Purpose: Due to their higher ionization density compared to photons, protons induce more clustered DNA lesions including clustered double strand breaks (DSBs), clustered single strand breaks (SSBs) and clustered base damages (BDs). Clustered SSBs and BDs can be converted into DSBs during attempted repair, increasing the number of DSBs. If not properly repaired, DSBs can generate micronuclei (MN), which are displaced chromosomes or chromosome fragments enclosed by the nuclear membrane and are known precursors of immune activation. Here we show that protons+PARP inhibitor (PARPi) sensitize cancer cells more than photons+PARPi, and that this combination might also induce antitumor immune signaling.
Methods: We obtained clonogenic cell survival for H460, H1299, PANC-1 and Panc 10.05 cell lines to quantify the effect of a PARPi on radiosensitization. We used immunohistochemistry in H1299, PANC-1, HCC1937 (BRCA mut) and HCC1937 (BRCA complemented) cell lines to measure MN and cGAS-positive MN (MN-cGAS+) numbers to quantify antitumor immune signaling at 24 and 72 h post-radiation. Cells were treated with 6 MV x-rays at 10 cm water equivalent depth or 100 MeV unmodulated protons at 4.3 water equivalent depth (9.9 keV/μm dose-weighted LET in water) alone or with a PARPi (0.1 or 1 μM, Olaparib).
Results: PARPi radiosensitizes multiple cell lines to both photons and protons. Relative to photons alone, protons+PARPi provided sensitizations of 1.70±0.04 (H460), 1.94±0.06 (H1299), 1.93±0.05 (PANC-1) and 1.70±0.09 (Panc 10.05). PARPi resulted in a modest increase (5%) or a significant decrease (-10%) in RBE. MN and MN-cGAS⁺ were increase by both photons and protons. However, protons+PARPi further amplified MN-cGAS⁺ compared to all other treatment combinations.
Conclusion: Protons+PARPi increase cell death to levels that are only achievable with carbon ions. Protons+PARPi also amplify precursors associated with antitumor immune signaling. Additional investigation is warranted to evaluate if these effects are seen in vivo.
Funding Support, Disclosures, and Conflict of Interest: This research was supported in part by RP170040, 1R21CA252411-01, Emerson Collective, and MD Anderson Cancer Center. Dr. Sawakuchi has research agreements with Alpha Tau Medical and Artios Pharma. Dr. Shaitelman has a research agreement with Exact Sciences.
Not Applicable / None Entered.