Targeting Triple Negative Breast Cancer with Combinatorial Kinase Inhibitors

Clinical and correlative studies have shown that targeting one signaling pathway alone is inefficient at controlling aggressive cancers, such as Triple Negative Breast Cancer (TNBC), whereas combinatorial strategies are more effective and necessary. While targeted therapies, such as kinase inhibitors are widely used, rewiring of cell signaling post treatment has been shown to reduce efficacy, which demands new methods of inhibiting proto-oncogenic kinases. CDK4/6 is one such kinase that is targeted and used in combination therapy with a high success rate. We have previously demonstrated that, in TNBC cells, co-targeting CDK4/6 and EGFR was synergistic. Thus, we hypothesized that inhibition of CDK4/6 with other kinases can be synergistic for treating TNBC.

As such, we hypothesized that there are other kinases, when inhibited in combination with a CDK4/6 inhibitor would synergize to effectively treat TNBC. To explore targeted combinatorial kinase therapy, we performed a pooled Kinome CRISPR knockout screen in TNBC cells under the treatment of CDK4/6 inhibitor (Abemaciclib).Using The Cancer Genome Atlas (TCGA) database, the hits were then ranked based on the prevalence of up-regulation in TNBC supporting clinical relevance and SRC is among the top six drop-out hits. Inhibition of SRC using Dasatinib, an FDA approved inhibitor for cancer therapy, synergize with Abemaciclib and decreased cell proliferation in a TNBC cell lines.

We also found that treatment with Abemaciclib, and a multi-kinase inhibitor, Lestaurtinib, rewires through the Receptor Tyrosine Kinase receptor (RTK), FAK in multiple TNBC cell lines. Based on this observed rewiring, and the known advantage of using sequential treatment to prevent reemergence of parallel signaling pathways, we theorized that sequential treatment would be more effective than concurrent combination treatment. Sequential treatment with Abemaciclib, Lestaurtinib and Defactinib, a FAK inhibitor, resulted in decreased cell proliferation and an increase in cell death. We demonstrate that sequential treatment induced apoptosis, as indicated by an increase in Cleaved Caspase 3(CC3) and Cleaved PARP. We further show that the induced apoptosis may be mediated by p38, as CC3 and cleaved PARP are increased while phospho-p38(p-p38) decreases under sequential treatment. Further we found that SRC, which is known to interact closely with FAK and is a well-known proto-oncogene, coordinates with FAK in the rewiring activation. Using either a SRC inhibitor, Dasatinib, or Defactinib the sequential treatment showed inhibition of the signaling interaction between FAK and SRC, as well as a decrease in the downstream signaling through p38. In summary, our results show that even with combination treatment, rewiring occurred through an RTK and thus, a sequential treatment scheme is a more effective approach and necessary to prevent reoccurrence and resistance.