This is a randomized, double-blind, placebo-controlled, single ascending dose phase I clinical study to evaluate the safety, tolerability, PK, preliminary PD and immunogenicity of SSS67 in healthy adult participants after single intravenous infusion.

The primary objective is to assess the safety and tolerability of SSS67 following single ascending doses.

The secondary objectives are to characterize the PK, PD, and anti-drug antibody (ADA) profiles of SSS67 after single administration.

The exploratory objective is to evaluate changes in body weight, waist circumference, and other related metabolic parameters following a single dose of SSS67.

Inclusion Criteria:

• Participants aged 18 to 65 years (inclusive), regardless of gender.
• Participate in the clinical trial voluntarily and provide informed consent form.
• At screening, body weight ≥ 50 kg for males and ≥ 45 kg for females. Body Mass Index (BMI) must be between 18.5 and 32 kg/m² (inclusive).
• At screening, the following must be within normal ranges or assessed by the investigator as abnormal but not clinically significant: Vital signs、Physical examination、12-lead electrocardiogram (ECG)、Chest X-ray (posteroanterior view) or lung computed tomography (CT)、Ultrasound (abdominal, urological, and gynecological [for females only])、Clinical laboratory tests (including hematology, urinalysis, blood biochemistry, coagulation, serum virolgy and thyroid function).
• Female participants with childbearing potential and male participants (and their female partners) must agree to take highly effective contraceptive measures and have no plan of childbearing, sperm donation, or egg donation from screening through at least 6 months after the last dose of investigational drug.
• Able to understand and comply with protocol requirements.

Exclusion Criteria:

• History of significant drug or food allergies; or known clinically significant hypersensitivity to the study intervention or related compounds.
• History of risk factors for torsades de pointes (e.g., heart failure/cardiomyopathy or family history of long QT syndrome), uncorrected hypokalemia or hypomagnesemia (one retest may be allowed per Investigator's judgment), or QT (QTcF) interval >450 ms.
• Any clinically significant medical condition or illness that could compromise participant safety or interfere with study results.
• History of any malignancy within the past 5 years, except for basal cell carcinoma, squamous cell carcinoma in situ, or cervical carcinoma in situ th with no evidence of metastatic disease for 3 years.
• Known concomitant conditions affecting skeletal muscle, or use of any medications or supplements known to influence muscle anabolism/catabolism within 3 months prior to screening.
• History of gastrointestinal surgery resulting in malabsorption, or chronic use of medications that s directly affect gastrointestinal motility. Examples: history of bariatric surgery/procedures (e.g., gastric banding), use of medications/products deemed by the investigator to cause weight change and affect weight assessment within 3 months prior to dosing, or intention to use weight-loss drugs during the study.
• Use of any prescription drugs or over-the-counter medications, herbal medicines, or dietary supplements within 4 weeks prior to screening (participants may be enrolled if the investigator confirms the time interval between last use and the first dose in this trial is >5 half-lives of the drug).
• Use of drugs potentially affecting follicle-stimulating hormone (FSH) levels within 90 days prior to screening, or planned use during the study.
• Regular weightlifting, bodybuilding, or strength training aimed at increasing muscle mass.

Due to the nature of the study and privacy concerns, de-identified individual participant data will not be made available to external researchers.