Improving immune checkpoint blockade efficacy using combination therapy with bioactive molecules

Immune checkpoint blockade (ICB) therapies, such as aPD-1 and aPD-L1, reactivate T cells to fight tumors. While these therapies have transformed cancer treatment, with nearly half of U.S. cancer patients eligible for ICB therapy, their efficacy is limited, with highly variable and generally low response rates. Significant investments aimed at improving ICB response rates have primarily focused on combining therapies with ICB, as demonstrated by the thousands of clinical trials testing anti-PD-1 or anti-PD-L1 with additional agents. However, this significant investment has only resulted in marginal improvements, underscoring a critical unmet need for an innovative, streamlined approach to identify compounds that enhance ICB efficacy. To address this gap, we developed a high-throughput screen (HTS) drug discovery platform that efficiently screens compounds for synergy with ICB. One primary mechanism that limits ICB efficacy is the complex tumor microenvironment (TME), which includes tumor-associated macrophages (TAMs) that take on an immunosuppressive (M2) phenotype and suppress T cell recruitment, proliferation, and function. The abundance of TAMs correlates with poor prognosis in numerous cancers, making TAMs a prime target for compounds that synergize with ICB. To identify potential targets that can reduce TAM immunosuppression, our validated HTS uses an ex vivo co-culture system of bone marrow-derived macrophages and spleen-derived CD4 + and CD8 + T cells. After incubation with small molecule libraries, cultures are evaluated for increased T cell activity; thus, this assay aims to identify small molecules that diminish myelomonocytic cell-dependent T cell suppression and enhance T cell activity when combined with ICB. Using this HTS, we screened over 3270 bioactive small molecules, identifying 128 that showed TAM-modulating activity. Of those 128 compounds, seven synergized with anti-PD1 and six with anti-PD-L1. One compound synergized with both anti-PD-1 and anti-PD-L1. These compounds were evaluated in vivo in syngeneic murine breast cancer models to assess anti-tumor effects. Indeed, combining these agents with anti-PD-1/anti-PD-L1 significantly slowed tumor growth in triple-negative breast cancer (TNBC) models (EMT6), which are generally resistant to anti-PD-1/anti-PD-L1 monotherapy. By tackling the challenges posed by the TME and immune evasion, in a target- and tumor-agnostic manner, our platform has the potential to uncover novel, transformative ICB combination therapies with broad translational potential across multiple malignancies.