Potential therapeutic radiotracers: preparation, biodistribution and metabolic characteristics of 177Lu-labeled cyclic RGDfK dimer

Evaluation of chelating agents based on pyridine-azacrown compounds H4PATA, PATAM, and H4PATPA for 68Ga and 177Lu

In this article, we present the synthesis and characterization of three macrocyclic chelators, H4PATA, PATAM, and H4PATPA, based on a pyridine-azacrown compound. Their complexation with 68Ga and 177Lu has been thoroughly investigated using MALDI TOF MS, 1H NMR spectroscopy, radiolabeling studies, and experiments in vitro with fetal bovine serum and a 1000-fold molar excess of H4EDTA. Our studies have shown that the chelators H4PATA and H4PATPA form complexes at room temperature with both radionuclides (RCY > 80 % and > 90 % for complexes with H4PATA and H4PATPA after 30 min, respectively). The chelator PATAM requires high temperature (95 °C) for complexation. In vitro stability assays in fetal bovine serum as well as H4EDTA-challenge revealed that transchelation occurs for all complexes with 68Ga. However, complexes of the ligands H4PATA and PATAM with 177Lu were found stable. Thus, taking into account the radiolabeling at room temperature and in vitro stability of the complex [177Lu]Lu·PATA, our investigations revealed the chelator H4PATA is a candidate for radiopharmaceutical use with 177Lu.

In Vivo Characterization of 4 68Ga-Labeled Multimeric RGD Peptides to Image αvβ3 Integrin Expression in 2 Human Tumor Xenograft Mouse Models

α_vβ₃ integrins play an important role in angiogenesis and cell migration in cancer and are highly expressed on the activated endothelial cells of newly formed blood vessels. Here, we compare the targeting characteristics of 4 ⁶⁸Ga-labeled multimeric cyclic arginine-glycine-aspartate (RGD)-based tracers in an α_vβ₃ integrin-expressing tumor model and a tumor model in which α_vβ₃ integrin is expressed solely on the neovasculature. Methods: Female BALB/c nude mice were subcutaneously injected with SK-RC-52 (α_vβ₃ integrin-positive) or FaDu (α_vβ₃ integrin-negative) tumor cells. ⁶⁸Ga-labeled DOTA-(RGD)₂, TRAP-(RGD)₃, FSC-(RGD)₃, or THP-(RGD)₃ was intravenously administered to the mice (0.5 nmol per mouse, 10-20 MBq), followed by small-animal PET/CT imaging and ex vivo biodistribution studies 1 h after injection. Nonspecific uptake of the tracers in both models was determined by coinjecting an excess of unlabeled DOTA-(RGD)₂ (50 nmol) along with the radiolabeled tracers. Results: Imaging and biodistribution data showed specific uptake in the tumors for each tracer in both models. Tumor uptake of ⁶⁸Ga-FSC-(RGD)₃ was significantly higher than that of ⁶⁸Ga-DOTA-(RGD)₂, ⁶⁸Ga-TRAP-(RGD)₃, or ⁶⁸Ga-THP-(RGD)₃ in the SK-RC-52 model but not in the FaDu model, in which ⁶⁸Ga-FSC-(RGD)₃ showed significantly higher tumor uptake than ⁶⁸Ga-TRAP-(RGD)₃ Most importantly, differences were also observed in normal tissues and in tumor-to-blood ratios. Conclusion: All tracers showed sufficient targeting of α_vβ₃ integrin expression to allow for tumor detection. Although the highest tumor uptake was found for ⁶⁸Ga-FSC-(RGD)₃ and ⁶⁸Ga-THP-(RGD)₃ in the SK-RC-52 and FaDu models, respectively, selection of the optimal tracer for specific diagnostic applications also depends on tumor-to-blood ratio and uptake in normal tissues; these factors should therefore also be considered.

Albumin-Templated Manganese Dioxide Nanoparticles for Enhanced Radioisotope Therapy

Although nanoparticle-based drug delivery systems have been widely explored for tumor-targeted delivery of radioisotope therapy (RIT), the hypoxia zones of tumors on one hand can hardly be reached by nanoparticles with relatively large sizes due to their limited intratumoral diffusion ability, on the other hand often exhibit hypoxia-associated resistance to radiation-induced cell damage. To improve RIT treatment of solid tumors, herein, radionuclide ¹³¹ I labeled human serum albumin (HSA)-bound manganese dioxide nanoparticles (¹³¹ I-HSA-MnO₂ ) are developed as a novel RIT nanomedicine platform that is responsive to the tumor microenvironment (TME). Such ¹³¹ I-HSA-MnO₂ nanoparticles with suitable sizes during blood circulation show rather efficient tumor passive uptake owing to the enhanced permeability and retention effect, as well as little retention in other normal organs to minimize radiotoxicity. The acidic TME can trigger gradual degradation of MnO₂ and thus decomposition of ¹³¹ I-HSA-MnO₂ nanoparticles into individual ¹³¹ I-HSA with sub-10 nm sizes and greatly improves intratumoral diffusion. Furthermore, oxygen produced by MnO₂ -triggered decomposition of tumor endogenous H₂ O₂ would be helpful to relieve hypoxia-associated RIT resistant for those tumors. As the results, the ¹³¹ I-HSA-MnO₂ nanoparticles appear to be a highly effective RIT agent showing great efficacy in tumor treatment upon systemic administration.

