Pretargeted Imaging beyond the Blood-Brain Barrier-Utopia or Feasible?

Head-to-Head Comparison of the in Vivo Performance of Highly Reactive and Polar 18F-Labeled Tetrazines

Pretargeted imaging harnessing tetrazine ligation has gained increased interest over recent years. Targeting vectors with slow pharmacokinetics may be visualized using short-lived radionuclides, such as fluorine-18 (18F) for positron emission tomography (PET), and result in improved target-to-background ratios compared to conventionally radiolabeled slowly accumulating vectors. We recently developed different radiochemical protocols enabling the direct radiofluorination of various tetrazine scaffolds, resulting in the development of various highly reactive and polar 18F-labeled tetrazines as lead candidates for pretargeted imaging. Here, we performed a direct head-to-head-comparison of our lead candidates to evaluate the most promising for future clinical translation. For that, all 18F-labeled tetrazine-scaffolds were synthesized in similar molar activity for improved comparability of their in vivo pretargeting performance. Intriguingly, previously reported dicarboxylic acid lead candidates with a net charge of -1 were outperformed by respective monocarboxylic acid derivatives bearing a net charge of 0, warranting further evaluation of such scaffolds prior to their clinical translation.

An 211At-labeled Tetrazine for Pretargeted Therapy

Pretargeted radioimmunoimaging has been shown to enhance tumor-to-background ratios by up to 125-fold at early time points, leading to more efficient and less toxic radionuclide therapies, particularly with shorter half-lives such as astatine-211 (211At). The tetrazine ligation is the most utilized bioorthogonal reaction in these strategies, making tetrazines ideal for 211At labeling and controlling the biodistribution. We developed a 211At-labeled pretargeting agent for alpha-radionuclide therapy, achieving a radiochemical yield of approximately 65% and purity over 99%. Our results showed higher tumor-to-blood ratios within the first 24 h compared to directly labeled monoclonal antibodies. This suggests that pretargeted therapy may deliver better tumor doses than conventional methods, although the deastatination observed will need to be addressed in future tetrazine developments.

Synthesis and evaluation of fluorine-18 labelled tetrazines as pre-targeting imaging agents for PET

BACKGROUND

The brain is a challenging target for antibody-based positron emission tomography (immunoPET) imaging due to the restricted access of antibody-based ligands through the blood-brain barrier (BBB). To overcome this physiological obstacle, we have previously developed bispecific antibody ligands that pass through the BBB via receptor-mediated transcytosis. While these radiolabelled ligands have high affinity and specificity, their long residence time in the blood and brain, typical for large molecules, poses another challenge for PET imaging. A viable solution could be a two-step pre-targeting approach which involves the administration of a tagged antibody that accumulates at the target site in the brain and then clears from the blood, followed by administration of a small radiolabelled molecule with fast kinetics. This radiolabelled molecule can couple to the tagged antibody and thereby make the antibody localisation visible by PET imaging. The in vivo linkage can be achieved by using the inverse electron demand Diels-Alder reaction (IEDDA), with trans-cyclooctene (TCO) and tetrazine groups participating as reactants. In this study, two novel 18F-labelled tetrazines were synthesized and evaluated for their potential use as pre-targeting imaging agents, i.e., for their ability to rapidly enter the brain and, if unbound, to be efficiently cleared with minimal background retention.

RESULTS

The two compounds, a methyl tetrazine [18F]MeTz and an H-tetrazine [18F]HTz were radiolabelled using a two-step procedure via [18F]F-Py-TFP synthesized on solid support followed by amidation with amine-bearing tetrazines, resulting in radiochemical yields of 24% and 22%, respectively, and a radiochemical purity of > 96%. In vivo PET imaging was performed to assess their suitability for in vivo pre-targeting. Time-activity curves from PET-scans showed [18F]MeTz to be the more pharmacokinetically suitable agent, given its fast and homogenous distribution in the brain and rapid clearance. However, in terms of rection kinetics, H-tetrazines are advantageous, exhibiting faster reaction rates in IEDDA reactions with dienophiles like trans-cyclooctenes, making [18F]HTz potentially more beneficial for pre-targeting applications.

CONCLUSION

This study demonstrates a significant potential of [18F]MeTz and [18F]HTz as agents for pre-targeted PET brain imaging due to their efficient brain uptake, swift clearance and appropriate chemical stability.

Reducing neonatal Fc receptor binding enhances clearance and brain-to-blood ratio of TfR-delivered bispecific amyloid-β antibody

