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http://hdl.handle.net/11375/25900
Title: | The Use of Bioorthogonal Chemistry to Create 99mTc and Micelle-based Diagnostic Agents |
Authors: | Ahmad, Zainab |
Advisor: | Valliant, John F. |
Department: | Chemical Biology |
Publication Date: | 2020 |
Abstract: | Technetium-99m (99mTc) radiopharmaceuticals are widely used in nuclear medicine to diagnose a variety of diseases, including cancer. While, 99mTc is a seemingly ideal radioisotope for imaging, there is an incompatibility between the isotope’s half-life (6 h) and the biological half-life of antibodies (multiple days), which are useful and prevalent targeting vectors. This misalignment in physical half-life and pharmacokinetics creates a barrier to the development of molecularly targeted 99mTc-based radiopharmaceuticals. To address this, bioorthogonal chemistry between tetrazine and trans-cyclooctene (TCO) derivatives labelled with 99mTc and a variety of different targeting molecules was explored to assess the feasibility of using pre-targeting versus conventional direct labeling strategies. Initially, a new family of tridentate chelate 99mTc-tetrazine ligands of varying polarity were synthesized and characterized. The 99mTc-tetrazine ligands were evaluated in vivo for bone targeting through a pre-targeting strategy with a (TCO)-functionalized bisphosphonate: TCO-BP. The hydrophobic 99mTc-tetrazine derivative demonstrated targeted shoulder and knee uptake of 4.62 ± 1.36 and 9.12 ± 1.88 % ID/g, respectively, in biodistribution studies at 6 h post injection. At the same time point, accumulation of the hydrophilic 99mTc-tetrazine derivative in the shoulder (2.10 ± 0.38 % ID/g) and knee (3.15 ± 0.33 % ID/g) joints, albeit to a lesser degree, is further evidence of successful targeting of 99mTc-tetrazine ligands to a TCO-tagged site in vivo. Next, the 99mTc-tetrazine ligands were evaluated in vivo in tumour xenografts through a pre-targeting strategy with a TCO-modified antibody targeting the A33 antigen in colon carcinomas. Biodistribution studies were performed using SPECT-CT imaging and tissue counting following necropsy. Unfortunately, low tumour uptake (~ 1 % ID/g) was observed for both the hydrophobic and hydrophilic 99mTc-tetrazine derivatives tested. The current hypothesis is that significant uptake in metabolizing organs, notably the liver and intestines, and short blood circulation time of the tetrazine ligands limited the coupling between the two components. To explore the impact of drug formulation on the distribution and accessibility of radiolabelled tetrazines, Pluronic F127 polymeric micelles were used where encapsulation was designed to modify the pharmacokinetics of the hydrophobic 99mTc-tetrazine. The micelles significantly increased the blood circulation time of the 99mTc-tetrazine while also significantly reducing liver uptake. Unfortunately, this did not translate into increased bone uptake of the encapsulated 99mTc-tetrazine when evaluated with the TCO-BP targeting vector through a pre-targeting strategy. This is likely because the tetrazine is not preferentially released at the sites with the high TCO-BP concentration. The micelles used in the initial study were non-targeted. To capitalize on the chemistry developed during the preparation of the 99mTc-tetrazines, the Pluronic F127 polymers previously used for encapsulation were modified with terminal TCO groups, creating a simple way of modifying the surface of micelles with targeting vectors. A F127-TCO polymer was synthesized in high yield and the corresponding micelles were visualized by DLS and TEM to be 20-25 nm in size. A bisphosphonate-derived tetrazine (TzBP) was prepared and radiolabelled with 99mTc and combined with the TCO-modified micelles to create a new class of targeted micelles. In vitro, active and pre-targeting studies showed the F127-TCO micelles preferentially bound to the hydroxyapatite salt in the presence of TzBP compared to controls. In vivo, F127-TCO micelles accumulated at bone at a 7-fold higher concentration when modified with [99mTc]TcTzBP compared to that seen when labelled with a non-targeted tetrazine. This approach ultimately represents a platform for the creation of targeted micelle encapsulated drugs. |
URI: | http://hdl.handle.net/11375/25900 |
Appears in Collections: | Open Access Dissertations and Theses |
Files in This Item:
File | Description | Size | Format | |
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Ahmad_Zainab_FinalSubmission2020SeptemberPhD.pdf | 28.31 MB | Adobe PDF | View/Open |
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