[99mTc]Tc-DB1 Mimics with Different-Length PEG Spacers: Preclinical Comparison in GRPR-Positive Models

Lessons learned in application driven imaging agent design for image-guided surgery

To meet the growing demand for intraoperative molecular imaging, the development of compatible imaging agents plays a crucial role. Given the unique requirements of surgical applications compared to diagnostics and therapy, maximizing translational potential necessitates distinctive imaging agent designs. For effective surgical guidance, exogenous signatures are essential and are achievable through a diverse range of imaging labels such as (radio)isotopes, fluorescent dyes, or combinations thereof. To achieve optimal in vivo utility a balanced molecular design of the tracer as a whole is required, which ensures a harmonious effect of the imaging label with the affinity and specificity (e.g., pharmacokinetics) of a pharmacophore/targeting moiety. This review outlines common design strategies and the effects of refinements in the molecular imaging agent design on the agent's pharmacological profile. This includes the optimization of affinity, pharmacokinetics (including serum binding and target mediated background), biological clearance route, the achievable signal intensity, and the effect of dosing hereon.

Engineering aspects of lipid-based delivery systems: In vivo gene delivery, safety criteria, and translation strategies

Defects in the genome cause genetic diseases and can be treated with gene therapy. Due to the limitations encountered in gene delivery, lipid-based supramolecular colloidal materials have emerged as promising gene carrier systems. In their non-functionalized form, lipid nanoparticles often demonstrate lower transgene expression efficiency, leading to suboptimal therapeutic outcomes, specifically through reduced percentages of cells expressing the transgene. Due to chemically active substituents, the engineering of delivery systems for genetic drugs with specific chemical ligands steps forward as an innovative strategy to tackle the drawbacks and enhance their therapeutic efficacy. Despite intense investigations into functionalization strategies, the clinical outcome of such therapies still needs to be improved. Here, we highlight and comprehensively review engineering aspects for functionalizing lipid-based delivery systems and their therapeutic efficacy for developing novel genetic cargoes to provide a full snapshot of the translation from the bench to the clinics. We outline existing challenges in the delivery and internalization processes and narrate recent advances in the functionalization of lipid-based delivery systems for nucleic acids to enhance their therapeutic efficacy and safety. Moreover, we address clinical trials using these vectors to expand their clinical use and principal safety concerns.

The Chemical Scaffold of Theranostic Radiopharmaceuticals: Radionuclide, Bifunctional Chelator, and Pharmacokinetics Modifying Linker

Therapeutic radiopharmaceuticals have been researched extensively in the last decade as a result of the growing research interest in personalized medicine to improve diagnostic accuracy and intensify intensive therapy while limiting side effects. Radiometal-based drugs are of substantial interest because of their greater versatility for clinical translation compared to non-metal radionuclides. This paper comprehensively discusses various components commonly used as chemical scaffolds to build radiopharmaceutical agents, i.e., radionuclides, pharmacokinetic-modifying linkers, and chelators, whose characteristics are explained and can be used as a guide for the researcher.

Radiolabeled Bombesin Analogs

Simple Summary

Recent medical advancements have strived for a personalized medicine approach to patients, aimed at optimizing therapy outcomes with minimum toxicity. In this respect, nuclear medicine methodologies have been playing increasingly important roles. For example, the overexpression of peptide receptors, such as the gastrin-releasing peptide receptor (GRPR), on tumor cells as opposed to their lack of expression in healthy surrounding tissues can be elegantly exploited with the aid of “smart” peptide carriers, such as the analogs of the amphibian 14-peptide bombesin (BBN). These molecules can bring clinically attractive radionuclides to malignant lesions in prostate, breast, and other human cancers, sparing healthy tissues. Depending upon the radionuclide in question, diagnostic imaging with single-photon emission computed tomography (SPECT) or positron emission tomography (PET) has been pursued, identifying patients who are eligible for peptide radionuclide receptor therapy (PRRT) in an integrated “theranostic” approach. In the present review, we (i) discuss the major steps taken in the development of anti-GRPR theranostic radioligands, with a focus on those selected for clinical testing; (ii) comment on the present status in this field of research; and (iii) reflect on the current limitations as well as on new opportunities for their broader and more successful clinical applications.

