This study aims to evaluate the analgesic effect and the safety of 4 doses of STR-324 administered as an infusion, with or without initial bolus, to patients suffering from post-operative pain.

The sensitivity of the pain model used in the trial will be evaluated with a standard group treated with morphine under the usual care conditions.

The first step (single-blind standard group) aims to verify the sensitivity of the study pain model in the proposed surgery to confirm the validity of the surgery according to their pain and enrolment rate. 12 patients are planned to be enrolled.

The second step (dose-finding study) is a randomized, double-blind, parallel groups, controlled clinical trial involving a total of 106 patients divided into two groups in which patients will be randomized between STR-324 and morphine HCl:

• Group 1: Titration initiated with a bolus
• Group 2: Titration without initial bolus.

Selection criteria:

1. Patient aged ≥18 to <65 years old, at screening;
2. Having signed an informed consent prior to any study-related procedure;
3. Patients planned to undergo a major laparoscopic abdominal or pelvic surgery;
4. Surgery to be performed without local or regional anaesthesia nor infiltration;
5. Body mass index (BMI) between 18 and 30 kg/m² inclusive at screening, and with a minimum weight of 50 kg;
6. Women of childbearing potential must agree to use at least one effective contraceptive method upon enrolment and for 1 cycle following the last dose of the investigational product.

Inclusion Criteria:

The criteria for definitive enrolment must be checked in the Post-Anaesthesia Care Unit (PACU) at the inclusion visit:

1. NRS-measured pain intensity ranging from 4 to 8 inclusive at baseline PACU measurement;
2. With a sedation score ≤ S1 (awake or intermittently sleepy) at the time of pain assessment.

Exclusion Criteria:

1. Patient contra-indicated for morphine administration;
2. Unstable or poorly controlled psychiatric condition (e.g., untreated PTSD, major anxiety, or depression). Subjects who take stable doses (same dose >30 days) of antidepressants and/or anti-anxiety drugs may be included;
3. Women who are pregnant or breastfeeding;
4. History of alcohol, opiate or other drug abuse.
5. Evidence of any active or chronic disease or condition that could interfere with, or for which the treatment might interfere with the conduct of the study, or that would pose an unacceptable risk to the subject in the opinion of the investigator. Minor deviations of values from the normal range may be accepted, if judged by the Investigator to have no clinical relevance;
6. Clinically significant abnormalities, as judged by the investigator, in laboratory test results;
7. Participation in an investigational drug or device study within 1 month prior to dosing.

Step 1 of the study is single blinded for participants only.