Targeting PARPi drug tolerant persistence and progression on treatment with clinical stage ATM inhibitors in advanced prostate cancer

Background: PARP inhibitors (PARPi) have improved prostate cancer management, but progression is inevitable. Drug tolerant persistence (DTP) is characterized by tumor cells which survive treatment and drive failure through transient acquisition of insensitivity. Cycling DTP cells may adapt and expand, giving rise to drug tolerant expanded persister (DTEP) populations which are thought to model progression on treatment. Emerging evidence supports a critical role for DTP in promoting PARPi progression. Identifying and targeting DTP and DTEP vulnerabilities may provide therapeutic strategies to combat disease progression. Methods: Viability assays, western blots, and additional assays defined treatment response in PARPi-sensitive C4-2B metastatic castration-resistant prostate cancer cells and C4-2B abiraterone-resistant derivative AbiR cells. DTP and DTEP models were developed through prolonged PARPi exposure. NGS profiled DTP cells. Clinical stage ATM inhibitors were tested for their effects on both DTP and DTEP populations. Results: Response to PARP inhibition is heterogeneous, characterized by cell death and emergence of a largely cytostatic, persistent population. C4-2B and AbiR cells exposed to clinically relevant PARPi dosing for 9 days followed by drug holiday regain normal, parental cell morphology and become re-sensitized to treatment in line with acquisition of a DTP phenotype. DTP cells display differential sensitivity to other classes of drugs compared to parental cells. DTP cells may be broadly stratified into two classes; 1) a minority which cycle and 2) those which don't. Prolonged PARPi treatment is observed to result in drug tolerant expanded persisters (DTEP) derived from cycling-DTP cells. DTP and DTEP cells display increased phospho-ATM levels suggesting constitutive DNA damage response activation. Utilization of clinical stage ATM inhibitors both prevent DTP progression into DTEPs and resensitize DTEPs to PARP inhibition. Conclusions: Our data suggest that drug tolerant persistence may mediate survival of tumor cells which drive progression on treatment. ATM inhibition may be used to prolong time to progression or to treat progressive disease. Future studies will focus on translating these strategies.