The bioinorganic and medicinal chemistry of carboranes: from new drug discovery to molecular imaging and therapy

The role of carboranes in medicinal chemistry has diversified in recent years and now extends into areas of drug discovery, molecular imaging, and targeted radionuclide therapy. An introduction to carborane chemistry is provided to familiarize the non-expert with some key properties of these molecules, followed by an overview of current medicinally-orientated research involving carboranes. The broad-ranging nature of this research is illustrated, with emphasis placed on recent highlights and advances in this field.

Aqueous Fluoride and the Preparation of [99mTc(CO)3(OH2)3]+ and 99mTc−Carborane Complexes

A method for the preparation of eta5-metallocarborane complexes of technetium-99m in water was developed. The key to the procedure is the use of aqueous sodium or potassium fluoride, which prevents premature degradation of the Tc(I) starting material used to prepare the carborane complexes. Solid-phase extraction was used to purify Tc-metallocarboranes derived from both ortho and meta isomers, which were isolated in good to excellent yields in high radiochemical purities. In conjunction with these studies, a series of fluoride-based "kits" were developed to produce the key precursor [99mTc(CO)3(H2O)3]+ in the absence of any other stabilizing ligand. Using this approach, [99mTc(CO)3(H2O)3]+ could be prepared directly from 99mTcO4- under a range of pH values, including neutral pH, which affords the opportunity to develop one-pot labeling procedures for base-sensitive targeting vectors.

Preparation of Re(I)− and 99mTc(I)−Metallocarboranes in Water under Weakly Basic Reaction Conditions

A new method for the preparation of Re- and (99m)Tc-metallocarboranes in water under mild reaction conditions was developed. Three nido-carborane ligands were reacted with [Re(CO)(3)Br(3)](2)(-) in the presence of aqueous potassium fluoride to give the corresponding eta(5)-Re(CO)(3)-carborane complexes. The use of KF as a base afforded the desired Re-metallocarboranes in good yields while avoiding the formation of Re clusters, which are byproducts commonly observed when reactions are carried out in the presence of strong aqueous bases. The reaction was also performed at the tracer level producing the first (99m)Tc-carborane complex, which was isolated in 80% radiochemical yield following a simple Sep-Pak purification process. The resulting organometallic complex was stable to cysteine and histidine challenges for more than 24 h.