Relaxation enhancement using liposomes carrying paramagnetic species

Area of Science:

• Biomedical Imaging
• Nanotechnology
• Pharmacology

Background:

• Magnetic Resonance Imaging (MRI) relies on contrast agents to enhance image quality.
• Manganese diethylenetriaminepentaacetic acid (Mn-DTPA) is a potential MRI contrast agent.
• Liposomes are versatile nanocarriers for drug and contrast agent delivery.

Purpose of the Study:

• To investigate the biodistribution and effects of Mn-DTPA encapsulated in multilamellar liposomes on mouse organ relaxation times.
• To compare the efficacy of liposomal Mn-DTPA with free Mn-DTPA as an MRI contrast agent.

Main Methods:

• Intravenous administration of Mn-DTPA entrapped in multilamellar liposomes, free Mn-DTPA, and controls (liposomes with 0.9% NaCl) in mice.
• Quantification of manganese concentrations in injectates and tissues using 54Mn radiotracer and atomic absorption spectroscopy.
• Measurement of T1 and T2 relaxation times of freshly excised organs using Nuclear Magnetic Resonance (NMR) spectroscopy at 20 MHz and 37°C with Inversion Recovery (IR) and Carr-Purcell-Meiboom-Gill (CPMG) sequences.

Main Results:

• Liposomal encapsulation significantly altered Mn-DTPA biodistribution: increased 54Mn accumulation in liver (+207%) and spleen (+1208%), while reducing it in heart (-20%) and kidney (-24%) compared to free Mn-DTPA.
• Statistically significant increases in tissue relaxation rates were observed.
• The relaxivity (increase in relaxation rate per unit concentration of Mn) was reduced by 31% in the liver and 62% in the spleen when Mn-DTPA was delivered via liposomes.

Conclusions:

• Liposomal encapsulation of Mn-DTPA modifies its biodistribution and tissue accumulation.
• While liposomal delivery affects manganese distribution, it reduces the efficiency of Mn-DTPA as a contrast agent in certain organs.
• These findings provide crucial insights for the rational design of novel liposome-based MRI contrast agents with optimized biodistribution and efficacy.