Matched-pair, 86Y/90Y-labeled, bivalent RGD/bombesin antagonist, [RGD-Glu-[DO3A]-6-Ahx-RM2], as a potential theranostic agent for prostate cancer

Two Targets, One Mission: Heterobivalent Metal-Based Radiopharmaceuticals for Prostate Cancer Imaging and Therapy

Prostate cancer (PCa) is a significant healthcare challenge, associated with considerable mortality and morbidity among men, particularly in developed countries. PCa mortality and morbidity are primarily related to its most advanced form, metastatic castration-resistant PCa (mCRPC), for which there is presently no cure. Therefore, novel therapeutic approaches to increase mCRPC survival are critically needed. Due to PCa tumor heterogeneity and a complex tumor microenvironment, the efficacy of single-target radiopharmaceuticals, such as the Food and Drug Administration-approved [177Lu]Lu-PSMA-617, is currently under reassessment. The design and development of PCa dual-target radiopharmaceuticals have garnered considerable attention, due to their benefits over single-target counterparts, namely increased therapeutic specificity and efficacy, as well as the ability to overcome the challenge of inconsistent tumor visualization caused by variable receptor expression across diverse lesions, thereby enabling more comprehensive imaging. Several PCa biomarkers are currently being investigated as potential targets for dual-target radiopharmaceuticals, including prostate-specific membrane antigen, gastrin-releasing peptide receptor, integrin αvβ3 receptor, fibroblast activation protein, sigma-1 receptor, as well as albumin, the radiosensitive cell nucleus, and mitochondria. This review explores recent advancements in heterobivalent metal-based radiopharmaceuticals for dual targeting in PCa, highlighting their significance in theranostic and personalized medicine.

Importance of integrin transmembrane helical interactions for antagonistic versus agonistic ligand behavior: Consequences for medical applications

Integrins are well-characterized receptors involved in cell adhesion and signaling. With six approved drugs, they are recognized as valuable therapeutic targets. Here, we explore potential activation mechanisms that may clarify the agonist versus antagonist behavior of integrin ligands. The reorganization of the transmembrane domain (TMD) in the integrin receptor, forming homooligomers within focal adhesions, could be key to the understanding of the agonistic properties of integrin ligands at substoichiometric concentrations. This has significant implications for medical applications. While we focus on the RGD peptide-recognizing integrin subfamily, we propose that these mechanistic insights may also apply to other integrin subtypes. For application of integrin ligands in medicine it is essential to consider this mechanism and its consequences for affinity and bioavailability.

Integrin Targeting and Beyond: Enhancing Cancer Treatment with Dual-Targeting RGD (Arginine-Glycine-Aspartate) Strategies

Integrins, an important superfamily of cell adhesion receptors, play an essential role in cancer progression, metastasis, and angiogenesis, establishing them as prime targets for both diagnostic and therapeutic applications. Despite their significant potential, integrin-targeted therapies have faced substantial challenges in clinical trials, including variable efficacy and unmet high expectations. Nevertheless, the consistent expression of integrins on tumor and stromal cells underscores their ongoing relevance and potential. Traditional RGD-based imaging and therapeutic agents have faced limitations, such as inconsistent target expression and rapid systemic clearance, which have reduced their effectiveness. To overcome these challenges, recent research has focused on advancing RGD-based strategies and exploring innovative solutions. This review offers a thorough analysis of the latest developments in the RGD-integrin field, with a particular focus on addressing previous limitations. It delves into new dual-targeting approaches and cutting-edge RGD-based agents designed to improve both tumor diagnosis and therapeutic outcomes. By examining these advancements, this review illuminates new pathways for enhancing the specificity and efficacy of integrin-targeted therapies, paving the way for more effective cancer diagnosis and treatment strategies.

