Targeting DNA Damage, Apoptosis, and the Cell Cycle for the Radiosensitization of Aggressive Forms of Breast Cancer

Abstract

Breast cancer is the most common invasive cancer diagnosed in women, and there is a critical need to identify novel treatment strategies. To that end, therapies designed to target molecular drivers specific to individual breast cancer subtypes have the potential to improve locoregional disease control, decrease rates of metastasis, and increase overall survival in patients with breast cancer. Thus, we sought to nominate and validate strategies for the radiosensitization of aggressive breast tumors in a subtype-specific manner. By focusing on clinical-grade pharmacological inhibitors of apoptosis, DNA repair, and the cell cycle, we proposed multiple novel preclinical combination therapies for inflammatory breast cancer (IBC), triple negative breast cancer (TNBC) and estrogen receptor positive (ER+) breast cancers. First, we demonstrated that pharmacological PARP1 inhibition using velparib or olaparib radiosensitized IBC models both in vitro and in vivo through the potentiation of DNA strand breaks and a delay in overall DNA repair capacity with the combination treatment. Next, we asked if the use of CDK4/6 inhibitors – which are currently only approved for the treatment of metastatic, ER+ breast cancer – would affect the radiation response when administered concurrently with radiation treatment. We demonstrated that repair of RT-induced DNA damage was suppressed with CDK4/6 inhibition and that radiosensitization occurred in a cell cycle-independent manner with palbociclib, ribociclib, or abemaciclib. In both ER+ and TNBC, efficacy of the combination therapy was predicted by the presence or absence of the retinoblastoma tumor suppressor (RB). Genetic or pharmacologic knockout of RB abrogated CDK4/6 inhibitor-mediated radiosensitization, but re-expression of RB in ER+ and TNBC models was sufficient to restore the radiosensitization phenotype. Combined RT and CDK4/6 inhibition caused clinically relevant levels of radiosensitization in vivo and led to the development of multiple clinical trials (Phase I/II) to test the safety, tolerability, and efficacy of this combination therapy in patients with ER+, RB-intact breast cancer. Finally, because there are very few targeted therapies available for the treatment of TNBC, we sought to nominate additional subtype-specific targets for radiosensitization in radioresistant models of TNBC. In this study, we demonstrated that Bcl-xL inhibition led to radiosensitization in TNBC cell lines with low Mcl-1 expression. While transient Mcl-1 knockdown was sufficient to sensitize radioresistant cell lines to Bcl-xL-mediated radiosensitization, overexpression of Mcl-1 or CRISPR-mediated knockout of the PTEN tumor suppressor induced radioresistance. Together, this data suggests that subtype-specific approaches for the radiosensitization of breast cancer can be an effective way to increase the efficacy of current treatment options for breast cancer patients. In addition, the use of multiple FDA-approved compounds has allowed a number of these approaches to move into early clinical trials for patients with breast cancer, increasing the translational relevance of these findings.