Abstract 1133: A B7-H4 targeting antibody-drug conjugate shows anti-tumor activity in PARPi and platinum resistant cancers with B7-H4 expression

Abstract Purpose: Platinum and PARP inhibitors (PARPi) demonstrate activity in breast and ovarian cancers, but drug resistance ultimately emerges. B7-H4 is over expressed in breast and ovarian cancer compared to normal tissues making it a promising target for therapies, however, to date it has not been evaluated if the expression is maintained in drug resistant cancer. Here we examine B7-H4 expression in high grade serous ovarian carcinoma (HGSOC) tumors at diagnosis and post-platinum or PARPi-resistance. We also evaluate the activity of a novel B7-H4-directed antibody-drug conjugate (ADC) bearing the pyrrolobenzodiazepine-dimer payload tesirine, in preclinical models of breast and ovarian cancer, including those resistant to standard of care therapies. Experimental Design: B7-H4 expression was measured by quantitative flow cytometry and immunohistochemistry. ADC efficacy was tested against multiple cell lines in vitro and patient-derived xenografts (PDX) in vivo. The effect of ADC treatment on cell cycle, DNA damage, and apoptosis was measured using flow cytometry. Results: B7-H4 was over-expressed in 92% of HGSOC tumors at diagnosis (n = 12), persisted in recurrent matched samples after platinum treatment, and was expressed at similar levels at multiple metastatic sites after acquired multi-drug resistance (n = 4 donors). Treatment with the ADC resulted in target-specific growth inhibition in a panel of ten B7-H4 expressing ovarian and breast cancer cell lines (IC50 6.18-273.9 pM, median 48.3 pM). Co-cultures of B7-H4 +/- lines support bystander killing effects leading to further tumor cell death in B7-H4- clones. In platinum- or PARPi-resistant ovarian cancer cells, ADC treatment significantly decreased viability and colony formation (P < 0.001) while increasing S or G2/M phase cell cycle arrest (P < 0.001) and DNA damage (P < 0.0001), ultimately leading to apoptosis (P < 0.01). A single 0.3 mg/kg dose of the ADC resulted in tumor regression in 61% of breast and ovarian PDX models tested (n = 23), where lower activity was identified in B7-H4 low or negative expressing models (P = 0.048). In PARPi- and platinum-resistant HGSOC models (n = 3), continuous B7-H4 ADC treatment (dosed once every 28 days) resulted in sustained anti-tumor activity, leading to complete (7 of 16) or partial responses (3 of 16) and increased survival (P < 0.004). Conclusions: These data support B7-H4 as an attractive ADC target for treatment of drug-resistant HGSOC. Citation Format: Sarah B. Gitto, Margaret Whicker, Gareth Davies, Sushil Kumar, Krista Kinneer, Arthur Lewis, Srinivas Mamidi, Sergey Medvedev, Judith Anderton, Jessica Tang, Benjamin Ferman, Steve Coats, Robert W. Wilkinson, Eric J. Brown, Daniel J. Powell, Fiona Simpkins. A B7-H4 targeting antibody-drug conjugate shows anti-tumor activity in PARPi and platinum resistant cancers with B7-H4 expression [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 1133.