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

Periodic Reporting for period 3 - Click-It (“In Vivo Click PET Imaging Agents”: Improving clinical companion diagnostics)

Reporting period: 2019-01-01 to 2020-06-30

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 aims 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. So far, only the fastest click reaction, the tetrazine ligation, has demonstrated potential in clinically relevant conditions. We have shown in a SPECT imaging study that this click reaction can be applied for non-internalizing nanomedicines in vivo.

This project aims 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 will expand our approach to short-lived, non-metal based, small molecule 18F-PET tracers, since PET offers a higher spatial and temporal resolution enabling quantitative decision making in disease diagnosis and management. Finally, the project aims to translate the developed click-pretargeting technology into a clinically applicable nanomedicine-based imaging approach.
Work Package 1 - Development of short-lived 18F-labeled TTAs
Several strategies have been established and evaluated. We have studied the structure activity relationship of 45 TTAs and observed a clear correlation between tumor uptake, lipophilicity and rate constant. Based on this, we have a detailed knowledge to effectively continue the work of setting up a library of click imaging agents.

Work Package 2 - Development of biodegradable/biocompatible clear agent
Several clearing agents (CA) have been synthesized and evaluated in vitro and in vivo. Based on promising experiments, a galactose-albumin-tetrazine clearing agent has been selected for upscaling and further development. Furthermore, a tetrazine-functionalized poly-Sar derivative shows promise. We consider this a suitable back-up for the further development toward the clinic.

Work Package 3 - Development of innovative tags: increasing reactivity and stability
Based on computational modelling several TCO precursors have been prepared and used. A family of allylically substituted TCO motifs has been found. They show some very promising characteristics. In vivo studies showed increased stability and that the TCO isomerization speed can be both decreased and modulated by conjugating different moieties on the TCO allylic position. We will continue pursuing this.

Work Package 4 - Translation of in vivo click imaging agents
The screening system established has been used to test approximately 100 compounds. More than 10 have been deemed appropriate for further evaluation. This is ongoing. Also, in vivo studies of clearing agents with cleavable linkers shows great promise. PDX cancer tumor models are currently being established for further in vivo testing.
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