Indium(III) and gallium(III) complexes of bis(aminoethanethiol) ligands with different denticities: Stabilities, molecular modeling, and in vivo behavior

Complexes of Ga(III) and In(III) radionuclides are widely used in diagnostic imaging. In this study, the following ligands of denticities 4, 5, and 6 respectively were prepared: N,N'-bis-(2,2-dimethyl-2- mercaptoethyl)ethylenediamine (4SS), 1-carboxy-N,N'-bis(2,2-dimethyl-2- mercaptoethyl)ethylenediamine (5SS), and N,N'-bis(2,2-dimethyl-2- mercaptoethyl)ethylenediamime-N,N'-diacetic acid (6SS). Syntheses of the two new ligands, 5SS and 6SS, are described. Equilibrium constants for their In(III) and Ga(III) complexes were determined by both direct and ligand- competitive potentiometric methods. The formation constant (K(ML) = [ML]/[M][L]) of In(III)-6SS in 0.100 M KNO₃ at 25.0 °C is 10^39.8, and its pM at physiological pH (7.4 with 100% excess of the ligand) is 30.9. These values are higher than those of any other previous reported ligand for In(III). The stability constants of the complexes of 4SS, 5SS, 6SS, and the analogous ligand EDDASS, N,N'-bis(2-mercaptoethyl)ethylenediamine-N,N'- diacetic acid, which does not contain gem-dimethyl groups, are compared. The thermodynamic stabilities of the In(III) complexes of all ligands except 6SS are greater than those of the corresponding Ga(III) complexes. The presence of the geminal dimethyl groups in 6SS increased the stability of the Ga(III) and In(III) complexes over those of EDDASS. The effects of the gem-dimethyl groups on complex stabilities are explained by molecular modeling. The serum stabilities and biodistributions out to 1 h postinjection of (67/68)Ga and ¹¹¹In chelates of 4SS, 5SS, and 6SS were measured and compared with those of EDDASS. The (67/68)Ga- and ¹¹¹In-ligand complexes with more donor atoms showed were more stable in serum, both in vitro and in vivo. The biodistributions of the (67/68)Ga- and ¹¹¹In-ligand complexes exhibited distinct trends. None of the (67/68)Ga- and ¹¹¹In-chelates demonstrated significant heart or brain uptake. The majority of uptake for all compounds was in the liver and kidney. The degree of clearance through the liver corresponded to the thermodynamic stability of the complex. Correlations between in vivo behavior, molecular modeling data, and thermodynamic stability of the complexes are discussed.