This study is an open-label first-in-human phase I clinical study to evaluate the safety, tolerability, and pharmacokinetic characteristics of HLX3902 in patients with mCRPC and other advanced solid tumours.

Inclusion Criteria

1. Voluntarily signed written informed consent and willing to comply with study procedures.
2. Age: ≥ 18 years, regardless of gender.
3. Histologically confirmed advanced or metastatic solid tumours (e.g., metastatic castration-resistant prostate cancer (mCRPC), non-small cell lung cancer, or gastric cancer) following failure of standard therapy.
4. mCRPC specifics: 1）Progression or refractory status after ≥ 1 novel anti-androgen agent and failure of 1-2 taxane-based regimens.

2）Ongoing surgical or medical castration (gonadotropin-releasing hormone agonist or antagonist) with serum testosterone ≤ 50 ng/dL.

3）Documented disease progression (prostate-specific antigen, nodal, visceral, or bone) .

5. Presence of at least one measurable lesion per RECIST criteria version 1.1. 6. ECOG Performance Status of 0-1. 7. Expected survival exceeding 3 months. 8. Agreement to provide archived or fresh tumour tissue. 9. Adequate organ function. 10. Agreement to use effective contraception for both genders and negative pregnancy test for females of childbearing potential.

Exclusion Criteria

1. Presence of histological types other than adenocarcinoma in mCRPC; or neuroendocrine or small cell differentiation in other solid tumours.
2. Active or symptomatic central nervous system metastases, carcinomatous meningitis, or spinal cord compression (stable treated brain metastases meeting protocol criteria are allowed).
3. Active malignancies within two years prior to the first dose, except cured carcinoma in situ or basal cell carcinoma of the skin.
4. Prior STEAP1-targeted therapy, or Radium-223/PSMA radionuclide therapy within 6 months.
5. Major surgery, radiotherapy, chemotherapy, biological therapy, immunotherapy, or endocrine therapy (excluding LHRH/GnRH analogues) within 28 days; small molecule drugs within 14 days.
6. Vaccination with live vaccines within 28 days.
7. Systemic corticosteroids (> 10 mg/day Prednisone equivalent) or other immunosuppressants within 14 days.
8. Currently participating in another interventional study or within 4 weeks of the end of treatment in such a study.
9. Adverse events from prior therapy not resolved to Grade ≤ 1, except for alopecia, ear toxicity, or stable Grade ≤ 2 taxane-related neurotoxicity.
10. History of Grade ≥ 2 immune-related pneumonitis or myocarditis, or severe/life-threatening immune-mediated adverse events during prior immunotherapy.
11. Poorly controlled cardiovascular disease within 6 months, unstable angina, stroke, thromboembolic events, or uncontrolled hypertension or arrhythmia.
12. Evidence of interstitial lung disease, or active non-infectious pneumonitis.
13. Active or suspected autoimmune disease, hypophysitis, or unstable pituitary dysfunction requiring systemic therapy.
14. Active systemic infectious diseases requiring intravenous antibiotics within 2 weeks, active tuberculosis, or positive for HIV, active HBV (HBV DNA ≥ 500 IU/mL), or HCV.
15. History of organ transplantation, central nervous system diseases within 12 months (e.g., seizures, dementia), or any condition that makes the participant unsuitable per Investigator.