Solid lipid nanoparticles (SLN) for controlled drug delivery - a review of the state of the art

Solid lipid nanoparticles (SLN) introduced in 1991 represent an alternative carrier system to traditional colloidal carriers, such as emulsions, liposomes and polymeric micro- and nanoparticles. SLN combine advantages of the traditional systems but avoid some of their major disadvantages. This paper reviews the present state of the art regarding production techniques for SLN, drug incorporation, loading capacity and drug release, especially focusing on drug release mechanisms. Relevant issues for the introduction of SLN to the pharmaceutical market, such as status of excipients, toxicity/tolerability aspects and sterilization and long-term stability including industrial large scale production are also discussed. The potential of SLN to be exploited for the different administration routes is highlighted. References of the most relevant literature published by various research groups around the world are provided.

Etherolytic cleavage of 4-(2,4-dichlorophenoxy)butyric acid and 4-(4-chloro-2-methylphenoxy)butyric acid by species of Rhodococcus and Aureobacterium isolated from an alkaline environment

Bacterial strains were isolated from the concrete rubble of a demolished herbicide production plant. The predominant feature of these strains was the etherolytic cleavage of 4-(2,4-dichlorophenoxy)butyric acid (DCPB)1) and 4-(4-chloro-2-methylphenoxy)butyric acid (MCPB) while liberating 2,4-dichlorophenol (DCP) and 4-chloro-2-methylphenol (MCP) respectively. Some of the isolates were identified by 16S rDNA sequence analysis and shown to belong to the genera Aureobacterium sp. (strain K2-17) and Rhodococcus (Rh. erythropolis K2-12). The other strains isolated clustered into these two groups according to fatty acid analysis. Etherolytic cleavage proceeded under neutral to alkaline conditions with an optimum at around pH 8.5. With Aureobacterium sp. No. K2-17, the degradation rate was zero at a pH of 6 but as much as 60% of the maximum activity was observed at pH 10.5. With Rh. erythropolis K2-12, by contrast, pronounced activity was detected at pH 6.5 while degradation was no longer observed at pH 10.5. The maximum rates of cleavage were about 1 mmol DCPB/h.g dry mass with Aureobacterium sp. No. K2-17 and about 0.6 mmol DCPB/h.g dry mass with Rh. erythropolis K2-12. DCPB and MCPB were utilized to the same extent. Substrate cleavage and product formation (DCP) proceeded at almost equal rates with Aureobacterium sp. No. K2-17 and Rh. erythropolis K2-12, which indicates that this compound was not further metabolized. Only phenoxybutyric acid compounds served as substrates; phenoxyacetic acid and phenoxypropionic acid derivatives were not utilized by these strains.

Degradation of various chlorophenols under alkaline conditions by gram-negative bacteria closely related to Ochrobactrum anthropi

From concrete debris of a demolished herbicide production plant several Gram-negative bacterial strains were isolated, which exhibit metabolic capabilities for the degradation of 2,4-dichlorophenol (DCP)l), 4-chloro-2-methylphenol (MCP) and 4-chlorophenol (4-CP), while 2-chlorophenol (2-CP) was degraded at a slower rate. Degradative activity was inducible and was impeded by adding of 100 mg/l of chloramphenicol to growing cultures. The strains displayed alkaliphilic properties with optimum DCP/MCP degradation at pH values around 8.5-9.5; activity was observed up to pH values of 11. Degradation was most likely complete according to chlorine balances; formation of intermediary products was observed with MCP some time. Specific activity of up to 380 mumol/h.g dry mass was found within the concentration range of 10-20 mg/l DCP; higher concentrations retarded the activity with complete inhibition at 200-400 mg/l. Some of the strains carry plasmids whose presence was not unambiguously correlated to the degradative properties. Ribotyping revealed a high degree of relationship between the strains. Preliminary taxonomic investigations showed close relationship to Ochrobactrum anthropi.

Investigation on the viscoelastic properties of lipid based colloidal drug carriers

The rheological behaviour of solid lipid nanoparticle dispersions (SLN) prepared by high pressure homogenization was investigated using a Haake RS-100 rheometer. Four preparations differing in their lipid content and macroscopic consistency were tested by continuous shear rheometry and oscillatory testing. Rheological data from continuous shear measurement reveal plastic flow for systems with low lipid content as well as for systems with high lipid content. By using oscillatory testing more detailed information concerning the structure could be achieved. Rheological measurements of 40% lipid dispersions show viscoelastic properties comparable to the data from standard dermal preparations. Therefore high concentrated lipid dispersions might constitute a promising vehicle for topical administration.

Heavy metal contamination of nanosuspensions produced by high-pressure homogenisation

High pressure homogenisation is a method for the production of nanosuspensions. In this process crystalline drug particles are pressed with high pressure through a narrow homogenisation gap. Due to the conditions in the gap it seems possible that metal erosion can occur. In this study the heavy metal (Fe) contamination of nanosuspensions produced by high pressure homogenisation was determined. Therefore nanosuspensions were analysed by atom absorption spectroscopy concerning their load of iron which is chosen as reference metal. The results show that the erosion of metal is below 1 ppm and will not cause any toxicological problems.

Nanoparticles with decreasing surface hydrophobicities: influence on plasma protein adsorption

The rapid uptake of i.v. injected nanoparticles by cells of the mononuclear phagocytic system (MPS) is a major obstacle for a long blood circulation time and a drug targeting to sites other than the MPS. The adsorption of proteins on the particles surface after i.v. administration depends on their surface characteristics and is regarded as key factor for the in vivo organ distribution. The objective of this study is to investigate changes in the plasma protein adsorption patterns in the course of surface hydrophobicity variation. Latex particles with decreasing surface hydrophobicity were synthesized as model colloidal carriers. Physicochemical characterization had been performed and considerable differences in the protein adsorption patterns on the particles could be detected by using two-dimensional polyacrylamide gel electrophoresis (2-D PAGE). Correlations between physicochemical characteristics and the protein adsorption patterns have been found and are discussed.

