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“In Vivo Click PET Imaging Agents”: Improving clinical companion diagnostics

Periodic Reporting for period 4 - Click-It (“In Vivo Click PET Imaging Agents”: Improving clinical companion diagnostics)

Reporting period: 2020-07-01 to 2021-06-30

Companion diagnostics are crucial for drug development and disease management with regard to patient selection, therapy planning and monitoring. Nanomedicines such as antibodies have been proven to be optimal disease-targeting agents because they generally exhibit superior target uptake and retention. However, to date, nuclear imaging of nanomedicines has been limited to the use of long-lived isotopes to be compatible with the slow pharmacokinetics of these large molecules. Major drawbacks are high radiation doses, precluding routine and repeated companion imaging procedures.

The Click-It consortium aimed to circumvent this issue by using pretargeting approach, which centers on the administration and target binding of a tagged nanomedicine followed by administration and binding of a small, fast-clearing, short-lived radiolabelled probe to the tag of the nanomedicine. This results in lower absorbed radiation doses and in a boost in target-blood ratios, which in turn leads to a superior imaging contrast. PET scan snapshots at multiple time-points provide long-term imaging information by applying short-lived nuclides. Before the project, only the fastest click reaction, the tetrazine ligation, had demonstrated potential in clinically relevant conditions.

This project aimed at expanding the scope of click-pretargeted imaging to intracellular targets, because a majority of nanomedicines internalize and is thus not accessible with the current approach. Furthermore, we wanted to expand our approach to short-lived, non-metal based, small molecule 18F-PET tracers, since PET offers a higher spatial and temporal resolution than SPECT enabling quantitative decision making in disease diagnosis and management. Finally, the project aimed to translate the developed click-pretargeting technology into a clinically applicable nanomedicine-based imaging approach.

The project has concluded with three promising leads that are now planned for translation into more advanced animal models. This was originally planned to take place during the project but was delayed due to COVID-19. It is still the consortium’s intention to deliver on this. Pretargeted probes have in animal studies been shown to outperform conventional monoclonal antibody based probes with respect to image contrast at early timepoints. Promising clearing agents have been discovered and further tests await to decide the optimal clearing agent for clinical translation.

The consortium has over the years built up knowledge and expertise. The project has resulted in expansion of the partners’ IP portfolios and resulted in new innovative project ideas, both for the partners to pursue individually and in collaboration.
Within this project, we elucidated the minimal structural features necessary for a tetrazine to reach and react in pretargeted vivo approaches. This knowledge enabled us to design tetrazines that are suitable to ligate rapidly in vivo at the tumor site. These efforts were complemented by the development of procedures to radiolabel highly reactive tetrazine scaffolds that have been out of reach so far, finally to obtain 18F-labeled bioorthogonal probes required for efficient pretargeted imaging. In this respect, we developed methods to radiolabel tetrazines at aliphatic and aromatic positions. This know-how in turn allowed us to develop two 18F-labeled tetrazines that display the best pharmacokinetic as well as imaging properties at early timepoints of all probes known and reported so far. Based on computational studies and comprehensive investigation of the underlying mechanisms we have designed bioorthogonal tools with substantially improved chemical performance and stability under physiological conditions. Structurally modified trans-cyclooctenes (TCOs) with unique molecular capabilities have been developed and shown to enable efficient bioorthogonal transformations in vivo. Methods for bioconjugation of monoclonal antibodies and the preparation of modified polymeric scaffolds were established, facilitating the design of improved targeting molecules and clearing agents. All compounds and tools have been evaluated in detail using various in vitro and in vivo model systems, overall confirming exceptional performance regarding stability, selectivity, reaction kinetics and biocompatibility. In particular, we have discovered and applied the significantly accelerated bioorthogonal reaction of TCO-modified polypeptide-graft-polypeptoid copolymers with radiolabeled tetrazines.

The results from Click-It have been presented at multiple conferences over the years. Fifteen articles have been published, one is under review and further six are planned for submission within the next six months. Furthermore, eight patents have been filed.
The ultimate goal of this consortium is to enable pretargeted imaging of tumor-associated targets with PET and fluorine-18. The possibility to perform this experiments will enable society to develop efficient companions diagnostics which display high contrast and low radiation burden. Pretargeted imaging is especially interesting for slow targeting vectors such as antibodies.
Pyramid model of Click-It's selection criteria during the technology development process