Technetium-99m radiolabeled ouabagenin-cysteine conjugate: biological evaluation in animal models

New 99mTc-Labeled Digitoxigenin Derivative for Cancer Cell Identification

In recent years, cardiac glycosides (CGs) have been investigated as potential antiviral and anticancer drugs. Digitoxigenin (DIG) and other CGs have been shown to bind and inhibit Na⁺/K⁺-adenosinetriphosphatase (ATPase). Tumor cells show a higher expression rate of the Na⁺/K⁺-ATPase protein or a stronger affinity towards the binding of CGs and are therefore more prone to CGs than non-tumor cells. Cancer imaging techniques using radiotracers targeted at specific receptors have yielded successful results. Technetium-99m (^99mTc) is one of the radionuclides of choice to radiolabel pharmaceuticals because of its favorable physical and chemical properties along with reasonable costs. Herein, we describe a new Na⁺/K⁺-ATPase targeting radiotracer consisting of digitoxigenin and diethylenetriaminepentaacetic acid (DTPA), a bifunctional chelating ligand used to prepare ^99mTc-labeled complexes, and its evaluation as an imaging probe. We report the synthesis and characterization of the radiolabeled compound including stability tests, blood clearance, and biodistribution in healthy mice. Additionally, we investigated the binding of the compound to A549 human non-small-cell lung cancer cells and the inhibition of the Na⁺/K⁺-ATPase by the labeled compound in vitro. The ^99mTc-labeled DTPA-digitoxigenin (^99mTc-DTPA-DIG) compound displayed high stability in vitro and in vivo, a fast renal excretion, and a specific binding towards A549 cancer cells in comparison to non-tumor cells. Therefore, ^99mTc-DTPA-DIG could potentially be used for non-invasive visualization of tumor lesions by means of scintigraphic imaging.

Tricarbonyltechnetium(I) and tricarbonylrhenium(I) complexes of amino acids: crystal and molecular structure of a novel cyclic dimeric Re(CO)3-amino acid complex comprised of the OON donor atom set of the tridentate ligand

Radiolabeled complexes of monoamino polycarboxylic, polyamino monocarboxylic and thiol containing amino acid ligands were prepared from a fac-[(99m)Tc(CO)3(H2O)3](+) precursor.The overall radiochemical yield was 94-98%. The complexes exhibited substantial in vitro and in vivo stability. The corresponding Re(I) complexes of the ligands DAPA, Asp and CysH were prepared and characterized by means of IR, NMR, and MS spectroscopic studies, as well as X-ray crystallography (for those containing D,L-DAPA and D,L-Asp). The rhenium complexes have been structurally correlated with the technetium complexes by means of HPLC studies. The reaction of Re(CO)5Cl with D,L-Asp in presence of triethylamine led to the formation of a new class of cyclic dimeric complexes formed by the OON donor atom set of the tridentate ligands. The amino carboxylate ligand system formed well defined complexes with a fac-[M(CO)3(H2O)3](+) core and shows good promise in (99m)Tc(CO)3 tracer development.

Preparation of 99mTc-labelled conjugates of ouabagenin and their biological evaluation in animal models

BACKGROUND

Ouabagenin and its 1,19-acetonide were conjugated with nitrilotriacetic acid (NTA) and diethylenetriaminepentaacetic acid (DTPA) through their respective anhydrides.

METHODS

The reaction mixtures were exhaustively purified by silica gel column chromatography and preparative high-performance liquid chromatography to furnish the ligands in good purity and moderate yield. These ligands were labelled with 99mTc to produce four chelates in 90-95% yield. Of these chelates the 99mTc-oubagenin-NTA conjugate and the corresponding acetonide exhibited appreciable myocardial uptake with respect to that of other vicinal organs in a guinea-pig model. However, all these 99mTc chelates exhibited poor heart-to-blood ratios, which could be attributed to the absence of a 3beta sugar residue in this molecule.

CONCLUSION

The result is in agreement with that previously reported in connection with radioiodinated digoxin and digoxigenin derivatives.