This is a Phase 1a, randomized, double-blind, placebo-controlled, single ascending dose (SAD) study to evaluate the safety, tolerability, pharmacokinetics (PK), and immunogenicity of STSP-0902.

Inclusion Criteria:

1. Healthy male volunteers, aged between 18 and 50 years inclusive, with a body weight of at least 50.0 kg, and a body mass index (BMI) between 19.0 and 28.0 kg/m² inclusive, and whose routine semen analysis results during the screening period meet the criteria of a total sperm count of less than 180 million (or a sperm concentration of less than 63 million) or a percentage of progressively motility sperm of less than 56%.
2. Participants (including the partners of the participants) must use effective non-drug contraceptive measures during the trial period and for four months after the end of administration, and must not have plans for pregnancy or sperm donation.
3. Subjects must give informed consent to this study before the study and voluntarily sign a written informed consent form.

Exclusion Criteria:

1. Participants with a history of severe diseases, including but not limited to conditions affecting the skeletal, neuropsychiatric, cardiovascular, hematologic, hepatic, renal, gastrointestinal, respiratory, metabolic, endocrine, immune, and reproductive systems (such as reproductive system infectious diseases, varicocele, reproductive tract obstruction, etc., except for oligoasthenzoospermia), as judged by the investigator, may endanger the safety of the participant or affect the study results.
2. Participants who have planned to receive treatments related to oligoasthenzoospermia, such as zinc sulfate, levocarnitine, escin, or pancreatic kallikrein, within 3 months prior to screening or during the trial period.
3. Participants with a history of treatment with nerve growth factor-like drug therapy (such as mouse nerve growth factor for injection) within 3 months prior to screening.
4. Subjects who have undergone any major surgery within 3 months prior to screening or have surgery planned during the trial period.
5. Pre-enrollment physical examination, electrocardiogram, vital signs, laboratory tests, and results of all tests related to the trial (except oligoasthenzoospermia), with abnormalities judged clinically significant by the investigator.
6. Subjects who are allergic to any component of the experimental drug or biological agent, or who, in the judgment of the investigator, are at risk of allergy as a result of participation in the study.
7. Subjects who are positive for any one of the hepatitis B surface antigen, hepatitis C antibody, treponema pallidum antibody, and HIV antigen/antibody combination test (primary screening).
8. Tattoos at the injection site or other skin conditions that interfere with observation of the skin.
9. Subjects who have smoked more than 5 cigarettes per day or an equivalent amount of tobacco in the 3 months prior to screening.
10. Subjects with frequent alcohol consumption within 6 months prior to screening, i.e., more than 2 units of alcohol per day (1 unit = 360 ml of beer or 45 ml of spirits of 40% alcohol by volume or 150 ml of wine); or those with a positive alcohol breath test.
11. Subjects who have habitual consumption of more than 5 cups of coffee, tea or cola, etc. per day (150 ml and above per cup) in the 3 months prior to screening.
12. Subjects who have a history of drug abuse within 1 year prior to screening or have a positive urine drug test.
13. Subjects who have participated in blood donation within 3 months prior to screening with a total blood donation of ≥ 400 mL or total blood loss of ≥ 400 mL, or who have history of blood transfusion within 4 weeks prior to enrollment.
14. Subjects who have taken any investigational product or participated in any clinical trial of drug, devices or vaccines intervention within 3 months prior to screening.
15. Vaccination within 1 month prior to screening or scheduled to be administered during the study period up to 2 months after completion of the study.
16. Subjects who have used any prescription, over-the-counter medications or herbal remedies within 14 days prior to screening.
17. Subjects with a history of fear of needles and homophobia.
18. Subjects with other factors that are not suitable for participation in this study as judged by the investigator.