Water-soluble lipophilic MR contrast agents for cell membrane labeling

Abstract

Long-term cell tracking using MR imaging necessitates the development of contrast agents that both label and are retained by cells. One promising strategy for long-term cell labeling is the development of lipophilic Gd(III)-based contrast agents that anchor into the cell membrane. We have previously reported the efficacy of monomeric and multimeric lipophilic agents and showed that the monomeric agents have improved labeling and contrast enhancement of cell populations. Here, we report on the synthesis, characterization, and in vitro testing of a series of monomeric lipophilic contrast agents with varied alkyl chain compositions. We show that these agents disperse in water, localize to the cell membrane, and label HeLa and MCF7 cells effectively. Additionally, these agents have up to tenfold improved retention in cells compared to clinically available ProHance(®).

Figure 1

Lipophilic MR contrast agents 1–5 contain alkyl chains of various lengths and branching. Complexes were synthesized with click chemistry and contain the same Gd(III) chelate. ProHance®, a clinically approved contrast agent, was used as a control.

Figure 2

Concentration dependent cell labeling of complexes 1–5 in a. HeLa and b. MCF7 cells. These results show that 2 is the most effective complex for cell labeling and low concentrations. Complex 5 is the least effective in both cell lines. The remaining complexes (1, 3, 4) label cells to a similar degree. Error bars represent ± the standard deviation of the mean of triplicate experiments.

Figure 3

T₁-weighted HeLa cell pellet images acquired at 7 T of 1–5 and ProHance®. Scale bar represents 1 mm. Error bars represent ± standard deviation of the mean of 4 slices. These images show that 2 and 3 produce the greatest reduction in T₁.

Figure 4

Cellular proliferation and retention of 1–5 and ProHance® in HeLa and MCF7 cells 72 hrs post-labeling. a: Cellular proliferation was determined by calculating the fold increase in cell count between t = 0 and 72 hrs. Data shows that complexes 1–5 do not slow proliferation. b: Cellular retention was determined by calculating the fold decrease in Gd(III) per cell between t = 0 and 72 hrs. These data show that retention is a cell line-dependent property with the lipophilic complexes outperforming ProHance® in HeLa cells but not in MCF7 cells.