Synthesis of Modular Desferrioxamine Analogues and Evaluation of Zwitterionic Derivatives for Zirconium Complexation

From Desferrioxamine B Umpolung to an Enantiopure Bifunctional Chelator for 89Zr-immunoPET

Promising results in preclinical diagnosis based on 89Zr-immunoPET have fostered the development of efficient chelators for this tetravalent metal. Leads in this area are octadentate ligands obtained by extension of the desferrioxamine B trihydroxamic acid DFO with a fourth bidentate ligand. In the approach reported here, the latter is a natural 6-membered cyclic hydroxamic acid deriving from (L)-ornithine. Its coupling to DFO via an (L)-lysine spacer required that the genuine amine function of the DFO terminus be changed to a carboxylic acid. Such a requirement prompted us to explore short sequences of chemical transformations that would challenge total syntheses leading to the same products. As a matter of fact, the target C-terminal DFO analogue was obtained in benzyl-protected form in three steps from commercially available DFO in 16% overall yield. Our short-step approach allowed us to implement other functionalities without DFO extension: -OH, -OTs, -CHO, -C(O)CH3, and -CH═CH2.

Isopeptidic Desferrioxamine Analogues: The Role of Hydroxamate Spacing for Chelation of Zr4

[89Zr]Zr4+ is a radionuclide of increasing clinical relevance for PET (positron emission tomography). However, an ideal chelator for stable Zr-chelation remains to be discovered. This study describes the solid-phase synthesis of octadentate Zr-chelators based on an isopeptidic (ip) scaffold derived from the natural siderophore desferrioxamine (DFOB). Several analogues with different spacers separating the chelating hydroxamates have been prepared and converted to [89Zr]Zr-complexes. The stability of these complexes was evaluated in human serum and in competition to excess of competing chelators. The assays revealed a beneficial effect of long hydroxamate spacing (9 atoms). Shorter spacing led to a decrease in complex stability. The most stable [89Zr]Zr-ipDFO complex had a high stability in challenging competition experiments with a large excess of EDTA for 72 h as determined by radio TLC and LC/MS. The straightforward synthesis, high complex stability and a modular character make ipDFO derivatives promising chelators for applications in targeted PET.

Decoration of 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) with N-oxides increases the T1 relaxivity of Gd-complexes

High complex stability and longitudinal relaxivity of Gd-based contrast agents are important requirements for magnetic resonance imaging (MRI) because they ensure patient safety and contribute to measurement sensitivity. Charged and zwitterionic Gd3+-complexes of the well-known chelator 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) provide an excellent basis for the development of safe and sensitive contrast agents. In this report, we describe the synthesis of DOTA-NOx, a DOTA derivative with four N-oxide functionalities via "click" functionalization of the tetraazide DOTAZA. The resulting complexes Gd-DOTA-NOx and Eu-DOTA-NOx are stable compounds in aqueous solution. NMR-spectroscopic characterization revealed a high excess of the twisted square antiprismatic (TSAP) coordination geometry over square antiprismatic (SAP). The longitudinal relaxivity of Gd-DOTA-NOx was found to be r1=7.7 mm-1 s-1 (1.41 T, 37 °C), an unusually high value for DOTA complexes of comparable weight. We attribute this high relaxivity to the steric influence and an ordering effect on outer sphere water molecules surrounding the complex generated by the strongly hydrated N-oxide groups. Moreover, Gd-DOTA-NOx was found to be stable against transchelation with high excess of EDTA (200 eq) over a period of 36 h, and it has a similar in vitro cell toxicity as clinically used DOTA-based GBCAs.

Medicinal Chemistry of Drugs with N-Oxide Functionalities

Molecules with N-oxide functionalities are omnipresent in nature and play an important role in Medicinal Chemistry. They are synthetic or biosynthetic intermediates, prodrugs, drugs, or polymers for applications in drug development and surface engineering. Typically, the N-oxide group is critical for biomedical applications of these molecules. It may provide water solubility or decrease membrane permeability or immunogenicity. In other cases, the N-oxide has a special redox reactivity which is important for drug targeting and/or cytotoxicity. Many of the underlying mechanisms have only recently been discovered, and the number of applications of N-oxides in the healthcare field is rapidly growing. This Perspective article gives a short summary of the properties of N-oxides and their synthesis. It also provides a discussion of current applications of N-oxides in the biomedical field and explains the basic molecular mechanisms responsible for their biological activity.