Lanthanide(III) complexes of rhodamine-DO3A conjugates as agents for dual-modal imaging.

Charlotte Rivas,Florencia Minuzzi,Nicholas J Long,Graeme J Stasiuk,Guy A Rutter,Juan Gallo

doi:10.1021/ic402233g

Abstract

Two novel dual-modal MRI/optical probes based on a rhodamine–DO3A conjugate have been prepared. The bis(aqua)gadolinium(III) complex Gd.L1 and mono(aqua)gadolinium(III) complex Gd.L2 behave as dual-modal imaging probes (r1 = 8.5 and 3.8 mM–1 s–1 for Gd.L1 and Gd.L2, respectively; λex = 560 nm and λem = 580 nm for both complexes). The rhodamine fragment is pH-sensitive, and upon lowering of the pH, an increase in fluorescence intensity is observed as the spirolactam ring opens to give the highly fluorescent form of the molecule. The ligands are bimodal when coordinated to Tb(III) ions, inducing fluorescence from both the lanthanide center and the rhodamine fluorophore, on two independent time frames. Confocal imaging experiments were carried out to establish the localization of Gd.L2 in HEK293 cells and primary mouse islet cells (∼70% insulin-containing β cells). Colocalization with MitoTracker Green demonstrated Gd.L2’s ability to distinguish between tumor and healthy cells, with compartmentalization believed to be in the mitochondria. Gd.L2 was also evaluated as an MRI probe for imaging of tumors in BALB/c nude mice bearing M21 xenografts. A 36.5% decrease in T1 within the tumor was observed 30 min post injection, showing that Gd.L2 is preferentially up taken in the tumor. Gd.L2 is the first small-molecule MR/fluorescent dual-modal imaging agent to display an off–on pH switch upon its preferential uptake within the more acidic microenvironment of tumor cells.

Highlights

Molecular imaging is a rapidly growing area of chemistry that aims to visualize cellular function and structure in a noninvasive manner with the in vivo use of specially designed imaging agents
This can be achieved by the in vivo use of a dual-modal magnetic resonance imaging (MRI)/fluorescent probe that responds to a characteristic trait of tumor cells.[7−9] It is known that the mitochondrial potential in cancer cells is greater than that of healthy cells,[10] and the design of a probe that can accumulate in these energized mitochondria will lead to a tumor-targeting agent
Removal of the tertbutyl ester groups was carried out using trifluoroacetic acid (TFA) to obtain L1

Summary

■ INTRODUCTION

Molecular imaging is a rapidly growing area of chemistry that aims to visualize cellular function and structure in a noninvasive manner with the in vivo use of specially designed imaging agents. Using time-gating techniques, Tb(III) emission is induced in both complexes upon excitation of the isolated phenyl ring of the rhodamine moiety at 310 nm by energy transfer to the Tb(III) metal center (Figures 2 and 3). These hydration states were confirmed by the lifetime measurements of the terbium analogues It is well-known that the presence of hydrophobic moieties within a probe is one of the basic structural requirements for ligand binding to human serum albumin (HSA).[51] HSA is the most abundant protein in blood, and binding to it results in an increase in intravascular retention and relaxivity (due to a decrease in τR of the complex).[52] In view of the fact that the rhodamine moiety has extended aromatic rings and hydrophobicity, a study was carried out to investigate possible interactions between Gd.L2 and HSA. The observed accumulation of Gd.L2 in the tumor area is consistent with the predicted effect of a lower pH environment on the probe; the rearrangement of the lactam ring at acidic pH introduces a positive charge in the molecule that can promote the intracellular uptake of the molecule responsible of the average T1 decrease observed in the tumor

■ CONCLUSIONS

■ ACKNOWLEDGMENTS

■ REFERENCES