid-induced hypoxemia with thiol-based drugs without compromising analgesia in goats). N-acetyl-L-cysteine (L-NAC), which readily enters central peripheral and cells upon systemic/oral administration, has many beneficial effects in humans/experimental animals and is approved for human use for numerous conditions. There are no reports that L-NAC overcomes OIRD although it is evident that L-NAC (a) provides reducing equivalents to cells, (b) increases intracellular concentrations of L-cysteine/ L-glutathione, and (c) exerts numerous other intracellular actions via multiple enzymatic pathways. We have begun studying the ability of our thiol compounds to overcome the OIRD elicited by continuous intravenous infusion of fentanyl in rats. Such infusions are used widely in adult/pediatric patients but their ability to provide pain relief is greatly compromised by their ability to depress respiration. This project will expand upon our findings that intravenous injection of L-NAC elicits an immediate and sustained reversal of the deleterious adverse effects of continuous fentanyl infusion on breathing and ABG chemistry in anaesthetized rats whereas it did not affect the analgesic effects of the opioid. It appears that continuous infusion of fentanyl somehow sets up a scenario that allows for L-NAC to modulate intracellular signaling cascades that mediate fentanyl-induced OIRD but not analgesia. Our findings raise the possibility that L-NAC could be readily evaluated for potential reversal of OIRD elicited by the infusion of fentanyl in human subjects. The Specific AIMS of this project are: AIM 1 – determine the efficacy of bolus injections of L-NAC to countermand fentanyl-induced OIRD: This will establish how effectively L-NAC reverses the deleterious effects of fentanyl infusion on breathing and ABG (but not analgesia) at early (e.g., 5 min) and prolonged ...