Marshalling the Potential of Auger Electron Radiopharmaceutical Therapy

Highlight selection of radiochemistry and radiopharmacy developments by editorial board

BACKGROUND

The Editorial Board of EJNMMI Radiopharmacy and Chemistry releases a biannual highlight commentary to update the readership on trends in the field of radiopharmaceutical development.

MAIN BODY

This selection of highlights provides commentary on 24 different topics selected by each coauthoring Editorial Board member addressing a variety of aspects ranging from novel radiochemistry to first-in-human application of novel radiopharmaceuticals.

CONCLUSION

Trends in radiochemistry and radiopharmacy are highlighted. Hot topics cover the entire scope of EJNMMI Radiopharmacy and Chemistry, demonstrating the progress in the research field in many aspects.

Advances in Radioligand Theranostics in Oncology

Theranostics with radioligands (radiotheranostics) has played a pivotal role in oncology. Radiotheranostics explores the molecular targets expressed on tumor cells to target them for imaging and therapy. In this way, radiotheranostics entails non-invasive demonstration of the in vivo expression of a molecular target of interest through imaging followed by the administration of therapeutic radioligand targeting the tumor-expressed molecular target. Therefore, radiotheranostics ensures that only patients with a high likelihood of response are treated with a particular radiotheranostic agent, ensuring the delivery of personalized care to cancer patients. Within the last decades, a couple of radiotheranostics agents, including Lutetium-177 DOTATATE (177Lu-DOTATATE) and Lutetium-177 prostate-specific membrane antigen (177Lu-PSMA), were shown to prolong the survival of cancer patients compared to the current standard of care leading to the regulatory approval of these agents for routine use in oncology care. This recent string of successful approvals has broadened the interest in the development of different radiotheranostic agents and their investigation for clinical translation. In this work, we present an updated appraisal of the literature, reviewing the recent advances in the use of established radiotheranostic agents such as radioiodine for differentiated thyroid carcinoma and Iodine-131-labeled meta-iodobenzylguanidine therapy of tumors of the sympathoadrenal axis as well as the recently approved 177Lu-DOTATATE and 177Lu-PSMA for differentiated neuroendocrine tumors and advanced prostate cancer, respectively. We also discuss the radiotheranostic agents that have been comprehensively characterized in preclinical studies and have shown some clinical evidence supporting their safety and efficacy, especially those targeting fibroblast activation protein (FAP) and chemokine receptor 4 (CXCR4) and those still being investigated in preclinical studies such as those targeting poly (ADP-ribose) polymerase (PARP) and epidermal growth factor receptor 2.

PARP-Targeted Radiotheranostics with Auger Electrons: An Updated Overview

Auger electrons (AEs) represent an intriguing topic in the field of radionuclide therapy. They are emitted by several radionuclides commonly used in nuclear medicine (indium-111, iodine-123, iodine-125), allowing for highly localized energy deposition and thus exerting a radiotoxic effect on specific cellular and sub-cellular targets. However, due to their short range in matter, AEs have had limited use in therapeutic applications so far. In recent years, the synthesis of various radiopharmaceuticals capable of binding to the enzyme poly(ADP-ribose) polymerase 1 has reignited interest in this type of therapy, laying the groundwork for a theranostic approach based on radionuclides emitting AEs. The enzyme PARP-1 operates enzymatically in close proximity to DNA that represents the prime target of radionuclide therapies. Following this trend, several PARP-targeted radiopharmaceuticals for AE-based theranostics have been developed. We provide an updated overview of preclinical studies focused on the applications of this new theranostic approach in glioblastoma, breast, prostate and ovarian carcinoma, and pancreatic adenocarcinoma.

DNA-Targeted Complexes of Tc and Re for Biomedical Applications

Due to their favorable chemical features, Re and Tc complexes have been widely used for the development of new therapeutic agents and imaging probes to solve problems of biomedical relevance. This review provides an update of the most relevant research efforts towards the development of novel cancer theranostic agents using Re and Tc-based compounds interacting with specific DNA structures. This includes a variety of homometallic complexes, namely those containing M(CO)3 (M=Re, Tc) moieties, that exhibit different modes of interaction with DNA, such as covalent binding, intercalation, groove binding or G-quadruplex DNA binding. Additionally, heterometallic complexes, designed to potentiate synergistic effects of different metal centers to improve DNA-targeting, cytotoxicity and fluorescence properties, are also reviewed. Particular attention is also given to 99m Tc- and 188 Re-labeled oligonucleotides that have been widely explored to develop imaging and therapeutic radiopharmaceuticals through the in vivo hybridization with a specific complementary DNA or RNA target sequence to provide useful molecular tools in precision medicine for cancer diagnosis and treatment. Finally, the need for further improvement of DNA-targeted Re and Tc-based compounds as potential therapeutic and diagnostic agents is highlighted, and future directions are discussed.

