A Phase II/III Study of F520 Combined With Paclitaxel Plus Carboplatin in the Treatment of Cancer of Unknown Primary (CUP)

Inclusion Criteria: * Age 18-75 years (inclusive), any gender Histopathologically confirmed adenocarcinoma, squamous cell carcinoma, poorly differentiated carcinoma, or undifferentiated carcinoma of metastatic lesions Diagnosis of CUP after standard evaluation (see Appendix 4) No prior systemic therapy for CUP At least one measurable lesion per RECIST 1.1 criteria Eastern Cooperative Oncology Group (ECOG) performance status score of 0-1 Investigator-assessed life expectancy ≥3 months Adequate organ function as follows (no blood products or hematopoietic growth factors within 14 days before first dose): Hematology: ANC ≥1.5×10⁹/L; Platelet count ≥90×10⁹/L; Hemoglobin (Hb) ≥90 g/L Liver Function: AST, ALT ≤2.5×ULN; Total bilirubin (TBIL) ≤1.5×ULN; If liver metastases present: AST and ALT ≤5×ULN, TBIL ≤3×ULN Renal Function: Serum creatinine (Cr) ≤1.25×ULN Coagulation: INR ≤1.5×ULN and APTT ≤1.5×ULN Understanding of study procedures and content, and voluntary signed informed consent Exclusion Criteria: * CUP patients who, in the investigator's judgment, are candidates for local curative treatment Histopathologically confirmed neuroendocrine carcinoma or germ cell tumor of metastatic lesions Prior genetic testing (including but not limited to NGS) showing NTRK fusion-positive, ALK fusion-positive, EGFR sensitizing mutations, BRAF mutations suitable for molecular targeted therapy, or MSI-H/dMMR Genetic testing (including but not limited to 90-gene assay) suggesting possible colorectal, renal, or breast cancer origin (excluding triple-negative breast cancer) Prior treatment with taxanes, platinum-based chemotherapy, and/or prior treatment with immune checkpoint inhibitors (e.g., anti-PD-1, anti-PD-L1) or any tumor immunotherapy Received chemotherapy, radiotherapy, biologic therapy, endocrine therapy, targeted therapy, or immunotherapy within 4 weeks or 5 half-lives (whichever is shorter) before enrollment Primary and/or metastatic central nervous system (CNS) malignancies or carcinomatous meningitis Required systemic corticosteroids (equivalent to \>10 mg prednisone/day) or other immunosuppressive drugs within 2 weeks before enrollment or during study. Exception: topical or inhaled corticosteroids, or short-term (≤7 days) corticosteroids for prevention or treatment of non-autoimmune, infrequent allergic diseases Prior anti-tumor treatment-related adverse events not recovered to NCI-CTCAE V5.0 (or later) Grade ≤1 or levels specified in inclusion/exclusion criteria (except toxicities judged by investigator to have no safety risk, such as alopecia, Grade 2 peripheral neuropathy, or stable hypothyroidism on hormone replacement) History of other malignancy within past 5 years, except locally curable cancers (melanoma in situ, cutaneous basal cell carcinoma, cutaneous squamous cell carcinoma, superficial bladder carcinoma in situ, superficial breast carcinoma in situ) Uncontrolled pleural effusion, pericardial effusion, or ascites requiring repeated drainage Known interstitial lung disease or non-infectious pneumonia, currently symptomatic or previously requiring systemic corticosteroids, which in the investigator's judgment may affect toxicity assessment or management related to study treatment History of organ transplantation or allogeneic bone marrow transplantation, or autologous stem cell transplantation within 3 months before first dose Major surgery within 4 weeks before first dose or not recovered from surgery before enrollment (excluding diagnostic surgery) Known positive HIV test history or known AIDS; Positive syphilis screening (specific antibody positive with non-specific antibody negative and clinically confirmed non-active infection excluded); HBsAg and/or HBcAb positive with HBV-DNA ≥200 IU/mL (or 1000 cps/mL); HCV antibody positive with detectable HCV-RNA indicating viral replication Uncontrolled or severe cardiovascular disease, NYHA Class II or higher congestive heart failure, unstable angina, myocardial infarction within 6 months before first dose; Uncontrolled hypertension (systolic ≥160 mmHg and/or diastolic ≥100 mmHg despite treatment) Serious concomitant diseases at screening that compromise patient safety or ability to complete the study \[e.g., active autoimmune disease, active infection (e.g., active tuberculosis), severe psychiatric disease, severe neurological disease, severe endocrine disease (e.g., Type 1 diabetes, drug-uncontrolled Type 2 diabetes), or any other condition\] Received any other investigational drug/device within 4 weeks before first dose History of drug abuse or alcohol abuse within 6 months before first dose Received live or attenuated live vaccines within 4 weeks before first dose or planned during study History of severe allergy, or known hypersensitivity to macromolecular protein preparations/monoclonal antibodies, or any study drug components Pregnant or lactating women; Women of childbearing potential or men with partners of childbearing potential who do not agree to use medically accepted effective contraception (e.g., intrauterine device or condoms) during study and for 6 months after last study drug administration Judged by investigator as unsuitable for enrollment