PARP-1–Targeted Radiotherapy in Mouse Models of Glioblastoma

Abstract

The DNA repair enzyme poly(ADP-ribose) polymerase 1 (PARP-1) is overexpressed in glioblastoma, with overall low expression in healthy brain tissue. Paired with the availability of specific small molecule inhibitors, PARP-1 is a near-ideal target to develop novel radiotherapeutics to induce DNA damage and apoptosis in cancer cells, while sparing healthy brain tissue. We synthesized anI-labeled PARP-1 therapeutic and investigated its pharmacology in vitro and in vivo. A subcutaneous tumor model was used to quantify retention times and therapeutic efficacy. A potential clinical scenario, intratumoral convection-enhanced delivery, was mimicked using an orthotopic glioblastoma model combined with an implanted osmotic pump system to study local administration of I-PARPi (PARPi is PARP inhibitor).I-PARPi is a 1(2H)-phthalazinone, similar in structure to the Food and Drug Administration-approved PARP inhibitor AZD-2281. In vitro studies have shown that I-PARPi and AZD-2281 share similar pharmacologic profiles.I-PARPi delivered 134.1 cGy/MBq intratumoral injected activity. Doses to nontarget tissues, including liver and kidney, were significantly lower. Radiation damage and cell death in treated tumors were shown by p53 activation in U87-MG cells transfected with a p53-bioluminescent reporter. Treated mice showed significantly longer survival than mice receiving vehicle (29 vs. 22 d, < 0.005) in a subcutaneous model. Convection-enhanced delivery demonstrated efficient retention of I-PARPi in orthotopic brain tumors, while quickly clearing from healthy brain tissue. Our results demonstrate I-PARPi's high potential as a therapeutic and highlight PARP's relevance as a target forradionuclide therapy. Radiation plays an integral role in brain tumor therapy, and radiolabeled PARP therapeutics could ultimately lead to improvements in the standard of care.

Targeted brain tumor radiotherapy using an Auger emitter

“…Na-123 I was used as a control and showed significantly lower uptake, Vmax = 2.5 ± 0.1. The two dominant targets for 127 I-PARPi are PARP1 and PARP2 (26), similar to what has been previously reported for Olaparib and other modified PARP inhibitors (15).…”

“…Pharmacological properties determined with 127 I-PARPi suggest that 123 I-MAPi ( fig. 1B, S1B), retains the same properties as 131 I-PARPi, which have been shown to be similar to the FDAapproved PARP inhibitor Olaparib (26,34).…”

“…ALZET® Osmotic Pumps were implanted subcutaneously into the mice to slowly deliver an infusion into the brain using the same coordinates as for the tumor cell injection as previously described (26…”

“…This central role has been successfully leveraged for the development of various PARP inhibitors, both as a monotherapy and in combination with other therapeutics (17,18). Modified PARP inhibitors have also been widely used for tumor detection and imaging due to their cancer specificity and their high target-to-background contrast (14,15,(19)(20)(21)(22)(23)(24)(25); more recently, they have found theranostic applications (26).…”

“…PARP1 primary antibody (Santa Cruz #sc-7150, 0.2 μg/mL) and goat anti-rabbit IgG-HRP secondary antibody (1:10000 dilution, sc-2004, SantaCruz). An anti-β-actin antibody (Sigma #A3854, 1:1000) was used as loading control.Synthesis of 123 I-MAPi123 I-MAPi was obtained similar to synthetic procedures reported before(26). Firstly, 2,5dioxopyrrolidin-1-yl 3-(iodo-123 I)benzoate was obtained by adding N-succinimidyl-3-(tributylstannyl) benzoate (250 μg, 0.5 μmol) in 10 μL of AcN to a solution containing methanol (40 μL), chloramine T (9 μg, 32 nmol), acetic acid (3 μL) and 123 I-NaI in NaOH 0.1 M (2.5 mCi).…”