Abstract P4-01-07: Insights into the compensation mechanism behind targeting PI3K and Wnt in triple-negative breast cancer
Authors: Solzak Jeffrey P , Wang Chao , Radovich Milan 
Published: March 2022

Abstract
Introduction: Triple-negative breast cancer (TNBC) is a devastating disease accompanied with substantial morbidity and mortality with very few tools in the clinical armamentarium to improve outcomes. The PI3K pathway is activated in the majority of TNBCs. However, single-agent inhibition of PI3K has seen limited clinical efficacy. We have previously demonstrated that the Wnt pathway is upregulated in response to certain PI3K inhibitors as a compensatory resistance mechanism. Dual targeting of the PI3K and Wnt pathways leads to a synergistic anti-tumor effect that has been observed preclinically and in clinical trial (NCT03243331). Recently, we have found that several isoform-specific PI3K inhibitors predictably induce PTK7. PTK7 is a tyrosine kinase protein in the noncanonical Wnt pathway found to be overexpressed in various cancers, including TNBC, and is target of an antibody drug conjugate (ADC) currently in clinical trials. Herein, we sought to determine the PI3K isoforms that are responsible for PTK7 induction to help guide rational drug combination design. Methods: Twelve PI3K inhibitors including alpelisib, buparlisib, idelalisib, duvelisib, GSK2636771, GSK2292767, and CAY10505 were tested. Each inhibitor targets different isoforms individually or combinations of PI3K isoforms. Using four TNBC cell lines with different PI3K pathway mutational status and PTK7 expression (MDA-MB-231, MDA-MB-453, MDA-MB-468, and Hs578T), IC50s were initially measured using a viability assay. Using these IC50s, cells were treated in triplicate and RNA was isolated after 72 hours of treatment. RT-PCR was then performed using vehicle controls looking at the change in PTK7 expression. PI3K δ and λ isoform inhibition was also performed using siRNAs individually and in combination for validation of PI3K inhibitors. Protein expression of PI3K isoforms and PTK7 was also observed. Endogenous levels of PTK7 to assess induction after PI3K inhibitor treatment was assessed using both RNA and protein expression. Results: Testing multiple PI3K inhibitors covering various combinations of isoforms, we found that inhibition of PI3K δ and λ or pan-PI3K inhibition caused significant increases in RNA and protein levels of PTK7. We also found that individually targeting α and β in these cell lines did not have any effect on PTK7. We validated this induction further using siRNA inhibition to rule out any off targets of PI3K inhibitors. Additionally, after PI3K inhibition, the fold change in PTK7 expression was observed to be significantly higher in those cells with low or no PTK7 expression, whereas those with high baseline PTK7 expression observed little or no increase, maintaining pre-treatment levels. Conclusion: Refining clinical trial combinations and therapies to optimize drug efficacy while decreasing toxicity is a vital part of precision medicine. Here we have sought to optimize our previous clinical experience of dual targeting of PI3K and PTK7 and found two potential FDA approved therapies that may provide better efficacy and toxicities. Taking the previous trial and this preclinical data, future trials are warranted in TNBC.
Citation Format: Jeffrey P Solzak, Chao Wang, Milan Radovich. Insights into the compensation mechanism behind targeting PI3K and Wnt in triple-negative breast cancer [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr P4-01-07.