The effect of size and aspect ratio of Fe-MIL-88B-NH₂ metal-organic frameworks on their relaxivity and contrast enhancement properties in MRI: in vitro and in vivo studies

Metal-organic frameworks (MOFs) have shown a great potential as a novel contrast agent in magnetic resonance imaging (MRI) due to their unique properties including versatile structures, composition, and surface chemistry. Fe-MIL-88B-NH₂ (MILs) was synthesized using Fe⁺³ ions and 2-aminoterephthalic acids (NH₂-BDC) in the presence of acetic acid and Pluronic F127 under a hydrothermal process. Three MILs in different sizes were synthesized by using variant CH₃COOH/Fe⁺³ molar ratios and denoted as MIL-S, MIL-M, and MIL-L having average sizes of 60, 350, and 730 nm, respectively. The effect of aspect ratio and particle size of MILs was investigated on proton relaxivity and contrast enhancement in aqueous, in vitro, and in vivo MR images. MRI study indicated that by increasing the MILs size, the incremental transverse relaxivity was achieved from 13.53 to 50.80 for r₂ and from 38.75 to 60.25 for r₂*, while the difference between r₂ and r₂* was decreased obviously. The r₂/r₁ ratios were found to be 5.80, 42.27, and 127.00 for MIL-S, MIL-M, and MIL-L, respectively. All MILs demonstrated T₂-weighted MR imaging performance. Furthermore, no susceptibility artifacts were observed on T₂-weighted images of MILs. By decreasing the size of MILs, r₂/r₁ ratio was reduced sharply and T₁ contrast was also observed for MIL-S. MIL-S was used for in vitro and in vivo study due to its capability in simultaneous T₁ and T₂ contrast enhancements. MTT and hemolysis assays showed no significant cytotoxicity and hemolysis for MIL-S against normal human breast cell line (MCF-10A) and red blood cells, respectively, at concentrations of Fe⁺³ up to 300 μg/mL (p < 0.05). T₁-weighted images acquired from in vitro study interestingly showed that the signal intensity of MIL-S is more than Dotarem. The in vivo experiment revealed that the signal intensity of mice liver decreased sharply after injection resulting in increase of negative contrast enhancement. The negative contrast enhancement decreased gradually by the time under T₂-weighted imaging mode confirming application of MIL-S as a T₂ contrast agent in biomedical applications (up to 12 h post-injection). Although a decrease of the size resulted in bright contrast in aqueous solution for MIL-S, it did not show proper efficiency for in vivo positive contrast enhancement. The advantages, including simple preparation and accessibility, acceptable potency and selectivity, blood compatibility, and suitable cytocompatibility as well as enough circulation time, make MIL-S as a dual MRI contrast agent to differentiate between low-contrast tissues for in vivo experiments.