Triple negative breast cancer (TNBC) is an aggressive disease, accounting for 15-20% of breast cancer cases, but disproportionally causing breast cancer-related death. TNBC does not express the targetable receptors ER, PR, and HER2 that are known to drive other forms of breast cancer. Due to lack of targetable driver the mainstay practice for TNBC in the clinic is therefore to treat with non-targeted cytotoxic chemotherapies. Long-term outcomes are however still poor, most likely owing to intrinsic drug resistance. Clinical trials have attempted to explore molecular targeted therapies to block dysregulated growth factor receptors in TNBC, for instance, epidermal growth factor receptor (EGFR), as it is overexpressed in ~80% of TNBC. Yet, considering the long-standing availability of clinically approved EGFR-targeted therapies, such as lapatinib, the clinical benefit of single agent therapy in TNBC is unsatisfactory, likely owing to the compensatory dysregulated signaling pathways. In our completed ERC – Advanced Grant Triple- BC (#322737) we have revealed that an inhibitor targeting cyclin-dependent kinase 9 (CDK9) strikingly synergizes with drugs targeting EGFR receptor family signaling inhibiting cell proliferation of TNBC cell lines and directing these cells towards apoptosis. Therefore, the overall goal of this PoC project was to exploit this novel finding and provide proof-of-concept evidence that stalling CDK9 signaling may enable successful targeted combination therapy in TNBC. Therefore, the specific objective of this PoC was to assess the pharmacological response of dual EGFR-CDK9 targeting in TNBC patient-derived xenograft (PDX) mouse models in vivo. We envisaged that this PoC project will translate our identified successful synergistic treatment into an effective future novel and safe combinatorial targeting strategy benefiting TNBC patients.
Indeed within this PoC project, we have provided additional support for our hypothesis. First, before initiating in vivo studies in mice, we have shown in collaboration with prof. Shudong Wang (Univ. Southern Australia) that the CDK9 inhibitor I-73 is the most optimal CDK9 inhibitor to be combined with lapatinib. Second, we have revealed that this combination therapy is effective not only in TNBC cell lines but also in TNBC organo-typic cultures derived from patient derived xenografts (PDX). Third, we showed that the combination treatment effectively targets TNBC cell cultures not only when these were grown in 2D but also in 3D cultures. Finally, we have provided evidence that this drug combination can also be effective in EGFR dependent cancers beyond TNBC (e.g. in colorectal and lung cancer). With regard to planned in vivo experiments in mice, we have shown that next to both treatments (lapatinib and CDK I-73) as monotherapy their combination treatment is safe in healthy mice. Secondly and importantly, we have determined in mice baring suitable TNBC xenograft models that the combination treatment was effectively inhibiting RNAPOL2 activity the direct target leading to downregulation of downstream target MCL1. This effect was not seen by treatment with either monotherapies. Currently, final in vivo experiments in mice bearing TNBC xenografts are on-going to show that this drug combination is an effective drug combination.
In conclusion, while we still have to complete the final in vivo experiments we can conclude that the PoC has broad this new combination treatment closer to the clinic since we have found that the combination treatment is effective and safe. Moreover we also provide evidence that combining two independent pathways which converge on the p-TEFb complex is effective not only in TNBC but also in other cancer type making this an effective strategy beyond breast cancer.
