• Signed the informed consent form voluntarily and agreed to follow the trial requirements

• Age ≥18 years

• Subject weighs more than 40 kg

• Has a life expectancy of ≥3 months

• Has documented locally advanced or metastatic SCLC, large cell neuroendocrine cancer (LCNEC), neuroendocrine prostate cancer (NEPC), high-grade gastrointestinal neuroendocrine tumors (GI-NET), Merkel cell carcinoma (MCC), or other neuroendocrine tumors (with neuroendocrine histology ≥10%) who have failed at least 1 line of standard therapy in the advanced/metastatic setting or are unable to receive standard treatment Notes: For SCLC, the subject must have failed at least 1 line of platinum therapy in the advanced/metastatic setting. No prior topoisomerase inhibitor-based antibody-drug conjugate (ADC) therapy is permitted

• Agree to provide archival tumor samples (FFPE tissue block or slides) from primary or metastatic sites:

∙ In dose escalation and dose finding: archival tissue obtained within 2 years or FFPE block from fresh biopsy. If no archival tissue is available, a fresh tissue biopsy is required

‣ In dose expansion: an FFPE block from fresh biopsy or archival tissue (within 6 months) is required NOTE: If no archival tissue is available and, a fresh tissue biopsy is clinically contraindicated, please consult the sponsor.

• Has at least one measurable lesion based on RECIST (Response Evaluation Criteria in Solid Tumors) V1.1

• Has an Eastern Cooperative Oncology Group performance status (ECOG PS) of 0 to 1

• Toxicity of previous antitumor therapy has returned to Grade ≤1 as defined by NCI CTCAE V5.0, except for alopecia and endocrinopathies controlled by replacement therapy

⁃ Has no serious cardiac dysfunction and left ventricular ejection fraction ≥50%

⁃ Has adequate organ function, defined as:

• Marrow function: Absolute neutrophil count (ANC) ≥1.5×109/L, platelet count ≥100×109/L, hemoglobin (Hb) ≥9.0 g/dL (blood transfusion, platelet transfusion, erythropoietin (EPO), hematopoiesis agents (such as thrombopoietin \[TPO\]), and G-CSF use are not allowed 1 week prior to screening)

∙ Hepatic function: Total bilirubin (TBIL) ≤1.5×ULN (≤3×ULN for subjects with Gilbert's syndrome or liver metastasis at baseline), AST and ALT without liver metastasis ≤3.0×ULN, AST and ALT with liver metastasis ≤5.0×ULN

∙ Renal function: Creatinine (Cr) clearance ≥60 mL/minute (Cockcroft-Gault equation)

⁃ Coagulation parameters: International normalized ratio (INR) ≤1.5×ULN, and activated partial thromboplastin time (aPTT) ≤1.5×ULN, unless receiving anticoagulation therapy with PT and aPTT levels within the intended therapeutic range

⁃ Urine protein ≤2+ or ≤1000 mg/24 hours

⁃ Sexually active fertile subjects and their partners must agree to use highly effective methods of contraception (defined in Appendix D) during the course of the study and for 7 months after the last dose of study treatment. An additional contraceptive method, such as a barrier method (eg, condom), is recommended.

⁃ Women of childbearing potential (WOCBP) must have a negative serum pregnancy test at screening and must be nonlactating. Female subjects are considered WOCBP unless one of the following criteria are met: documented permanent sterilization (hysterectomy, bilateral salpingectomy, or bilateral oophorectomy) or documented postmenopausal status (defined as 12 months of amenorrhea in a woman \>45 years old in the absence of other biological or physiological causes. In addition, females \<55 years old must have a serum follicle stimulating hormone (FSH) level \>40 mIU/mL to confirm menopause.