(A to F) Mice bearing SCLC LU64 (A and D), SCLC LU86 (B and E), or LCNEC LU37 (C and F) PDX tumors were treated with IgG1LD6.5 or SC16LD6.5 (1 mg/kg) (A to C) on a Q4D×3 regimen, or vehicle (saline) or SOC chemotherapy (D to F). (G) DLL3 surface expression quantified by IHC (H-score) correlated with dTTP in 10 SCLC and 1 LCNEC PDX model. (H and I) Mice bearing SCLC LU64 PDX tumors were treated with C/E and, upon tumor recurrence (35 days after initial C/E treatment), were randomized and treated again either with (H) IgG1LD6.5 or SC16LD6.5 (1 mg/kg) on a Q4D×3 regimen or with (I) vehicle or C/E.