Lung cancer is the most common type of cancers and the leading cause of cancer-related mortality worldwide, with 2.5 million new cases and 1.8 million deaths in 2022. Small-cell lung cancer (SCLC) accounts for ~15% of all lung tumors, representing the most aggressive high-grade neuroendocrine carcinoma with a five-year survival rate of less than 4%. The majority of patients are diagnosed with extensive stage SCLC (ES-SCLC) with brain metastases in ~80% of cases, yielding a grim 5-year survival rate of under 2%. Many patients experience relapse following front-line therapies, and current second-line treatment options remain limited. Delta-like ligand 3 (DLL3) is highly expressed in SCLC, neuroendocrine tumors, and glioblastoma with minimally or no expression in normal tissues, making it an attractive target for SCLC. B7-H3, an immunoregulatory protein, is overexpressed in several tumor types, including SCLC. Both DLL3 and B7-H3 are validated targets for SCLC. Tarlatamab, a T cell engager targeting DLL3, was recently approved for SCLC. ADCs targeting DLL3 have shown robust anti-tumor activities in clinical trials. Similarly, anti-B7-H3 ADCs have also shown promising clinical potential.

We constructed a series of bispecific ADC (bsADC) that target both DLL3 and B7-H3 (DLL3xB7H3) simultaneously for SCLC and other tumors expressing these markers. We hypothesized that dual targeting could enhance selectivity and potency, address tumor heterogeneity in tumoral DLL3 and B7-H3 expression levels and allow a broader patient population for treatment. The bispecific antibody (bsAb) moieties of the bsADCs were designed in a1+1 format and were conjugated to a novel TOPO1 inhibitor payload with potent bystander effect (DAR 8) and a highly hydrophilic, enzyme-cleavable linker. In vitro, the bsAbs showed superior internalization in double positive cell lines than the parental monovalent arms and outperformed the naked antibodies of DS-7300 and rovalpituzumab (Rova). In vivo studies using a SCLC CDX model, the bsADCs demonstrated robust anti-tumor efficacy that was superior to DS-7300 ADC when conjugated to BLD-1102. The specificity of DLL3 targeting was validated in DLL3 negative CDX and PDX models in vivo. The bsAbs for our lead bsADCs exhibited excellent stability under stress conditions, suggesting their suitability for CMC development. In summary, our DLL3xB7H3 bsADCs showed promising and superior in vitro and in vivo preclinical activities with first-in-class potential for SCLC treatment.