Targeting prostate cancer cells with DNA repair inhibitors presents an opportunity to improve outcomes in patients with poor prognosis cancers. There are multiple reasons which rationalise targeting DNA repair in prostate cancer, including the enrichment of DNA repair genetic alterations in men with poor prognosis disease, as well as clinical reports of PARP inhibitor efficacy in patients with and without defective homologous recombination repair defects, when combined with standard-of-care antiandrogen therapies. However, the dominant molecular mechanisms underlying tumour cell responses to DNA repair and antiandrogen combination treatment remain unascertained.

Within this thesis, I review the rationale supporting the clinical utility of DNA repair inhibitors in prostate cancer, as well as the exploration of mechanistic hypotheses behind the combinatorial efficacy of antiandrogen and DNA repair-targeted therapies. Using prostate cancer cell line models, the effect of multiple drug combinations on cell viability was profiled, highlighting synergistic interactions between the AR signalling inhibitor Enzalutamide (Enza) when combined with either PARP inhibitor Olaparib or ATM inhibition (ATMi).

Olaparib and Enzalutamide combination treatment were determined to transcriptionally downregulate cell cycle progression signatures. Cell cycle analysis demonstrated G2/M accumulation, providing indications of delayed cell cycle progression due to DNA damage in the combination treatment.

With regards to ATM inhibition, AR-null prostate cancer cell lines were observed to be highly sensitive to single-agent treatment. In AR-dependent prostate cancer cells, single-agent Enzalutamide or ATMi resulted in mild inductions of yH2AX levels, whereas ATMi and Enzalutamide combination treatment resulted in yH2AX suppression. This indicates an interaction between these therapies which impairs DNA damage response signalling.

A whole genome CRISPR-Cas9 screen was performed to identify candidate genes that may drive sensitivity or resistance to combination treatment strategies. The findings from this work provide insights into the potential for targeting DNA repair in prostate cancer and possible mechanisms underlying the observed synergistic interactions in combination treatments. Novel groups of factors were indicated to drive exceptional sensitivity to each combination treatment. These gene sets were enriched for chromatin remodelling factors, transcriptional regulators and modulators of TGF-β signalling, presenting novel mechanisms which drive DNA repair combination therapy responses in prostate cancer.

Finally, exploratory studies were conducted which leveraged patient tissue samples. This included the characterisation of ex vivo organoid cultures derived from high-risk localised prostate cancers, as well as preliminary tumour transcriptomic analysis for the CaNCaP03 Olaparib vs Olaparib with androgen deprivation window-of-opportunity trial. Transcriptomic analysis of patients treated with these therapies showed alignment with in vitro-based hypotheses. Overall, this work provides novel insight into the mechanistic underpinnings of DNA repair-targeted therapies in prostate cancer and highlights critical cell signalling pathways for further exploratory work.