High relaxivity contrast agents for magnetic resonance imaging (MRI)

The development of Gd(III)-based contrast agents for MRI applications has intensified in recent years due to the paramagnetic ion’s long electron spin relaxation time and large effective magnetic moment, µeff. Secondly, the exploitation of the long lived luminescent properties of the Ln(III) ions has lead to the development of luminescent lanthanide probes for sensing, time resolved immunoassay and imaging applications. Herein a series of novel Ln-DO3A based complexes are reported. Modulation of relaxivity, r1, (Gd) and emission intensity (Eu, Tb, Sm and Dy) has been achieved in three ways: Firstly, mono- and bis-methyl Ln-dpp-DO3A based complexes have been prepared, where dpp is a pendant diphenylphosphinamide moiety. These show pH responsive relaxivity (Gd) and luminescence (Eu) with calculated pKa values of 8.65 (± 0.09) and 8.59 (± 0.14). Sensitised emission of Eu(III), Tb(III), Dy(III) and Sm(III) has been observed following excitation of the dpp antenna at λex ~ 270 nm. Relaxivities have been measured as r1 = 7.9 mMˉ¹sˉ¹and r1 = 8.2 mMˉ¹sˉ¹ in acidic media, q = 2 and r1 = 5.4 mMˉ¹sˉ¹ and r1 = 4.4 mMˉ¹sˉ¹ in basic media, q = 1 for the mono- and bis-methyl Gd-dpp-DO3A complexes respectively. The pH responsive behaviour has been attributed to the reversible ligation of the dpp moiety. Secondly, non-covalent attachment of the mono- and bis-methyl Gd-dpp-DO3A-based complexes to Human Serum Albumin (HSA) at pH 7.4 resulted in a 64% (r1 = 11.7 mMˉ¹sˉ ¹) and a 146% (r1 = 16.0 mMˉ¹sˉ¹) enhancement in relaxivity, with binding affinities, K, determined from luminescence studies as K = 22,268 ± 12% Mˉ¹and K = 20,059 ± 14% Mˉ¹ for the mono- and bis-methyl dpp Eu-dpp-DO3A complexes respectively. The negatively charged [Gd-dpp-aDO3A]3ˉ complex was developed in order to improve the observed relaxivity of the HSA bound species: r1 = 16.0 mMˉ¹sˉ¹, K = 17,915 (± 14%) Mˉ¹. Competitive binding studies with the fluorescent probes dansylsarcosine and warfarin showed each of the dpp complex analogues to bind preferentially to HSA site II, only the S-enantiomer of the mono-methyl Gd-dpp-DO3A showed an affinity for site I. Finally, an accumulation and activation strategy following enzyme activity has been demonstrated. Neutral q = 2 Gd(III) ethyl and acetoxymethyl ester Ln-DO3MA based complexes have shown decreased relaxivity in the presence of carbonate due to the inner sphere water molecule displacement by bidentate anion binding. The binding is suppressed by the introduction of negative charge to the complex following enzymatic hydrolysis of the ester groups, resulting in ~ 84% relaxivity enhancement (Gd) as well as Eu luminescence quenching. The high observed relaxivity of the ethyl ester model: r1 = 10.2 mMˉ¹sˉ¹ is attributed to the extremely short observed water exchange lifetime, τm = 7.9 ns.