Paulina M. Getsy ¹ Walter J. May ² Alex P. Young ² Santhosh M. Baby ³ Gregory A. Coffee ¹ James Bates ⁴ Yee-Hsee Hsieh ⁵ Stephen J. Lewis ^6,7*

• ¹ Department of Pediatrics, School of Medicine, Case Western Reserve University, Cleveland, United States
• ² Department of Pediatrics, School of Medicine, University of Virginia, Charlottesville, Virginia, United States
• ³ Galleon Pharmaceuticals, Inc. Horsham, United States, Horsham, Pennsylvania, United States
• ⁴ The University of Iowa, Iowa City, Iowa, United States
• ⁵ Division of Pulmonary, Critical Care and Sleep Medicine, University Hospital Case Medical Center, Case Western Reserve University, Cleveland, Georgia, United States
• ⁶ Department of Pharmacology, School of Medicine, Case Western Reserve University, Cleveland, Ohio, United States
• ⁷ Functional Electrical Stimulation Center, Case Western Reserve University, Cleveland, Georgia, United States

We are determining the efficacy profiles of tropine analogues against opioid-induced respiratory depression (OIRD). Our companion manuscript reports that the cell-permeant tropeine, tropine ester (Ibutropin), produces a rapid and sustained reversal of the deleterious actions of fentanyl on breathing, Alveolar-arterial (A-a) gradient (i.e., index of alveolar gas-exchange), and arterial blood-gas (ABG) chemistry in freely-moving male Sprague Dawley rats while not compromising fentanyl analgesia. We report here that in contrast to Ibutropin, the injection of the parent molecule, tropine (200 mol/kg, IV), worsens the adverse actions of fentanyl (75 g/kg, IV) on ventilatory parameters (e.g., frequency of breathing, tidal volume, minute ventilation, peak inspiratory and expiratory flows, and inspiratory and expiratory drives), A-a gradient, ABG chemistry (e.g., pH, pCO2, pO2, sO2) and sedation (i.e., the righting reflex) while not affecting fentanyl antinociception (i.e., the tail-flick latency) in freely-moving male Sprague Dawley rats. These data suggest that tropine augments opioid receptor-induced signaling events that mediate the actions of fentanyl on breathing and alveolar gas-exchange. The opposite effects of Ibutropin and tropine may result from the ability of Ibutropin to readily enter peripheral and central cells. Of direct relevance is that tropine, resulting from the hydrolysis of Ibutropin, would combat Ibutropin-induced reversal of the adverse effects of fentanyl. Since numerous drug classes, such as cocaine, atropine, and neuromuscular blocking drugs, contain a tropine moiety, it is possible that their hydrolysis to tropine has unexpected/unintended consequences. Indeed, others have found that tropine exerts the same behavioral profile as cocaine upon central administration. Together, these data add valuable information about the pharmacological properties of tropine.