Evaluating BL-M14D1 in Subjects With Locally Advanced or Metastatic Small Cell Lung Cancer and Neuroendocrine Tumors

Inclusion Criteria: * Documented locally advanced or metastatic SCLC, large cell neuroendocrine cancer of the lung (LCNEC), neuroendocrine prostate cancer (NEPC), poorly differentiated gastroenteropancreatic neuroendocrine carcinomas (GEP-NEC) or other extrapulmonary neuroendocrine carcinomas (EP-NECs), Merkel cell carcinoma (MCC), or other poorly differentiated and/or high-grade neuroendocrine neoplasms with evidence of DLL3 expression 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 participant must have failed at least 1 line of platinum therapy in the advanced/metastatic setting. * No prior topoisomerase inhibitor-based ADC therapy is permitted. * In the dose expansion part, Cohort 6 (DLL3-Positive NEN Subgroup): participants will be eligible based on documented positive DLL3 expression. * At least one measurable lesion based on RECIST (Response Evaluation Criteria in Solid Tumors) v1.1 * 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 National Cancer Institute (NCI) CTCAE v5.0, except for alopecia and endocrinopathies controlled by replacement therapy * No serious cardiac dysfunction and left ventricular ejection fraction ≥50% * Adequate organ function Exclusion Criteria: * Chemotherapy, biological therapy, immunotherapy, , targeted therapy (including small molecule inhibitor of tyrosine kinase), and other antitumor therapy within 4 weeks or 5 half-lives (whichever is shorter) prior to the first administration; radical radiotherapy, major surgery within 4 weeks prior to the first administration; mitomycin and nitrosoureas treatment within 6 weeks prior to the first administration; oral fluorouracil drugs such as tegafur, capecitabine, or palliative radiotherapy within 2 weeks prior to initial administration. * Participants who have received prior topoisomerase inhibitor-based ADC therapy * Participants with other prior or concurrent malignancies except for basal cell carcinoma of the skin, squamous cell carcinoma of the skin and/or carcinoma in situ after adequate resection, or other malignancy treated with curative intent with a disease-free interval of at least 3 years * Participants with advanced/ clinically significant lung diseases, such as poorly controlled chronic obstructive pulmonary disease (COPD) and asthma, restrictive lung disease, pulmonary hypertension etc. * Participants with primary neoplasms in the central nervous system (CNS), active or untreated CNS metastases, spinal cord compression, or carcinomatous meningitis should be excluded. Active brain metastases are defined as untreated symptomatic brain metastases or untreated brain metastases requiring systemic corticosteroids and/or anticonvulsants to control CNS-related symptoms. Patients with brain metastases are eligible if they meet any of the following criteria: 1. untreated, asymptomatic brain metastases 2. previously treated brain metastases may participate provided they are clinically stable with local therapy (eg, surgery or radiotherapy, currently asymptomatic, no longer requiring corticosteroid treatment, and recovered from all local therapy-related adverse events, as determined by the investigator * Participated in another clinical trial within 4 weeks prior to first dose of study treatment * Participants who are pregnant or breastfeeding, or planning to become pregnant during the study * Other conditions that the Investigator or Sponsor believes are not suitable for participating in this clinical trial