Martin Brandl

Filter extrusion of liposomes using different devices: comparison of liposome size, encapsulation efficiency, and process characteristics

Liposomes were prepared by stepwise extrusion through 5, 1, 0.4, 0.2, 0.1 and 0.05 microm pore sizes using two different filter-extruders, the continuous high pressure device Dispex Maximator (CE) or alternatively the discontinuous Avestin LiposoFast (DE). The liposome dispersions obtained were compared in terms of particle size, lamellarity and encapsulation efficiency of calcein. The liposomes were smaller with CE than DE at all stages due to higher flow rates and pressure drops, except for final filter pore size (0.05 microm) where both preparations had similar sizes. The particle size analysis technique itself had a strong influence on the liposome sizes measured. For bigger liposomes (extruded through 0.4 microm filters) the Nicomp 370 revealed bigger volume-based mean particle sizes along with more stringent differences between volume-based and number-based diameters than the Malvern Zetasizer. In contrast, for small liposomes extruded through 0.05 microm filters, similar liposome sizes were found no matter which of the two PCS techniques or cryo-transmission electron microscopy was used. In congruence to the liposome sizes measured, encapsulation efficiencies were smaller for CE than DE at all filter stages except the final (0.05 microm). No lipid loss occurred and lyso-phosphatidylcholine formation was negligible irrespective of which extrusion technique was used.

Liposomes with nifedipine and nifedipine-cyclodextrin complex: calorimetrical and plasma stability comparison

Abstract Inclusion complexes of nifedipine with 2-hydroxypropyl-s-cyclodextrin (HPβCD) were formed by the spray- and freeze-drying methods. Nifedipine or its inclusion complexes (Nifedipine-CD complex I and II) were incorporated into liposomes prepared by the ethanol injection method. The highest entrapment value (77.7% of the starting material) was achieved for liposomes with N-CD complex II. The interaction of nifedipine with lipid bilayers was measured calorimetrically. DPPC liposomes mixed with nifedipine or N-CD complex II showed a slight shift of the transition temperature of DPPC towards lower temperatures compared to DPPC liposomes alone or mixed with HPsCD. However, with nifedipine, an additional transition peak was seen at lower temperatures in the second and all subsequent scans which could not be detected for the N-CD complex. Plasma stability studies showed that liposomes containing N-CD complex II are more stable than liposomes containing nifedipine. Encapsulation of drug-cyclodextrin complexes into liposomes can increase the entrapment of the lipophilic drug and reduce its release from the carrier.

Detection of Lipopolysaccharides in Phospholipids and Liposomes Using the Limulus Test

AbstractLPS (lipopolysaccharides) represent a feared pyrogenic impurity in parenterals and raw materials used for their production. In liposome dispersions detection of LPS via the standard Limulus amoebocyte lysate (LAL) test was proved unreliable in presence of phospholipids (liposomes). Attempts were made either to eliminate or inactivate disturbances of the LAL test by phospholipid(s). Common methods to overcome inhibition of the test, such as dilution of the sample, removal of the inhibiting substance by centrifugation or its inactivation by addition of detergents, were found not successful when LPS was present in liposome membrane-bound form. Another means to remove inhibiting substances is ultrafiltration. Ultrafiltration of aqueous lipid dispersions cannot be performed due to clogging of the filter membrane. Ultrafiltration upon addition of organic solvents turned out to be difficult due to the very limited resistance of celluloseacetate filter membranes against these solvents. Nevertheless a test...