QLC5508 vs. Investigator's Choice Chemotherapy in Advanced Extrapulmonary Neuroendocrine Carcinoma

1. Voluntarily consent to participate in this study and sign the informed consent form.
2. Males and females aged ≥18 years old;
3. Eastern Cooperative Oncology Group Performance Status (ECOG PS) score of 0 or 1;
4. Estimated survival time of more than 3 months.
5. A serum pregnancy test must be performed within 7 days prior to randomization for premenopausal women of childbearing potential, and the result must be negative and must not be lactating; All enrolled patients and the partners should take adequate barrier contraception during the entire treatment cycle and for 6 months after the end of treatment.
6. Capable of understanding trial requirements, willing and able to comply with trial and follow-up procedures.
7. Toxicity of previous antineoplastic therapy has returned to ≤ grade 1 defined by National Cancer Institute-Common Terminology Criteria for Adverse Events (NCI-CTCAE) version 6.0 (v6.0).
8. Voluntarily sign the written informed consent form and comply with the protocol requirements.
9. Histologically or cytologically confirmed advanced extrapulmonary neuroendocrine carcinoma (EP-NEC)
10. Has at least 1 measurable lesion according to Response Evaluation Criteria in Solid Tumors (RECIST) version 1.1 (v1.1) as assessed by the investigator.
11. Sufficient bone marrow and organ function.

Exclusion Criteria:

1. Diagnosis of other primary malignancies within 5 years prior to signing the informed consent form.
2. Patients diagnosed with Merkel cell carcinoma or medullary thyroid carcinoma.
3. Brain metastases (unless asymptomatic and no progression confirmed by imaging ≥4 weeks prior to randomization); Presence of leptomeningeal metastases or brainstem metastases; Spinal cord compression (identified via imaging, regardless of symptoms).
4. Previous or ongoing treatment with topoisomerase I inhibitors
5. Having previously received B7-H3-targeted therapy.
6. Insufficient washout of prior anticancer therapies prior to randomization.
7. Having underwent major organ surgery (excluding biopsy) or significant trauma within 4 weeks prior to randomization; Requiring elective surgery during the study.
8. Having received live vaccine or live attenuated vaccine within 4 weeks before study randomization.
9. Having received treatment with systemic corticosteroids (prednisone at >10 mg/day, or similar drugs at equivalent dose) or other immunosuppressive agents within 14 days prior to randomization.
10. Moderate to severe pulmonary disease significantly impairing lung function, including idiopathic pulmonary fibrosis, autoimmune/connective tissue disorders with lung involvement, or prior pneumonectomy.
11. Having a history of interstitial lung disease (ILD)/ non-infectious pneumonitis that required corticosteroids, current ILD/ non-infectious pneumonitis, or suspected ILD/ non-infectious pneumonitis that cannot be ruled out by imaging at screening;
12. Active tuberculosis; Autoimmune diseases not in clinical remission, other acquired or congenital immunodeficiency diseases; Having a history of allogeneic stem cell, bone marrow, or organ transplantation.
13. Serious infections (e.g., bacteremia, or severe pneumonia) within 4 weeks prior to randomization; Active infection requiring systemic antibiotic therapy within 1 weeks prior to randomization;
14. Having positive results in virus serology tests (hepatitis B virus infection participants receiving antiviral treatment other than interferon are allowed to be enrolled).
15. Uncontrolled or significant cardiovascular disease.
16. Clinically uncontrolled third-space effusion.
17. Drug abuse.
18. Having alcohol or drug dependence.
19. Poor compliance as per investigator discretion; Any other medical conditions that may interfere with study participation or the results of the clinical study as per the discretion of investigator.