Abstract 4078: Acquired resistance to PARP inhibitors evolves from drug-tolerant cells vulnerable to AsiDNA

Purpose: PARP inhibitors (PARPi) resistance remains a major clinical hurdle as virtually all patients treated with PARPi will eventually relapse. We already demonstrated that co treatment with AsiDNA abrogates and reverses PARPi-acquired resistance. However, the mechanisms by which AsiDNA prevents genetic adaptations driving resistance and allows long-term efficacy of PARPi remain unknown. Here, we describe that PARPi resistance occurs through emergence of drug-tolerant cells (DTC) preventing PARPi from achieving long-term efficacy. Emerging evidence implicates a survival of residual DTC that constitutes a reservoir from which drug-resistant proliferative cells may emerge. We demonstrate in this study that AsiDNA is able to target specifically DTC and therefore represents a therapeutic opportunity to impede tumor relapse under PARPi treatment. Experimental design: AsiDNA is a double stranded (DS) DNA molecule (decoy oligonucleotide) that mimics DS DNA breaks to interfere with DNA repair, by over-activating a false DNA damage signaling through DNA-PK and PARP enzymes. We used different treatment protocols, cyclic or continuous, to select resistance to the PARPi Olaparib and Talazoparib and assessed the impact of AsiDNA addition on resistance abrogation, durability and irreversibility. We also addressed the mechanisms underlying the survival and evolution of the residual DTC under PARPi treatment, and how AsiDNA could impede their outgrowth. Results: Long-term treatment of BRCA2-mutated cells initially highly sensitive to PARPi leads to acquired resistance in all independently treated populations. Addition of AsiDNA to PARPi completely and irreversibly abolishes resistance emergence. This AsiDNA-dependent resistance abrogation is maintained for at least twelve weeks after AsiDNA removal (nine weeks after treatment start), indicating that AsiDNA maintains PARPi efficacy. PARPi-acquired resistance does not occur through selection of pre-existing BRCA2-wt and/or HR proficient sub-clones, but rather via (epi)-genetic evolution toward an active HR pathway and high genetic instability. We hypothesize that PARPi resistance occurs through a DTC stage as it is described for other targeted therapies like tyrosine kinase inhibitors (TKi). DTC are characterized by a transient senescent phenotype and evolve progressively into proliferative state. We showed that repeated treatments with PARPi lead to the apparition of a DTC-like population displaying a senescent phenotype evolving over time into proliferative cells. The co-treatment with AsiDNA and PARPi prevents DTC regrowth. Conclusion: Several groups have demonstrated that resistance to TKi can emerge from DTC. Our findings provide for the first time the evidence that PARPi-resistant cells can evolve from DTC, and point to the therapeutic opportunity of combining AsiDNA and PARPi to prevent or overcome resistance in clinical situation. Citation Format: Wael Jdey, Olivier Calvayrac, Orlane Maloudi, Florent Ubelmann, Gilles Favre, Francoise Bono. Acquired resistance to PARP inhibitors evolves from drug-tolerant cells vulnerable to AsiDNA [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 4078.