Inclusion criteria:
* Men or women
* Age between 40 and 75 years (inclusive)
* Body weight between 50 and 90 Kg
* Post-surgical patient of major orthopedic surgery, with opioid treatment (oxycodone) at discharge by treating physician
* Physical status of ASA I to III
* Be able and willing to read, understand, sign, and date the Informed
* Consent Form prior to entering the study.
* Woman of childbearing age with effective contraception, not pregnant, not on breastfeeding, not pregnancy plans during the development of the study, postmenopausal, defined by cessation of menstruation for at least 12 consecutive months and confirmed by serum FSH levels \> 40 mIU/mL and estradiol \< 20 pg/mL or surgically sterile (with hysterectomy, bilateral salpingectomy, or bilateral oophorectomy) and not undergoing in vitro fertilization or other fertility treatments)
* Men agreed to use contraception or remain abstinent (when this is the subject's usual and preferred lifestyle)Be willing to complete all study visits and procedures.
Exclusion criteria:
* History of respiratory depression, severe bronchial asthma, chronic obstructive pulmonary disease, or acute and/or severe hypercepnia and cor pulmonale.
* History of gastrointestinal motility disorders (e.g., severe reflux esophagitis, gastroparesis, severe irritable bowel syndrome, active inflammatory bowel disease, and paralytic ileus).
* History of depression, seizures, and epilepsy.
* Uncontrolled hypertension: systolic BP ≥160 mmHg or diastolic BP ≥100 mmHg confirmed by at least two measurements.
* History of allergic reactions, anaphylactic reactions, severe systemic hypersensitivity, or any allergic reaction that, in the opinion of the Principal Investigator or his /her delegate, is likely to be exacerbated by the study drug (RAP-103 and oxycodone).
* History of human immunodeficiency virus or positive HIV test result (HIV 1-2 antibodies).
* History of hepatitis C infection or positive hepatitis C virus (HCVAb) antibody screening result.
* Current hepatitis B (HBV) infection (defined as positive for HepBsAg and/or HepBcAb). Subjects with immunity to hepatitis B from prior natural infection (defined as HepBsAg negative, HepBcAb positive, and HepBsAb positive) or vaccination (defined as HepBsAg negative, HepBCAb negative, and HepBSAb positive) are eligible to participate in the study.
* Transcutaneous oxygen saturation (SpO₂) \<90%.
* Patients with severe liver or kidney disease.
* Patients with immunosuppression, neoplasms, or HIV infection.
* Active substance use disorder (SUD) or within the previous 12 months, including opioid, alcohol, benzodiazepines, or any other substance with potential for abuse.
* Patients with the use of antipsychotic medications.
* Any clinical condition that makes it difficult to self-assess pain.
* Recent opioid use (14 days prior recruitment).
* Previous exposure to RAP-103 or DAPTA (T-Peptide) at any time in your life.
* Abnormal and clinically significant results at the discretion of the Principal investigator (or his/her delegate) in the laboratory tests and electrocardiogram of visit 0. Laboratory values out of range and determined to be not clinically significant at the discretion of the Principal Investigator or his/her delegate may be repeated on one occasion, the subject may be enrolled if the repeated value is within the normal range.
* Participation in any other investigational study with drugs, biologics, medical devices, or treatment with an investigational product or therapy approved for investigational use within 30 days or 5 halflives at visit 0.
* Any other condition that, in the judgment of the PI or Sponsor, determines that the subject is not suitable to participate in the study.
