Abstract 3797: Tumor-targeted CD28 bispecific antibodies enhance the antitumor efficacy of CD3-bispecifics and mitigate T cell exhaustion

Bispecific antibodies (bsAbs) that stimulate CD3 (bsAbsCD3) have emerged as promising therapeutic compounds and effectively induce T cell responses by providing “signal 1" for T cell activation. However, the efficacy of these reagents is often limited by T cell exhaustion that is caused by excessive TCR/CD3 signaling in the absence of a costimulatory "signal 2". We recently developed a PSMAxCD3 bsAb (CC-1) and currently evaluate this bsAbCD3 in patients with metastatic, castration-resistant prostate carcinoma (NCT04104607). While treatment with CC-1 significantly reduced PSA levels in the heavily pretreated patients, the induced T cell activity as well as PSA reduction was rather transient. Using blood samples of our study patients in ex vivo analyses, we here show that T cells loose responsiveness and cytotoxic capacity over time upon exposure to PSMAxCD3 treatment. To overcome this limitation, we generated novel bispecific costimulatory antibodies for tumor-restricted activation of CD28 (BiCos) to sustain the efficacy of bsAbCD3 such as CC-1. A lead construct directed to Endoglin (BiCo-1) was identified based on producibility, minimal aggregation tendency and, most importantly, fully target cell-restricted CD28-stimulatory activity. We demonstrate in various in vitro and in vivo models that BiCo-1 alone exhibits neither therapeutic effects nor toxicity. However, in combination with bsAbCD3 including CC-1, BiCo-1 induced sustained T cell proliferation and memory formation, and profoundly increased the antitumor efficacy of bsAbCD3. Finally, and perhaps most importantly, BiCo-1 also reverted T cell exhaustion induced by CC-1 single agent treatment in patient-derived T cells ex vivo. Taken together, these data demonstrate that the combinatorial use of bsAbCD3 and Bicos may profoundly enhance the sustainability and specificity of therapeutically induced T cell responses against tumors. The GMP-compliant production of BiCo-1 as well as preclinical evaluation in non-human primates has been completed, and a first-in-human clinical trial in prostate cancer patients is will begin recruitment in 2025. Citation Format: Latifa Zekri, Nisha Prakash, Monika Engel, Timo Manz, Stefanie Müller, Jonas Heitmann, Sebastian Hörner, Ilona Hagelstein, Martin Pflügler, Gundram Jung, Helmut R. Salih. Tumor-targeted CD28 bispecific antibodies enhance the antitumor efficacy of CD3-bispecifics and mitigate T cell exhaustion [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 3797.