Nanosuspensions of poorly soluble drugs--reproducibility of small scale production

The major problem of many newly developed pharmaceutical drugs is their poor solubility in water and simultaneously in organic media. To solve these problems formulation as nanosuspensions is an attractive alternative. During the drug development process screening for an optimal formulation by homogenisation is essential. Time and cost effective production in an initial phase of R&D can be conducted on lab scale by using the Micron Lab 40 in its discontinuous version. In this report reproducibility of small scale production parameters (particle size, size distribution, content of microparticles) was exemplary studied for the drug RMKP22.

Influence of different parameters on reconstitution of lyophilized SLN

Drug-loaded solid lipid nanoparticles (SLN) suitable for parenteral administration were freeze-dried. The lipid matrix Imwitor 900 (concentration, 2.5%) was stabilized with Lipoid E 80 and sodium glycocholate. The influence of different parameters of lyophilization like the protective effect of cryoprotectants, freezing velocity, and thermal treatment was investigated. The results of this study demonstrate that, by optimizing critical process parameters, i.v.-injectable SLN-dispersions can be freeze-dried, preserving their small particle size.

Preserved solid lipid nanoparticles (SLN) at low concentrations do cause neither direct nor indirect cytotoxic effects in peritoneal macrophages

In order to investigate the interaction of preserved solid lipid nanoparticles (SLN) with murine peritoneal macrophages (Mpsi), cytotoxicity and proinflammatory effects of two different solid lipid nanoparticles (SLN) preparations consisting of either compritol (CO) or cetyl palmitate (CP) preserved with thiomersal were analyzed. Concentration-dependent cytotoxic effects were observed using the 3-(4,5-dimethylthiazol-2-yl)-2, 5-diphenyl-tetrazolium bromide (MTT) assay. Secretion of interleukin-6 by Mpsi following incubation with CO and CP SLN did not differ from secretion by untreated cells; proinflammatory cytokines interleukin-12 and tumor-necrosis-factor-alpha as further indicators of immunomodulatory effects were not detectable. These findings paralleled our previous findings that unpreserved CO and CP SLN did not induce immunomodulatory effects but cytotoxicity at higher concentrations. There were no synergistic cytotoxic effects of preservative and SLN. Thus, preservation of SLN using thiomersal does not appear to cause increased cytotoxicity and immunomodulatory effects following incubation with Mpsi.

Interactions of nanoparticles with body proteins--improvement of 2D-PAGE-analysis by internal standard

Two-dimensional polyacrylamide gel electrophoresis (2D-PAGE) is the method of choice to investigate protein adsorption of blood proteins (opsonization) onto nanoparticular drug carriers. In general the reproducibility of the obtained adsorption patterns is satisfying. However, direct comparison between the amounts of single protein spots from gels obtained in different runs is difficult, because 2D-PAGE is a multistep procedure. A possible solution of the problem is to establish a protein as internal standard. Therefore, selected proteins (Bio-rad) were under investigation. Due to its molecular weight and isoelectric point, soybean trypsin inhibitor (TI) does not interfere with plasma components. Therefore, a method was established to use TI as an internal standard protein to improve comparability between the 2D-PAGE gels obtained in different analytical runs.

Influence of high pressure homogenisation equipment on nanodispersions characteristics

In this study a comparison of the influence of the homogenising equipment supplied by different manufacturers on the quality of the lipid nanodispersions is given. An Avestin EmulsiFlex-B3 (B3) and APV Micron Lab 40 (LAB 40) were used for high pressure homogenisation. Particle size and particle size distribution were chosen as quality parameters. The influence of different process parameters was evaluated. The two homogenisers were compared in their quality of nanoparticles-production by hot and cold homogenisation technique and in processing nanoemulsions. Working with the B3 appeared as useful for preformulation studies and processing of expensive or rare drugs and excipients. This first scaling up within laboratory scale is evaluated and the problems and remarkable aspects working with the B3 are pointed out.

Nanosuspensions as a new approach for the formulation for the poorly soluble drug tarazepide

Poorly soluble drugs are often a challenging problem in drug formulation, especially when the drug is not soluble in either aqueous media or organic solvents. Attempts to overcome the solubility problem are, e.g. solubilisation with mixed micelles or forming a complex using cyclodextrines, but these approaches are of limited success. Another problem with new high potential drug is that these drugs often show bioavailability problems. One tried to improve the in vivo performance of poorly soluble drugs by reducing the particles size of the drug thus leading to an increased surface area and an increased dissolution velocity (Müller et al., 1994, 1999). Some of these problems occurred with tarazepide and therefore it was tried to create a formulation with this drug as nanosuspension which is suitable for intravenous administration.

The influence of alkali fatty acids on the properties and the stability of parenteral O/W emulsions modified with solutol HS 15

Arachis oil based parenteral O/W emulsions were prepared using soya bean phosphatidylcholine (SPC) and different combinations of co-emulsifiers containing polyethylene glycol fatty acid esters (Solutol HS 15) and alkali fatty acids (sodium laurate, sodium stearate). The parameters measured were droplet size (both by photon correlation spectroscopy and laser diffractometry), pH and zeta potential. All emulsions were subjected to autoclaving. The addition of polyethylene glycol 12-hydroxy stearate (Solutol HS 15) led to a significant decrease of mean oil droplet size. For long-term stability the amount added turned out to be the most important factor. With increased amounts of Solutol HS 15 the packing density of the emulsifier layer and the zeta potential decreased leading to instability. The optimum load of Solutol HS 15 was found to be 15 micromol/ml. Alkali fatty acids markedly improved the physical stability of the emulsions. Improved stability properties conferred to emulsions by alkali fatty acids could be attributed to the zeta potential increase even in the presence of Solutol HS 15. Consequently a mixed emulsifier film was established in which the ionized fatty acids determined the interface charge. In addition to this a strengthening of the molecular interactions occurring between phospholipid and Solutol HS 15 emulsifier in the presence of ionized fatty acids at the O/W interface can be assumed (L. Rydhag, The importance of the phase behaviour of phospholipids for emulsion stability, Fette Seifen Anstrichm. 81 (1979) 168-173). Different co-emulsifier mixtures were shown to have a pronounced impact on the plasma protein adsorption onto emulsion droplets.

