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Direct Cu-mediated aromatic 18F-labeling of highly reactive tetrazines for pretargeted bioorthogonal PET imaging† † Electronic supplementary information (ESI) available. See DOI: 10.1039/d1sc02789a

ABSTRACT: Pretargeted imaging can be used to visualize and quantify slow-accumulating targeting vectors with short-lived radionuclides such as fluorine-18 – the most popular clinically applied Positron Emission Tomography (PET) radionuclide. Pretargeting results in higher target-to-background ratios compared to conventional imaging approaches using long-lived radionuclides. Currently, the tetrazine ligation is the most popular bioorthogonal reaction for pretargeted imaging, but a direct 18F-labeling strategy for highly reactive tetrazines, which would be highly beneficial if not essential for clinical translation, has thus far not been reported. In this work, a simple, scalable and reliable direct 18F-labeling procedure has been developed. We initially studied the applicability of different leaving groups and labeling methods to develop this procedure. The copper-mediated 18F-labeling exploiting stannane precursors showed the most promising results. This approach was then successfully applied to a set of tetrazines, including highly reactive H-tetrazines, suitable for pretargeted PET imaging. The labeling succeeded in radiochemical yields (RCYs) of up to approx. 25%. The new procedure was then applied to develop a pretargeting tetrazine-based imaging agent. The tracer was synthesized in a satisfactory RCY of ca. 10%, with a molar activity of 134 ± 22 GBq μmol−1 and a radiochemical purity of >99%. Further evaluation showed that the tracer displayed favorable characteristics (target-to-background ratios and clearance) that may qualify it for future clinical translation. A simple, scalable and reliable direct 18F-labeling procedure has been developed and applied to obtain a pretargeting tetrazine-based imaging agent with favorable in vivo characteristics.

SUBMITTER: Garcia-Vazquez R

PROVIDER: S-EPMC8442695 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Project description:Background: The pretargeted imaging strategy using inverse electron demand Diels-Alder (IEDDA) cycloaddition between a trans-cyclooctene (TCO) and tetrazine (Tz) has emerged and rapidly grown as a promising concept to improve radionuclide imaging and therapy in oncology. This strategy has mostly relied on the use of radiolabeled Tz together with TCO-modified targeting vectors leading to a rapid growth of the number of available radiolabeled tetrazines, while only a few radiolabeled TCOs are currently reported. Here, we aim to develop novel and structurally diverse 18F-labeled cis-dioxolane-fused TCO (d-TCO) derivatives to further expand the bioorthogonal toolbox for in vivo ligation and evaluate their potential for positron emission tomography (PET) pretargeted imaging. Results: A small series of d-TCO derivatives were synthesized and tested for their reactivity against tetrazines, with all compounds showing fast reaction kinetics with tetrazines. A fluorescence-based pretargeted blocking study was developed to investigate the in vivo ligation of these compounds without labor-intensive prior radiochemical development. Two compounds showed excellent in vivo ligation results with blocking efficiencies of 95 and 97%. Two novel 18F-labeled d-TCO radiotracers were developed, from which [18F]MICA-214 showed good in vitro stability, favorable pharmacokinetics, and moderate in vivo stability. Micro-PET pretargeted imaging with [18F]MICA-214 in mice bearing LS174T tumors treated with tetrazine-modified CC49 monoclonal antibody (mAb) (CC49-Tz) showed significantly higher uptake in tumor tissue in the pretargeted group (CC49-Tz 2.16 ± 0.08% ID/mL) when compared to the control group with nonmodified mAb (CC49 1.34 ± 0.07% ID/mL). Conclusions: A diverse series of fast-reacting fluorinated d-TCOs were synthesized. A pretargeted blocking approach in tumor-bearing mice allowed the choice of a lead compound with fast reaction kinetics with Tz. A novel 18F-labeled d-TCO tracer was developed and used in a pretargeted PET imaging approach, allowing specific tumor visualization in a mouse model of colorectal cancer. Although further optimization of the radiotracer is needed to enhance the tumor-to-background ratios for pretargeted imaging, we anticipate that the 18F-labeled d-TCO will find use in studies where increased hydrophilicity and fast bioconjugation are required.

