Stabilization of small, unilamellar phospholipid vesicles by sucrose during freezing and dehydration

Solid lipid nanoparticles: a versatile approach for controlled release and targeted drug delivery

Solid Lipid Nanoparticles (SLN), the first type of lipid-based solid carrier systems in the nanometer range, were introduced as a replacement for liposomes. SLN are aqueous colloidal dispersions with solid biodegradable lipids as their matrix. SLN is produced using processes like solvent diffusion method and high-pressure homogenization, among others. Major benefits include regulated release, increased bioavailability, preservation of peptides and chemically labile compounds like retinol against degradation, cost-effective excipients, better drug integration, and a broad range of applications. Solid lipid nanoparticles can be administered via different routes, such as oral, parenteral, pulmonary, etc. SLN can be prepared by using high shear mixing as well as low shear mixing. The next generation of solid lipids, nanostructured lipid carriers (NLC), can reduce some of the drawbacks of SLN, such as its restricted capacity for drug loading and drug expulsion during storage. NLC are controlled nanostructured lipid particles that enhance drug loading. This review covers a brief introduction of solid lipid nanoparticles, manufacturing techniques, benefits, limitations, and their characterization tests.

Effect of sucrose as a lyoprotectant on the integrity of paclitaxel-loaded liposomes during lyophilization

CONTEXT

The stability of liposomes in the form of dispersion is a major concern due to drug leakage and fusion or aggregation. The stability can be improved by lyophilization, but the stress induced by the lyophilization process can affect the integrity of liposomes.

OBJECTIVE

The objective of this study was to evaluate the lyoprotective effect of sucrose on drug leakage and vesicle size of paclitaxel-loaded liposomes during the lyophilization process.

MATERIALS AND METHODS

Paclitaxel-loaded liposomal dispersions were prepared with sucrose at several lipid-sugar ratios, and internal-external sugar ratios, and then lyophilized. The entrapped paclitaxel content and vesicle size were monitored before and after lyophilization. The stability of the formulation was evaluated at 5 and 25 °C.

RESULTS

A significant increase in free paclitaxel and vesicle size was observed with the liposomes lyophilized without sucrose. Inclusion of sucrose in the formulation significantly reduced the free paclitaxel concentration and minimized the changes in vesicle size. The extent of protection improved further when sucrose was also present in the internal portion of the bilayer. The lyophilized formulation was stable at 5 °C for 3 months.

DISCUSSION

A significant (p < 0.05) correlation was observed between free paclitaxel content and the average diameter of the liposomes with respect to sucrose concentrations in the formulation. Sucrose protected the liposomes from drug leakage and aggregation/fusion induced by the lyophilization process in a concentration dependent manner.

CONCLUSION

The integrity of paclitaxel-loaded liposomes was preserved during lyophilization by optimal levels of lyoprotectant sucrose in the formulation.

Investigating lyophilization of lipid nanocapsules with fluorescence correlation spectroscopy

This paper describes characterization of lyophilized lipid nanocapsules loaded with Alexa 488 by fluorescence correlation spectroscopy (FCS). Fluorimetry analysis of nanocapsules containing self-quenching concentrations of 5- and 6-carboxyfluorescein was performed to establish a point of reference for FCS. FCS results complemented the results obtained by fluorimetry for a bulk nanocapsule solution and provided additional information about the size and dye retention by individual nanocapsules. Using this method, we determined that nanocapsules composed of the thiol-functionalized lipids showed the best dye retention and the most consistent results. Dye retention, size, and photolysis efficiency of these thiol-functionalized nanocapsules doped with a far-red photosensitizer did not change substantially upon lyophilization and storage at -20 degrees C for up to 2 months, making lyophilization a suitable method for the long-term storage of nanocapsules with the appropriate lipid composition.

Biomembrane model interaction and percutaneous absorption of papaverine through rat skin: effects of cyclodextrins as penetration enhancers

The effects of different concentrations of beta-cyclodextrin (beta-CyD), hydroxypropyl-beta-cyclodextrin (HP-beta-CyD) and 2,6-di-O-methyl-beta-cyclodextrin (DM-beta-CyD) on percutaneous absorption of papaverine hydrochloride (PAP) were investigated. Abdominal rat skin mounted in Franz cells was used for in vitro experiments. To evaluate CyD interaction with a bilayer structure model, dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and DPPC-Chol (8:2 mole ratio) vesicles were used. CyD vesicle interaction was evaluated by differential scanning calorimetry. Permeation through rat skin and calorimetric experiments demonstrated that at low concentrations DM-beta-CyD shows higher enhancer activity as a possible result of a perturbing action on the skin by a complexation of its lipid components, but at higher concentrations HP-beta-CyD is the most effective. By considering that HP-beta-CyD presents a very moderate destabilizing action on the skin, we conclude that a 10% aqueous solution of this macrocycle appears to be the most suitable transdermal absorption enhancer for PAP.

