Tumour heterogeneity during the progression of metastatic breast cancer and anti-tumour effects of the novel FAK inhibitor BI 853520 in breast cancer

Abstract

Over the past decades, reasonable progress has been made in the understanding of breast cancer biology and the treatment of the primary tumour. However, the molecular contribution of multiple cancer cell clones on the various steps of tumour progression is still poorly understood. Aside from that, standard of care treatments, like the chemotherapeutic reagents cyclophosphamide and docetaxel, are rarely able to cure breast cancer patients and the overall survival rates for metastatic disease remain poor. Only in 2018, more than 620,000 women lost their live to breast cancer, mostly due to the presence of tumour heterogeneity, an emerging drug resistance and the formation of secondary lesions. This exemplifies the unmet medical need to an in-depth understanding of tumour heterogeneity during the progression of metastatic breast cancer to finally develop new targeted therapies for this presently incurable disease. The first project has aimed to assess clonal heterogeneity during tumour progression using the MMTV-PyMT mouse model of metastatic breast cancer expressing the Confetti lineage reporter. For this purpose, mammary epithelial cells have been induced to express one of the four Confetti reporter fluorescent proteins. The outgrowth of clonal cell populations has been analysed when the maximum tumour volume comprising all stages (normal, hyperplasia, adenoma, carcinoma, pulmonary metastases) had been reached. The Confetti lineage tracing system initially visualized the emergence of clonal heterogeneity, which culminated in clonal restriction during carcinogenesis and pointed towards a polychromatic metastatic spread. Laser capture microdissection, RNA sequencing and comparative gene expression analysis of various clonal lesions indicated a substantial level of heterogeneity across and also within the various stages of tumour progression. This intra-stage tumour heterogeneity manifested by differences in proliferation, oxidative phosphorylation and cell death and could also be observed in human breast cancer biopsies. This novel understanding of clonal variation and intra-stage heterogeneity needs to be implemented in diagnosis and therapeutic options. In the past years, rising efforts have been made to develop agents targeting molecules and signalling pathways that are specifically present in breast cancer cells. Previous studies have linked an overexpression of focal adhesion kinase (FAK) – a cytoplasmic tyrosine kinase – with the initiation and progression of a wide variety of malignancies, including breast cancer. This correlation of FAK and cancer, together with its role in cell migration, invasion, and proliferation, propose FAK as an attractive target for cancer therapy. In collaboration with a pharmaceutical company, we have assessed and characterized the therapeutic potential and the biological effects of BI 853520, a novel, potent and selective small chemical inhibitor of FAK, in vitro and in several preclinical mouse models of breast cancer. We observed a significant reduction in primary tumour growth driven by an anti-proliferative effect of BI 853520. In contrast, dissecting its influence on metastasis revealed heterogenous effects at different levels of the metastatic cascade. Hence, manipulation of FAK activity with the novel FAK-inhibitor BI 853520 offers a promising anti-tumour approach for breast cancer therapy. In summary, my Ph.D. work delivered new insights into: - The existence of an intra-stage tumour heterogeneity, which is conferred by clonal variations in proliferation, oxidative phosphorylation and cell death. This novel understanding of an intra-stage heterogeneity could have a significant impact on a patient’s diagnosis and therapeutic response and should be implemented in clinical decision-making. - The therapeutic potential and biological effects of the novel FAK-inhibitor BI 853520 in vitro and in preclinical mouse models of breast cancer. This highlighted BI 853520 as a promising anti-proliferative approach for cancer therapy.