Synthesis and99mTc-labelling of bz-MAG3-triprolinyl-peptides, their radiochemical evaluation and in vitro receptor-binding

99mTc-labeled FAPI compounds for cancer and inflammation: from radiochemistry to the first clinical applications

BACKGROUND

In recent years, fibroblast activating protein (FAP), a biomarker overexpressed by cancer-associated fibroblasts, has emerged as one of the most promising biomarkers in oncology. Similarly, FAP overexpression has been detected in various fibroblast-mediated inflammatory conditions such as liver cirrhosis and idiopathic pulmonary fibrosis. Along this trajectory, FAP-targeted positron emission tomography (PET), utilizing FAP inhibitors (FAPi) labeled with positron emitters, has gained traction as a powerful imaging approach in both cancer and inflammation. However, PET represents a high-cost technology, and its widespread adoption is still limited compared to the availability of gamma cameras. To address this issue, several efforts have been made to explore the potential of [99mTc]Tc-FAPi tracers as molecular probes for imaging with gamma cameras and single photon emission computed tomography (SPECT).

MAIN BODY

Several approaches have been investigated for labeling FAPi-based compounds with 99mTc. Specifically, the mono-oxo, tricarbonyl, isonitrile, and HYNIC strategies have been applied to produce [99mTc]Tc-FAPi tracers, which have been tested in vitro and in animal models. Overall, these labeling approaches have demonstrated high efficiency and strong binding. The resulting [99mTc]Tc-FAPi tracers have shown high specificity for FAP-positive cells and xenografts in both in vitro and animal model studies, respectively. However, the majority of [99mTc]Tc-FAPi tracers have exhibited variable levels of lipophilicity, leading to preferential excretion through the hepatobiliary route and undesirable binding to lipoproteins. Consequently, efforts have been made to synthesize more hydrophilic FAPi-based compounds to improve pharmacokinetic properties and achieve a more favorable biodistribution, particularly in the abdominal region. SPECT imaging with [99mTc]Tc-FAPi has yielded promising results in patients with gastrointestinal tumors, demonstrating comparable or superior diagnostic performance compared to other imaging modalities. Similarly, encouraging outcomes have been observed in subjects with gliomas, lung cancer, breast cancer, and cervical cancer. Beyond oncological applications, [99mTc]Tc-FAPi-based imaging has been successfully employed in myocardial and idiopathic pulmonary fibrosis.

CONCLUSIONS

This overview focuses on the various radiochemical strategies for obtaining [99mTc]Tc-FAPi tracers, highlighting the main challenges encountered and possible solutions when applying each distinct approach. Additionally, it covers the preclinical and initial clinical applications of [99mTc]Tc-FAPi in cancer and inflammation.

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.

A Picture of Modern Tc-99m Radiopharmaceuticals: Production, Chemistry, and Applications in Molecular Imaging

Even today, techentium-99m represents the radionuclide of choice for diagnostic radio-imaging applications. Its peculiar physical and chemical properties make it particularly suitable for medical imaging. By the use of molecular probes and perfusion radiotracers, it provides rapid and non-invasive evaluation of the function, physiology, and/or pathology of organs. The versatile chemistry of technetium-99m, due to its multi-oxidation states, and, consequently, the ability to produce a variety of complexes with particular desired characteristics, are the major advantages of this medical radionuclide. The advances in technetium coordination chemistry over the last 20 years, in combination with recent advances in detector technologies and reconstruction algorithms, make SPECT’s spatial resolution comparable to that of PET, allowing 99mTc radiopharmaceuticals to have an important role in nuclear medicine and to be particularly suitable for molecular imaging. In this review the most efficient chemical methods, based on the modern concept of the 99mTc-metal fragment approach, applied to the development of technetium-99m radiopharmaceuticals for molecular imaging, are described. A specific paragraph is dedicated to the development of new 99mTc-based radiopharmaceuticals for prostate cancer.

Preparation and evaluation of bombesin peptide derivatives as potential tumor imaging agents: effects of structure and composition of amino acid sequence on in vitro and in vivo characteristics

OBJECTIVES

Among the many clinically relevant peptide receptor systems, bombesin (BN) receptors have attracted enormous attraction due to their overexpression in various frequently occurring human tumors including breast and prostate, thus making such receptors promising targets with radiolabeled BN analogs. The present study describes the preparation and evaluation of a series of new BN derivatives as potential tumor imaging agents.

METHODS

Several new BN derivatives with the common structure MAG(3)-X-BN(1-14 or 6-14), where X=Asp or Asp-Asp, were synthesized by solid-phase peptide synthesis. S-benzoylmercaptoacetic acid was incorporated at the end of synthesis via manual conjugation to yield MAG(3)-BN conjugates. Radiolabeling with (99m)Tc was accomplished by ligand exchange method. The receptor-binding affinity assays were performed in MDA-MB-231, MCF-7, T47-D and PC-3 cancer cell lines. In vivo biodistribution and clearance kinetics were assessed in Balb/c mice, and tumor targeting efficacy was determined in nude mice bearing breast tumor xenografts.

RESULTS

The peptides were prepared conveniently and radiolabeled efficiently with (99m)Tc (up to 95% labeling efficiency). In vitro cell binding assays demonstrated high affinity (values in the nanomolar range) of (99m)Tc peptides towards breast and prostate cancer cell lines. In addition, the radioconjugates displayed significant internalization (values ranged between 19% and 35%) in tumor cells. In vivo biodistribution and biokinetics are characterized by efficient clearance from the blood and variable degrees of excretion through the renal pathway. In vivo tumor targeting studies displayed variable uptake capacity of different BN derivatives, underlining the influence of specific amino acid sequence on tumor targeting profiles. Tumor uptake was always higher than the radioactivity in the blood and muscle, with good tumor retention and good tumor-to-blood and tumor-to-muscle ratios, indicating the potential of these agents for targeting tumors in vivo.

