Abstract 3056: Alpha particle radiobiology and the treatment of prostate adenocarcinoma
Authors: Mello Barbara P. , Revskaya Ekaterina , Scheinberg David A. , McDevitt Michael R. 
Published: July 2021

Background: Castration-resistant prostate cancer (CRPC) is treated with the androgen receptor (AR) antagonist Enzalutamide (ENZA). However, early positive outcomes are later reversed by acquired resistance, ultimately leading to lethal AR-independent disease progression. High linear energy transfer (LET) alpha particle emitting drugs targeting Prostate Specific Membrane Antigen (e.g., [Actinium-225]-PSMA617) are potent and specific, yielding radiological and biomarker disease regression, but are not curative. DNA damage responses (DDR) in irradiated cells repair double strand breaks (DSB) allowing CRPC to escape. We are exploring the pharmacological effect of AR signaling on alpha particle therapy. AR signaling has a significant role in DDR and a better understanding of the alpha radiobiology is necessary to better deploy these potent drugs in combination.

Methods: Alpha particle-induced DDR was investigated using readouts of clonogenic survival, cell cycle, apoptosis, and gene expression in AR-positive LNCaP-AR and AR-negative DU145 cells that were also treated with ENZA and an ATM inhibitor (ATMi, KU59403). Briefly, cells were seeded on mylar, treated with ENZA and ATMi and subsequently irradiated with alpha particles. Clonogenic survival was quantified at 10 days post alpha-irradiation. DNA damage was confirmed by imaging gamma-H2AX foci. Flow cytometry was used to analyze cell cycle arrest and apoptosis. Relative gene expression was quantified from RNA extracted immediately after and 10 days post-irradiation via RT-qPCR.

Results: We observed the formation of increased DNA DSB (gamma-H2AX foci) versus controls after irradiation. ENZA radiosensitized LNCaP-AR versus untreated cells as evidenced by the D0 values obtained from clonogenic assays. However, combination of ENZA and alpha particles also upregulated expression of DDR genes. LNCaP-AR were in G2 arrest with increased apoptosis following irradiation. An ATMi was added to the regimen to address the upregulated DDR and increased apoptosis was observed. Altogether, these results indicate that cells are radiosensitized by ENZA but concomitant activation of DDR requires ATMi to improve therapy.

Conclusion: DNA damage caused by alpha particles alone does not eradicate CRPC because some cells can repair DSB. Cell control from ENZA treatment alone is transient, leading to development of acquired resistance and disease relapse. Combining alpha particles and ENZA radiosensitizes AR-dependent prostate cancer cells, but DDR rescues a fraction of cells. Inhibiting the ATM pathway in AR-antagonized cells, in combination with high LET alpha irradiation, is a better approach to overcome prostate cancer cell resistance mechanisms. Combination ENZA/ATMi/alpha particles is a strategy designed to address the radioresistant subpopulation and could avoid CRPC progression to the more lethal, non-treatable stage of this disease.

Citation Format: Barbara P. Mello, Ekaterina Revskaya, David A. Scheinberg, Michael R. McDevitt. Alpha particle radiobiology and the treatment of prostate adenocarcinoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 3056.