Abstract A021: Pre and post treatment transcriptomic analysis provide new insights on the mechanisms underlying the efficacy of PARPi and Androgen blockage in prostate cancer
Authors: Reis Ana Filipa Palma dos , Milne-Clark Toby , Amin Amit Dipak , Nazlamova Liliya , Pacey Simon , Dev Harveer 
Published: January 2024

Abstract
Aim: Recent phase 3 trials (PROPEL, TALAPRO2) have demonstrated survival benefits when combining PARP inhibitors (PARPi) with androgen pathway inhibitors (API) in prostate cancer (PC) treatment. Importantly, this benefit is observed regardless of homologous repair deficiency (HRD) status, suggesting alternative underlying mechanisms beyond synthetic lethality. However, the complete identification of all the PC patients who can benefit from this combination therapy remains an unmet need. Therefore, further investigation into the distinct molecular pathways involved and identification of biomarkers of response is critical and is the aim of this study. Methods: We have performed an exploratory transcriptomic analysis including RNA-seq, differential expression analysis and gene set enrichment analyses (GSEA) of pre and post treatment biopsies of localized PC patients (pts) with higher risk of relapse and that were included in a window of opportunity (WoO) study. We have included pts recruited to the CANCAP-3 study that were exposed to either two weeks of olaparib alone or in combination with degarelix (1:1 randomization). A corresponding transcriptomic analysis of pre and post enzalutamide biopsies of patients recruited to the WoO study DARANA was also included. Results: GSEA results have shown p53 hallmark upregulation more pronounced in the combination cohort. In the combination cohort the androgen response genes were downregulated. Additionally, canonical cell cycle progression hallmarks (E2F and G2M checkpoint) were suppressed with a significant downregulation of the Prolaris signature. In the olaparib monotherapy cohort pts with the greatest PSA decline demonstrated large reductions in the Prolaris score. Furthermore, in the olaparib monotherapy cohort the two patients that relapsed within 4 years of follow up demonstrated greater Prolaris scores. Interestingly, a significant positive correlation between HR deficiency signatures and reduced prostate cancer proliferation index was observed across the study cohorts, although there was only minimal overlap in the genes constituting these two signatures. This correlation was maintained excluding HRD pts (n=2). Finally, this relationship was also observed following analysis of the DARANA trial. Conclusions: Tissue transcriptomic data analysis of PC pts can provide further insights on the possible biological mechanisms that underlie the effect of this drug combination. In this study the expression of HRD signature appeared to be in direct proportion to the reduction in cellular proliferation (Prolaris). This was seen in the combination treatment cohort, as well as in a separate API monotherapy study (DARANA). This suggests that androgen inhibition is the main driver for the reduction in HR signature. However, this phenomenon may represent a consequence of cell cycle arrest occurring more readily in cells with the greatest HR defect. Further studies are needed to further explain the unique upstream processes that contribute to an AR-supported DNA damage response, in the context of PARP inhibition.
Citation Format: Ana Filipa Palma dos Reis, Toby Milne-Clark, Amit Dipak Amin, Liliya Nazlamova, Simon Pacey, Harveer Dev. Pre and post treatment transcriptomic analysis provide new insights on the mechanisms underlying the efficacy of PARPi and Androgen blockage in prostate cancer [abstract]. In: Proceedings of the AACR Special Conference in Cancer Research: DNA Damage Repair: From Basic Science to Future Clinical Application; 2024 Jan 9-11; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2024;84(1 Suppl):Abstract nr A021.