TY - JOUR T1 - PARP1 as a molecular target for the delivery of theranostic Auger emitters to cancer chromatin JF - Journal of Nuclear Medicine JO - J Nucl Med SP - 59 LP - 59 VL - 59 IS - supplement 1 AU - Mehran MaKvandi AU - Harrison Winters AU - Chi-Chang Weng AU - Laura Puentes AU - Chia-Ju Hsieh AU - David Mankoff AU - Daniel Pryma AU - Robert Mach Y1 - 2018/05/01 UR - http://jnm.snmjournals.org/content/59/supplement_1/59.abstract N2 - 59Objectives: Auger emitting radionuclides possess favorable therapeutic properties when targeted to sub-cellular regions that are in close proximity to DNA. Poly(ADP-ribose) Polymerase (PARP) 1 is the second highest expressed nuclear protein after histones and has DNA and chromatin binding properties. In this work, we have investigated the theranostic potential of a small molecule PARP inhibitor, KX1, radiolabeled with iodine-125 ([125I]) and iodine-123 ([123I]). We hypothesized the sub-cellular location of PARP-1 would provide a direct target for delivering Auger emitters to DNA and allow imaging by single-photon emission tomography (SPECT) as a theranostic agent. Methods: We mediated the deletion of PARP1 using CRISPR/Cas9 in a DNA repair deficient ovarian cancer cell line to test the specificity of [123/125I]KX1 induced DNA damage. Next, using immunofluorescent cell microscopy we tested if [123/125I]KX1 caused a dose dependent increase in DNA damage markers γH2AX and 53BP1 in wildtype vs. PARP1 knockout (KO) cells. In addition, we tested whether [123/125I]KX1 altered the synthesis of Poly(ADP-ribose) (PAR), a marker of PARP-1 enzymatic activity. We then evaluated the therapeutic activity of [123/125I]KX1 in vitro using two DNA repair deficient ovarian cancer cell lines with 4 isogenic matches that were PARP1 KO’s or DNA repair proficient. Finally, we performed SPECT imaging and ex vivo autoradiography with correlative histology to test the feasibility of [123I]KX1 as a theranostic. Results: PARP1 was successfully deleted in OVCAR8 cells and was used as a negative control to test the specificity of [123/125I]KX1 induced DNA damage. We found PARP1 KO cells showed a significant reduction in DNA damage compared to wildtype cells after treatment with [123/125I]KX1. In addition, we showed that [123/125I]KX1 did not effect PAR demonstrating that the DNA damage was independent from catalytic inhibition. In vitro, we observed similar effects with reduced sensitivity to [125I]KX1 in PARP1 KO cells. Sensitivity to [125I]KX1 was also dependent on DNA repair deficiency and we showed that restoration of DNA repair blunts the therapeutic effect. SPECT imaging showed good agreement with ex vivo autoradiography and similarly DNA damage measured in tumors was elevated compared to muscle. Conclusions: We have demonstrated PARP-1 can serve as a feasible target for the delivery of Auger emitters as theranostics. We showed that [123/125I]KX1 induces DNA damage that is PARP-1 specific and independent of catalytic inhibition. This proves [123/125I]KX1 induces DNA damage through Auger electrons without altering physiological processes therefore maintaining the radiotracer principle. Future studies are underway to evaluate the potential as a theranostic in vivo using pre-clinical models of ovarian cancer. ER -