Targeted Brain Tumor Radiotherapy Using an Auger Emitter

Abstract

Purpose: Glioblastoma multiforme is a highly aggressive form of brain cancer whose location, tendency to infiltrate healthy surrounding tissue, and heterogeneity significantly limit survival, with scant progress having been made in recent decades.

Experimental design: ¹²³I-MAPi (Iodine-123 Meitner-Auger PARP1 inhibitor) is a precise therapeutic tool composed of a PARP1 inhibitor radiolabeled with an Auger- and gamma-emitting iodine isotope. Here, the PARP inhibitor, which binds to the DNA repair enzyme PARP1, specifically targets cancer cells, sparing healthy tissue, and carries a radioactive payload within reach of the cancer cells' DNA.

Results: The high relative biological efficacy of Auger electrons within their short range of action is leveraged to inflict DNA damage and cell death with high precision. The gamma ray emission of ¹²³I-MAPi allows for the imaging of tumor progression and therapy response, and for patient dosimetry calculation. Here we demonstrated the efficacy and specificity of this small-molecule radiotheranostic in a complex preclinical model. In vitro and in vivo studies demonstrate high tumor uptake and a prolonged survival in mice treated with ¹²³I-MAPi when compared with vehicle controls. Different methods of drug delivery were investigated to develop this technology for clinical applications, including convection enhanced delivery and intrathecal injection.

Conclusions: Taken together, these results represent the first full characterization of an Auger-emitting PARP inhibitor which demonstrate a survival benefit in mouse models of GBM and confirm the high potential of ¹²³I-MAPi for clinical translation.

Fig. 1.

¹²³I-MAPi Synthesis, binding, and efficacy in vitro. (A) ¹²³I-MAPi binding to PARP1 can deliver lethal Auger radiation within ~100 Å, enough to affect DNA in cancer cells when bound. (B) RP-HPLC coelution of ¹²³I-MAPi with nonradioactive I-127 analog. (C) Synthetic scheme of ¹²³I-MAPi, (a) 0.1 M NaOH, Chloramine T, AcOH, MeOH 20 m, RT; (b) 4-(4-fluoro-3-(piperazine-1-carbonyl)benzyl)phthalazin-1(2H)-one, HBTU, DMAP, 2,6-Lutidine, DMSO, ACN, 2 h, 65 °C. (D) Cellular uptake of ¹²³I-MAPi in U251 GBM cell line. Blocking performed with 100-fold incubation of Olaparib prior to treatment. Sodium Iodine-123 represented by red line. Michaelis-Menten curve fitting. (E) Alamar Blue assay comparing ¹²³I-MAPi efficacy with Olaparib at equal molar concentrations. ¹²³I-MAPi EC₅₀ = 69 nM. Nonlinear fit four parameters variable slope. (F) DNA damage characterization in vitro showing immunofluorescence of γ-H2AX (red), DAPI nuclear staining (blue), PARP1 expression (green), and merged images. (G) Quantification of the number of γ-H2AX foci within the nucleus of cells after treatment with ¹²³I-MAPi (n = 85), ¹²⁷I-PARPi (n = 125), and vehicle control (n = 140). ***p-value < 0.001, Kruskal-Wallis test.

Fig. 2.

Imaging and biodistribution of ¹²³I-MAPi in subcutaneous TS543 mouse model. (A) Specific tumor uptake of ¹²³I-MAPi at 18 h after local injection. Blocking was performed intravenously 1 h before injection. T = tumor, Th = thyroid. (B) Ex vivo biodistribution of ¹²³I-MAPi at 18 h after local injection. (C) Uptake of PARPi-FL in tumor and liver of TS543 tumor-bearing mice show nuclear uptake for tumor and cytoplasmic uptake for liver tissue. (D) Quantification of PARPi-FL uptake in the nucleus and cytoplasm of tumor and liver tissue. Tumor control was performed by IV injection of blocking Olaparib dose. Liver control was performed by injection of vehicle. Nuclear vs cytoplasmic uptake was automatically detected using an ImageJ script to detect colocalization of Hoechst (IV injected 5 minutes before extracting the organs) and PARPi-FL. Student t-test, **p-value < 0.01, ***p-value < 0.001.

Fig. 3.

TS543 glioblastoma stem-cell mouse model and ¹²³I-MAPi efficacy. (A) MRI monitoring of TS543 xenograft disease progression. (B) SPECT/CT imaging of GBM with ¹²³I-MAPi single injection. Images were taken at 18 h after local injection. T = tumor, Cl = clearing organs. (C) Kaplan-Meier curve of mice injected with ¹²³I-MAPi (local single injection 370 kBq - 1.11 MBq, n = 10) compared to vehicle injection (n = 12). Log-Rank (Mantel-Cox) test, **p-value < 0.01. (D) Cytology staining of untreated mice at Weeks 3 and 7 after tumor implantation and cytology staining and MRI imaging of treated mouse brain at 14 weeks post implantation.

Fig. 4.

Improved delivery of ¹²³I-MAPi with an in vivo CED model. (A) Kaplan-Meier survival study of pump implanted mice shows an improvement of survival of ¹²³I-MAPi treated mice (n = 8) when compared to control (n = 8). Treatment mice osmotic pumps were loaded with 481 ± 111 kBq. Log-Rank (Mantel-Cox) test, *p-value < 0.05. (B) Organ-level and (C) 3D dosimetry estimates calculated by Monte Carlo simulation for subcutaneous pump administration. Equivalent doses assume the measured deterministic RBE of 48.4 for ¹²³I-MAPi tumor tissue and assume RBE of 1.0 elsewhere. T = tumor, P = pump.