Abstract
Immune checkpoint inhibitors have shown limited success in breast cancer, the most common and deadly cancer in women worldwide. Novel immune therapies, such as CD3-engaging bispecific antibodies, have shown clinical promise in hematologic malignancies. However, developing CD3 bispecifics for solid tumors has been challenging due to the difficulty in identifying tumor-specific antigens. B7-H4 is proposed as an attractive tumor-associated antigen for breast cancer therapeutics with comprehensive coverage regardless of breast cancer molecular subtype. We designed a B7-H4-targeting CD3 bispecific molecule, PF-07260437, and demonstrated B7-H4-dependent pharmacology in vitro by directing cytotoxic T-cell killing to breast cancer cell lines. Treatment of cell line- and patient-derived xenograft in vivo models of human breast cancer with PF-07260437 induced substantial tumoricidal activity, often resulting in complete responses. Mechanistically, PF-07260437 increased T-cell number and activation, leading to efficient tumor killing. Additionally, combining PF-07260437 with standard of care (palbociclib plus fulvestrant) and a checkpoint inhibitor (anti-PD-1) showed combinatorial benefits in an immune-competent in vivo model. Clinically relevant noninvasive PET/CT imaging with a CD8-targeting tracer demonstrated PF-07260437-mediated increases in intratumoral CD8 T cells, highlighting the utility of CD8-PET technology to potentially assess biomarker changes in the clinic. Finally, the manageable toxicity profile of PF-07260437 was highlighted in an exploratory toxicology study in cynomolgus monkeys. These data support the clinical testing of PF-07260437 for treating B7-H4-expressing solid tumors, including breast cancer.