Abstract 3473: High throughput screening of combination therapies to improve response to checkpoint inhibitors in solid tumors using a myelomonocytic and T cell co-culture assay
Authors: Kumar Sushil , Tailor Dhanir , Dheeraj Arpit , Li Wenqi , Lee Jee Min , Stefan Kirsten , Nelson Dylan , Kummar Shivaani , Coussens Lisa M. , Malhotra Sanjay V. 
Published: April 2023

Abstract
Background: Immune checkpoint inhibitors (ICI) targeting the PD-1/PD-L1 pathway are part of the new “standard of care” treatment for a number of advanced cancers. However, for many types of cancer, targeting either PD-1 or PD-L1 is associated with clinical benefit in a minority of patients. Combination strategies are needed to improve clinical benefit rates in ‘immune responsive’ cancers, and to expand treatment options for non-responsive cancers. Preclinical models have revealed that tumor-associated macrophages (TAMs) possess T cell-suppressive activities associated with resistance to PD-1/PD-L1 targeted therapies. To reveal therapeutic targets to relieve this suppression, we developed a robust cell-based high-throughput screening (HTS) assay to identify small molecules that can relieve myelomonocytic cell-dependent T cell-suppression, and that enhance T cell activities when combined with PD-1/PD-L1 ICI.
Methods: Bone marrow-derived myelomonocytic cells and splenic T cells were used to develop the HTS assay. After exposure to libraries of small molecules, co-cultures were evaluated for enhanced T cell activity, followed by in vivo investigations of emerging small molecules in syngeneic murine models of breast cancer to reveal anti-tumor activity.
Results: Over 1,400 FDA-approved drugs, used for both cancer or non-cancer indications, were screened in the HTS assay, 66 of which demonstrated activity in the HTS assay. Approximately 50% of these were known anti-inflammatory agents, with the highest density reflecting various COX-2 inhibitors (COX-2i). In vivo, combination therapy of the COX-2i with PD-1/PD-L1 ICI resulted in a significant reduction in tumor growth kinetics in both 4T1 and EMT6 breast cancer models, both of which are resistant to PD-1/PD-L1 inhibitors as monotherapy. In the 4T1 model, slowed tumor kinetics was associated with ICI increased T cell activation as compared to controls or monotherapy anti-PD-1/PD-L1 therapy.
Conclusions: A myelomonocytic cell-dependent T cell-suppression assay can be used to develop a HTS platform for discovering combination therapies showing antitumor responses in preclinical models. COX-2 inhibitors can be repurposed to enhance therapeutic benefits of PD-1/PD-L1 targeting agents in breast cancer patients.
Keywords: Myelomonocytic cells, T-cell, immunotherapy, PD-1, PD-L1, breast cancer
Citation Format: Sushil Kumar, Dhanir Tailor, Arpit Dheeraj, Wenqi Li, Jee Min Lee, Kirsten Stefan, Dylan Nelson, Shivaani Kummar, Lisa M. Coussens, Sanjay V. Malhotra. High throughput screening of combination therapies to improve response to checkpoint inhibitors in solid tumors using a myelomonocytic and T cell co-culture assay [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 3473.