Development Of Solid Lipid Nanoparticle-Based Magnetic Resonance Colonography

PurposeWe attempted to develop a novel nanoparticle-based magnetic resonance (MR) colonography technique, which enabled to generate contrast-enhanced MRI of the colonic walls and demonstrate the colorectal carcinoma via a transrectal administration of solid lipid nanoparticle (SLN) loaded with gadolinium-diethylenetriaminepentaacetic acid (Gd-DTPA).Materials and Methods1. We synthesized three different SLNs, including blank SLNs, Gd-DTPA-loaded SLNs (Gd-DTPA-SLNs), and Gd-DTPA-SLNs conjugated with FITC (FITC-Gd-DTPA-SLNs) by solvent diffusion method. Physicochemical properties of different SLNs were characterized by using Zetasizer, fluorescence spectrophotometer and MR. In vitro release study of Gd-DTPA was performed.2. Validation of the feasibility of the novel concept—nanoparticle-based MR colonography in normal mice and histological confirmation of the ability of transrectally-infused SLNs infiltrating to the colonic walls in normal mice.The sigmoid-rectal colons of 30 normal mice were locally infused with various SLNs and control agents via a transrectal enema approach. The SLNs and control agents included Gd-DTPA-SLNs (n=6), FITC-Gd-DTPA-SLNs (n=6), blank SLNs (n=6), Gd-DTPA (n=6), and water (n=6). In the first phase of the study to validate the feasibility of MR colonography, the mice in each group were imaged with colonic MR imaging using a series of T1-weighted fluid-attenuated inversion-recovery (FLAIR) sequences, which were performed before, during and 20 min after transrectal enema with different SLNs and control agents. Then, the quantitative analyses were performed. The quantitative criteria were T1 relaxation times of the colorectal wall, colorectal wall-to-surrounding muscle contrast (C) and colorectal wall-to-surrounding muscle contrast-to-noise ratio (CNR). MR colonography in 2 normal mice were performed with intravenous injection of Gd-DTPA. In the second phase of the study, all mice were scarified to histologically confirm the novel concept after MR examination.3. Validation of the feasibility of the novel nanoparticles-based MR colonography in DMH-induced colorectal tumor model.Two mice in DMH-induced colorectal tumor model were imaged with colonic MR imaging using a T1-weighted fluid-attenuated inversion-recovery (FLAIR) sequence, which were performed before, during and 20 min after transrectal enema with Gd-DTPA-SLNs. Then, the mice were scarified for histological examination.Results1. Gd-DTPA-SLNs, FITC-Gd-DTPA-SLNs and blank SLNs displayed a size distribution of 40.8-300.8 nm with high Gd-DTPA entrapment efficiency at 55%-55.8%. In vitro MR showed that the T1 relaxation times of Gd-DTPA-SLN and FITC-Gd-DTPA-SLN were similar to that of Gd-DTPA. 32% Gd-DTPA was released from Gd-DTPA-SLNs at PH 7.4 buffer in 1 h. The release o Gd-DTPA reached a balance in 8 h.2. The bright lumen MR Colonography was obtained when a T1-weighted FLAIR sequence was performed during transrectal enema with Gd-DTPA-SLNs and FITC-Gd-DTPA-SLNs. T1-weighted FLAIR MRI showed bright enhancement of colorectal wall, with the decrease of T1 relaxation time and the increase of colorectal wall-to-surrounding muscle C and CNR in all mice 20 min after transrectal enema with Gd-DTPA-SLNs and FITC-Gd-DTPA-SLNs. Of the colonic tissues treated with FITC-Gd-DTPA-SLNs, confocal laser-scanning microscope and fluorescence microscope demonstrated highly-concentrated green fluorescence (due to FITC emission), which localized in both extracelluar spaces and cytoplasms at various layers of the colorectal walls, including mucosa, submucosa, tunica muscularis, and serosa. These histological and MRI findings were not visualized in the control animal groups.3. The bright lumen MR Colonography showed the masses when a T1 -weighted FLAIR sequence was performed during transrectal enema with Gd-DTPA-SLNs. MR colonography showed marked homogeneous enhancement of normal colorectal walls and mild inhomogeneous enhancement of the mass. The signal intensity in the lesion was much lower than that in the colorectal walls. There was a great contrast between the tumor and surrounding colorectal walls.Conclusion1. SLNs loaded Gd-DTPA by solvent diffusion method exhibit intestinal absorbablity, a suitable particle size, high loading efficiency, stability of paramagnetic properties of Gd-DTPA.2. This study establishes the "proofs-of-principle" of a novel imaging technique—nanoparticle-based MR colonography by combining the advantages of both MR technology and nanotechnology. The properties of the novel MR colonography include that (i) MR colonography is based on transrectal enema with SLNs loaded Gd-DTPA without introvenous administration; (ii) the bright lumen MR Colonography and the similar dark lumen MR Colonography were obtained after transrectal enema with SLNs loaded Gd-DTPA; (iii) a useful imaging tool is provided for basic science to precisely establish MRI and histological correlation by using the Gd-FITC-SLNs we have produced; (iv) nanoparticles can penetrate into the cells of the colorectal walls in large quantities and cellular imaging is obtainable; and (v) a functional MRI can be potentially developed to assess colorectal absorption.3. Validation of the feasibility of the novel nanoparticles-based MR colonography in DMH-induced colorectal tumor model demonstrates that the visualization of colorectal carcinoma based on colorectal absorption of Gd-DTPA-carrying nanoparticles on MR colonography is possible.4. Nanoparticle-based MR colonography can become a novel in vivo evaluation method of colon delivery of drug.5. Abdominal magnetic resonance imaging in mice using a clinical 3T MR scanner with a small coil is feasible and provides images of high quality.