First line treatment of cancer currently involves all kinds of non-selective strategies such as surgery, chemotherapy and external beam irradiation therapy. Due to the major side effects of these non-selective treatments, the focus of cancer treatment is shifting more and more to personalized, targeted therapies. Amongst those targeted therapies, Targeted Radionuclide Therapy (TRNT) has emerged as an efficient tool for cancer treatment.
In the past few years, several therapeutic radionuclides have been evaluated for their potential application in TRNT. Among them, rhenium-188 is one of the promising radionuclides for TRNT applications. It emits β--particles with a maximum energy of 2.12 MeV. These β--particles have a maximum range of about 11 mm in soft tissue. Therefore, rhenium-188 is particularly well suited for radiotherapy when effective deep tissue penetration is required. An additional benefit of rhenium-188 is that it is easily obtained from a 188W/188Re generator as a perrhenate solution. The parent radionuclide, W-188 is prepared by a double neutron capture: W-186 (2n,γ) W-188 from enriched W-188 targets in a high flux reactor, such as the Belgian Reactor 2 (BR2) at SCK•CEN.
Rhenium offers complex coordination chemistry due to its numerous oxidation states. These oxidation states allow for a wide spectrum of chelating ligands from pure electron donors, like oxo- or imido-ligands, to systems with back-donation, like carbonyls or isocyanides. Conjugating rhenium-188 to a vector is a crucial step in the development of rhenium-188 radiopharmaceuticals to avoid off target accumulation of rhenium-188 due to release of the free radio‑metal from the complex. For transition metals like rhenium, conjugation is achieved using a chelator. Typically, bifunctional chelators such as MAG3, HYNIC or DTPA have been used to form complexes with Re-188. Often, these complexes suffer from a lack of water solubility and in vivo stability and the development of new and novel chelators for Re is of outmost importance for the progress of the field of Re-188 based radiopharmaceuticals.
Therefore, in this collaborative project between SCK•CEN and UCT, we aim to develop novel water soluble chelators which are specifically adapted to the more complex coordination chemistry of rhenium-188. To increase the water solubility of the new chelating ligands we will incorporate a sulfonate moiety. Our interest lies in bidentate chelating ligands and we will focus our research to include the synthesis of SHYNIC-, triazole- and α‑diimine-containing ligands. In addition to this we will synthesize ligands which are bifunctional chelators that not only form strong and stable bonds with rhenium-188, but also allow to easily couple the rhenium complex to a vector molecule. Development of these bifunctional chelators will enable radiolabeling of biomolecules (small proteins, antibodies, nanobodies,…) allowing targeting of any kind of tumor with rhenium-188.
The overall objective of this PhD study is to improve current practice in radiolabeling radiopharmaceuticals using rhenium-188. Since biomolecules such as nanobodies and antibodies are rapidly emerging in the field of radiopharmaceuticals, the emphasis of the project will be to develop bifunctional chelators that allow easy conjugation of the rhenium-chelator complex to biomolecules.
The project can be divided in three major work packages (WPs).
WP1: Design, synthesis and characterization of novel bifunctional chelators and their rhenium complexes.
One of the greatest challenges of developing rhenium-188 based radiopharmaceuticals is the lack of suitable commercially available chelating ligands for rhenium. Thus in collaboration with the Synthetic Organometallic & Bioorganometallic Research Lab of Gregory Smith at the University of Cape Town (UCT) in South Africa, we aim to design and synthesize new bifunctional water-soluble chelating ligands for rhenium-188.
WP2: Optimization of radiolabeling conditions for the ligands of WP1 and the radiochemical stability of rhenium-188 complexes.
After initial screening of all novel chelators, we will focus on the optimization of radiolabeling techniques with rhenium-188. The influence of different parameters (pH, type of co-ligand, reaction time, solvents, presence of metals, …) on the radiolabeling will be evaluated for every synthesized ligand. Labeling efficiency will be monitored by instant thin-layer chromatography (iTLC) or radio-HPLC. Following radiolabeling optimization, the stability of the rhenium-188 complex needs to be investigated in storage and physiological (in vitro and in vivo) conditions.
WP3: Radiolabeling of heat sensitive biomolecules.
After obtaining a functional bifunctional chelator with optimal binding characteristics and stability (WP 1 and 2), a proof of concept study will be performed to label a heat sensitive biomolecule. The specificity and cell internalization of the 188Re‑construct will be monitored in vitro on tumor cells. The biodistribution of the 188Re-construct will be monitored in both healthy mice and nude mice bearing implanted xenografts of tumors.