We have synthesized new magnetic resonance imaging (MRI) T1
contrast agents (CA1 and CA2) that permit the activatable recognition of the cellular vicinal
thiol motifs of the
protein thioredoxin. The
contrast agents showed MR relaxivities typical of
gadolinium complexes with a single water molecule coordinated to a Gd3+ center (i.e., ~4.54 mM-1s-1) for both CA1 and CA2 at 60 MHz. The
contrast agent CA1 showed a ~140% relaxivity enhancement in the presence of
thioredoxin, a finding attributed to a reduction in the flexibility of the molecule after binding to
thioredoxin. Support for this rationale, as opposed to one based on preferential binding, came from 1H-15N-HSQC NMR spectral studies; these revealed that the binding affinities toward
thioredoxin were almost the same for both CA1 and CA2. In the case of CA1, T1-weighted phantom images of
cancer cells (MCF-7, A549) could be generated based on the expression of
thioredoxin. We further confirmed
thioredoxin expression-dependent changes in the T1-weighted contrast via knockdown of the expression of the
thioredoxin using
siRNA-transfected MCF-7 cells. The nontoxic nature of CA1, coupled with its relaxivity features, leads us to suggest that it constitutes a first-in-class MRI T1
contrast agent that allows for the facile and noninvasive monitoring of vicinal
thiol protein motif expression in live cells.