PARP-1–Targeted Auger Emitters Display High-LET Cytotoxic Properties In Vitro but Show Limited Therapeutic Utility in Solid Tumor Models of Human Neuroblastoma

Abstract

The currently available therapeutic radiopharmaceutical for highrisk neuroblastoma, 131 I-metaiodobenzylguanidine, is ineffective at targeting micrometastases because of the low-linear-energy-transfer (LET) properties of high-energy β-particles. In contrast, Auger radiation has high-LET properties with nanometer ranges in tissue, efficiently causing DNA damage when emitted near DNA. The aim of this study was to evaluate the cytotoxicity of targeted Auger therapy in preclinical models of high-risk neuroblastoma. Methods: We used a radiolabled poly(adenosine diphosphate ribose) polymerase (PARP) inhibitor called 125 I-KX1 to deliver Auger radiation to PARP-1, a chromatin-binding enzyme overexpressed in neuroblastoma. The in vitro cytotoxicity of 125 I-KX1 was assessed in 19 neuroblastoma cell lines, followed by in-depth pharmacologic analysis in a sensitive and resistant pair of cell lines. Immunofluorescence microscopy was used to characterize 125 I-KX1-induced DNA damage. Finally, in vitro and in vivo microdosimetry was modeled from experimentally derived pharmacologic variables. Results: 125 I-KX1 was highly cytotoxic in vitro across a panel of neuroblastoma cell lines, directly causing double-strand DNA breaks. On the basis of subcellular dosimetry, 125 I-KX1 was approximately twice as effective as 131 I-KX1, whereas cytoplasmic 125 I-metaiodobenzylguanidine demonstrated low biological effectiveness. Despite the ability to deliver a focused radiation dose to the cell nuclei, 125 I-KX1 remained less effective than its α-emitting analog 211 At-MM4 and required significantly higher activity for equivalent in vivo efficacy based on tumor microdosimetry. Conclusion: Chromatin-targeted Auger therapy is lethal to high-risk neuroblastoma cells and has the potential to be used in micrometastatic disease. This study provides the first evidence for cellular lethality from a PARP-1-targeted Auger emitter, calling for further investigation into targeted Auger therapy.

Targeted Brain Tumor Radiotherapy Using an Auger Emitter

“…Cell-level dosimetry analysis showed a noteworthy increase of cell sterilization as a function of absorbed dose with 123 I-MAPi in comparison to external photon irradiation, resulting in high estimates of relative biological effectiveness and suggesting that intranuclearly delivered radiation doses from PARP1bound 123 I-MAPi are nearly 50 times as potent as that from externally directed photons. This estimation, despite dependent on the parameters of the Monte Carlo simulation geometry, is in agreement with the expected equivalent dose for Auger electron emitters, which is comparable to that of intracellularly incorporated 5.3 MeV alpha particles(35,48). The dose was calculated to the tumor cells using PARaDIM v1.0, a Monte Carlo method, using as input the time-integrated activity coefficients from measured data(49).…”

“…Previous attempts to use Auger emitters as cancer therapies have not been successful, due to the limited range of the radiation emitted and the difficulty of reliably delivering the lethal electrons close enough to the DNA target (< 100 Å) (31,(33)(34)(35).…”

Improved radiosynthesis of 123I-MAPi, an auger theranostic agent

“…Auger emitters are an extremely potent radioactive source for targeted radiotherapy, and are characterized by their greater linear energy transfer, incredibly short range and ability to cause more complex, lethal DNA damage as compared to traditional X-rays or b-particles (Daghighian et al 1996;Welt et al 1996;Buchegger et al 2006;Kiess et al 2015;Gill et al 2017;Bavelaar et al 2018). Previous attempts to use Auger emitters as cancer therapies have not been successful (Lee et al 2020), due to the limited range of the radiation emitted and the difficulty of reliably delivering the lethal electrons close enough to the DNA target (<100 Å) (Hofer and Hughes 1971;Kassis 2003;Bavelaar et al 2018). Iodine-123 is a particularly suitable Augeremitter because it also emits a 159 keV c-ray, which can be used for SPECT/CT imaging and disease monitoring.…”

Advancements in PARP1 Targeted Nuclear Imaging and Theranostic Probes

“…The cytotoxic efficiency in neuroblastoma cell lines treated with 125 I-KX1 was 10 4 -10 6 times higher than its non-radioactive precursor KX1. DNA damage induction was significantly higher compared to veliparib treatment as measured by pH2AX fluorescence intensity [51]. Its PARP1 specificity was demonstrated by Makvandi et al, where PARP1 KO ovarian cancer cell lines showed reduced 125 I-KX1 uptake [46].…”

“…In neuroblastoma 3D solid tumor models, comparison of the cytotoxic efficiencies of 125 I-KX1 with other PARP1-targeted ( 211 At-MM4) or non-PARP1-targeted ( 125 I-MIBG) radiopharmaceuticals showed that 125 I-KX1 was less effective compared to 211 At-MM4 in terms of concentration, tumor dosage (350x lower per decay), and tumor-cell nuclei dosage (150x lower per decay) [51]. The theranostic ability of 211 At is comparably advantageous to 125 I due to its superior cytotoxicity, and thereby low dosage requirement in addition to its favorable physical half-life.…”