Abstract

The gastrin-releasing peptide receptor (GRPR) is expressed in high numbers in a variety of human tumors, including the frequently occurring prostate and breast cancers, and therefore provides the rationale for directing diagnostic or therapeutic radionuclides on cancer lesions after administration of anti-GRPR peptide analogs. This concept has been initially explored with analogs of the frog 14-peptide bombesin, suitably modified at the N-terminus with a number of radiometal chelates. Radiotracers that were selected for clinical testing revealed inherent problems associated with these GRPR agonists, related to low metabolic stability, unfavorable abdominal accumulation, and adverse effects. A shift toward GRPR antagonists soon followed, with safer analogs becoming available, whereby, metabolic stability and background clearance issues were gradually improved. Clinical testing of three main major antagonist types led to promising outcomes, but at the same time brought to light several limitations of this concept, partly related to the variation of GRPR expression levels across cancer types, stages, previous treatments, and other factors. Currently, these parameters are being rigorously addressed by cell biologists, chemists, nuclear medicine physicians, and other discipline practitioners in a common effort to make available more effective and safe state-of-the-art molecular tools to combat GRPR-positive tumors. In the present review, we present the background, current status, and future perspectives of this endeavor.

[99mTc]Tc-DB15 in GRPR-Targeted Tumor Imaging with SPECT: From Preclinical Evaluation to the First Clinical Outcomes

Diagnostic imaging and radionuclide therapy of prostate (PC) and breast cancer (BC) using radiolabeled gastrin-releasing peptide receptor (GRPR)-antagonists represents a promising approach. We herein propose the GRPR-antagonist based radiotracer [^99mTc]Tc-DB15 ([^99mTc]Tc-N₄-AMA-DGA-_DPhe⁶,Sar¹¹,LeuNHEt¹³]BBN(6-13); N₄: 6-carboxy-1,4,8,11-tetraazaundecane, AMA: aminomethyl-aniline, DGA: diglycolic acid) as a new diagnostic tool for GRPR-positive tumors applying SPECT/CT. The uptake of [^99mTc]Tc-DB15 was tested in vitro in mammary (T-47D) and prostate cancer (PC-3) cells and in vivo in T-47D or PC-3 xenograft-bearing mice as well as in BC patients. DB15 showed high GRPR-affinity (IC₅₀ = 0.37 ± 0.03 nM) and [^99mTc]Tc-DB15 strongly bound to the cell-membrane of T-47D and PC-3 cells, according to a radiolabeled antagonist profile. In mice, the radiotracer showed high and prolonged GRPR-specific uptake in PC-3 (e.g., 25.56 ± 2.78 %IA/g vs. 0.72 ± 0.12 %IA/g in block; 4 h pi) and T-47D (e.g., 15.82 ± 3.20 %IA/g vs. 3.82 ± 0.30 %IA/g in block; 4 h pi) tumors, while rapidly clearing from background. In patients with advanced BC, the tracer could reveal several bone and soft tissue metastases on SPECT/CT. The attractive pharmacokinetic profile of [^99mTc]DB15 in mice and its capability to target GRPR-positive BC lesions in patients highlight its prospects for a broader clinical use, an option currently being explored by ongoing clinical studies.

The Influence of a Polyethylene Glycol Linker on the Metabolism and Pharmacokinetics of a 89Zr-Radiolabeled Antibody

Most experimental work in the space of bioconjugation chemistry focuses on using new methods to construct covalent bonds between a cargo molecule and a protein of interest such as a monoclonal antibody (mAb). Bond formation is important for generating new diagnostic tools, yet when these compounds advance to preclinical in vitro and in vivo studies, and later for translation to the clinic, understanding the fate of potential metabolites that arise from chemical or enzymatic degradation of the construct is important to obtain a full picture of the pharmacokinetic performance of a new compound. In the context of designing new bioconjugate methods for labeling antibodies with the positron-emitting radionuclide ⁸⁹Zr, we previously developed a photochemical process for making ⁸⁹Zr-mAbs. Experimental studies on [⁸⁹Zr]ZrDFO-PEG₃-azepin-mAb constructs revealed that incorporation of the tris-polyethylene glycol (PEG₃) linker improved the aqueous phase solubility and radiochemical conversion. However, the use of a PEG₃ linker also has an impact on the whole-body residence time of the construct, leading to a more rapid excretion of the ⁸⁹Zr activity when compared with radiotracers that lack the PEG₃ chain. In this work, we investigated the metabolic fate of eight possible metabolites that arise from the logical disconnection of [⁸⁹Zr]ZrDFO-PEG₃-azepin-mAb at bonds which are susceptible to chemical or enzymatic cleavage. Synthesis combined with ⁸⁹Zr-radiolabeling, small-animal positron emission tomography imaging at multiple time points from 0 to 20 h, and measurements of the effective half-life for whole-body excretion are reported. The conclusions are that the use of a PEG₃ linker is non-innocent in terms of its impact on enhancing the metabolism of [⁸⁹Zr]ZrDFO-PEG₃-azepin-mAbs. In most cases, degradation can produce metabolites that are rapidly eliminated from the body, thereby enhancing image contrast by reducing nonspecific accumulation and retention of ⁸⁹Zr in background organs such as the liver, spleen, kidney, and bone.

