Self-assembled Tetrahedral Iron(III) Cage for MRI Guided Delivery of a Gold Drug

A T1 MRI probe based on a self-assembled tetrahedral iron(III) cage acts as a host for a gold(I) anticancer drug, which is added as Au(PEt3)Cl. 1H NMR characterization of the gold complex within the Ga(III) analog of the tetrahedral cage is consistent with loss of chloride to give a mixture of Au(PEt3)(OH2) and Au(PEt3)(OH) within the cage. This aqua species may be converted to encapsulated Au(PEt3)CN by treatment with KCN. The binding constant of the gold aqua complex is estimated as ≈ 103 M-1 by displacement experiments with various ammonium cations. Studies show that the iron cage with encapsulated gold complex binds tightly to serum albumin and that there is little dissociation of iron or gold complexes over 24 hours. The iron-gold host-guest complex shows enhanced contrast of the vasculature, consistent with strong binding of the cage to serum proteins. Uptake of the iron cage into C26 tumors as shown by MRI corresponds to the deposition of gold as measured by ICP-MS. These data suggest that the tetrahedral iron cage may serve as a host and carrier for gold drugs towards applications as theragnostic agents.