Synthesis and Preclinical Evaluation of PSMA-Targeted 111In-Radioconjugates Containing a Mitochondria-Tropic Triphenylphosphonium Carrier

Nuclear DNA is the canonical target for biological damage induced by Auger electrons (AE) in the context of targeted radionuclide therapy (TRT) of cancer, but other subcellular components might also be relevant for this purpose, such as the energized mitochondria of tumor cells. Having this in mind, we have synthesized novel DOTA-based chelators carrying a prostate-specific membrane antigen (PSMA) inhibitor and a triphenyl phosphonium (TPP) group that were used to obtain dual-targeted 111In-radioconjugates ([111In]In-TPP-DOTAGA-PSMA and [111In]In-TPP-DOTAGA-G3-PSMA), aiming to promote a selective uptake of an AE-emitter radiometal (111In) by PSMA+ prostate cancer (PCa) cells and an enhanced accumulation in the mitochondria. These dual-targeted 111In-radiocomplexes are highly stable under physiological conditions and in cell culture media. The complexes showed relatively similar binding affinities toward the PSMA compared to the reference tracer [111In]In-PSMA-617, in line with their high cellular uptake and internalization in PSMA+ PCa cells. The complexes compromised cell survival in a dose-dependent manner and in the case of [111In]In-TPP-DOTAGA-G3-PSMA to a higher extent than observed for the single-targeted congener [111In]In-PSMA-617. μSPECT imaging studies in PSMA+ PCa xenografts showed that the TPP pharmacophore did not interfere with the excellent in vivo tumor uptake of the "golden standard" [111In]In-PSMA-617, although it led to a higher kidney retention. Such kidney retention does not necessarily compromise their usefulness as radiotherapeutics due to the short tissue range of the Auger/conversion electrons emitted by 111In. Overall, our results provide valuable insights into the potential use of mitochondrial targeting by PSMA-based radiocomplexes for efficient use of AE-emitting radionuclides in TRT, giving impetus to extend the studies to other AE-emitting trivalent radiometals (e.g., 161Tb or 165Er) and to further optimize the designed dual-targeting constructs.

Adjuvant Auger Electron-Emitting Radioimmunotherapy with [111In]In-DOTA-Panitumumab in a Mouse Model of Local Recurrence and Metastatic Progression of Human Triple-Negative Breast Cancer

Triple-negative breast cancer (TNBC) has a high risk for recurrence and metastasis. We studied the effectiveness of Auger electron (AE) radioimmunotherapy (RIT) with antiepidermal growth factor receptor (EGFR) panitumumab conjugated with DOTA complexed to 111In ([111In]In-DOTA-panitumumab) for preventing metastatic progression after local treatment of 231/LM2-4 Luc+ human TNBC tumors in the mammary fat pad of NRG mice. Prior to RIT, the primary tumor was resected, and tumor margins were treated with X-irradiation (XRT; 5 days × 6 Gy/d). RIT was administered 1 day post-XRT by intravenous injection of 26 MBq (15 μg) or 2 × 10 MBq (15 μg each) separated by 7 d. These treatments were compared to tumor resection with or without XRT combined with DOTA-panitumumab (15 μg) or irrelevant [111In]In-DOTA-IgG2 (24 MBq; 15 μg), and efficacy was evaluated by Kaplan-Meier survival curves. The effect of [111In]In-DOTA-panitumumab (23 MBq; 15 μg) after tumor resection without local XRT was also studied. Tumor resection followed by XRT and RIT with 26 MBq [111In]In-DOTA-panitumumab significantly increased the median survival to 35 d compared to tumor resection with or without XRT (23-24 d; P < 0.0001). Local treatment with tumor resection and XRT followed by 2 × 10 MBq of [111In]In-DOTA-panitumumab, DOTA-panitumumab, or [111In]In-DOTA-IgG2 did not significantly improve median survival (26 days for all treatments). RIT alone with [111In]In-DOTA-panitumumab postresection of the tumor without XRT increased median survival to 29 days, though this was not significant. Despite significantly improved survival in mice treated with tumor resection, XRT, and RIT with [111In]In-DOTA-panitumumab, all mice eventually succumbed to advanced metastatic disease by 45 d post-tumor resection. SPECT/CT with [111In]In-DOTA-panitumumab, PET/MRI with [64Cu]Cu-DOTA-panitumumab F(ab')2, and PET/CT with [18F]FDG were used to detect recurrent and metastatic disease. Uptake of [111In]In-DOTA-panitumumab at 4 d p.i. in the MFP tumor was 26.8 ± 9.7% ID/g and in metastatic lymph nodes (LN), lungs, and liver was 34.2 ± 26.9% ID/g, 17.5 ± 6.0% ID/g, and 9.4 ± 2.4%ID/g, respectively, while uptake in the lungs (6.0 ± 0.9% ID/g) and liver (5.2 ± 2.9% ID/g) of non-tumor-bearing NRG was significantly lower (P < 0.05). Radiation-absorbed doses in metastatic LN, lungs, and liver were 9.7 ± 6.1, 6.4 ± 2.1, and 10.9 ± 2.7 Gy, respectively. In conclusion, we demonstrated that RIT with [111In]In-DOTA-panitumumab combined with tumor resection and XRT significantly improved the survival of mice with recurrent TNBC. However, the aggressive nature of 231/LM2-4 Luc+ tumors in NRG mice may have contributed to the tumor recurrence and progression observed.