This is a randomized, double-blind, placebo-controlled, single ascending dose phase I clinical study to evaluate the safety, tolerability, PK, PD and immunogenicity of SSS68 in healthy adult participants following a single subcutaneous administration.

This is the first-in-human (FIH) clinical study of SSS68. Seven dose groups are planned for dose escalation in this study: 2 mg, 50 mg, 200 mg, 400 mg, 600 mg, 900 mg and 1200 mg. The study will be conducted sequentially from low dose to high dose. Participants in all dose groups will receive a single subcutaneous injection.

The study consists of a screening period (no more than 4 weeks), a single-dose administration period (Day1), and a follow-up period (Day2 to Day113), with duration of approximately 20 weeks.

Inclusion Criteria:

1. Able to understand and comply with protocol requirements, voluntarily participate in the trial and sign a written informed consent form (ICF).
2. Male or female participants aged 18-45 years (inclusive) when signing the ICF.
3. Male participants with body weight ≥ 50 kg, and female participants with body weight ≥ 45 kg.
4. Participants whose vital signs, physical examination results, laboratory test results, 12-lead electrocardiogram (ECG) and other test results are normal, or abnormal without clinical significance as determined by the investigator.
5. IgG/IgM/IgA > upper limit of normal value at the screening Visit.
6. 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 dose of study drug.

Exclusion Criteria:

1. History of severe allergy, or with a history of allergy to the investigational drug, any of its components, or related excipients.
2. With any clinically significant diseases (including but not limited to gastrointestinal, hepatic, renal, neurological, hematological, endocrine, tumor, pulmonary, immunological, psychiatric, or cardiovascular diseases) prior to the screening visit.
3. Had any confirmed or suspected immunosuppressive or immune-deficient condition, including but not limited to history of opportunistic infections, asplenia, or primary immunodeficiency.
4. History of recurrent or chronic infections, or any infection requiring hospitalization or intravenous antiviral/antibiotic treatment within 6 months before dosing.
5. Participants with positive test results for hepatitis B surface antigen (HBsAg), anti-hepatitis C virus (HCV) antibody, anti-human immunodeficiency virus (HIV) antibody.
6. Participants who have undergone major surgery within 6 months before dosing, or plan to undergo major surgery during the study.
7. Participants who have received any marketed or investigational biological agent within 5 half-lives or 12 weeks (whichever is longer) before dosing.
8. Participants who have used any prescription drugs, over-the-counter medications, vitamin supplements, or herbal products within 4 weeks or 5 half-lives (whichever is longer) before dosing.
9. Participants who smoke more than 5 cigarettes per day within 3 months prior to screening.
10. History of significant alcohol abuse or regular use of alcohol within 6 months prior to the screening visit.
11. History of significant drug abuse, or any use of soft drugs within 3 months prior to screening, or any use of hard drugs within 1 year prior to screening, or positive result for urine drug screen test at screening.
12. Participants who have been vaccinated with live or live-attenuated vaccines within 30 days before dosing, or plan to receive vaccination during the study.
13. Participation in another interventional clinical trial within 3 months prior to screening.
14. Participants who have donated blood (≥ 400 mL) or lost a lot of blood (≥ 400 mL) within 3 months before screening.
15. QTcF ≥450 ms at the screening visit.
16. Participants who have other factors making participation in this trial inappropriate, as judged by the investigator.