Targeting highly aggressive ductal prostate tumours with poly ADP ribose polymerase inhibitor (PARPi) and androgen receptor signaling inhibitor (ARSi) combination therapy.

420 Background: Prostatic ductal adenocarcinoma (DAC) is an aggressive prostate cancer variant, prone to early metastasis at low PSA levels and showing poor response to androgen blockade despite androgen receptor expression. Although DACs do not have any efficacious therapies, DACs frequently harbour DNA damage repair (DDR) mutations. We investigate the efficacy of combined PARP inhibitor (PARPi) and androgen signalling inhibitor (ARSi) therapy in DDR-proficient DAC tumours. Methods: To model DAC, organoids were developed in Matrigel from patient-derived xenografts (PDXs) originating from DDR-proficient radical prostatectomy (287R, 275R) and BRCA2 heterozygous mutant metastatic (201.1) tumours, retaining key histologic and genomic features. These organoids were exposed to different PARP inhibitors (Talazoparib, Saruparib) and androgen signalling inhibitors (Enzalutamide, Apalutamide, Darolutamide) for up to 11 days. Cell viability and growth responses were assessed using PrestoBlue and CellTiter-Glo assays and automated imaging analysis. SynergyFinder software calculated synergy scores for each treatment combination. Results were further validated in vivo using DDR-proficient 287R PDXs. Mechanistic insights were explored through RNA sequencing of 20 DAC and ten acinar radical prostatectomy samples, and four matched DAC and acinar PDXs, with γH2AX immunohistochemistry to examine DNA damage response. Results: Overall, PARPi/ARSi combination treatment significantly reduced organoid viability compared to PARPi alone or ARSi in DDR-proficient and heterozygous BRCA2 -mutant DAC tumours. The levels of synergy were comparable regardless of which PARPi and ARSi agents were combined. In vivo results using DDR-proficient PDX 287R confirmed the efficacy of PARPi + ARSi combination by reducing tumour volume by 58% compared to control ( p =0.0198) and by 40% versus PARPi alone ( p = 0.0326). Mechanistically, RNA sequencing demonstrated upregulation of multiple DDR pathways in DACs compared to acinar prostate tumours, regardless of the DDR status. After treatment with PARPi + ARSI combination, there was an increase in γH2AX compared to the control in the DDR-proficient PDX 287R (mean 1.42 vs 0.5, p=0.0056), suggesting enhanced DNA damage may contribute to the efficacy of the combination treatment. Conclusions: Our results show that PARPi increases the efficacy of ARSi in DAC. This is notable given the poor response of DAC to AR-directed treatments and provides the rationale for a pre-planned phase 1/2 study.