Re and Tc Complexes with Pyrazolyl-Containing Chelators: from Coordination Chemistry to Target-Specific Delivery of Radioactivity

The design of novel target-specific imaging agents based on 99mTc requires a considerable development of its coordination chemistry. Among all oxidation states available for technetium (-I to VII), the V oxidation state has been the most extensively studied in radiopharmaceutical chemistry, and the majority of the 99mTc-radiopharmaceuticals in clinical use contain the core [99mTc(O)]3+. More recently, the remarkable features of the organometallic precursor fac- [M(CO)3(H2O)3]+ (M = Re, Tc), introduced by Alberto et al., brought renewed interest in the design of innovative lowoxidation 99mTc-based radiopharmaceuticals. Owing to our interest on the design of innovative target-specific radioactive probes, we have been recently involved in the study of the chemistry of [M(O)]3+ and fac-[M(CO)3]+ (M = Re, Tc) with chelators combining a pyrazolyl unit with aliphatic amines and/or carboxylic acids or thioethers. Such research efforts are reviewed herein, where we present an overview of the chemistry, radiochemistry and biological properties of Re and 99mTc complexes anchored by those pyrazolyl-containing chelators with relevance in radiopharmaceutical research. The revised work focuses mainly on tricarbonyl M(I) complexes but M(V) oxocomplexes are also covered. This contribution intends to highlight the potential of pyrazolyl-containing chelators for the labeling of biologically active molecules with 99mTc(I), being presented a variety of examples which include peptides, peptide nucleic acids, inhibitors/substrates of enzymes and DNA-binders.