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Pretargeting imaging has gained a lot of prominence, due to its excellent bioorthogonality and improved imaging contrast compared to conventional imaging. A new iodo tetrazine (Tz) derivative has been synthesized and further developed into the corresponding iodine-125 ( 125 I) analog (12), via the trimethylstannane precursor. Radiolabeling with either N-chlorosuccinimide or chloramine-T, in either MeCN or MeOH proceeded with a radiochemical conversion (RCC) of >80%. Subsequent deprotection only proved successful, among the tested conditions, when the radiolabeled Tz was stirred in 6-M HCl (aq.) at 60°C for 2.5 h. To the best of our knowledge, this is the first H-tetrazine labeled with iodine. In vivo investigations on the pretargeting ability of 12 are currently under way.

Tucatinib is a selective human epidermal growth factor receptor 2 (HER2) tyrosine kinase inhibitor approved by the U.S. Food and Drug Administration (FDA) in April 2020 for HER2-positive lesions in metastatic breast cancer patients, including CNS metastases. In this article, we attempted to develop the first small molecule, blood-brain-barrier (BBB) penetrant HER2 PET imaging probe based on tucatinib. [

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.

Pretargeted PET imaging is an emerging and fast-developing method to monitor immuno-oncology strategies. Currently, tetrazine ligation is considered the most promising bioorthogonal reaction for pretargeting in vivo. Recently, we have developed a method to

Tucatinib is a selective human epidermal growth factor receptor 2 (HER2) tyrosine kinase inhibitor approved by the U.S. Food and Drug Administration (FDA) in April 2020 for HER2-positive lesions in metastatic breast cancer patients, including CNS metastases. In this article, we attempted to develop the first small molecule, blood–brain-barrier (BBB) penetrant HER2 PET imaging probe based on tucatinib. [11C]tucatinib was synthesized via a Stille-coupling from the respective trimethylstannyl precursor and its biodistribution was evaluated in NMRI nude mice bearing HER2-overexpressing human ovarian cancer cells (SKOV-3). No significant tumor accumulation was observed despite its high affinity for HER-2 receptors (IC50 = 6.9 nM). High liver and intestinal uptake indicate that [11C]tucatinib is too lipophilic to be used as a tumor targeting PET tracer. Therefore, chemical modifications of [11C]tucatinib are needed to increase the polarity for tumor imaging. Tucatinib as an FDA approved drug is still an interesting platform to develop the first small molecule HER2-selective PET tracer. The study highlights the differences between a drug, which needs to be effective, and an imaging agent, which is dependent on contrast.

Pretargeted imaging of nanomedicines have attracted considerable interest because it has the potential to increase imaging contrast while reducing radiation burden to healthy tissue. Currently, the tetrazine ligation is the fastest bioorthogonal reaction for this strategy and, consequently, the state-of-art choice for

Pretargeting imaging of nanomedicines have attracted considerable interest in nuclear medicine since it has the potential to increase imaging contrast while simultaneously reducing radiation burden to healthy tissue. Currently, the tetrazine ligation is the fastest bioorthogonal reaction available for this strategy and consequently, the state-of-art choice for in vivochemistry. We have recently identified key properties for tetrazines to be applied in pretargeting. We have also developed a method to 18F-label highly reactive tetrazines using an aliphatic nucleophilic substitution strategy. In this study, we combined this knowledge and developed an 18F-labeled tetrazine for pretargeted imaging. In order to develop this ligand, a small structure-property study was carried out. The most promising compound - with respect to reactivity, hydrophilicity and ex vivo blocking effect - was selected for labeling and subsequent PET in vivo imaging. Radiolabeling was achieved in satisfying radiochemical yields, molar activities as well as in high radiochemical purities. The tracer displayed favorable pharmacokinetics and remarkable target-to-background ratios in pretargeted experiments - already one hour post injection. We believe that the developed pretargeting imaging agent is a promising candidate for translation into clinical studies.

Pretargeted bioorthogonal imaging can be used to visualize and quantify slow accumulating targeting vectors with short-lived radionuclides such as fluorine-18 - the most 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 direct18 F-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 18 F-labeling procedure has been developed and applied to obtain a pretargeting tetrazine-based imaging agent with favorable characteristics (target-to-background ratios and clearance) that may qualify it for future clinical translation.

The development of highly selective and fast biocompatible reactions for ligation and cleavage has paved the way for new diagnostic and therapeutic applications of pretargeted

The development of highly selective and fast biocompatible reactions for ligation and cleavage has paved the way for new diagnostic and therapeutic applications of in vivo chemistry. The concept of bioorthogonal pretargeting has attracted considerable interest, in particular for the targeted delivery of radionuclides and drugs. In nuclear medicine, pretargeting can provide increased target-to-background ratios at early time-points compared to traditional approaches. This reduces the radiation burden to healthy tissue and, depending on the selected radionuclide, enables better imaging contrast or higher therapeutic efficiency. Moreover, bioorthogonally triggered cleavage of pretargeted antibody-drug conjugates represents an emerging strategy to achieve controlled release and locally increased drug concentrations. The toolbox of bioorthogonal reactions has significantly expanded in the past decade, with the tetrazine ligation being the fastest and one of the most versatile in vivo chemistries. Progress in the field, however, relies heavily on the development and evaluation of (radio)labeled compounds, preventing the use of compound libraries for systematic studies. The rational design of tetrazine probes and triggers has thus been impeded by the limited understanding of the impact of structural parameters on the in vivo ligation performance. In this work, we describe the development of a pretargeted blocking assay that allows for the investigation of the in vivo fate of a structurally diverse library of 45 unlabeled tetrazines and their capability to reach and react with pretargeted trans-cyclooctene (TCO)-tagged antibodies in tumor-bearing mice. This study enabled us to assess the correlation of click reactivity and lipophilicity of tetrazines with their in vivo performance. In particular, high rate constants (>50,000 M-1s-1) for the reaction with TCO and low calculated logD7.4 values (below -3) of the tetrazine were identified as strong indicators for successful pretargeted in vivo click chemistry. Click-radiolabeling gave access to a set of selected 18F-labeled tetrazines, including highly reactive scaffolds, which were used in pretargeted PET imaging studies to confirm the results from the blocking study. These insights thus enable the rational design of tetrazine probes for in vivo application and will thereby assist the clinical translation of bioorthogonal pretargeting.