Peptide receptor radionuclide therapy: an overview

Peptide receptor radionuclide therapy (PRRT) is a site-directed targeted therapeutic strategy that specifically uses radiolabeled peptides as biological targeting vectors designed to deliver cytotoxic levels of radiation dose to cancer cells, which overexpress specific receptors. Interest in PRRT has steadily grown because of the advantages of targeting cellular receptors in vivo with high sensitivity as well as specificity and treatment at the molecular level. Recent advances in molecular biology have not only stimulated advances in PRRT in a sustainable manner but have also pushed the field significantly forward to several unexplored possibilities. Recent decades have witnessed unprecedented endeavors for developing radiolabeled receptor-binding somatostatin analogs for the treatment of neuroendocrine tumors, which have played an important role in the evolution of PRRT and paved the way for the development of other receptor-targeting peptides. Several peptides targeting a variety of receptors have been identified, demonstrating their potential to catalyze breakthroughs in PRRT. In this review, the authors discuss several of these peptides and their analogs with regard to their applications and potential in radionuclide therapy. The advancement in the availability of combinatorial peptide libraries for peptide designing and screening provides the capability of regulating immunogenicity and chemical manipulability. Moreover, the availability of a wide range of bifunctional chelating agents opens up the scope of convenient radiolabeling. For these reasons, it would be possible to envision a future where the scope of PRRT can be tailored for patient-specific application. While PRRT lies at the interface between many disciplines, this technology is inextricably linked to the availability of the therapeutic radionuclides of required quality and activity levels and hence their production is also reviewed.

Anti-tumor effect of integrin targeted (177)Lu-3PRGD2 and combined therapy with Endostar

PURPOSE

Targeted radiotherapy (TRT) is an emerging approach for tumor treatment. Previously, 3PRGD2 (a dimeric RGD peptide with 3 PEG4 linkers) has been demonstrated to be of advantage for integrin αvβ3 targeting. Given the promising results of (99m)Tc-3PRGD2 for lung cancer detection in human beings, we are encouraged to investigate the radiotherapeutic efficacy of radiolabeled 3PRGD2. The goal of this study was to investigate and optimize the integrin αvβ3 mediated therapeutic effect of (177)Lu-3PRGD2 in the animal model.

EXPERIMENTAL DESIGN

Biodistribution, gamma imaging and maximum tolerated dose (MTD) studies of (177)Lu-3PRGD2 were performed. The targeted radiotherapy (TRT) with single dose and repeated doses as well as the combined therapy of TRT and the anti-angiogenic therapy (AAT) with Endostar were conducted in U87MG tumor model. The hematoxylin and eosin (H&E) staining and immunochemistry (IHC) were performed post-treatment to evaluate the therapeutic effect.

RESULTS

The U87MG tumor uptake of (177)Lu-3PRGD2 was relatively high (6.03 ± 0.65 %ID/g, 4.62 ± 1.44 %ID/g, 3.55 ± 1.08 %ID/g, and 1.22 ± 0.18 %ID/g at 1 h, 4 h, 24 h, and 72 h postinjection, respectively), and the gamma imaging could visualize the tumors clearly. The MTD of (177)Lu-3PRGD2 in nude mice (>111 MBq) was twice to that of (90)Y-3PRGD2 (55.5 MBq). U87MG tumor growth was significantly delayed by (177)Lu-3PRGD2 TRT. Significantly increased anti-tumor effects were observed in the two doses or combined treatment groups.

CONCLUSION

The two-dose TRT and combined therapy with Endostar potently enhanced the tumor growth inhibition, but the former does not need to inject daily for weeks, avoiding a lot of unnecessary inconvenience and suffering for patients, which could potentially be rapidly translated into clinical practice in the future.

Integrin targeted delivery of radiotherapeutics

Targeted radionuclide therapy, which is based on the selective delivery of a sufficient radiation dose to tumors without significantly affecting normal tissues, is a promising therapeutic approach for the treatment of a wide variety of malignancies. Integrins, a family of cell adhesion molecules, play key roles during tumor angiogenesis and metastasis. Among all the integrins, αvβ3 seems to be the most important in the process of tumor angiogenesis. Integrin αvβ3 is highly expressed on activated endothelial cells, new-born vessels as well as some tumor cells, but is not present in resting endothelial cells and most normal organ systems, making it a suitable target for anti-tumor therapy. In this review, we summarize the current development and applications of antibody-, peptide-, and other ligand-based integrin targeted radiotherapeutics for tumor radiation therapy.