Abstract 3215: Combination BB2r targeted radiotherapy with PARP inhibitors in human prostate cancer cells: An in vitro evaluation
Authors: Rold Tammy , White Reneise , Okoye Nkemakonam C. , Hoffman Timothy J. 
Published: August 2018

Introduction: Although recent studies employing 177Lu-PSMA beta emitting targeted radiotherapy (TRT) have demonstrated efficacy in treating patients with CRPC, there remains significant opportunities for improvement. Since the TRT mode of action involves DNA damage, inhibiting DNA damage repair with a PARP inhibitor (PARPi) may lead to increased cell death. This pilot in vitro study aims to evaluate the potential synergy of two FDA approved PARPi's, Olaparib and Rucaparib, when used in combination with 177Lu-RM2 TRT to prevent repair of DNA damage caused by TRT in PC3 cells. 177Lu-RM2 TRT, a beta emitting TRT agent targeting the BB2 receptor frequently expressed in CRPC.

Methods: 177Lu-RM2 was prepared via automated radiosynthesis with HPLC QC validation of product purity &gt;90%. PC3 cells cultured overnight were treated with media or 177Lu-RM2 TRT (5uCi to 40uCi/160uL). Cells were incubated with TRT for 4 h prior to addition of fresh media with or without the PARP inhibitors, Olaparib (0.25, 0.5, 1.0, 1.5uM) or Rucaparib (0.5, 1.0, 1.5, 2.0uM). DNA damage was assessed at 24 hrs after TRT addition by enumeration of 53BP1 and gamma H2AX foci. Clonogenic survival was evaluated in cells treated with TRT and Olaparib at 0.25, 0.5, 1.0, 1.5uM as single and combination treatments with media refreshed every 4-7 days. CalcuSyn software (Biosoft™) was used to evaluate synergism.

Results: At 24 hrs after TRT, the addition of either Olaparib or Rucaparib to the cell culture media showed significant increases in DNA damage markers. Combined with TRT dose levels of 10-20uCi/160uL, the addition of Olaparib significantly increased the number of both markers at 0.5-1.25uM as compared to TRT alone. Treatment with Rucaparib resulted in extreme significance (P &lt; 0.0001) being observed in the number of 53BP1 foci at 13/16 drug combinations as compared to TRT alone. In 7/16 drug combinations gamma H2AX foci were noted as extremely significant (P &lt; 0.0001), and the single combination of 40uCi + 1.5uM was noted as very significant (P &lt; 0.005) as compared to TRT alone. Synergy assessment of TRT plus Olaparib showed increasing levels of synergy (synergism to very strong synergism) with increasing dose levels of TRT.

Conclusions: This in vitro investigation supports the use of either Olaparib or Rucaparib in combination with beta emitting TRT to increase PC cell death resulting from beta particle induced DNA damage. Additional in vitro and in vivo preclinical therapeutic evaluation is warranted to support the clinical translation of TRT/PARPi combination therapy.

Acknowledgements: US Veterans Administration (VA) BX001699 and USVA Research Career Scientist (TJH)

Citation Format: Tammy Rold, Reneise White, Nkemakonam C. Okoye, Timothy J. Hoffman. Combination BB2r targeted radiotherapy with PARP inhibitors in human prostate cancer cells: An in vitro evaluation [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 3215.