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. Methods: We synthesized an ¹³¹I-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 ¹³¹I-PARPi (PARPi is PARP inhibitor). Results:¹³¹I-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 ¹³¹I-PARPi and AZD-2281 share similar pharmacologic profiles. ¹³¹I-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, P < 0.005) in a subcutaneous model. Convection-enhanced delivery demonstrated efficient retention of ¹³¹I-PARPi in orthotopic brain tumors, while quickly clearing from healthy brain tissue. Conclusion: Our results demonstrate ¹³¹I-PARPi's high potential as a therapeutic and highlight PARP's relevance as a target for radionuclide therapy. Radiation plays an integral role in brain tumor therapy, and radiolabeled PARP therapeutics could ultimately lead to improvements in the standard of care.

Keywords: 131I; 131I-PARPi; PARP; convection enhanced delivery (CED); radiotherapeutic.

FIGURE 1.

(A) In vitro studies of PARP inhibitors. Structure of ¹³¹I-PARPi (14). (B) Heat map of binding characteristics of synthetic inhibitors (at 100 nM) to PARP family enzymes in percentage enzyme inhibition. (C) Cellular uptake of ¹³¹I-PARPi alone and blocked with mass excess dose of olaparib.

FIGURE 2.

¹³¹I-PARPi binding specificity. (A) Experimental outline of immuno-PET/CT imaging after injection of nonradioactive version of ¹³¹I-PARPi (¹²⁷I-PARPi). (B) ¹⁸F-PARPi PET tracer injected intravenously 30 min before intratumoral injection of ¹²⁷I-PARPi. PET scan of mice 2 h after receiving ¹⁸F-PARPi with and without ¹²⁷I-PARPi before injection. (C) Ex vivo biodistribution of ¹⁸F-PARPi 1 h after injection in selected tissue. ****P < 0.0001. Error bars represent SD.

FIGURE 3.

In vivo binding, biodistribution, and pharmacokinetics. (A) SPECT/CT images of subcutaneously U87-p53–xenografted mice intratumorally injected with (11.1 MBq) ¹³¹I-PARPi at time of injection and 6 h after injection. (B) Blocked mice received a systemic blocking dose of Olaparib 1 h before ¹³¹I-PARPi. ¹³¹I-PARPi (14.8 MBq) biodistribution in key organs after intratumoral injection at 1, 6, and 24 h (n = 4 per time point). (C) ¹³¹I-PARPi biodistribution in key organs at 1, 6, and 24 h postintratumoral injection after systemic blocking via intravenous injection of mass excess olaparib (n = 4 per time point).

FIGURE 4.

In vivo therapeutic effect of ¹³¹I-PARPi therapy in subcutaneous mouse model. (A) Difference of tumor growth percentage at beginning of study and at 2 wk. (B) Kaplan–Meier plot and table of treatment groups with median survival. Nonparametric Student t test was used to calculate statistics for A. *P < 0.05. Error bars represent SD. P values were calculated by Mantel-Cox test for B, P = 0.0001.

FIGURE 5.

In vivo CED in orthotopic glioblastoma mouse model. (A) Three-dimensional model of orthotopic glioblastoma mouse model with CED-mimicking osmotic pump. (B) SPECT/CT of orthotopic U87-p53 tumors during osmotic pump treatment of ¹³¹I-PARPi at 72 h. (C) Calculated absorbed dose to brain* during treatment. *Brain and tumor have been considered together as a single organ in organ-level dose calculation.