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Comparative evaluation of two membrane-based liposomal MRI contrast agents
S Karlik1, E Florio, C W Grant
1Department of Diagnostic Radiology, University Hospital, London, Canada.
Abstract:
Two phospholipids--one bearing a nitroxide free radical covalently attached to its polar headgroup, and the other bearing a similarly attached chelating agent with bound gadolinium--were compared in vivo as liposomal contrast agents for MR imaging. In each case the phospholipid contrast agent was incorporated into the membranes of sonicated unilamellar vesicles. The agent with bound gadolinium proved to be considerably more potent at highlighting regions of liposome biodistribution than did its spin-labelled analogue. Injected intramuscularly into rats, the spin-label liposomes produced local tissue contrast that persisted for 1 h; while under similar conditions, the liposomal gadolinium persisted for over 24 h. By comparison, water-soluble, nonliposomal DTPA-Gd3+ (dimeglumine), was rapidly cleared from the same intramuscular sites--appearing in kidney bladder within 15 min of injection. When delivered intravenously, maximum effect from both liposomal agents was observed in liver and spleen within 1-2 h, although the spin-label agent produced only marginal contrast. The concomitant use of fat suppression proved a valuable adjunct to liposomal contrast for imaging organs of the reticuloendothelial system.
Insights
Liposomal gadolinium contrast agents offer superior MRI enhancement compared to spin-label agents. These gadolinium-based liposomes provide prolonged imaging contrast, especially in intramuscular injections.
Area of Science:
- Biomedical Imaging
- Materials Science
- Pharmacology
Background:
- Liposomes are utilized as drug delivery vehicles and contrast agents.
- Developing effective liposomal contrast agents for Magnetic Resonance Imaging (MRI) is crucial for diagnostic imaging.
- Gadolinium and spin-label agents are explored for their potential in liposomal formulations.
Purpose of the Study:
- To compare the in vivo efficacy of two novel liposomal contrast agents for MRI.
- To evaluate the biodistribution and contrast enhancement properties of spin-label and gadolinium-bound phospholipids.
- To assess the duration of contrast enhancement following different administration routes.
Main Methods:
- Two types of phospholipids were synthesized: one with a nitroxide free radical and another with a gadolinium-chelating agent.
- These phospholipids were incorporated into sonicated unilamellar vesicles to form liposomal contrast agents.
- In vivo studies were conducted in rats, involving intramuscular and intravenous injections.
- MRI was performed to assess contrast enhancement and biodistribution.
- Fat suppression techniques were employed as an adjunct imaging method.
Main Results:
- Liposomal gadolinium demonstrated significantly higher potency in highlighting liposome biodistribution compared to the spin-label analogue.
- Intramuscular injection of liposomal gadolinium resulted in contrast lasting over 24 hours, whereas spin-label liposomes provided contrast for only 1 hour.
- Water-soluble DTPA-Gd3+ showed rapid clearance from intramuscular sites within 15 minutes.
- Intravenous administration of both liposomal agents led to maximum effect in the liver and spleen within 1-2 hours, with spin-label agents yielding marginal contrast.
- Fat suppression improved imaging of reticuloendothelial system organs.
Conclusions:
- Liposomal contrast agents containing gadolinium are more effective and provide longer-lasting contrast than spin-label analogues for MRI.
- The biodistribution and clearance profiles differ significantly between liposomal and non-liposomal agents, as well as between gadolinium and spin-label liposomes.
- Liposomal gadolinium represents a promising contrast agent for enhanced diagnostic imaging, particularly for tracking liposome distribution over extended periods.