Studies on the molecular and functional properties of exosomes in the metastatic prostate cancer bone microenvironment

Abstract

Prostate cancer is the most common cancer type and the second leading cancer related cause of death worldwide in men. Prostate cancer patients initially respond to standard treatment (e.g., hormonal, surgery/irradiation) but about 30% of them will develop resistance and progress to metastatic Castration Resistant Prostate Cancer (mCRPC). The cornerstone therapy selection for these patients is chemotherapy (e.g., taxanes). Bone is the most common metastatic site in prostate cancer (mCRPC) and the most frequent cause of death in mCRPC. One of the main bone targeted therapies for patients with bone metastatic CRPC is Radium223, an alpha emitter that has been shown to prolong survival as a single agent. Monitoring the bone tumor microenvironment (bone-TME) is challenging and is based on invasive bone biopsies that cannot be readily performed longitudinally. Liquid biopsies are an attractive approach to monitor the bone-TME including circulating tumor cells, ctDNA and extracellular vesicles (EVs). EVs have a lipid bilayer, contain nucleic acids and proteins. They have been shown to play an important role in homotypic and heterotypic intercellular communication, as well as serve as a source of biomarkers for response and resistance to therapy. In this thesis, the molecular properties of EVs were studied, co-clinically (in vitro, in vivo and in patient samples).

In paper I, we characterized the EV transcriptome changes in response to Radium-223 in vivo and patients’ samples. We identified changes in genes related to bone, DNA repair and immune in both the pre-clinical models and patient samples. Treatment with Radium-223 showed a downregulation of bone related transcripts and an upregulation of DNA repair pathways (pharmacodynamic measure). Furthermore, changes in the immune system that are associated with immunosuppression and immune checkpoint activation were identified in patients with unfavorable overall survival. The data obtained indicate that EVs can detect changes in the bone-TME that were functionalized by combining Radium-223 with immunotherapy that improved treatment efficacy.

In paper II, we characterized the EV transcriptome for patients treated with Cabazitaxel. Pathway and gene enrichment analysis identified several pathways and associated genes that were enriched in patients that did not respond to Cabazitaxel. Furthermore, at baseline EVs derived from the plasma of Non-responders (NR) were enriched in transcripts encoding genes that are related to oncogenesis, cytoskeleton and immune regulation. Two genes identified to be enriched in NR are Stathmin-1 and ITSN-1 both of which have been previously associated with resistance to Cabazitaxel. Further studies are needed to determine whether longitudinal monitoring of these and other genes identified in the EVs correlates with treatment response and clinical outcome.

Taken together, our studies demonstrate that plasma derived EVs could be a useful tool in monitoring the bone TME as well as treatment responses and acquisition of resistance that correlate with clinical outcome.