Purpose: To compare the competing radiosensitizing effects of DNA-repair inhibitors and proton LET in vitro.
Methods: We treated H460 and H1299 non-small cell lung cancer and PANC-1 and Panc 10.05 pancreatic adenocarcinoma cells with varying concentrations (0.1-10 μM) of DNA repair inhibitors (DRis) targeting the DNA repair proteins Rad51 (BO2), DNA-PKcs (NU7441), ATM (KU5593), ATR (ADZ6738) and PARP (AZD2281), or with DMSO vehicle as a control. We irradiated the cells with 6 MV x-rays or 9.9 keV/μm protons (dose-weighted LET in water) and performed clonogenic assays to characterize their survival. To parameterize cell radiosensitivity, we estimated the dose required to achieve 10% survival, D10%. To quantify drug sensitization at a given radiation quality, Q, we calculated the sensitization enhancement ratio, SER(Q)=D10%(DMSO,Q)/D10%(drug,Q). To quantify the proton LET effect, we calculated the RBE for each drug combination as: RBE=D10%(drug,x-rays)/D10%(drug,protons). To quantify the combined sensitizing effect of protons and drugs, we calculated the total radiosensitivity enhancement, TRE=D10%(DMSO,x-rays)/D10%(drug,protons).
Results: The SERs varied greatly across drug combinations and cell lines, ranging from 1.00±0.02 (H460, 5 μM BO2, x-rays) to 3.23±0.13 (H1299, 1 μM ADZ6738, x-rays). Between radiation qualities, the SERs were positively correlated (r=0.8807), suggesting that DRis that are effective for x-rays are similarly effective for protons. However, the RBE values were negatively correlated with SER(x-ray) (r=-0.6160), suggesting that the greater the DRi-induced sensitization, the less benefit there will be from the proton LET effect. Nevertheless, the TREs from combining DRis with protons were generally larger than SER(x-ray) values (30±3% on average), and were positively correlated with SER(x-ray) (r=0.8911), suggesting that even when DRis reduce proton RBE, there is still a net radiosensitivity gain when combining them with protons.
Conclusion: Although protons+DRi may reduce proton RBE, the DRi-induced sensitization outweighs the RBE loss, resulting in a substantial net sensitization beyond that achieved with DRi+photons.
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.