A theoretical study on the metabolic requirements resulting from alpha-ketoglutarate-dependent cleavage of phenoxyalkanoates

The etherolytic cleavage of phenoxyalkanoic acids in various bacteria is catalyzed by an alpha-ketoglutarate-dependent dioxygenase. In this reaction, the electron acceptor is oxidatively decarboxylated to succinate, whereas the proper substrate is cleaved by forming the oxidized alkanoic acid and the phenolic intermediate. The necessity of regenerating alpha-ketoglutarate and the consequences for the overall metabolism were investigated in a theoretical study. It was found that the dioxygenase mechanism is accompanied by a significant loss of carbon amounting to up to 62.5% in the assimilatory branch, thus defining the upper limit of carbon conversion efficiency. This loss in carbon is almost compensated for in comparison to a monooxygenase-catalyzed initial step when the dissimilatory efforts of the entire metabolism are included: the yield coefficients become similar. The alpha-ketoglutarate-dependent dioxygenase mechanism has more drastic consequences for microorganisms which are restricted in their metabolism to the first step of phenoxyalkanoate degradation by excreting the phenolic intermediate as a dead-end product. In the case of phenoxyacetate derivatives, the cleavage reaction would quickly cease due to the exhaustion of alpha-ketoglutarate and no growth would be possible. With the cleavage products of phenoxypropionate and phenoxybutyrate herbicides, i.e., pyruvate and succinate(semialdehyde), respectively, as the possible products, the regeneration of alpha-ketoglutarate will be guaranteed for stoichiometric reasons. However, the maintenance of the cleavage reaction ought to be restricted due to physiological factors owing to the involvement of other metabolic reactions in the pool of metabolites. These effects are discussed in terms of a putative recalcitrance of these compounds.

Preparation of a clofazimine nanosuspension for intravenous use and evaluation of its therapeutic efficacy in murine Mycobacterium avium infection

Clofazimine nanosuspensions were produced by high pressure homogenization and the formulation was optimized for lyophilization. Characterization of the product by photon correlation spectroscopy, laser diffraction and Coulter counter analysis showed that the clofazimine nanosuspensions were suitable for iv injection with a particle size permitting passive targeting to the reticuloendothelial system. Following iv administration to mice of either the nanocrystalline or a control liposomal formulation at a dose of 20 mg clofazimine/kg bodyweight, drug concentrations in livers, spleens and lungs reached comparably high concentrations, well in excess of the MIC for most Mycobacterium avium strains. When C57BL/6 mice were experimentally infected with M. avium strain TMC 724, nanocrystalline clofazimine was as effective as liposomal clofazimine in reducing bacterial loads in the liver, spleen and lungs of infected mice. Nanocrystalline suspensions of poorly soluble drugs such as riminophenazines are easy to prepare and to lyophilize for extended storage and represent a promising new drug formulation for intravenous therapy of mycobacterial infections.

Adsorption kinetics of plasma proteins on oil-in-water emulsions for parenteral nutrition

The plasma protein adsorption patterns on colloidal drug carriers are regarded as an important factor for their in vivo fate. In this study the adsorption kinetics on oil-in-water emulsions were determined and compared to the adsorption kinetics on polystyrene particles. In addition, the adsorption kinetics on the same systems after surface-modification with a hydrophilic polymer were also investigated. The protein adsorption was determined by means of two dimensional polyacrylamide gel electrophoresis (2D-PAGE). The determination of the plasma protein adsorption kinetics was carried out using different concentrations of human plasma in the incubation medium to prolong the residence time of the more abundant plasma proteins on the surface. Proteins which are likely to be displaced in a split second are thus accessible to analysis. The oil-in-water emulsion showed a distinctly different adsorption behavior from the one previously described for solid surfaces, where initially adsorbed proteins are displaced by others, having a higher affinity to the surface ('Vroman effect'). No competitive protein adsorption could be observed on the emulsions. Moreover, the predominantly adsorbed apolipoproteins A-I, A-IV, C-II and C-III increase in amount with increasing plasma concentration. The knowledge of the adsorption kinetics of colloidal carriers might be helpful for a better understanding of the in vivo behavior of such systems and for the transfer of principles already known from other carrier systems to the controlled development of emulsions for site specific drug delivery.

Comamonas acidovorans strain MC1: a new isolate capable of degrading the chiral herbicides dichlorprop and mecoprop and the herbicides 2,4-D and MCPA

A gram-negative prototrophic bacterial species, strain MC1, was isolated from the vicinity of herbicide-contaminated building rubble and identified by 16S rDNA sequence analysis, its physiological properties, GC content, and fatty acid composition as Comamonas acidovorans. This strain displays activity for the productive degradation of the two enantiomers of dichlorprop [(RS)-2-(2,4-dichlorophenoxy-)propionate; (RS)-2,4-DP] and mecoprop [(RS)-2-(4-chloro-2-methyl-) phenoxypropionate; (RS)-MCPP] in addition phenoxyacetate herbicides, i.e. 2,4-dichlorophenoxyacetate (2,4-D) and 4-chloro-2-methylphenoxyacetate (MCPA), and various chlorophenols were utilized. Rates amounted to 1.2 mmoles/h g dry mass (2,4-D) and 2.7 mmoles/h g dry mass [(RS)-2,4-DP]. Degradation of (RS)-2,4-DP was not inhibited up to concentrations of 500 mg/l, nor of 2,4-D up to 200 mg/l. The optimum pH value of (RS)-2,4-DP degradation was around 8. The application of respective primers for PCR amplification revealed the presence of tfdB and tfdC genes.

Complement activation by model drug carriers for intravenous application: determination by two-dimensional electrophoresis

The interactions of intravenously injected drug carriers with blood proteins are considered as an important factor for the fate of the particles after their administration. Protein adsorption on latex particles applied as model for intravenous drug carriers was analysed using two-dimensional electrophoresis (2-DE). The particles were incubated in citrated plasma, serum and heat-inactivated serum, respectively. Incubation in the various media resulted in clear differences in the protein adsorption patterns. Two characteristic protein spots were determined to be enriched on the 2-DE gels only after incubation of the particles in serum. Employing N-terminal microsequencing these protein spots were identified to be fragments of the complement protein C3. Enrichment of these particular spots was most likely a result of complement activation by the particles. Mechanism of C3 binding to the particle surface and subsequent inactivation by cleavage are discussed in order to explain the results. It could be demonstrated that 2-DE analysis provides the possibility to distinguish between adsorption and covalent attachment of C3 to particulate surfaces. The findings indicate that complement activation was caused by covalent binding of the C3 component C3b to the particles' surface. The influence of the incubation medium on the in vitro protein adsorption of particulate drug carriers has to be considered when a correlation between the protein adsorption pattern and the in vivo behaviour of the particles is approached.

