Targeted therapies for the treatment of solid cancers

Abstract

Cancer is the leading cause of the burden of disease and injury in Australia, accounting for approximately 19 percent of the total disease burden. In 2010, the risk of developing cancer before the age of 85 years was 1 in 2 in men and 1 in 3 in women and as such, its burden on society as a whole is immense. In recent years, considerable advances have been made in developing new therapeutics for cancer, particularly therapies targeted to specific molecular subtypes, which have resulted in significantly improved patient survival. However, for some types of solid cancers there have been no significant recent improvements in patient survival or development of targeted therapies. Examples are the triple negative (estrogen and progesterone receptor, and human epidermal growth factor receptor 2 negative) subgroup of breast carcinomas and Ewing sarcomas. Herein, we explore two biological pathways that are frequently dysregulated in these cancers and thus provide opportunity for therapeutic intervention. Section I of this thesis explores the proteasome as a therapeutic target and the potential use of proteasome inhibitors in the treatment of solid cancers and in particular triple negative breast cancers. In general, cancer is associated with increased proteasome activity, and is therefore an attractive target for therapy. Bortezomib, carfilzomib and ixazomib are proteasome inhibitors with FDA approval, but their current use is limited to multiple myeloma and mantle cell lymphoma. Chapter 1 summarizes our current understanding of the structure and function of proteasome variants, their dysregulation in solid cancers, as well as the rationale for 2 proteasome inhibitor based therapy. Chapter 2 assesses the potential for the use of proteasome inhibitors for the treatment of breast cancers. Initially, the proteasome of breast cancer subgroups was characterized in detail by analysis of the The Cancer Genome Atlas breast cancer RNA-sequencing database. Analysis of these data revealed biologically meaningful insights which were subsequently confirmed by experiments in breast cancer cell lines. These results revealed that patients with basal-like and HER2+ breast cancer subgroups express significantly higher levels of the immuno-proteasome variant compared to luminal subgroups. Based on the presented data, it was concluded that this subgroup of breast cancers are likely to respond to proteasome inhibitor based therapy. Proteasome inhibitors in clinical use have several shortcomings including side-effects and lack of efficacy for treatment of solid cancers. In chapter 3, 4 and 5, by exploiting a cross-disciplinary collaboration with Chemistry, we explore the efficacy of new synthesized compounds which can inhibit the proteasome and novel strategies to improve efficacy and/or decrease side effects associated with the current clinical used FDA approved proteasome inhibitors. Section II of this thesis explores the potential of new therapeutic approaches for the treatment of Ewing sarcoma. Chapter 6 evaluates the potential of exploiting the p53 pathway as a targeted therapy for Ewing sarcoma. p53 is a critical tumour suppressor that is involved in a multitude of cellular processes including cell cycle regulation and apoptosis. It is frequently deactivated in the majority of cancers by either direct mutation or by up-regulation of its negative regulators MDM2 and MDM4. Ewing sarcoma is atypical among cancer types as the 3 more than 90% of cases retain a wild-type p53 with functionally intact downstream pathways. Chapter 7 and 8 assess two drugs that engage p53 signaling to elicit their anti-tumourigenic effects. Chapter 7 explores whether the RNA polymerase I inhibitor, CX-5461, has potential for treatment of Ewing sarcoma. RNA Polymerase I is a cellular enzyme that regulates ribosomal synthesis and thus controls the rate of cellular growth and proliferation. To attain accelerated growth, cancer cells up-regulate RNA Polymerase I activity and therefore it is an attractive therapeutic target. It has previously been shown that inhibition of RNA Polymerase I by CX-5461 causes cell death or cell cycle arrest in a variety of cancers both in vitro and in vivo in a p53-dependent manner. Our results reveal that Ewing sarcoma cell lines are acutely sensitive to RNA Polymerase I inhibition by CX-5461, with cell lines with wild-type p53 exhibiting cytotoxic LD50 values in low nanomolar figures (<3nM). This study provides encouraging pre-clinical results for the application of CX-5461 for Ewing sarcoma treatment and warrants further in vivo evaluation. Chapter 8 assesses the potential of XI-006, a pharmacological inhibitor of MDM2, a p53 antagonist, for the treatment of Ewing sarcoma. While results were not consistent with targeting the activity of p53, XI-006 was found to be a potent inducer of apoptosis in Ewing sarcoma.