Improved Access to Linear Tetrameric Hydroxamic Acids with Potential as Radiochemical Ligands for Zirconium(iv)-89 PET Imaging

Abstract

Two new linear tetrameric hydroxamic acid ligands (3 and 4) have been prepared as potential radioligands for immunological ZrIV-89 positron emission tomography (PET) imaging. The ligands were prepared by conjugating endo-hydroxamic acid amino carboxylic acid (endo-HXA) monomers 5-[(5-aminopentyl)(hydroxy)amino]-5-oxopentanoic acid (PPH) or 2-(2-((2-(2-aminoethoxy)ethyl)(hydroxy)amino)-2-oxoethoxy)acetic acid (PPHNOCO) to trimeric desferrioxamine B (DFOB). The properties of DFOB-PPH (3) and DFOB-PPHNOCO (4) were compared with the first-in-class ligand DFO* (named DFOB-PBH (2) in the present work) and DFOB (1). In the initial phase of an FeIII : ZrIV competition experiment, 1 preferentially formed FeIII-1, with ZrIV-1 becoming dominant after 48 h. Tetrameric 2–4 selected ZrIV above FeIII at all times. The initial rates of formation of ZrIV-3 and ZrIV-4 were greater than ZrIV -2, which could reflect a better match between the ZrIV ionic radius and the increased volume of the coordination sphere provided by 3 and 4. In the presence of excess EDTA, ZrIV-4 dissociated more rapidly than ZrIV-2 and ZrIV-3, which indicated that any beneficial increase in water solubility conferred by the presence of ether oxygen atoms in 4 could be offset by a reduction in complex stability. Outer-sphere solvation of the ether oxygen atoms in ZrIV-4 may increase the entropic contribution to the dissociation of the complex. The rank order of the initial rate of ZrIV complexation in the presence of equimolar FeIII (highest to lowest) 4 > 3 > 2 >>> 1 together with the rate of the dissociation of the ZrIV complex (lowest to highest) 2 ≈ 3 > 4 >>> 1 identifies 3 as a ligand with potential value for immunological ZrIV-89 PET imaging.