CONCLUSIONS

The combination of favorable in vitro and in vivo properties may render these BN peptides as potential candidates for targeting BN/GRP receptor-positive tumors. They deserve further evaluation to determine their real strength. The present data indeed provide useful information regarding peptide structure-pharmacologic activity relationship, which might be useful in designing and developing new BN-like peptides for efficient targeting of tumors in vivo.

In vitro and in vivo characterization of novel 18F-labeled bombesin analogues for targeting GRPR-positive tumors

The gastrin-releasing peptide receptor (GRPR) is overexpressed on a number of human tumors and has been targeted with radiolabeled bombesin analogues for the diagnosis and therapy of these cancers. Seven bombesin analogues containing various linkers and peptide sequences were designed, synthesized, radiolabeled with (18)F, and characterized in vitro and in vivo as potential PET imaging agents. Binding studies displayed nanomolar binding affinities toward human GRPR for all synthesized bombesin analogues. Two high-affinity peptide candidates 6b (K(i) = 0.7 nM) and 7b (K(i) = 0.1 nM) were chosen for further in vivo evaluation. Both tracers revealed specific uptake in GRPR-expressing PC-3 tumors and the pancreas. Compared to [(18)F]6b, compound [(18)F]7b was characterized by superior tumor uptake, higher specificity of tracer uptake, and more favorable tumor-to-nontarget ratios. In vivo PET imaging allowed for the visualization of PC-3 tumor in nude mice suggesting that [(18)F]7b is a promising PET tracer candidate for the diagnosis of GRPR-positive tumors in humans.

Direct one-step 18F-labeling of peptides via nucleophilic aromatic substitution

Methods for the radiolabeling molecules of interest with [18F]-fluoride need to be rapid, convenient, and efficient. Numerous [18F]-labeled prosthetic groups, e.g., N-succinimidyl 4 [18F]-fluorobenzoate ([18F]-SFB), 4-azidophenacyl-[18F]-fluoride ([18F]-APF), and 1-(3-(2-[18F]fluoropyridin-3-yloxy)propyl)pyrrole-2,5-dione ([18F]-FpyMe), for conjugating to biomolecules have been developed. As the synthesis of these prosthetic groups usually requires multistep procedures, there is still a need for direct methods for the nucleophilic [18F]-fluorination of biomolecules. We report here on the development of a procedure based on the trimethylammonium (TMA) leaving group attached to an aromatic ring and activated with different electron-withdrawing groups (EWGs). A series of model compounds containing different electron-withdrawing substituents, a trimethylammonium leaving group, and carboxylic functionality for subsequent coupling to peptides were designed and synthesized. The optimal model compound, 2-cyano-4-(methoxycarbonyl)-N,N,N-trimethylbenzenaminium trifluoromethanesulfonate, was converted to carboxylic acid and coupled to peptides. The results of the one-step [18F]-fluorination of tetrapeptides and bombesin peptides show that the direct 18F-labeling of peptides is feasible under mild conditions and in good radiochemical yields.

New directions in the coordination chemistry of 99mTc: a reflection on technetium core structures and a strategy for new chelate design

Bifunctional chelates offer a general approach for the linking of radioactive metal cations to macromolecules. In the specific case of 99mTc, a variety of technologies have been developed for assembling a metal-chelate-biomolecule complex. An evaluation of these methodologies requires an appreciation of the coordination characteristics and preferences of the technetium core structures and oxidation states, which serve as platforms for the development of the imaging agent. Three technologies, namely, the MAG3-based bifunctional chelates, the N-oxysuccinimidylhydrazino-nicotinamide system and the recently described single amino acid chelates for the {Tc(CO)3}1+ core, are discussed in terms of the fundamental coordination chemistry of the technetium core structures. In assessing the advantages and disadvantages of these technologies, we conclude that the single amino acid analogue chelates (SAAC), which are readily conjugated to small peptides by solid-phase synthesis methods and which form robust complexes with the {Tc(CO)3}1+ core, offer an effective alternative to the previously described methods.

Peptide-based radiopharmaceuticals: Future tools for diagnostic imaging of cancers and other diseases

Small synthetic receptor-binding peptides are the agents of choice for diagnostic imaging and radiotherapy of cancers due to their favorable pharmacokinetics. Molecular modification techniques permit the synthesis of a variety of bioactive peptides with chelating groups, without compromising biological properties. Various techniques have been developed that allow efficient and site-specific labeling of peptides with clinically useful radionuclides such as (99m)Tc, (123)I, (111)In, and (18)F. Among them, (99m)Tc is the radionuclide of choice because of its excellent chemical and imaging characteristics. Recently, many (99m)Tc-labeled peptides have proven to be useful imaging agents. Beside (99m)Tc-labeled peptides, several peptides radiolabeled with (111)In and (123)I have been prepared and characterized. In addition, (18)F-labeled peptides hold clinical potential due to their ability to quantitatively detect and characterize a variety of human diseases using positron-emission tomography. The availability of this wide range of peptides labeled with different radionuclides offers multiple diagnostic and therapeutic applications. Various receptors are over-expressed in particular tumor types and peptides binding to these receptors can be used to visualize tumor lesions scintigraphically. Thus, radiolabeled peptides have potential use as carriers for the delivery of radionuclides to tumors, infarcts, and infected tissues for diagnostic imaging and radiotherapy. Many radiolabeled peptides are currently under investigation to determine their potential as imaging agents. These peptides are designed mainly for thrombus, tumor, and infection/inflammation imaging. This article presents recent developments in small synthetic peptides for imaging of thrombosis, tumors, and infection/inflammation.