Poly(ADP-ribose) polymerase 1 (PARP-1) targeted Auger emitter is highly cytotoxic across a panel of ovarian cancer cell lines

Objectives: Small molecule inhibitors of the nuclear enzyme PARP-1 constitute an emerging class of therapy for ovarian cancer. PARP-1 is also an attractive target for specific delivery of radionuclides directly to DNA (1). Such radiotherapeutic strategy was examined in a panel of ovarian cancer cell lines in vitro using [¹²⁵I]KX1, a novel small molecule PARP-1 inhibitor labeled with the Auger electron-emitting radionuclide iodine-125 (2).

Methods: Seven ovarian cancer cell lines were each treated with various concentrations of [¹²⁵I]KX1, non-radioactive PARP-1 inhibitor talazoparib, and non-DNA-targeting Auger emitter ([¹²⁵I]MIBG) ± equimolar KX1. The survival fraction post-treatment was quantified with ATP bioluminescence assay. For each cell line and treatment condition, the effective drug concentration for 50% cell death (EC₅₀) was calculated using a sigmoidal dose response curve. Dosimetry was then performed on four cell lines based on radiopharmacology and cellular geometry properties specific to each cell line. With Monte Carlo simulation using MIRDCell V2.1 (3), the radiation dose to the cell nucleus at 50% cell death (D₅₀) was calculated.

Results: The EC₅₀ for [¹²⁵I]KX1 for the seven cell lines ranged from 20 pM to 930 pM. [¹²⁵I]KX1 caused cell death at concentrations 10²-10³ times lower than talazoparib and 10¹-10² times lower than [¹²⁵I]MIBG ± equimolar KX1, demonstrating no cell kill by biochemical inhibition of PARP-1 at therapeutic doses. The sensitivity to [¹²⁵I]KX1 was positively correlated with sensitivity to talazoparib (R²=0.64, p<0.05). Compared to EC₅₀, the D₅₀ values for [¹²⁵I]KX1 for the four cell lines were relatively uniform at 1.5 - 4.4 Gy, with 63% less variability than EC₅₀. The reduction in variability was mainly due to differences in target binding rather than cellular geometry. Sensitivity to [¹²⁵I]KX1 was relatively unaffected by BRCA1 status: when BRCA1 was restored in UWB1.289 cell line, D₅₀ and EC₅₀ increased only by a factor of 1.3 and 2.1 respectively (p<0.01).

Conclusions: In ovarian cancer cell lines, delivery of an Auger emitter to the cell nucleus by the radiolabeled PARP-1 inhibitor [¹²⁵I]KX1 results in enhanced cytotoxicity without requiring enzymatic inhibition of PARP-1. Variable sensitivity to [¹²⁵I]KX1 is explained by differential target binding rather than intrinsic radiosensitivity. This promising efficacy in vitro has potential to be examined in vivo in both BRCA mutant and wild-type tumors.