Solid lipid nanoparticles (SLN/Lipopearls)--a pharmaceutical and cosmetic carrier for the application of vitamin E in dermal products

Solid lipid nanoparticles (SLN, Lipopearls) are nanoparticles made from solid lipids by high pressure homogenization. Incorporation of chemically labile active ingredients into the solid lipid matrix protects against chemical degradation, which is shown for vitamin E. The SLN are physically stable in aqueous dispersions and also after incorporation into a dermal cream as proven by photon correlation spectroscopy and differential scanning calorimetry. Electron microscopy and atomic force microscopy data reveal the spherical shape of the SLN and the detailed structure of the particle surface. Ultrafine particles form an adhesive film leading to an occlusive effect on the skin. The occlusion promotes the penetration of vitamin E into the skin, as shown by the stripping test. In addition to chemical stabilization of active ingredients, occlusive effects on the skin and subsequent enhanced penetration of compounds, the SLN also possess a pigment effect covering undesired colours leading to an increased aesthetic acceptance by the customer.

High resolution size determination of 20 nm colloidal gold particles by SedFFF

PURPOSE

Assessment of lower size limit of Sedimentation Field-Flow Fractionation (SedFFF), specifically to evaluate if the method is suitable to determine the size and size distribution of 20 nm colloidal gold particles with high resolution.

METHODS

Sedimentation Field-Flow Fractionation was used to determine the size of the colloidal particles. Due to the high density of gold it was possible to extend the lower size limit of SedFFF well below 20 nm. The size distribution of a gold colloid was obtained from the peak broadening caused by the polydispersity of the sample. The peak broadening due to instrumental imperfections was determined. For comparison purpose the particles were also sized using SEM and PCS.

RESULTS

The mean diameter of the particles was determined to be (20.87+/-0.05) nm, the standard deviation in size being 1.04 nm (about 5%). SEM could confirm that the particles are about 20 nm in diameter. A sizing with PCS was not possible. The particles have a strong tendency to aggregate and PCS yields a diameter that is much too large.

CONCLUSIONS

At optimized analytical parameters Sedimentation Field-Flow Fractionation is an effective method to measure the size of gold particles as small as 15 nm with an accuracy of about 0.1 nm. The polydispersity of the sample can easily be determined.

Lab-scale production unit design for nanosuspensions of sparingly soluble cytotoxic drugs

Cytotoxic agent processing assumes certain prerequisites in order to guarantee a high level of security for both personnel and the material used. A lab-scale unit designed for the production and analysis of nanosuspensions of cytotoxic agents has been in use for more than three years. This design encompasses equipment, exhaust systems, materials used and measures for staff security, and this lab suite may also be used in applications that involve production in aseptic conditions.

An investigation into the distribution of lecithins in nanosuspension systems using low frequency dielectric spectroscopy

A range of nanosuspensions comprising a model drug (RMKP 22) with varying concentrations of Phospholipon 90 were prepared using high pressure homogenization and analyzed using low frequency dielectric spectroscopy as a novel means of characterizing the distribution of Phospholipon 90 within the suspensions. A corresponding range of aqueous Phospholipon 90 suspensions were also studied for the purpose of comparison. The dielectric responses were interpreted using a modification of the Maxwell-Wagner approach, whereby the systems were considered to comprise a high frequency response corresponding to the bulk layer serially connected to a lower frequency response corresponding to an electrode barrier layer. The low frequency responses of both the Phospholipon 90 dispersions and the nanosuspensions were found to be essentially independent of phospholipid concentration, indicating the presence of a barrier layer covering the electrode surfaces. In contrast, the high frequency (bulk) loss response was found to increase with Phospholipon 90 concentration for the surfactant suspensions while a maximum in response was seen with Phospholipon 90 concentration for the nanosuspensions; this behaviour was attributed to the presence of impurities within the phospholipids. Based on this investigation, a model is proposed with which the dielectric response may be related to the surface coverage of the suspended drug particles.

Visualization of in vitro protein-rejecting properties of PEGylated stealth polycyanoacrylate nanoparticles

The in vitro protein-rejecting properties of PEG-coated polyalkylcyanoacrylate (PACA) nanoparticles were for the first time visualized after freeze-fracture of the nanoparticles pre-incubated with fibrinogen as a model blood protein. The reduced protein association to the nanoparticles was evidenced also by two-dimensional PAGE after incubation of the nanoparticles with human plasma. In vivo experiments showed the 'stealth' long-circulating properties of the PEGylated nanoparticles after intravenous administration to mice. Thus, the images obtained after nanoparticle-protein incubation were predictive of the behavior observed in vivo. In conclusion, freeze-fracture analysis represents a novel and original qualitative approach to investigate the interactions between proteins and particulate systems.

Enzymatic degradation of SLN-effect of surfactant and surfactant mixtures

Solid lipid nanoparticles (SLN) show different degradation velocities by the lipolytic enzyme pancreatic lipase as a function of their composition (lipid matrix, stabilizing surfactant). In combination with pancreatic colipase a degradation assay has been developed for studying the degradation behavior. As a measure to follow the degradation the formed free fatty acids have been analyzed using an enzymatic test. In the studies SLN degradation showed dependencies in relation to the length of the fatty acid chains in the triglycerides and the surfactants used for SLN production. The longer the fatty acid chains in the glycerides, the slower the degradation. The influence of surfactants can be degradation accelerating (e.g. cholic acid sodium salt) or a hindering, degradation slowing down effect due to steric stabilization (e.g. Poloxamer 407). As a second steric stabilizer, Tween 80 has been used and the results showed a less pronounced effect on hindering the degradation process than for Poloxamer 407. This result seems to be correlated to the number of ethyleneoxide chains in the molecule. The longer the ethyleneoxide chains are in the molecule, the more hindered is the anchoring of the lipase/colipase complex and consequently the degradation of the SLN. The result can be used to adjust degradation of SLN and consequently drug release in a controlled way.

