Abstract 2099: PELP1 inhibition enhances the therapeutic efficacy of topoisomerase inhibitors in triple-negative breast cancer

Abstract BACKGROUND: Triple negative breast cancers (TNBCs) account for 15-24% of breast cancer deaths and are characterized by aggressive clinical course, a worse prognosis, and a higher propensity to metastasize. Proline, glutamic acid-, and leucine-rich protein 1 (PELP1) is an oncogene, its expression is deregulated in TNBC, and its status is a prognostic indicator of poor TNBC survival. Recently, a small molecule inhibitor of PELP1 (SMIP34) which binds to and degrades PELP1 was developed. The Objective of this proposal is to define the mechanism of action of PELP1 inhibitor SMIP34 in TNBC progression and to pave the path for a novel combination therapy. METHODS: We tested the effectiveness of PELP1 inhibitor, SMIP-34, in combination with 140 FDA-approved medications on cell survival of multiple TNBC cell lines. The effect of SMIP34 combination therapy on cell survival was measured using MTT, colony formation, and Annexin V apoptosis assays. Mechanistic studies were done using RT-qPCR, immunoprecipitation, proximity ligation, mass spectrometry, gene reporter, immunofluorescence, and comet assays. Furthermore, the preclinical utility of combination therapy was evaluated using MDA-MB-231 xenografts, patient-derived organoids (PDOs) and explants (PDEs). RESULTS: We performed an in vitro screening of 140 FDA-approved drugs in combination with SMIP34. Results revealed that SMIP34 sensitized TNBC cells to five FDA approved drugs. Of the five drugs, the three drugs Gemcitabine, Valrubicin, and Mitoxantrone, induced DNA damage by inhibiting topoisomerase, a protein that cleaves and reconnects DNA during replication. We validated the ability of SMIP34 to enhance the efficacy of TIs using two additional TNBC cell lines and confirmed their synergistic activity using several in vitro assays including cell viability, colony formation, and apoptosis. In agreement with the role of PELP1 in DNA damage response, Western blotting analyses showed higher levels of γ-H2AX in SMIP34+TI combination therapy when compared to monotherapy. Results from the comet assays further demonstrate increased DNA damage caused by the SMIP-34 + TIs combination therapy. Mechanistic studies using Proximity labelling followed by immunoprecipitation identified topoisomerase 2A and 2B (TOP2A and TOP2B) as potential interactors of PELP1. Furthermore, gene correlation analysis using TCGA datasets revealed that PELP1 expression is positively correlated with TOP2A, and TOP2B in TNBC. SMIP34 plus Mitoxantrone (TI) combination treatment significantly reduced the growth of TNBC PDEXs, and PDOs in vitro, and MDA-MB-231 xenograft tumor growth in vivo, compared to vehicle or monotherapy treatment. CONCLUSION: Together, these results suggest a novel targeted therapy for the treatment of TNBC, involving the combination of topoisomerase inhibitors and the PELP1 inhibitor SMIP34. Citation Format: Khaled Mohamed Nassar, John R. Sanchez, Durga Meenakshi Panneerdoss, Behnam Ebrahimi, Yang Xue, Uday P. Pratap, Salvador C. Alejo, Gangadhara R. Sareddy, Suryavathi Viswanadhapalli, Ratna K. Vadlamudi. PELP1 inhibition enhances the therapeutic efficacy of topoisomerase inhibitors in triple-negative breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 2099.