Advances in Development of Radiometal Labeled Amino Acid-Based Compounds for Cancer Imaging and Diagnostics

Radiolabeled biomolecules targeted at tumor-specific enzymes, receptors, and transporters in cancer cells represent an intensively investigated and promising class of molecular tools for the cancer diagnosis and therapy. High specificity of such biomolecules is a prerequisite for the treatment with a lower burden to normal cells and for the effective and targeted imaging and diagnosis. Undoubtedly, early detection is a key factor in efficient dealing with many severe tumor types. This review provides an overview and critical evaluation of novel approaches in the designing of target-specific probes labeled with metal radionuclides for the diagnosis of most common death-causing cancers, published mainly within the last three years. Advances are discussed such traditional peptide radiolabeling approaches, and click and nanoparticle chemistry. The progress of radiolabeled peptide based ligands as potential radiopharmaceuticals is illustrated via novel structure and application studies, showing how the molecular modifications reflect their binding selectivity to significant onco-receptors, toxicity, and, by that, practical utilization. The most impressive outputs in categories of newly developed structures, as well as imaging and diagnosis approaches, and the most intensively studied oncological diseases in this context, are emphasized in order to show future perspectives of radiometal labeled amino acid-based compounds in nuclear medicine.

Preclinical Evaluation of 99mTc-Labeled GRPR Antagonists maSSS/SES-PEG2-RM26 for Imaging of Prostate Cancer

BACKGROUND

Gastrin-releasing peptide receptor (GRPR) is an important target for imaging of prostate cancer. The wide availability of single-photon emission computed tomography/computed tomography (SPECT/CT) and the generator-produced ^99mTc can be utilized to facilitate the use of GRPR-targeting radiotracers for diagnostics of prostate cancers.

METHODS

Synthetically produced mercaptoacetyl-Ser-Ser-Ser (maSSS)-PEG₂-RM26 and mercaptoacetyl-Ser-Glu-Ser (maSES)-PEG₂-RM26 (RM26 = d-Phe-Gln-Trp-Ala-Val-Gly-His-Sta-Leu-NH₂) were radiolabeled with ^99mTc and characterized in vitro using PC-3 cells and in vivo, using NMRI or PC-3 tumor bearing mice. SPECT/CT imaging and dosimetry calculations were performed for [^99mTc]Tc-maSSS-PEG₂-RM26.

RESULTS

Peptides were radiolabeled with high yields (>98%), demonstrating GRPR specific binding and slow internalization in PC-3 cells. [^99mTc]Tc-maSSS-PEG₂-RM26 outperformed [^99mTc]Tc-maSES-PEG_2-RM26 in terms of GRPR affinity, with a lower dissociation constant (61 pM vs 849 pM) and demonstrating higher tumor uptake. [^99mTc]Tc-maSSS-PEG₂-RM26 had tumor-to-blood, tumor-to-muscle, and tumor-to-bone ratios of 97 ± 56, 188 ± 32, and 177 ± 79, respectively. SPECT/CT images of [^99mTc]Tc-maSSS-PEG₂-RM26 clearly visualized the GRPR-overexpressing tumors. The dosimetry estimated for [^99mTc]Tc-maSSS-PEG₂-RM26 showed the highest absorbed dose in the small intestine (1.65 × 10^-3 mGy/MBq), and the effective dose is 3.49 × 10^-3 mSv/MBq.

CONCLUSION

The GRPR antagonist maSSS-PEG₂-RM26 is a promising GRPR-targeting agent that can be radiolabeled through a single-step with the generator-produced ^99mTc and used for imaging of GRPR-expressing prostate cancer.

Radioactive Molecules 2019-2020

It is with great pleasure that I have accepted the challenge of reviewing and summarizing the articles published in Molecules through 2019 and 2020 on radioactive molecules [...].