Correlation between long-term stability of solid lipid nanoparticles (SLN) and crystallinity of the lipid phase

Aqueous dispersions of solid lipid nanoparticles (SLN) are usually physically stable for more than 3 years. However, in some systems gelation occurred leading to solid gels due to an unknown mechanism. To elucidate this mechanism, Compritol SLN were stored at different temperatures, varying light exposure, in different packing materials and stressed by shear forces in short-term tests and a long-term study of 3 years. The SLN were analyzed by differential scanning calorimetry and sizing techniques. After production by hot homogenization of the melted lipid, the Compritol SLN crystallize in a mixture of stable beta' with unstable polymorphs (alpha, sub alpha). The destabilizing factors light, temperature and shear forces cause a distinct increase in the recrystallization index by transformation of the lipid to the beta' modification being accompanied by gel formation. Physically stable SLN remain as a mixture of modifications, increase in crystallinity index during storage is slow and crystallization occurs mainly in unstable modifications. From this, stabilization of physically critical SLN dispersions seems possible by inhibition of the transformation of the lipid to the stable modification.

Stability determination of solid lipid nanoparticles (SLN) in aqueous dispersion after addition of electrolyte

The contribution of mono-, di- and trivalent ions to the destabilization of solid lipid nanoparticle (SLN) dispersions was investigated, i.e. particle growth and subsequent formation of semi-solid gels. Sodium, calcium and aluminium chloride were added in varying concentrations to a Compritol formulation which had proved to be highly sensitive towards destabilizing effects. Dispersions containing up to 10(-3) M sodium chloride remained stable for 14 days. The same concentrations of calcium or aluminium induced slight and rapid particle growth, respectively. Generally, a pronounced destabilizing effect was observed with increasing electrolyte concentration and increasing valence. Higher concentrations of electrolyte (10(-2), 10(-1) M) induced gelation of the systems. The extent of solidification was highly dependent on the crystallinity of the lipid phase. The recrystalization indices of the gels were distinctly higher compared to the liquid systems. Additionally, unstable modifications, being present in liquid dispersions, were transformed into stable ones with increasing solidification. The mechanisms of the destabilizing effect of the electrolytes are reduced electrostatic repulsion and transformation of the lipid Compritol to the beta' modification promoting gel formation.

HPLC determination of clofazimine in tissues and serum of mice after intravenous administration of nanocrystalline or liposomal formulations

A simple HPLC method is described for the determination of clofazimine in mouse tissues and in serum. The main application of the method was the determination of the drug in mouse tissues after i.v. administration of nanocrystalline suspensions or liposomal encapsulated clofazimine. Tissues were extracted with a 10-fold (w/v) volume of an extraction solution consisting of methanol/glacial acetic acid 9:1 (v/v). Serum proteins were precipitated with a 2-fold volume of acetonitrile. Isocratic chromatography was performed using an anion exchange column (Nucleosil 100-5 SA, Macherey & Nagel) for separation. The mobile phase was a mixture of acetonitrile and 0.1 mol/l aqueous phosphoric acid (75:25, v/v), adjusted to pH 2.9 with sodium hydroxide solution. Absorption of the eluate was monitored at 495 nm. The assay was precise, simple to perform and fast. Recovery from tissues was > or = 98%, from nanoparticles > or = 98%, and from liposomes > or = 96%. No interference was observed in extracts from mouse liver, spleen, lungs and human serum.

Plasma protein adsorption on biodegradable microspheres consisting of poly(D,L-lactide-co-glycolide), poly(L-lactide) or ABA triblock copolymers containing poly(oxyethylene). Influence of production method and polymer composition

Biodegradable particulate systems have been considered as parenteral drug delivery systems. The adsorption of plasma proteins on micro- and nanoparticles is determined by the surface properties and may, in turn, strongly influence the biocompatibility and biodistribution of both carriers. In the present study the influence of the polymer composition and the production method of microspheres on the in vitro plasma protein adsorption were investigated using two-dimensional electrophoresis (2-DE). Microparticles were prepared from poly(l-lactide) (l-PLA), poly(d,l-lactide-co-glycolide) (PLGA), and ABA triblock copolymers containing hydrophilic poly(oxyethylene) (B-blocks) domains connected to hydrophobic polyesters (A-blocks). Two different microencapsulation methods were employed, namely the w/o/w emulsion solvent evaporation method and the spray-drying technique. It could be demonstrated that the polymer composition and, especially, the encapsulation technique, influenced the interactions with plasma proteins significantly. For example, the percentages of several apolipoproteins in the plasma protein adsorption patterns of spray-dried PLGA- and l-PLA-particles were distinctly higher when compared to the adsorption patterns of the particles produced by the w/o/w-technique. Some adsorbed proteins were found to be characteristic or even specific for particles produced by the same method or consisting of identical polymers. Polyvinyl alcohol used as stabilizer in the w/o/w-technique may decisively influence the surface properties relevant for protein adsorption. The plasma protein adsorption on particles composed of ABA copolymers was drastically reduced when compared to microspheres made from pure polyesters. The adsorption patterns of ABA-particles were dominated by albumin. The plasma protein adsorption patterns detected on the different microspheres are likely to affect their in vivo performance as parenteral drug delivery systems.

The influence of the sample preparation on plasma protein adsorption patterns on polysaccharide-stabilized iron oxide particles and N-terminal microsequencing of unknown proteins

The in vivo organ distribution of i.v. injected drug carriers is strongly influenced by the adsorption of plasma proteins after i.v. injection, e.g. uptake by the mononuclear phagocytic system (MPS). 2-D PAGE could be established to analyze plasma protein adsorption patterns on polysaccharide-stabilized aqueous iron oxide dispersions used as contrast agents in Magnetic Resonance Imaging (MRI). After incubation in human plasma, centrifugation, a washing procedure and a solubilization step were carried out to obtain the proteins adsorbed onto these ultrasmall particles (65 nm in diameter). Patterns of adsorbed proteins were analyzed in dependence on the washing medium used, i.e. highly purified water, phosphate buffered saline and Krebs buffer pH 7.4. Conductivity and composition of the washing medium influenced the adsorption of IgG onto the particles, but had little effect on the other proteins present. IgG was strongly reduced when using the relatively high conductive buffers. The more stabilizing polysaccharide was desorbed the larger was the total amount of adsorbed proteins. Appearance of two unknown chains of spots in the range of appr. 92 kDa, accounting for appr. 10% and 2% of the overall detected protein amount, was observed only when using Krebs buffer during the washing process. Performing N-terminal microsequencing one unknown chain of spots could be identified as a dimer of fibrinogen gamma chains.

