Manganese hydroxylapatite as a potential magnetic resonance contrast agent for liver imaging

Investigation of lanthanide-based starch particles as a model system for liver contrast agents

Gadolinium and dysprosium diethylenetriamine pentaacetic acid-labeled starch microparticles (Gd-DTPA-SP and Dy-DTPA-SP) were investigated as model liver contrast agents. The liver contrast efficacy of particles with low and high metal contents was compared in two imaging models: in vivo rat liver and ex vivo perfused rat liver. The biodistribution of intravenously injected particles was also assessed by ex vivo relaxometry and inductively coupled plasma atomic emission spectrophotometry of tissues. All particles reduced the liver signal intensity on T2-weighted spin-echo and gradient-recalled echo images as a result of susceptibility effects. Because of their higher magnetic susceptibility, the Dy-DTPA-SP were more effective negative contrast enhancers than the Gd-DTPA-SP. On T1-weighted spin-echo images, only the Gd-DTPA-SP with low metal content significantly increased the liver signal intensity. In addition, these low-loading Gd-DTPA-SP markedly reduced the blood T1. The two latter observations were not consistent with the anticipated blood circulation time of microparticles, but were a result of the lower stability of these particles in blood compared with Gd-DTPA-SP, which has a high metal content. Regardless of stability or imaging conditions, the paramagnetic starch particles investigated showed potential as negative liver contrast enhancers. However, the observed accumulation of particles in the lungs represented a biological limitation for their use as contrast agents.

Preliminary evaluation of a polyethyleneglycol-stabilized manganese-substituted hydroxylapatite as an intravascular contrast agent for MR angiography

A blood-persistent particulate paramagnetic contrast agent has been formulated via size stabilization of manganese-substituted hydroxylapatite by a polyethylene glycol (PEG) bearing a terminal diphosphonate. At high PEG surface densities (35-40 mol%), particles with mean diameter 8 +/- 2 nm were obtained. Relaxivities of autoclaved samples (at 20 MHz proton Lamor frequency) were R1 = 18.7 +/- .8 mM-1 sec-1 and R2 = 22.3 +/- .7 mM-1 sec-1. The formulation persisted in rabbit blood with a biphasic clearance profile. Half-lives (with amplitudes in parenthesis) were 4 +/- 1 minutes (55%), and 49 +/- 3 minutes (45%), respectively, for the two phases. A dose of 40 mumol Mn/kg body weight enhanced the signal from rabbit vasculature for more than 45 minutes on MR angiograms. Thus, PEG-modified MnHA particles may find use as T1 agents for MR angiography.