This is a first-in-human (FIH), open-label, Phase 1 study designed to evaluate the safety, pharmacokinetics (PK), and preliminary efficacy of ML261, an autologous potency enhanced anti-DLL3 CAR T cell therapy, in participants with R/R SCLC or select NECs

Inclusion Criteria

• ≥18 years of age at the time of signing the ICF
• Have been previously treated with at least one line of systemic standard of care (SOC) anti-cancer therapy for their respective cancer indication. Participants with locally advanced disease who are eligible for curative resection will be excluded.
• Have documented radiological disease progression/relapse during or after their most recent line of anti-cancer therapy with measurable disease on imaging, as assessed by RECIST v1.1
• Have histologically and/or cytologically confirmed diagnosis of select advanced or metastatic R/R solid tumor malignancy in one of the following: R/R SCLC, R/R GEP-NEC, R/R high-grade NEPC, R/R epNEC with biopsy-documented DLL3 expression on archival tissue or fresh biopsy by local or central assessment. CNS NEC is excluded. Participants with mixed histologies for any of these indications qualify if the small cell/neuroendocrine tumor cell percentage is > 50%, except for high-grade NEPC where neuroendocrine component must be > 20%.
• Eastern Cooperative Oncology Group (ECOG) Performance Score (PS) 0 or 1
• Life expectancy ≥12 weeks
• Have adequate hematologic and end-organ function

Exclusion Criteria:

• Previous systemic anti-cancer therapies within the timeframes, as specified in the protocol.
• Prior exposure/treatment with DLL3-targeted CAR T therapy or any other genetically engineered adoptive T cell therapy.
• Prior allogeneic organ transplant (including allogeneic bone marrow transplant).
• Major surgical procedure within 4 weeks of the first dose of any study drug administration or anticipated to be in need of a major surgical procedure during the course of study.
• Participants with toxicities (as a result of prior anti-cancer therapy) which have not recovered to baseline or CTCAE v5.0 \