Radiopharmaceuticals Targeting Gastrin-Releasing Peptide Receptor for Diagnosis and Therapy of Prostate Cancer

The high incidence and heavy disease burden of prostate cancer (PC) require accurate and comprehensive assessment for appropriate disease management. Prostate-specific membrane antigen (PSMA) positron emission tomography (PET) cannot detect PSMA-negative lesions, despite its key role in PC disease management. The overexpression of gastrin-releasing peptide receptor (GRPR) in PC lesions reportedly performs as a complementary target for the diagnosis and therapy of PC. Radiopharmaceuticals derived from the natural ligands of GRPR have been developed. These radiopharmaceuticals enable the visualization and quantification of GRPR within the body, which can be used for disease assessment and therapeutic guidance. Recently developed radiopharmaceuticals exhibit improved pharmacokinetic parameters without deterioration in affinity. Several heterodimers targeting GRPR have been constructed as alternatives because of their potential to detect tumor lesions with a low diagnostic efficiency of single target detection. Moreover, some GRPR-targeted radiopharmaceuticals have entered clinical trials for the initial staging or biochemical recurrence detection of PC to guide disease stratification and therapy, indicating considerable potential in PC disease management. Herein, we comprehensively summarize the progress of radiopharmaceuticals targeting GRPR. In particular, we discuss the impact of ligands, chelators, and linkers on the distribution of radiopharmaceuticals. Furthermore, we summarize a potential design scheme to facilitate the advancement of radiopharmaceuticals and, thus, prompt clinical translation.

Carrier systems of radiopharmaceuticals and the application in cancer therapy

Radiopharmaceuticals play a vital role in cancer therapy. The carrier of radiopharmaceuticals can precisely locate and guide radionuclides to the target, where radionuclides kill surrounding tumor cells. Effective application of radiopharmaceuticals depends on the selection of an appropriate carrier. Herein, different types of carriers of radiopharmaceuticals and the characteristics are briefly described. Subsequently, we review radiolabeled monoclonal antibodies (mAbs) and their derivatives, and novel strategies of radiolabeled mAbs and their derivatives in the treatment of lymphoma and colorectal cancer. Furthermore, this review outlines radiolabeled peptides, and novel strategies of radiolabeled peptides in the treatment of neuroendocrine neoplasms, prostate cancer, and gliomas. The emphasis is given to heterodimers, bicyclic peptides, and peptide-modified nanoparticles. Last, the latest developments and applications of radiolabeled nucleic acids and small molecules in cancer therapy are discussed. Thus, this review will contribute to a better understanding of the carrier of radiopharmaceuticals and the application in cancer therapy.

The effects of novel macrocyclic chelates on the targeting properties of the 68Ga-labeled Gastrin releasing peptide receptor antagonist RM2

BACKGROUND

The gastrin-releasing peptide receptor (GRPr) is a molecular target for the visualization of prostate cancer. Bombesin (BN) analogs are short peptides with a high affinity for GRPr. RM2 is a bombesin-based antagonist. It has been demonstrated that RM2 have superior in vivo biodistribution and targeting properties than high-affinity receptor agonists. This study developed new RM2-like antagonists by introducing the novel bifunctional chelators AAZTA5 and DATA5m to RM2.

RESULTS

The effects of different macrocyclic chelating groups on drug targeting properties and the possibility of preparing 68Ga-radiopharmaceuticals in a kit-based protocol were investigated using 68Ga-labeled entities. Both new RM2 variants were labelled with 68Ga3+ resulting in high yields, stability, and low molarity of the ligand. DATA5m-RM2 and AAZTA5-RM2 incorporated 68Ga3+ nearly quantitatively at room temperature within 3-5 min, and the labelling yield for 68Ga-DOTA-RM2 was approximately 10% under the same conditions. 68Ga-AAZTA5-RM2 showed stronger hydrophilicity according to partition coefficient. Although the maximal cellular uptake values of the three compounds were similar, 68Ga-AAZTA5-RM2 and 68Ga-DATA5m-RM2 peaked more rapidly. Biodistribution studies showed high and specific tumor uptake, with a maximum of 9.12 ± 0.81 percentage injected activity per gram of tissue (%ID/g) for 68Ga-DATA5m-RM2 and 7.82 ± 0.61%ID/g for 68Ga-AAZTA5-RM2 at 30 min after injection.