Surface-modified amikacin-liposomes: organ distribution and interaction with plasma proteins

Amikacin-loaded liposomes were produced and surface-modified by adsorption of PEG 4000, Tween 80, poloxamer 407 and gelatin. The organ distribution was studied in mice by analysing the amikacin content in liver, spleen, lung, kidneys and serum. Highest serum levels were obtained with the PEG- and Tween 80 modified liposomes (at 2 hours p.inj.). Modification of the liposomes with gelatin as opsonization promoting agent distinctly increased the amikacin concentration in the liver from 36 to 66 mg/kg. Highest spleen concentrations were observed with non-modified and poloxamer 407 liposomes (242 mg/kg and 248 mg/kg, respectively). The data suggest that modification by a simple adsorption process is sufficient to effectively alter the organ distribution. The liposomes differing in organ distribution exhibited also different plasma protein adsorption patterns, up to 115 spots were detected by 2-D PAGE. Hydrophilic albumin was present in a conc. of appr. 80% on liposomes modified with ethoxylated compounds. On the gelatin liposomes, 14% of alpha-2-Macroglobulin were adsorbed which is a protein typically found on particles rapidly cleared by the RES. IgM, Apo A-I, Apo C-II and alpha-1-Antitrypsin were other detected proteins.

The influence of the sample preparation on plasma protein adsorption patterns on polysaccharide-stabilized iron oxide particles and N-terminal microsequencing of unknown proteins

The in vivo organ distribution of i.v. injected drug carriers is strongly influenced by the adsorption of plasma proteins after i.v. injection, e.g. uptake by the mononuclear phagocytic system (MPS). 2-D PAGE could be established to analyze plasma protein adsorption patterns on polysaccharide-stabilized aqueous iron oxide dispersions used as contrast agents in Magnetic Resonance Imaging (MRI). After incubation in human plasma, centrifugation, a washing procedure and a solubilization step were carried out to obtain the proteins adsorbed onto these ultrasmall particles (65 nm in diameter). Patterns of adsorbed proteins were analyzed in dependence on the washing medium used, i.e. highly purified water, phosphate buffered saline and Krebs buffer pH 7.4. Conductivity and composition of the washing medium influenced the adsorption of IgG onto the particles, but had little effect on the other proteins present. IgG was strongly reduced when using the relatively high conductive buffers. The more stabilizing polysaccharide was desorbed the larger was the total amount of adsorbed proteins. Appearance of two unknown chains of spots in the range of appr. 92 kDa, accounting for appr. 10% and 2% of the overall detected protein amount, was observed only when using Krebs buffer during the washing process. Performing N-terminal microsequencing one unknown chain of spots could be identified as a dimer of fibrinogen gamma chains.

Analysis of plasma protein adsorption on polymeric nanoparticles with different surface characteristics

Plasma protein adsorption patterns on colloidal drug carriers acquired after i.v. administration depend on their surface characteristics and are regarded as key factors for their in vivo organ distribution. Polymeric latex particles with strongly differing surface properties were synthesized as models for colloidal drug carriers for tissue-specific drug targeting via the intravenous route. Physicochemical characterization was performed for size, surface charge density, zeta potential, and surface hydrophobicity. The interactions with human plasma proteins were studied by way of two-dimensional polyacrylamide gel electrophoresis (2-D PAGE). Considerable differences in protein adsorption on the latex particles were detected with regard to the total amount of surface-bound protein on the various particle types as well as specific proteins adsorbed, for example, fibrinogen, albumin, and a recently identified plasma glycoprotein. Possible correlations between protein adsorption patterns and the physicochemical characteristics and topography of the polymeric surfaces are shown and discussed. Knowledge about protein-nanoparticle interactions can be utilized for the rational design of colloidal drug carriers and also may be useful for optimizing implants and medical devices.

Plasma protein adsorption patterns on emulsions for parenteral administration: establishment of a protocol for two-dimensional polyacrylamide electrophoresis

The two-dimensional polyacrylamide electrophoresis (2-D PAGE) of the plasma protein adsorption pattern previously established for polymeric nanoparticles was modified and transferred to oil in water emulsions for intravenous administration. The emulsions were incubated with citrated plasma, and separation from excess plasma was performed by centrifugation under optimized conditions: 15000 g and three washing steps with 0.05 M phosphate buffer, pH 7.4. With this sample preparation, coalescence of droplets could be avoided and an unchanged surface area maintained, in addition the phosphate buffer minimized artificial IgG adsorption. Critical factors affecting sensitivity were contamination of the sample by oil residues and the use of thiourea in the immobilized pH gradients. Changes in the protein adsorption pattern caused by altered surface properties of the emulsion (i.e. adsorbed Poloxamer 407) were detectable when applying the optimized protocol. Knowledge of the protein adsorption patterns and their correlation to in vivo behavior opens the perspective for the development of intravenous emulsions for controlled drug delivery.

Identification of plasma proteins facilitated by enrichment on particulate surfaces: analysis by two-dimensional electrophoresis and N-terminal microsequencing

Plasma protein adsorption on intravenously injectable drug carriers is regarded as an important factor for the fate of the particles in the body after their administration. Therefore, the plasma protein adsorption patterns on a number of different carrier systems were analyzed in vitro employing two-dimensional electrophoresis (2-DE). The particulate systems presented in this study were polystyrene (PS) model particles, PS nanoparticles surface-modified by adsorption of a surfactant, a commercial fat emulsion, and magnetic iron oxide particles used as contrast agents in magnetic resonance imaging. Most of the spots in the plasma protein adsorption patterns could be identified by matching the resulting 2-DE gels with a reference map of human plasma proteins. Several other proteins that indicated preferentially adsorbed proteins on the surface of the particles investigated have either not been identified on the reference map, or their identity was found to be ambiguous. The relevant proteins are all present in plasma in low abundance. Since these proteins were strongly enriched on the surface of the particles, the resulting spots on the 2-DE gels were successfully identified by N-terminal microsequencing. With this approach, two chains of spots, designated PLS:6 and PLS:8, were determined on a plasma reference map: inter-alpha-trypsin inhibitor family heavy chain-related protein (also named PK-120) and a dimer of fibrinogen gamma, respectively. Plasma gelsolin is presented in a 2-DE adsorption pattern of PS model particles. One of the main proteins adsorbed by droplets of a commercial fat emulsion was identified as apoliprotein H. Moreover, the positions of apolipoproteins apoC-II and apoC-III were also verified on the 2-DE protein map of human plasma. Thus, protein adsorption experiments of the kind presented in this study are increasing our insight into human plasma proteins.

