Enhancing Effects of Olaparib by Alpha- and Beta-Emitting Radionuclides, X-Rays, and Ultraviolet A Light in Combination with Ortho-IodoHoechst in a Prostate Cancer Cell Model.

New therapeutic strategies for metastatic castration-resistant prostate cancer (mCRPC) have been developed in the past to achieve the best response rates. Most recently, the use of combination therapies has been explored to optimize patient outcomes. Poly(ADP-ribose) polymerase inhibitors (PARPi) may help to treat mCRPC more effectively. This study aimed to determine whether the combination of a PARPi with different radiation qualities results in different levels of radiosensitization of PC-3 cells. The radiosensitizing potential of Olaparib in combination with Lu, Ra, X-rays and photodynamic therapy (PDT) using the UVA light-activated photosensitizer ortho-iodoHoechst33258 (oIH) was evaluated by determining the clonogenic survival, DNA damage and cell cycle analysis. Here, we show that this combination strategy differentially sensitized PC-3 cells to different radiation qualities. The combination of Lu with Olaparib increased the numbers of persistent double-strand breaks (DSBs) by a factor of 3.3 and cell death in PC-3 cells. Overall, the β-emitter Lu indicated a higher radiosensitization efficacy compared to Ra, with X-rays corresponding to dose modification factors (DMF) of 1.77, 1.17 and 1.16 respectively. Even in the case of the α-emitter Ra, the effects were much less pronounced than for Lu. PARPi also showed a slight potentiation of the cytotoxic effects both in co-treatment with X-rays and with PDT. The results of our study indicate a potential role for Olaparib in further optimizing the PSMA radioligand therapy (PRLT) outcomes. However, further evaluation of the combination of PARPi with PRLT is needed to gain more insights into improving the benefit to patients suffering from mCRPC.