CONCLUSIONS

The conditions for complexation of DATA5m-RM2 and AAZTA5-RM2 with gallium-68 are milder, faster and require less amount of precursors than DOTA-RM2. Chelators had an evident influence on the pharmacokinetics and targeting properties of 68Ga-X-RM2 derivatives. Positively charged 68Ga-DATA5m-RM2 provided a high tumor uptake, high image contrast and good capability of targeting GRPr.

Peptide-based positron emission tomography probes: current strategies for synthesis and radiolabelling

In medical imaging, techniques such as magnetic resonance imaging, contrast-enhanced computerized tomography, and positron emission tomography (PET) are extensively available and routinely used for disease diagnosis and treatment. Peptide-based targeting PET probes are usually small peptides with high affinity and specificity to specific cellular and tissue targets opportunely engineered for acting as PET probes. For instance, either the radioisotope (e.g., 18F, 11C) can be covalently linked to the peptide-probe or another ligand that strongly complexes the radioisotope (e.g., 64Cu, 68Ga) through multiple coordinative bonds can be chemically conjugated to the peptide delivery moiety. The main advantages of these probes are that they are cheaper than classical antibody-based PET tracers and can be efficiently chemically modified to be radiolabelled with virtually any radionuclide making them very attractive for clinical use. The goal of this review is to report and summarize recent technologies in peptide PET-based molecular probes synthesis and radiolabelling with the most used radioisotopes in 2022.

Multimeric RGD-Based Strategies for Selective Drug Delivery to Tumor Tissues

RGD peptides have received a lot of attention over the two last decades, in particular to improve tumor therapy through the targeting of the αVβ3 integrin receptor. This review focuses on the molecular design of multimeric RGD compounds, as well as the design of suitable linkers for drug delivery. Many examples of RGD-drug conjugates have been developed, and we show the importance of RGD constructs to enhance binding affinity to tumor cells, as well as their drug uptake. Further, we also highlight the use of RGD peptides as theranostic systems, promising tools offering dual modality, such as tumor diagnosis and therapy. In conclusion, we address the challenging issues, as well as ongoing and future development, in comparison with large molecules, such as monoclonal antibodies.

Copper-67-Labeled Bombesin Peptide for Targeted Radionuclide Therapy of Prostate Cancer

The gastrin-releasing peptide receptor (GRPR) is a promising molecular target for imaging and therapy of prostate cancer using bombesin peptides that bind to the receptor with high affinity. Targeted copper theranostics (TCTs) using copper radionuclides, ⁶⁴Cu for imaging and ⁶⁷Cu for therapy, offer significant advantages in the development of next-generation theranostics. [⁶⁴Cu]Cu-SAR-BBN is in clinical development for PET imaging of GRPR-expressing cancers. This study explores the therapeutic efficacy of [⁶⁷Cu]Cu-SAR-BBN in a pre-clinical mouse model. The peptide was radiolabeled with ⁶⁷Cu, and specific binding of the radiolabeled peptide towards GRPR-positive PC-3 prostate cancer cells was confirmed with 52.2 ± 1.4% total bound compared to 5.8 ± 0.1% with blocking. A therapy study with [⁶⁷Cu]Cu-SAR-BBN was conducted in mice bearing PC-3 tumors by injecting 24 MBq doses a total of six times. Tumor growth was inhibited by 93.3% compared to the control group on day 19, and median survival increased from 34.5 days for the control group to greater than 54 days for the treatment group. The ease and stability of the radiochemistry, favorable biodistribution, and the positive tumor inhibition demonstrate the suitability of this copper-based theranostic agent for clinical assessment in the treatment of cancers expressing GRPR.

86Y PET imaging

Yttrium-86 is a non-standard positron emitter that can provide dosimetry information prior to therapy with yttrium-90 radiopharmaceuticals and be used to follow biochemical processes. In this chapter, we discuss the production, purification and applications of ⁸⁶Y for PET imaging. More specifically, ⁸⁶Y radiolabeling is highlighted and protocols to determine the radiochemical purity of ⁸⁶Y-DOTA and ⁸⁶Y-DTPA are presented.

