Naldemedine (S-297995) is a peripherally acting μ-opioid receptor antagonist for the treatment of opioid-induced constipation, a common side effect of opioid therapy. We determined the safety, tolerability, and pharmacokinetic profiles of oral naldemedine in healthy volunteers in 2 randomized, double-blind, placebo-controlled, phase 1 studies. In the single ascending dose study, subjects received a single dose of naldemedine (0.1-100 mg; n = 42) or placebo (n = 14). In the multiple ascending dose study, subjects received once-daily naldemedine (3-30 mg; n = 27) or placebo (n = 9) for 10 days. On day 1 of the single ascending dose studies and day 10 of the multiple ascending dose studies, respectively, the maximum plasma concentration levels of naldemedine were 1.98 to 2510 ng/mL and 73.8 to 700 ng/mL, peaked at 0.5 hours and 0.5 to 0.75 hours, and the fraction excreted in urine was 15.9% to 20.5% and 19.7% to 19.1%. There were no major safety or tolerability concerns even at naldemedine doses 150 to 500 times the therapeutic dose of 0.2 mg. The incidence of adverse events was not dose dependent. Gastrointestinal adverse events occurred more frequently with naldemedine vs placebo, and all of these were considered treatment related. Overall, naldemedine was rapidly absorbed, and no safety or tolerability issues were noted at the doses evaluated.Keywords naldemedine, opioid-induced constipation, peripherally acting μ-opioid receptor antagonist, pharmacokinetics, safety Opioid-induced constipation (OIC) is a frequent and debilitating side effect of opioid analgesics, which are widely used to manage moderate to severe chronic pain. Unlike other side effects of opioids, OIC is generally not self-limiting. 1,2 OIC is characterized by reduced frequency of bowel movements, development or worsening of straining, a sense of incomplete evacuation, and hard stools after the initiation of opioid therapy. 3,4 Opioids act on the μ-, δ-, and κ-opioid receptors distributed widely in the central and peripheral nervous systems. Although the role of δ-and κ-opioid receptors in causing gastrointestinal adverse events (AEs) is less clear, the importance of μ-opioid receptors is better understood. The stimulation of μ-opioid receptors, which are expressed throughout the gastrointestinal tract, alters neural activity in the submucosal and myenteric plexuses of the enteric nervous system. These alterations can lead to impairment of gastrointestinal fluid secretion and motility, along with increased fluid absorption, resulting in OIC. 4,5 Laxatives are often used as first-line treatment for OIC, but many patients do not achieve satisfactory relief of constipation. 4,6 A targeted pharmacological treatment for OIC is peripherally acting μ-opioid receptor antagonists (PAMORAs). PAMORAs aim to reverse OIC by blocking the action of opioids at peripheral μ-opioid receptors in the gastrointestinal tract without affecting analgesia. 4 Naldemedine This is an open access article under the terms of the Creative Commons Attribution-NonCommerc...