One-Pot Radiosynthesis and Biological Evaluation of a Caspase-3 Selective 5-[123,125I]iodo-1,2,3-triazole derived Isatin SPECT Tracer

Development of caspase-3-selective activity-based probes for PET imaging of apoptosis

BACKGROUND

The cysteine-aspartic acid protease caspase-3 is recognized as the main executioner of apoptosis in cells responding to specific extrinsic and intrinsic stimuli. Caspase-3 represents an interesting biomarker to evaluate treatment response, as many cancer therapies exert their effect by inducing tumour cell death. Previously developed caspase-3 PET tracers were unable to reach routine clinical use due to low tumour uptake or lack of target selectivity, which are two important requirements for effective treatment response evaluation in cancer patients. Therefore, the goal of this study was to develop and preclinically evaluate novel caspase-3-selective activity-based probes (ABPs) for apoptosis imaging.

RESULTS

A library of caspase-3-selective ABPs was developed for tumour apoptosis detection. In a first attempt, the inhibitor Ac-DW3-KE (Ac-3Pal-Asp-βhLeu-Phe-Asp-KE) was 18F-labelled on the N-terminus to generate a radiotracer that was incapable of adequately detecting an increase in apoptosis in vivo. The inability to effectively detect active caspase-3 in vivo was likely attributable to slow binding, as demonstrated with in vitro inhibition kinetics. Hence, a second generation of caspase-3 selective ABPs was developed based on the Ac-ATS010-KE (Ac-3Pal-Asp-Phe(F5)-Phe-Asp-KE) with greatly improved binding kinetics over Ac-DW3-KE. Our probes based on Ac-ATS010-KE were made by modifying the N-terminus with 6 different linkers. All the linker modifications had limited effect on the binding kinetics, target selectivity, and pharmacokinetic profile in healthy mice. In an in vitro apoptosis model, the least hydrophilic tracer [18F]MICA-316 showed an increased uptake in apoptotic cells in comparison to the control group. Finally, [18F]MICA-316 was tested in an in vivo colorectal cancer model, where it showed a limited tumour uptake and was unable to discriminate treated tumours from the untreated group, despite demonstrating that the radiotracer was able to bind caspase-3 in complex mixtures in vitro. In contrast, the phosphatidylethanolamine (PE)-binding radiotracer [99mTc]Tc-duramycin was able to recognize the increased cell death in the disease model, making it the best performing treatment response assessment tracer developed thus far.

CONCLUSIONS

In conclusion, a novel library of caspase-3-binding PET tracers retaining similar binding kinetics as the original inhibitor was developed. The most promising tracer, [18F]MICA-316, showed an increase uptake in an in vitro apoptosis model and was able to selectively bind caspase-3 in apoptotic tumour cells. In order to distinguish therapy-responsive from non-responsive tumours, the next generation of caspase-3-selective ABPs will be developed with higher tumour accumulation and in vivo stability.

One-pot two-step radioiodination based on copper-mediated iododeboronation and azide-alkyne cycloaddition reaction

This methodology demonstrates the ability to sequentially regulate copper-mediated radioiododeboronation and an azide-alkyne cycloaddition reaction, which facilitates the continuous incorporation of reagents into the reaction system and mediates the integration of the purification steps into the final process. Additionally, this reaction is suited to be conducted under mild conditions and yields target compounds through potent radiochemical conversions.

Radionuclide imaging of apoptosis for clinical application

Apoptosis was a natural, non-inflammatory, energy-dependent form of programmed cell death (PCD) that can be discovered in a variety of physiological and pathological processes. Based on its characteristic biochemical changes, a great number of apoptosis probes for single-photon emission computed tomography (SPECT) and positron emission tomography (PET) have been developed. Radionuclide imaging with these tracers were potential for the repetitive and selective detection of apoptotic cell death in vivo, without the need for invasive biopsy. In this review, we overviewed molecular mechanism and specific biochemical changes in apoptotic cells and summarized the existing tracers that have been used in clinical trials as well as their potentialities and limitations. Particularly, we highlighted the clinic applications of apoptosis imaging as diagnostic markers, early-response indicators, and prognostic predictors in multiple disease fields.