Recent development of amyloid-β (Aβ)-targeted immunotherapies for Alzheimer's disease (AD) have highlighted the need for accurate diagnostic methods. Antibody-based positron emission tomography (PET) ligands are well suited for this purpose as they can be directed toward the same target as the therapeutic antibody. Bispecific, brain-penetrating antibodies can achieve sufficient brain concentrations, but their slow blood clearance remains a challenge, since it prolongs the time required to achieve a target-specific PET signal. Here, two antibodies were designed based on the Aβ antibody bapineuzumab (Bapi) - one monospecific IgG (Bapi) and one bispecific antibody with an antigen binding fragment (Fab) of the transferrin receptor (TfR) antibody 8D3 fused to one of the heavy chains (Bapi-Fab8D3) for active, TfR-mediated transport into the brain. A variant of each antibody was designed to harbor a mutation to the neonatal Fc receptor (FcRn) binding domain, to increase clearance. Blood and brain pharmacokinetics of radiolabeled antibodies were studied in wildtype (WT) and AD mice (AppNL-G-F). The FcRn mutation substantially reduced blood half-life of both Bapi and Bapi-Fab8D3. Bapi-Fab8D3 showed high brain uptake and the brain-to-blood ratio of its FcRn mutated form was significantly higher in AppNL-G-F mice than in WT mice 12 h after injection and increased further up to 168 h. Ex vivo autoradiography showed specific antibody retention in areas with abundant Aβ pathology. Taken together, these results suggest that reducing FcRn binding of a full-sized bispecific antibody increases the systemic elimination and could thereby drastically reduce the time from injection to in vivo imaging.

Development of 11 C-labeled CRANAD-102 for positron emission tomography imaging of soluble Aβ-species

CRANAD-102, a selective near-infrared fluorescent tracer targeting soluble amyloid-β (Aβ) species, has recently attracted attention due to its potential to be used as a diagnostic tool for early stages of Alzheimer's disease (AD). Development of a positron emission tomography (PET) tracer based on CRANAD-102 could as such allow to noninvasively study soluble and protofibrillar species of Aβ in humans. These soluble and protofibrillar species are thought to be responsible to cause AD. Within this work, we successfully 11 C-labeled CRANAD-102 via a Suzuki-Miyaura reaction in a RCС of 48 ± 9%, with a RCP of >96% and a molar activity (Am ) of 25 ± 7 GBq/μmol. Future studies have to be conducted to evaluate if [11 C]CRANAD-102 can be used to detect soluble protofibrils in vivo and if [11 C]CRANAD-102 can be used to detect AD earlier as possible with current diagnostics.

Click Reactions in Medicinal Chemistry

Two decades after K [...].

Preclinical Evaluation of Novel PET Probes for Dementia

The development of novel PET imaging agents that selectively bind specific dementia-related targets can contribute significantly to accurate, differential and early diagnosis of dementia causing diseases and support the development of therapeutic agents. Consequently, in recent years there has been a growing body of literature describing the development and evaluation of potential new promising PET tracers for dementia. This review article provides a comprehensive overview of novel dementia PET probes under development, classified by their target, and pinpoints their preclinical evaluation pathway, typically involving in silico, in vitro and ex/in vivo evaluation. Specific target-associated challenges and pitfalls, requiring extensive and well-designed preclinical experimental evaluation assays to enable successful clinical translation and avoid shortcomings observed for previously developed 'well-established' dementia PET tracers are highlighted in this review.

Applications of radiolabeled antibodies in neuroscience and neuro-oncology

Positron emission tomography (PET) is a powerful tool in medicine and drug development, allowing for non-invasive imaging and quantitation of biological processes in live organisms. Targets are often probed with small molecules, but antibody-based PET is expanding because of many benefits, including ease of design of new antibodies toward targets, as well as the very strong affinities that can be expected. Application of antibodies to PET imaging of targets in the central nervous system (CNS) is a particularly nascent field, but one with tremendous potential. In this review, we discuss the growth of PET in imaging of CNS targets, present the promises and progress in antibody-based CNS PET, explore challenges faced by the field, and discuss questions that this promising approach will need to answer moving forward for imaging and perhaps even radiotherapy.

Recent Advances in Bioorthogonal Click Chemistry for Enhanced PET and SPECT Radiochemistry

Due to their high reaction rate and reliable selectivity, bioorthogonal click reactions have been extensively investigated in numerous research fields, such as nanotechnology, drug delivery, molecular imaging, and targeted therapy. Previous reviews on bioorthogonal click chemistry for radiochemistry mainly focus on ¹⁸F-labeling protocols employed to produce radiotracers and radiopharmaceuticals. In fact, besides fluorine-18, other radionuclides such as gallium-68, iodine-125, and technetium-99m are also used in the field of bioorthogonal click chemistry. Herein, to provide a more comprehensive perspective, we provide a summary of recent advances in radiotracers prepared using bioorthogonal click reactions, including small molecules, peptides, proteins, antibodies, and nucleic acids as well as nanoparticles based on these radionuclides. The combination of pretargeting with imaging modalities or nanoparticles, as well as the clinical translations study, are also discussed to illustrate the effects and potential of bioorthogonal click chemistry for radiopharmaceuticals.

ImmunoPET Directed to the Brain: A New Tool for Preclinical and Clinical Neuroscience

Immuno-positron emission tomography (immunoPET) is a non-invasive in vivo imaging method based on tracking and quantifying radiolabeled monoclonal antibodies (mAbs) and other related molecules, such as antibody fragments, nanobodies, or affibodies. However, the success of immunoPET in neuroimaging is limited because intact antibodies cannot penetrate the blood-brain barrier (BBB). In neuro-oncology, immunoPET has been successfully applied to brain tumors because of the compromised BBB. Different strategies, such as changes in antibody properties, use of physiological mechanisms in the BBB, or induced changes to BBB permeability, have been developed to deliver antibodies to the brain. These approaches have recently started to be applied in preclinical central nervous system PET studies. Therefore, immunoPET could be a new approach for developing more specific PET probes directed to different brain targets.