Paramagnetic complexes of manganese(II), iron(III), and gadolinium(III) as contrast agents for magnetic resonance imaging. The influence of stability constants on the biodistribution of radioactive aminopolycarboxylate complexes.

Paramagnetic complexes of manganese(II), iron(III), and gadolinium(III) with many ligands appear to undergo ligand substitution in vivo, producing biodistribution data similar to the hydrated metal ions. To identify ligands likely to be valuable in the preparation of paramagnetic contrast agents, a series of aminopolycarboxylate complexes with stability constants increasing in the order iminodiacetic acid (IDA) less than nitrilotriacetic acid (NTA) less than EDTA less than CDTA less than or equal to DTPA was prepared with 54Mn(II), 59Fe(III), and 153Gd(III) at both tracer and carrier levels. Biodistribution studies in mice suggested that complexes remained unchanged in vivo if their stability constants (K1) were approximately greater than 10(16) for Mn(II) and Gd(III) and greater than 10(22) for Fe(III) complexes at tracer levels. Metal complexes with added carrier appeared to be effectively more stable in vivo, possibly due to dissociation and saturation of metal-binding sites. To avoid the accumulation of metal ions in tissues, new paramagnetic contrast agents containing these metal ions will require stability constants equal to or greater than those identified here.