Comparative evaluation of two membrane-based liposomal MRI contrast agents

Paramagnetic liposomes for molecular MRI and MRI-guided drug delivery

Liposomes are a versatile class of nanoparticles with tunable properties, and multiple liposomal drug formulations have been clinically approved for cancer treatment. In recent years, an extensive library of gadolinium (Gd)-containing liposomal MRI contrast agents has been developed for molecular and cellular imaging of disease-specific markers and for image-guided drug delivery. This review discusses the advances in the development and novel applications of paramagnetic liposomes in molecular and cellular imaging, and in image-guided drug delivery. A high targeting specificity has been achieved in vitro using ligand-conjugated paramagnetic liposomes. On targeting of internalizing cell receptors, the effective longitudinal relaxivity r1 of paramagnetic liposomes is modulated by compartmentalization effects. This provides unique opportunities to monitor the biological fate of liposomes. In vivo contrast-enhanced MRI studies with nontargeted liposomes have shown the extravasation of liposomes in diseases associated with endothelial dysfunction, such as tumors and myocardial infarction. The in vivo use of targeted paramagnetic liposomes has facilitated the specific imaging of pathophysiological processes, such as angiogenesis and inflammation. Paramagnetic liposomes loaded with drugs have been utilized for therapeutic interventions. MR image-guided drug delivery using such liposomes allows the visualization and quantification of local drug delivery.

Approaches and agents for imaging the vascular system

Several classes of vascular imaging agents are described: (1) liposome-based blood cell mimetics; (2) plasma protein mimetics; (3) small molecules that bind to plasma proteins in the circulation. The characteristic features of the different agents are described and critically compared, including the advantages and potential pitfalls of each individual type.

Invited review: biophysical properties and clinical applications of magnetic resonance imaging contrast agents

Contrast enhanced magnetic resonance imaging (MRI) is a very versatile and effective technique for detecting and characterizing lesions, for identifying a variety of patho-physiological abnormalities, and for providing perfusion and functional information. The application of contrast enhanced MRI to many clinical and research indications has emerged because of the rapid evolution in imaging techniques, improved methodology, and the development of efficient and specific contrast agents. Problems related to optimizing parameters and dosage have been due to complex interplay of relaxation times, biophysical mechanisms and acquisition parameters. A knowledge of basic biophysical aspects is therefore essential for a full understanding of the results obtained for different organs under different conditions, and for optimizing the image parameters and dosage of contrast agents. This article underlines the biophysical basis of the effects of contrast agents in MRI, identifies the problems involved in optimizing the parameters for maximum efficiency, and presents a general overview of the clinical studies and research applications in the central nervous system, perfusion abnormalities, hepatobiliary system, musculoskeletal system and the gastrointestinal tract. The section on perfusion studies includes a discussion of quantitative analysis and kinetic models describing the effects of contrast agents. Finally, a critical evaluation of the scope and limitations of contrast enhanced MRI is presented.

Evaluation of a nitroxyl fatty acid as liver contrast agent for magnetic resonance imaging

In this study, we report the synthesis and the evaluation as MRI contrast agent of a new compound (nitroxyl fatty acid, NFA), where a pyrrolidinoxyl radical (3-carboxy-proxyl, PCA) is linked to a fatty acid moiety. Fatty acids were selected as vector because they present a high affinity for the liver, their efficient cellular uptake being the result of a specific interaction with a transmembrane transporter (liver plasma membrane-fatty acid binding protein). The uptake of 3H-oleic acid is inhibited after the injection of 1 mmol/kg of NFA, suggesting that NFA recognizes the same transmembrane transporter as the natural fatty acids. The higher relaxivity R1 of NFA in albumin solutions, compared with PCA, was explained by the immobilization of the nitroxyl radical in the protein. MR imaging was performed using T1-weighted images on mice in order to compare the contrast effect obtained after the injection of 1 mmol/kg of radical. The % signal enhancement in the liver 5 min after intravenous injection was 49 +/- 4 and 14 +/- 5 for NFA and PCA, respectively. NFA allowed a better delimitation of some necrotic tumors (Novikoff hepatocarcinoma) due to its preferential uptake by the nontumorous tissue.

