• Are willing to participate in this clinical study, understand the study procedures, and are able to sign the written ICF.

• Subjects with histologically or cytologically confirmed extensive-stage SCLC neuroendocrine carcinoma (NEC), and other DLL3+ tumors, are eligible per protocol. Subjects must have radiologically progressed or recurred after previous standard treatment, For SCLC, this includes platinum-based therapy and programmed death-1/programmed death-ligand 1 inhibitors (except for subjects who refuse or are judged by the Investigator to be unsuitable for immunotherapy). No more than 2 lines of previous systemic chemotherapy in any setting and no more than 3 total lines of systemic therapy in the recurrent or metastatic setting will be allowed.

• Subjects will be required to provide blood/tumor tissue samples for biomarker testing.

• Have at least 1 measurable lesion according to RECIST version 1.1.

• Have Eastern Cooperative Oncology Group (ECOG) Performance Status (PS) score of 0 or 1.

• Have life expectancy \> 3 months.

• Have adequate bone marrow and organ function.

• Women of childbearing potential must agree to take highly effective contraceptive measures from signing of consent through 8 months after the last dose of IDE849; men with partners of child-bearing potential must use effective contraception through 5 months after the last dose.