Determination of plasma protein adsorption on magnetic iron oxides: sample preparation

PURPOSE

The purpose of this study was to investigate the influence of the sample preparation on the plasma protein adsorption pattern of polysaccharide-stabilized iron oxide particles by two-dimensional polyacrylamide gel electrophoresis (2-D PAGE).

METHODS

The iron oxide particles were incubated in vitro in human plasma for five minutes. Thereafter, four different methods for particle recovery, including adsorbed proteins from surplus plasma, were investigated: centrifugation, magnetic separation, gel filtration and membrane-based static microfiltration. Adsorbed proteins were desorbed from the particle surfaces by surfactants and analyzed by 2-D PAGE, as described elsewhere (1,2).

RESULTS

All the techniques investigated were able to separate small-size iron oxides (approx. 110 nm) and adsorbed proteins from excess plasma. The gels obtained by the different separation procedures displayed almost identical adsorption patterns. Major proteins identified were: fibrinogen, IgG, albumin and an unclassified protein of about 70 kDa with a pI value of 6.5-7.5.

CONCLUSIONS

Centrifugation was regarded as the most suitable separation method due to its speed and ease of use. In contrast to gel filtration, any washing media can be used. The magnetic separation process is restricted to particles with high inducible magnetic saturation, in particular, to iron oxides with overall sizes > 50 nm.

Cytotoxicity of solid lipid nanoparticles as a function of the lipid matrix and the surfactant

PURPOSE

Assessment of the in vitro cytotoxicity of solid lipid nanoparticles (SLNs) as a function of lipid matrix (Dynasan 114, Compritol ATO 888), and stabilizing surfactant (poloxamers, Tween 80, soya lecithin, and sodium dodecyl sulphate). Comparison with other colloidal carriers should determine their potential use in the clinic.

METHODS

SLNs were produced by high pressure homogenisation. Cytotoxicity was assessed by measuring the viability of HL60 cells and human granulocytes after incubation with SLNs. Particle internalisation was quantified by chemiluminescence measurements.

RESULTS

The nature of the lipid had no effect on viability; distinct differences were found for the surfactants. Binding to the SLN surface reduced markedly the cytotoxic effect of the surfactants, e.g., up to a factor of 65 for poloxamer 184. The permanent HL60 cell line-differentiated from cells with granulocyte characteristics by retinoic acid treatment-yielded results identical to freshly isolated human granulocytes. In general, the SLNs showed a lower cytotoxicity compared to polyalkylcyanoacrylate and polylactic/glycolic acid (PLA/ GA) nanoparticles.

CONCLUSIONS

Because the results are identical when using human granulocytes, differentiated HL60 cells can be used as an easily accessible in vitro test system for i.v. injectable SLN formulations. The SLNs appear suitable as a drug carrier system for potential intravenous use due to their very low cytotoxicity in vitro.

Influence of fluorescent labelling of polystyrene particles on phagocytic uptake, surface hydrophobicity, and plasma protein adsorption

PURPOSE

To investigate the influence of fluorescent labelling of polystyrene particles on phagocytic uptake, surface hydrophobicity and protein adsorption.

METHODS

Phagocytic uptake was analysed using chemiluminescence. Hydrophobicity was quantified by adsorption measurements of a hydrophobic dye. Protein adsorption was evaluated by two-dimensional electrophoresis.

RESULTS

Commercially available fluorescently labelled particles showed marked differences when compared to unlabelled particles: phagocytic uptake and surface hydrophobicity of labelled particles were diminished. Also the plasma protein adsorption pattern was found to be different from the unlabelled particles: for example, the amount of fibrinogen adsorbed was strongly reduced on the labelled particles. On the other hand, some unknown proteins could be detected on the fluorescently marked particles. In contrast, plain polystyrene particles and labelled ones could be successfully synthesised by Paulke which did not show any considerable differences in phagocytic uptake, surface hydrophobicity and protein adsorption. Polysorbate 20 added as stabilizer to particle suspensions led to completely different behaviour of the particles: the particles showed altered protein adsorption patterns, dominated by immunoglobulins and especially by apolipoproteins. Furthermore, these particles were not phagocytized at all.

CONCLUSIONS

Surface hydrophobicity and phagocytic uptake in vitro as well as the interactions with plasma proteins of commercially available polystyrene particles were strongly affected by fluorescent labelling. Particles synthesised by Paulke remained unchanged after labelling. The results show the importance of thorough surface characterization for using particles in test systems in vitro and in vivo.

Fat emulsions based on structured lipids (1,3-specific triglycerides): an investigation of the in vitro interaction with plasma proteins

Structured lipids (1,3-specific triglycerides) are new chemical entities made by enzymatic transesterification of the fatty acids in the 1,3-positions of the triglyceride. The purpose of this study was to investigate the in vitro interaction of fat emulsions based on either structured lipids or vegetable oils with human plasma proteins employing two-dimensional polyacrylamide gel electrophoresis (2-D PAGE). The structured lipids are triglycerides of the SLS and MLM types, where S is short-chain fatty acids (C4), M is medium-chain fatty acids (C8-10) and L is long-chain fatty acids (C16-18). The vegetable oil-based fat emulsions were the commercially available product, Intralipid, and a soybean oil (LLL) emulsion made de novo identically as the emulsions containing structured lipids. The SLS emulsion was found to adsorb a different protein pattern than the MLM and LLL emulsions. The protein pattern of the SLS emulsion was similar to the protein pattern of Intralipid. These findings might explain the in vivo difference in elimination found in another study, where the emulsion based on structured lipids with short-chain fatty acids in the 1,3-positions was removed more slowly from the general blood circulation compared to emulsions based on lipids with long-chain fatty acids in the 1,3-positions (LLL).