Project description:The specificity of antibodies have made immunoconjugates promising vectors for the delivery of radioisotopes to cancer cells; however, their long pharmacologic half-lives necessitate the use of radioisotopes with long physical half-lives, a combination that leads to high radiation doses to patients. Therefore, the development of targeting modalities that harness the advantages of antibodies without their pharmacokinetic limitations is desirable. To this end, we report the development of a methodology for pretargeted PET imaging based on the bioorthogonal Diels-Alder click reaction between tetrazine and transcyclooctene.A proof-of-concept system based on the A33 antibody, SW1222 colorectal cancer cells, and (64)Cu was used. The huA33 antibody was covalently modified with transcyclooctene, and a NOTA-modified tetrazine was synthesized and radiolabeled with (64)Cu. Pretargeted in vivo biodistribution and PET imaging experiments were performed with athymic nude mice bearing A33 antigen-expressing, SW1222 colorectal cancer xenografts.The huA33 antibody was modified with transcyclooctene to produce a conjugate with high immunoreactivity, and the (64)Cu-NOTA-labeled tetrazine ligand was synthesized with greater than 99% purity and a specific activity of 9-10 MBq/?g. For in vivo experiments, mice bearing SW1222 xenografts were injected with transcyclooctene-modified A33; after allowing 24 h for accumulation of the antibody in the tumor, the mice were injected with (64)Cu-NOTA-labeled tetrazine for PET imaging and biodistribution experiments. At 12 h after injection, the retention of uptake in the tumor (4.1 ± 0.3 percent injected dose per gram), coupled with the fecal excretion of excess radioligand, produced images with high tumor-to-background ratios. PET imaging and biodistribution experiments performed using A33 directly labeled with either (64)Cu or (89)Zr revealed that although absolute tumor uptake was higher with the directly radiolabeled antibodies, the pretargeted system yielded comparable images and tumor-to-muscle ratios at 12 and 24 h after injection. Further, dosimetry calculations revealed that the (64)Cu pretargeting system resulted in only a fraction of the absorbed background dose of A33 directly labeled with (89)Zr (0.0124 mSv/MBq vs. 0.4162 mSv/MBq, respectively).The high quality of the images produced by this pretargeting approach, combined with the ability of the methodology to dramatically reduce nontarget radiation doses to patients, marks this system as a strong candidate for clinical translation.

Project description:Neurodegenerative biomarkers support diagnosis and measurement of disease progression in the Alzheimer's disease (AD) continuum. 18F-Fluorodeoxyglucose Positron Emission Tomography (18F-FDG-PET), which measures glucose metabolism, is one of the most commonly used biomarkers of neurodegeneration, but is expensive and requires exposure to ionizing radiation. Arterial Spin Labeled (ASL) perfusion Magnetic Resonance Imaging (MRI) provides non invasive quantification of cerebral blood flow (CBF), which is believed to be tightly coupled to glucose metabolism. Here we aimed to compare the performances of ASL derived CBF and 18F-FDG-PET derived standardized uptake value ratio (SUVR) in discriminating patients with mild cognitive impairment (MCI) from older Controls. 2D pseudo continuous ASL and 18F-FDG-PET data with adequate scan quality from 50 MCI study participants (age=73.0 ± 7.0 years, 16 female) and 35 older controls (age=70.2 ± 6.9 years, 20 female), acquired in close temporal proximity, usually on the same day, were considered for this study. We assessed Control-patient group differences both at voxel level and within a priori regions of interest (ROIs). We also compared their area under receiver operating characteristic curves (AUC) with mean CBF or SUVR in a priori selected posterior cingulate cortex (PCC). CBF and 18F-FDG-PET showed abnormalities in similar areas, particularly in medial temporoparietal regions, consistent with the typically observed pattern of prodromal AD. The hypoperfusion pattern with relative CBF (obtained by normalizing voxel CBF values with mean CBF in putamen) was more localized than with absolute CBF. Pearson's correlation coefficients between the T-scores corresponding to the group-differences obtained with 18F-FDG-PET SUVR and absolute and relative ASL CBF were 0.46 and 0.43 (p<0.001), respectively. ROI analyses were also consistent, with the strongest differences observed in PCC (p<0.01). 18F-FDG-PET SUVR, absolute and relative CBF in the PCC ROI demonstrated moderate and similar discriminatory power in predicting MCI status with AUC of 0.71 ± 0.12, 0.77 ± 0.12 and 0.74 ± 0.13, respectively. In conclusion, ASL CBF may be a reasonable, less expensive and safer substitute for 18F-FDG-PET in clinical research.

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Direct Cu-mediated aromatic 18F-labeling of highly reactive tetrazines for pretargeted bioorthogonal PET imaging† † Electronic supplementary information (ESI) available. See DOI: 10.1039/d1sc02789a

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