Liposomal delivery of a 30-mer antisense oligodeoxynucleotide to inhibit proopiomelanocortin expression

An oligodeoxynucleic sequence of 30 bases (30-mer ODN), complementary to a region of beta-endorphin mRNA, was synthesized to have an antisense effect with regard to the expression of this oligopeptide. Following the solid-phase synthesis of the oligodeoxynucleotide, the 30-mer ODN was encapsulated within liposomes to provide a higher resistance against DNases and an improved entrance into cells. The most suitable liposome formulation as a 30-mer ODN carrier consisted of small unilamellar vesicles (50 nm) with an encapsulation capacity of 4.76 microL/micromol. The liposomal formulations containing dipalmitoyl-DL-alpha-phosphatidyl-L-serine presented fusogenic properties, which are of great importance for the delivery of antisense compounds. The antisense activity of 30-mer ODN-loaded liposomes was evaluated by the determination of beta-endorphin levels in AtT-20 cells. The free 30-mer ODN did not provide any lowering of the beta-endorphin production, whereas the liposomally entrapped compound elicited a concentration-dependent inhibition. The inhibition was determined by a sequence-specific binding of the 30-mer ODN with the target mRNA.

Entrapment of haemoglobin into liposomes by the dehydration-rehydration method: vesicle characterization and in vivo behaviour

Haemoglobin (Hb) was isolated from human erythrocytes under conditions which maintained NADH-cytochrome-b5 reductase activity and suppressed oxidation of Hb during storage at 4 degrees C (methaemoglobin values < 3% after 29 days). Hb was entrapped into liposomes composed of hydrogenated egg phosphatidylcholine and equimolar cholesterol according to the dehydration/rehydration procedure of Kirby and Gregoriadis ((1984) Biotechnology 2, 979). However, encapsulation of Hb in its intact form was poor (< 5%) as a result of its oxidation and denaturation during freeze-drying. The addition of cryoprotectants and the use of both, higher initial concentrations of Hb and very small void vesicles resulted in Hb-rich dehydration/rehydration vesicles (phospholipid/Hb molar ratio of about 200:1) of the preferred size of 110 nm (mean). Highly homogeneous and small void vesicles as starting material were prepared using the one-step method of Brandl et al. ((1990) Drug Dev. Ind. Pharm. 16, 2167). The cryoprotectants were chosen with respect to their sufficient protection of Hb without affecting its loading into vesicles during freeze-drying and rehydration. 51Cr-labelling of Hb was used for the in vivo monitoring of the fate of Hb-containing vesicles rather than 125I-labelling since the latter induced strong interactions of Hb with liposomes. Upon intravenous administration into rats, liposomal 51Cr-Hb showed greater blood levels and prolonged circulation times in the blood compared to free Hb. The present approach provides high yield entrapment of labile molecules into vesicles of small size known to exhibit long circulation time.

Perturbation of the chain melting transition of DPPC by galactose, agarose and Laurdan as determined by differential scanning calorimetry

Differential scanning calorimetry was used to determine the effects of agarose hydrogel, galactose and the fluorophore Laurdan on the thermal behavior of multilamellar liposomes of DPPC. Although the effect of agarose on the phase transition was found to be negligible at low concentrations (< or = 2.5%), higher concentrations result in an endotherm that broadens and splits into two calorimetric events, one of which is at a higher temperature than that of hydrated DPPC. Equal weight fractions of galactose produce similar effects, although both fractions have raised melting temperatures. The higher melting components may be produced by osmotically-driven dehydration of the inner liposomal monolayers, a physical interaction between the carbohydrates and lipid headgroups, or a combination of both. Laurdan has little effect on the phase transition of DPPC vesicles at the concentration used in the sensor (0.67 mol%); concentrations up to 5.4 mol% only slightly lowered the melting temperature.