Rapid assessment of liposomal stability in blood by an aqueous nitroxide spin label

Characterization of the interaction between liposomal formulations and Pseudomonas aeruginosa

The interactions between three liposomal formulations and Pseudomonas aeruginosa cells were evaluated by a lipid mixing assay and electron paramagnetic resonance (EPR) spectroscopy. The effect of the bacteria on the liposomal phase characteristics, the release of the liposomes' content, and the uptake rate of gentamicin by bacteria were monitored as a function of time, using EPR spectroscopy. The [16-DSA uptake](Total) from DPPC (1,2-dipalmitoyl-sn-glycero-3-phosphocholine) liposomes reached 93 +/- 12% over a 3-hour assay period, of which 9% crossed the bacterial inner membrane. A small amount of 16-DSA uptake from DPPC/Chol (cholesterol) vesicles was found throughout the 3-hour period of time. Although DPPC/DMPG (dimyristoylphosphatidylglycerol) vesicles showed a smaller value of [16-DSA uptake](Total) with respect to that of DPPC vesicles, they appeared to be effective in disrupting the bacterial membrane, resulting in a greater accumulation of 16-DSA inside the inner membrane. Exposure to bacteria caused the DPPC/Chol, DPPC, and DPPC/DMPG formulations to release 4.6 +/- 1.5, 17.6 +/- 1.2, and 34 +/- 3.7% of their content, respectively. Time-dependent fluid regions were developed within the vesicles when mixed with bacteria, and their growth over time depended on liposomal formulations. Incubation of gentamicin with bacteria for 3 hours resulted in 87 +/- 3% of the drug crossing the bacterial inner membrane. In conclusion, interaction between the liposome drug carriers and the bacterial cells result in vesicle fusion, disruption of the bacterial membrane, release of the liposomal content in the close vicinity of the bacteria cells, and the subsequent intracellular uptake of the released liposomal content.

Cellular uptake of electron paramagnetic resonance imaging probes through endocytosis of liposomes

Electron paramagnetic resonance imaging (EPRI) allows detection and localization of paramagnetic spin probes in vivo and in real time. We have shown that nitroxide spin probes entrapped in the intracellular milieu can be imaged by EPRI. Therefore, with the development of a tumor-targetable vehicle that can efficiently deliver nitroxides into cells, it should be possible to use nitroxide spin probes to label and image cells in a tumor. In this study, we assess the potential of liposomes as a delivery vehicle for imaging probes. We demonstrate that liposomes can stably encapsulate nitroxides at very high concentrations (>100 mM), at which nitroxides exhibit concentration-dependent quenching of their EPR signal-a process analogous to the quenching of fluorescent molecules. The encapsulating liposomes thus appear spectroscopically "dark". When the liposomes are endocytosed and degraded by cells, the encapsulated nitroxides are liberated and diluted into the much larger intracellular volume. The consequent relief of quenching generates a robust intracellular nitroxide signal that can be imaged. We show that through endocytosis of nitroxide-loaded liposomes, CV1 cells can achieve intracellular nitroxide concentrations of approximately 1 mM. By using tissue phantom models, we verify that this concentration is more than sufficient for in vivo EPR imaging.

In vivo EPR spectroscopic imaging for a liposomal drug delivery system

We used the membrane-impermeable nitroxyl radical 4-trimethylammonium-2,2,6,6-tetramethylpiperidine-1-oxyliodide (CAT-1) as a model drug encapsulated in liposomes in order to separately map the 2D distribution of both liposomal-encapsulated CAT-1 and free CAT-1. Phantoms were prepared with a CAT-1 solution and a liposomal CAT-1 suspension. Spectral-spatial images were obtained along several polar-arranged spatial axes through the phantom. The 1D spatial distributions (projections) of each signal component, reflecting the concentration of CAT-1, were then extracted from the spectral-spatial images. 2D EPR images of liposomal-encapsulated CAT-1 and free CAT-1 were separately reconstructed from the resulting projection data sets. 2D mapping of each component exhibited good agreement with respect to the phantom. Separate maps were generated from separate injections of free CAT-1 and liposomal CAT-1 injected into the femoral muscle of a living mouse. The EPR signal of the free CAT-1 gradually decreased during data acquisition. Because of this decay, we calibrated the image intensity by extrapolating the signal intensity to that detected at the beginning of data sampling. Both the position and size of the individual images were in very good agreement with those of the mouse thigh obtained by MRI.

Spectroscopic techniques in the study of membrane solubilization, reconstitution and permeabilization by detergents

This review focuses on the use of spectroscopic techniques for the study of membrane solubilization, reconstitution, and permeabilization by detergents. Turbidity and light scattering, visible and infrared spectroscopic methods, fluorescence, nuclear magnetic resonance, electron spin resonance and X-ray diffraction are examined from the point of view of their applicability to the above detergent-mediated phenomena. A short introduction is provided about each of the techniques, and references are given for further study.

Liposomes as MR contrast agents: pros and cons

Liposomes are generally thought of as being useful for entrapping drugs within their internal aqueous space. When used with MR contrast agents, this has the drawback that water flux across the membrane bilayer is limiting to contrast enhancement. This can be partially overcome by making the liposomes very small, such that surface area is relatively great compared to internal volume, thereby facilitating water exchange. Alternatively the membranes can be designed to be permeable to water but this may render the vesicles unstable in serum. Another approach is to incorporate the contrast agents into the lipid bilayer. By designing novel complexes of manganese with the ligands incorporated onto dual acyl chains, we have achieved R1 and R2 values of over 20/mmol sec-1 or more than five times higher than Gd-DTPA. Hepatic metastasis detection is significantly improved in rats at doses of only 10 mumol/kg manganese. Membrane-bound manganese complexes function as highly effective liver imaging agents and merit further study for development as agents to undergo human clinical trials.