Phagocytic uptake and cytotoxicity of solid lipid nanoparticles (SLN) sterically stabilized with poloxamine 908 and poloxamer 407

Solid lipid nanoparticles (SLN) as alternative intravenous colloidal drug carriers were produced by high pressure homogenisation of melted lipids (glycerolbehenate, cetylpalmitate). Their surface was modified by using hydrophilic poloxamine 908 and poloxamer 407 blockcopolymers in order to reduce the phagocytic uptake by the reticuloendothelial system (RES) after i. v. injection. The phagocytosis reducing effect of the polymers was investigated in vitro in cultures of human granulocytes, uptake was quantified by chemiluminescence. Modification of the SLN with poloxamine 908 and poloxamer 407 reduced the phagocytic uptake to appr. 8-15% compared to the phagocytosis of hydrophobic polystyrene particles. The modified SLN proved more efficient in avoiding phagocytic uptake than polystyrene particles surface-modified with these blockcopolymers (48% and 38%, respectively). Viability determinations revealed the SLN to be 10 fold less cytotoxic than polylactide nanoparticles and 100 fold less than butylcyanoacrylate particles.

Colloidal carriers for intravenous drug targeting: plasma protein adsorption patterns on surface-modified latex particles evaluated by two-dimensional polyacrylamide gel electrophoresis

Targeting to specific sites of the body via colloidal carriers is sought in order to reduce drug side effects. The adsorption of plasma proteins on intravenously injected particles is regarded as the key factor in explaining their organ distribution: total bound protein, or, more likely, the presence of specific proteins and their conformation, are expected to influence macrophage uptake. Polystyrene beads, 60 nm in diameter, were used as model carriers; their surface was differentially modified by adsorption of increasingly hydrophilic block copolymers, poloxamers 184, 188 and 407. After incubation in plasma, the patterns of protein adsorption onto coated beads were analyzed by high-resolution two-dimensional polyacrylamide gel electrophoresis (2-D PAGE). The behavior of some representative proteins was monitored, including albumin, fibrinogen, IgG, factor B and the apolipoproteins, A-I, A-IV, C-III, E and J. The more hydrophobic the particles, the larger the total amount of bound protein. However, this correlation was not valid for all of the analyzed protein species, which proves that it is insufficient to look only at physicochemical data to predict organ distribution. On the contrary, it is essential to use 2-D PAGE to establish the correlation between adsorbed proteins and carrier behavior in vivo.

Fat emulsions for parenteral nutrition II: Characterisation and physical long-term stability of Lipofundin MCTLCT

Lipofundin MCT LCT emulsions for parenteral nutrition were characterized in terms of particle size (bulk population), presence of large particles ( 1 mum) and droplet charge (zeta potential as measure of electrostatic stabilization). Photon Correlation Spectroscopy (PCS), microscopy and Laser Diffractometry (LD) were employed as sizing techniques and compared to each other. Zeta potentials were determined by Laser Doppler Anemometry (LDA). The characterization data showed a good batch-to-batch reproducibility. The physical stability of Lipofundin MCT LCT was monitored over a period of 2 years. The mean diameter of the bulk droplet population did not increase. No formation of larger droplets was found by microscopy and LD. A negligible coalescence could only be detected by the very sensitive PCS measurements (two time window analysis). The zeta potential stayed above -45 mV providing sufficient electrostatic stabilization during the whole storage period.

Fat emulsions for parenteral nutrition. I: Evaluation of microscopic and laser light scattering methods for the determination of the physical stability

Droplet size distribution was studied as a measure of the physical stability of fat emulsions. Test emulsions with increasing fractions of larger particles were prepared and investigated by conventional light microscopy and laser light scattering techniques (Photon Correlation Spectroscopy (PCS), Laser Diffractometer). Light microscopy proved to be most sensitive for detecting a few single large particles. All the techniques placed the test emulsions in the same order of increasing content of larger droplets. The application of a two time window analysis for PCS markedly improved the ability to differentiate between emulsions which were very similar in size distribution. In contrast to the applied semi-quantitative microscopic method, the laser techniques characterise the emulsion in absolute figures. Additional zeta potential measurements by Laser Doppler Anemometry (LDA) can quantify reduced electrostatic repulsion as a possible cause for increased formation of larger droplets.

In vitro model for the degradation of alkylcyanoacrylate nanoparticles

A photometric assay was developed to study the surface erosion of polymeric nanoparticles. The hydrolytic degradation of polyalkylcyanoacrylate particles was studied in different environments (NaOH, buffer, cell culture medium and serum). The influence of particle modification on the degradation rate was assessed. Particularly, the effect of polymer coating for particle targeting and fluorescence labelling was investigated. From the absorption data, a t50% and t100% can be calculated for fast degrading particles and obtained by an extrapolation in case of a slow degradation process. The degradation rate was found to decrease with increasing alkyl chain length from methyl-, ethyl-, isobutyl- to isohexylcyanoacrylate particles. Polymer coating and fluorescent labelling had little effect on the rate of degradation.

Surface characteristics and the interaction of colloidal particles with mouse peritoneal macrophages

The effect of surface characteristics on the interaction of sterically stabilized polystyrene particles with mouse peritoneal macrophages was studied using a range of poloxamers and poloxamine as coating agents. The coated particles were characterized in terms of thickness of coating layer, surface charge and critical flocculation temperature. The relative phagocytic uptake was found to decrease with increasing adsorbed layer thickness i.e. longer hydrophilic polymer chains of the coating agent and consequently a greater steric stabilization effect.

The organ distribution and circulation time of intravenously injected colloidal carriers sterically stabilized with a block copolymer--poloxamine 908

Colloidal carriers injected intravenously are normally removed rapidly and efficiently by the liver and this represents a major barrier to drug targeting. By coating model microspheres and emulsions with a block co-polymer (poloxamine) it has been possible to keep the carrier circulating in the vascular compartment with little or no uptake by the reticuloendothelial system.

Emulsifier formation with Acinetobacter: search for an excretion-reduced mutant of Acinetobacter calcoaceticus 69-V

After growth on acetate three groups of Acinetobacter strains could be identified with respect to the excretion of a bioemulsifier. Mutants of A. calcoaceticus 69-V were selected which produced reduced amounts of emulsifier.