A new nanoparticulate inorganic-organic hybrid-type positive contrast agent (CA), PGP/dextran-K01, was synthesized based on a GdPO4 inorganic core as a relaxation-time-shortening moiety and a dextran-coating, which generates monodispersibility in water, a high relaxation-time-shortening effect by retaining a large number of water molecules in proximity of the core and toxicity reduction in in-vivo studies. This PGP/dextran-K01 nanoparticle has high r1 and r2 values and a significantly low r2/r1 value, 1.1, which is unprecedented and which is the lowest value among existing nanoparticulate CAs indicating that PGP/dextran-K01 is a positive contrast agent. Because of this low r2/r1 value and the nanoparticulate shape, PGP/dextran-K01 will be a useful clinical substitute for negative CAs based on iron oxides.
Novel gadolinium complexes (KMR-series: KMR-K and KMR-Mg), which have a bis-15-crown-5 ether or a charged beta-diketone structure as a recognition site, have been designed, synthesized and applied for the detection of K(+) or of Mg(2+) and Ca(2+) using MRI or NMR techniques. The measurements are based on the modulation of the longitudinal relaxation time (T(1)) of water protons in proximity of the gadolinium complexes. Relaxivity measurements of KMR-K1 in aqueous solution showed that the initial longitudinal relaxivity value (r(1)) of 5.05 mM(-1) s(-1) is monotonously decreasing with increasing K(+) concentrations, reaching a final value of 4.78 mM(-1) s(-1). This decrease is attributed to a change in the second sphere of hydration of the gadolinium (Gd(3+)) complex (KMR-K), resulting in a K(+) concentration-dependent contrast in MR images. From stoichiometric analysis using mass spectrometry and UV/VIS spectrometry, a 1 : 1 complex formation between KMR-K1 and K(+) in a sandwich-type manner with a log K of 3.20 was confirmed. In the case of KMR-Mg, the initial r(1) value of 4.98 mM(-1) s(-1) is monotonously decreasing with increasing Mg(2+) or Ca(2+) concentrations, reaching a final value of 3.95 or 4.16 mM(-1) s(-1), respectively, resulting in Mg(2+) and Ca(2+) concentration-dependent contrast in MR images. The formation of a 1:1 complex with a log K of 2.33 for Mg(2+) and 1.91 for Ca(2+) was confirmed. KMR-K1 and KMR-Mg are the first ion-selective or ion-sensitive gadolinium complexes for K(+) or Mg(2+) and Ca(2+), respectively.
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