Rational design of a B7-H3/PD-L1 bsADC combining checkpoint blockade with targeted cytotoxicity for improved antitumor efficacy

B7-H3 (CD276) is a transmembrane glycoprotein of the B7 family aberrantly overexpressed across diverse solid tumors with limited expression in normal tissues, making it an appealing therapeutic target. In addition to potential immunosuppressive effects on T cells, B7-H3 possesses non-immune regulatory functions in enhancing tumor proliferation, angiogenesis, and therapy resistance, which contribute to aggressive disease progression and poor prognosis. PD-L1, another member of the B7 family with clinically validated immune checkpoint function, is frequently co-expressed with B7-H3 in multiple tumor types. Although B7-H3-targeted antibody-drug conjugates (ADCs) and PD-1/PD-L1 checkpoint inhibitors have shown clinical benefit, therapeutic resistance and limited efficacy in “cold” tumors remain challenges. Co-expression of B7-H3 and PD-L1 creates an opportunity to pair targeted cytotoxic delivery with checkpoint blockade. Moreover, the ability of ADCs to induce immunogenic cell death provides additional rationale for integrating ADCs with immune-oncology therapy to enhance antitumor immunity. Guided by this rationale, we developed a bispecific ADC (bsADC), JLC062, that engages both B7-H3 and PD-L1. Several anti-B7-H3 antibodies were identified from our fully human naïve phage library, recognizing different epitopes with varying affinities and internalization rates. Fast-internalizing anti-B7-H3 antibodies were formatted as IgGs to enable efficient intracellular delivery, which results in potent in vitro cytotoxicity across cancer cell lines with varying B7-H3 expression. A high-affinity PD-L1 binder, also isolated from our library, was fused to the C-terminus of the anti-B7-H3 IgG Fc as an scFv. It exhibited robust PD-1/PD-L1 blocking activity and induced IL-2 and IFN-gamma secretion levels comparable to the clinical benchmark avelumab in a mixed lymphocyte reaction assay. Importantly, this anti-PD-L1 arm was intentionally selected as an antibody with minimal internalization to limit toxicity toward immune cells. In a syngeneic mouse model, JLC062 demonstrated significantly greater antitumor efficacy than either parental monotherapy, anti-PD-L1 antibody or anti-B7-H3 ADC, with no observed body weight loss. These findings highlight JLC062 as a promising therapeutic modality that integrates immune checkpoint inhibition with targeted cytotoxic delivery to yield deep and durable antitumor responses.