Abstract 1486: Pharmacodynamic response to HER2/CD3 bispecific antibody (HER2-TDB): evidence of T-cell recruitment and further rationale for combination treatment with anti-PD-L1

Abstract Based on the recent clinical success of tumor immunotherapies that block immune suppressive mechanisms to restore T cell function, there is a profound interest in the clinical development of T cell targeted therapies. We have produced a trastuzumab-based HER2 T cell dependent bispecific antibody (HER2/CD3; HER2-TDB; Junttila et al Cancer Res 2014, 74:5561) that conditionally activates T cells resulting in lysis of HER2 expressing cancer cells at low picomolar concentrations. In vivo, HER2/CD3 can inhibit growth of established mammary tumors in mice at doses as low as 0,01 mg/kg. Due to its unique mechanism of action, which is unrelated to HER2 signaling or sensitivity to chemotherapeutic agents, HER2/CD3 can eliminate cells refractory to currently approved HER2 therapies and inhibit growth of Kadcyla insensitive mammary tumor models. Treatment with HER2-TDBs also results in tumor regression as a second line treatment for Kadcyla relapsed mammary tumors in mice. Spontaneous mammary tumors arising in MMTV-huHER2 transgenic animals are poorly immunogenic or “immunological deserts” due to their low mutational load and low T cell infiltration. Despite these attributes, HER2/CD3 shows remarkable activity in this model characterized by a robust increase in the number of infiltrating lymphocytes and CD8+ T cells as well as a significant enhancement of tumor CD8+ cytotoxic effector function as measured by increased IFNg production. In addition to inducing T cell proliferation, we further show that HER2/CD3 strongly recruits new T cells from the periphery and this leads to significantly increased T cell numbers in the mammary tumor tissue. Another notable feature of the response to HER2/CD3 treatment is a significant upregulation of PD-1 expression on tumor infiltrating CD8+ T cells. This observation adds a strong rationale for combining HER2/CD3 with anti-PDL1 treatment. Our in vitro experiments further demonstrate that PD-1/PD-L1 signaling can inhibit killing mediated by CD3 bispecific antibodies and that combining two immune therapies - direct polyclonal recruitment of T cell activity together with inhibiting the T cell suppressive PD-1/PD-L1 axis results in enhanced and durable long term responses in vivo. Citation Format: Ji Li, Maria Hristopoulos, Robyn Clark, Jennifer Johnston, Dion Slaga, Bu-Er Wang, Rafael Cubas, Klara Totpal, Melissa R. Junttila, Teemu T. Junttila. Pharmacodynamic response to HER2/CD3 bispecific antibody (HER2-TDB): evidence of T-cell recruitment and further rationale for combination treatment with anti-PD-L1. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 1486.