Recent advances in synthesis and anticancer potential of triazole containing scaffolds

Cancer is the most lethal disease that may be found anywhere on the globe. Approximately 10% of individuals die as a result of cancer of various types, with 19.3 million new cancer cases and 10 million deaths expected in 2020. More than 100 medications are commercially available for the treatment of cancer, but only a few candidates have high specificity, resulting in several side effects. The scientific community has spent the past decades focusing on drug discovery. Natural resources are used to isolate pharmaceutically active candidates, which are then synthesized in laboratories. More than 60% of all prescribed drugs are made from natural ingredients. Unique five-membered heteroaromatic center motifs with sulfur, oxygen and nitrogen atoms are found in heterocyclic compounds such as indazole, thiazole, triazole, triazole, and oxazole, and are used as a core scaffold in many medicinally important therapies. Triazole possesses a wide range of pharmacological activities including anticancer, antibacterial, antifungal, antibiotic antiviral, analgesic, anti-inflammatory, anti-HIV, antidiabetic, and antiprotozoal activities. Novel Triazole motifs with a variety of biological characteristics have been successfully synthesized using versatile synthetic methods. We intend here to facilitate the rational design and development of innovative triazole-based anti-cancer medicines with increased selectivity for various cancer cell lines by providing insight into various ligand-receptor interactions.

Molecular Imaging of Apoptosis: The Case of Caspase-3 Radiotracers

The molecular imaging of apoptosis remains an important method for the diagnosis and monitoring of the progression of certain diseases and the evaluation of the efficacy of anticancer apoptosis-inducing therapies. Among the multiple biomarkers involved in apoptosis, activated caspase-3 is an attractive target, as it is the most abundant of the executioner caspases. Nuclear imaging is a good candidate, as it combines a high depth of tissue penetration and high sensitivity, features necessary to detect small changes in levels of apoptosis. However, designing a caspase-3 radiotracer comes with challenges, such as selectivity, cell permeability and transient caspase-3 activation. In this review, we discuss the different caspase-3 radiotracers for the imaging of apoptosis together with the challenges of the translation of various apoptosis-imaging strategies in clinical trials.

Recent advances in stimuli-responsive in situ self-assembly of small molecule probes for in vivo imaging of enzymatic activity

Stimuli-responsive in situ self-assembly of small molecule probes into nanostructures has been promising for the construction of molecular probes for in vivo imaging. In the past few years, a number of intelligent molecular imaging probes with fluorescence, magnetic resonance imaging (MRI), positron electron tomography (PET) or photoacoustic imaging (PA) modality have been developed based on the in situ self-assembly strategy. In this minireview, we summarize the recent advances in the development of different modality imaging probes through controlling in situ self-assembly for in vivo imaging of enzymatic activity. This review starts from the brief introduction of two different chemical approaches amenable for in situ self-assembly, including (1) stimuli-mediated proteolysis and (2) stimuli-triggered biocompatible reaction. We then discuss their applications in the design of fluorescence, MRI, PET, PA, and bimodality imaging probes for in vivo imaging of different enzymes, such as caspase-3, furin, gelatinase and phosphatase. Finally, we discuss the current and prospective challenges in the stimuli-responsive in situ self-assembly strategy for in vivo imaging.

[18F]FET-βAG-TOCA: The Design, Evaluation and Clinical Translation of a Fluorinated Octreotide

The success of Lutathera™ ([¹⁷⁷Lu]Lu-DOTA-TATE) in the NETTER-1 clinical trial as a peptide receptor radionuclide therapy (PRRT) for somatostatin receptor expressing (SSTR) neuroendocrine tumours (NET) is likely to increase the demand for patient stratification by positron emission tomography (PET). The current gold standard of gallium-68 radiolabelled somatostatin analogues (e.g., [⁶⁸Ga]Ga-DOTA-TATE) works effectively, but access is constrained by the limited availability and scalability of gallium-68 radiopharmaceutical production. The aim of this review is three-fold: firstly, we discuss the peptide library design, biological evaluation and clinical translation of [¹⁸F]fluoroethyltriazole-βAG-TOCA ([¹⁸F]FET-βAG-TOCA), our fluorine-18 radiolabelled octreotide; secondly, to exemplify the potential of the 2-[¹⁸F]fluoroethylazide prosthetic group and copper-catalysed azide-alkyne cycloaddition (CuAAC) chemistry in accessing good manufacturing practice (GMP) compatible radiopharmaceuticals; thirdly, we aim to illustrate a framework for the translation of similarly radiolabelled peptides, in which in vivo pharmacokinetics drives candidate selection, supported by robust radiochemistry methodology and a route to GMP production. It is hoped that this review will continue to inspire the development and translation of fluorine-18 radiolabelled peptides into clinical studies for the benefit of patients.

Guest Edited Collection: Radioisotopes and radiochemistry in health science

Radioisotopes can be produced artificially from stable nuclei through the interaction with particles or highly energetic photons. In combination with modern detection and counting techniques, radioisotopes and radiochemical methods uniquely contribute to the health sciences. This Collection showcases salient aspects of medical radioisotope science ranging from the production, recovery and purification of radioisotopes to the methods used to attach them to biomolecules. The Collection also presents studies that highlight the importance of radiochemistry in the assessment of environmental radioactivity.