Baseline consideration of liposomal contrast agent. CNS transport by macrophages in experimental allergic encephalomyelitis

The purpose of this study was to investigate a specialized liposomal contrast agent for magnetic resonance imaging (MRI), as part of a program to examine infiltrating immune cells in lesions of experimental allergic encephalomyelitis (EAE). A potent investigational liposomal contrast agent, phosphatidylethanolamine-DTPA-gadolinium, was chosen which has been shown to remain tightly liposomal-associated, with long persistence in vivo. Europium (Eu3+), a fluorescent paramagnetic metal, was also utilized in these experiments in place of gadolinium. This material is avidly taken up by monocytes in vivo. Thirty-four animals received some form of liposomal material either before or during the opening of the blood-brain barrier (BBB). Twenty-seven Hartley guinea pigs were inoculated for EAE with homogenized brain and Complete Freunds Adjuvant (CFA) and seven control animals received CFA alone. Eighty-two percent of the experimental animals exhibited degeneration of the BBB with inflammation and edema in the brain, while all control animals had normal brain scans. T1-weighted MRI, performed to detect the presence of liposomal contrast material in experimental animals, was not different from untreated animals. Fluorescent microscopy revealed no characteristic changes associated with Eu3+ presence in the brains of treated or control animals. Therefore, it would seem that insufficient material crosses the disrupted BBB, either in free form or subsequent to macrophage ingestion, to be detected by MRI or fluorescent microscopic examination.

Liposomes in drug delivery. Clinical, diagnostic and ophthalmic potential

Liposomes (phospholipid-based vesicles) have been investigated since 1970 as a system for the delivery or targeting of drugs to specific sites in the body. Because of their structural versatility in terms of size, composition, surface charge, bilayer fluidity and ability to incorporate almost any drug regardless of solubility, or to carry on their surface cell-specific ligands, liposomes have the potential to be tailored in a variety of ways to ensure the production of formulations that are optimal for clinical use. This includes controlled retention of entrapped drugs in the presence of biological fluids, controlled vesicle residence in the blood circulation or other compartments in the body, and enhanced vesicle uptake by target cells. Accumulated in vivo evidence, particularly in areas such as cancer chemotherapy, antimicrobial therapy, vaccines, diagnostic imaging and the treatment of ophthalmic disorders has indicated clearly that some liposome-entrapped drugs and vaccines exhibit superior pharmacological properties to those observed with conventional formulations. Such work has encouraged the application of liposomes in the treatment of diseases in humans. A large number of trials in patients with cancer or infections suggest that certain liposomal drug formulations are likely to prove clinically useful.

Selective MR imaging of labeled human peripheral blood mononuclear cells by liposome mediated incorporation of dextran-magnetite particles

Human peripheral blood mononuclear cells (PBMCs) were incubated with large unilamellar vesicles (LUV) containing encapsulated dextran-magnetite particles (DMP). This resulted in an efficient incorporation of DMP. Electron microscopy revealed the presence of DMP in cells mainly in phagosomes and secondary lysosomes. DMP-labeled PBMCs showed a strong increase of the transverse relaxation rate (up to 16.6 s-1 for 5 x 10(7) cells/ml) and, accordingly, a great loss of signal intensity in MR imaging. The fraction of DMP containing PBMCs could be enriched by magnetic cell separation. The major population of the DMP containing cells proved to be monocytes. When PBMCs depleted of monocytes were used for labeling, DMP uptake was observed also in the peripheral blood lymphocytes. The labeling of PBMCs presented here may be used in future studies of selective MR imaging of in vivo cell migration in a variety of immunologically compromised tissue states, e.g., tumors, transplantations, and abscesses.