New Frontiers in Molecular Imaging Using Peptide-Based Radiopharmaceuticals for Prostate Cancer

The high incidence of prostate cancer (PCa) increases the need for progress in its diagnosis, staging, and precise treatment. The overexpression of tumor-specific receptors for peptides in human cancer cells, such as gastrin-releasing peptide receptor, natriuretic peptide receptor, and somatostatin receptor, has indicated the ideal molecular basis for targeted imaging and therapy. Targeting these receptors using radiolabeled peptides and analogs have been an essential topic on the current forefront of PCa studies. Radiolabeled peptides have been used to target receptors for molecular imaging in human PCa with high affinity and specificity. The radiolabeled peptides enable optimal quick elimination from blood and normal tissues, producing high contrast for positron emission computed tomography and single-photon emission computed tomography imaging with high tumor-to-normal tissue uptake ratios. Owing to their successful application in visualization, peptide derivatives with therapeutic radionuclides for peptide receptor radionuclide therapy in PCa have been explored in recent years. These developments offer the promise of personalized, molecular medicine for individual patients. Hence, we review the preclinical and clinical literature in the past 20 years and focus on the newer developments of peptide-based radiopharmaceuticals for the imaging and therapy of PCa.

Current State of Radiolabeled Heterobivalent Peptidic Ligands in Tumor Imaging and Therapy

Over the past few years, an approach emerged that combines different receptor-specific peptide radioligands able to bind different target structures on tumor cells concomitantly or separately. The reason for the growing interest in this special field of radiopharmaceutical development is rooted in the fact that bispecific peptide heterodimers can exhibit a strongly increased target cell avidity and specificity compared to their corresponding monospecific counterparts by being able to bind to two different target structures that are overexpressed on the cell surface of several malignancies. This increase of avidity is most pronounced in the case of concomitant binding of both peptides to their respective targets but is also observed in cases of heterogeneously expressed receptors within a tumor entity. Furthermore, the application of a radiolabeled heterobivalent agent can solve the ubiquitous problem of limited tumor visualization sensitivity caused by differential receptor expression on different tumor lesions. In this article, the concept of heterobivalent targeting and the general advantages of using radiolabeled bispecific peptidic ligands for tumor imaging or therapy as well as the influence of molecular design and the receptors on the tumor cell surface are explained, and an overview is given of the radiolabeled heterobivalent peptides described thus far.

64Cu-labeled melanin nanoparticles for PET/CT and radionuclide therapy of tumor

Melanin is a group of natural pigments found in living organism. It can be used for positron emission tomography (PET) imaging due to its inherent chelating ability to radioactive cupric ion. This study was to prepare ⁶⁴Cu-labeled PEGylated melanin nanoparticles (⁶⁴Cu-PEG-MNPs), and to further take advantage of the enhanced permeability and retention (EPR) effect of radiolabeled nanoparticles to realize the integration of tumor diagnosis and treatment. We successfully synthesized PEG-MNPs. Saline and serum stability experiments demonstrated good stability. PET/CT showed high tumor aggregation. Moreover, ⁶⁴Cu-PEG-MNPs resulted in a therapeutic effect on the A431 tumor-bearing mice in the treatment group. The pathological results further confirmed that the therapeutic doses of ⁶⁴Cu-PEG-MNPs cause pathological changes of tumor tissues while showing minimal toxicity to normal tissues. Our data successfully demonstrate the good imaging performance of ⁶⁴Cu-PEG-MNPs on A431 tumors and further proved its therapeutic effect, highlighting a great potential in targeted radionuclide therapy.

Radiometals for imaging and theranostics, current production, and future perspectives

The aim of this review is to make the reader familiar with currently available radiometals, their production modes, capacities, and quality concerns related to their medical use, as well as new emerging radiometals and irradiation technologies from the perspective of their diagnostic and theranostic applications. Production methods of ¹⁷⁷ Lu serve as an example of various issues related to the production yield, specific activity, radionuclidic and chemical purity, and production economy. Other radiometals that are currently used or explored for potential medical applications, with particular focus on their theranostic value, are discussed. Using radiometals for diagnostic imaging and therapy is on the rise. The high demand for radiometals for medical use prompts investigations towards using alternative irradiation reactions, while using existing nuclear reactors and accelerator facilities. This review discusses these production capacities and what is necessary to cover the growing demand for theranostic nuclides.