The purpose of the study is to evaluate the safety, tolerability, pharmacokinetics, and efficacy of CS-1103 (1,000 mg), given by intravenous (IV) infusion in healthy participants in the presence of a clinically relevant dose of fentanyl (200 μg, IV), with naloxone blockade (400 μg, IV).

Major Inclusion Criteria:

1. Healthy participants aged 18 to 55 years, inclusive;
2. Have prior experience with opioids, such as from a dental procedure, chronic pain management, or previous inpatient surgical procedure;
3. A body mass index between 18.0 to 30.0 kg/m2, inclusive, and a minimum body weight of 56 kg;
4. Females must not be lactating and must have a negative pregnancy test during screening and admission.

Major Exclusion Criteria:

1. Estimated glomerular filtration rate <90 mL/min/1.73 m2;
2. History of cardiovascular disease;
3. History of any clinically important disease or disorder which, in the opinion of the Investigator, may interfere with safe study participation.