Increase In Encapsulation Efficiency Research Articles

Preparation and evaluation of liposomes encapsulating synthetic MMP inhibitor (Ro 28-2653)—cyclodextrin complexes

In this study, preparation and evaluation of liposomes, intended for intravenous administration, encapsulating synthetic MMP inhibitor (Ro 28-2653) – cyclodextrin complexes were realized. An increase in Ro solubility, via formation of binary (Ro/HPβCD) or ternary (Ro/HPβCD/L-lysine) complexes, permitted a similar increase in encapsulation efficiency of liposomes (Table 1). Moreover, Ro release kinetics depend on the encapsulation efficiency.

Effect of surfactant HLB and different formulation variables on the properties of poly-D,L-lactide microspheres of naltrexone prepared by double emulsion technique

The aim of this work was to investigate the role of HLB of emulsifier as well as volume of the internal aqueous phase (W1) and presence of salt in the external aqueous phase (W2) on the morphology, size and encapsulation efficiency of poly(D,L-lactide) microspheres containing naltrexone HCl. PLA microparticles containing naltrexone HCl, an effective opiate antagonist, were prepared by a water-in-oil-in-water emulsification–solvent evaporation procedure. One of the five different emulsifiers: span 80, span 20, tween 85, tween 80 and tween 20, with HLB values from 4–17 were added to W1. Presence of emulsifier in W1 resulted in smaller particles with a more dense and uniform internal structure. Incorporation of span 80 (HLB 4.3, suitable for W/O emulsions) yield the highest encapsulation efficiency. Increasing the HLB value to 8 or 11 (span 20 or tween 85) decreased the efficiency of naltrexone HCl-loading. HLB values higher than 15 (tween 80 or tween 20) increased encapsulation efficiency unexpectedly, which could be attributed to migration of these emulsifiers to the O/W2 interface and modifying the surface properties of microparticles. Increasing the internal water phase volume from 0.2–1.8 ml resulted in larger particle size with poor encapsulation efficiency. Addition of 10% w/w NaCl to the W2 changed the surface morphology of microspheres from a porous form to a smooth surface. It was shown that, by selecting the appropriate HLB value of emulsifier in W1, addition of salt to W2 and controlling the volume of W1, one can control the encapsulation efficiency, size and morphology of microspheres.

Optimization of Satellite Access Lower Layers for the Transport of IP Datagrams

This paper presents a novel solution optimizing satellite access lower layers and system performances called variable size packets (VSPs) designed for the transport of Internet protocol (IP) datagrams or any packet type from the network layers characterized by a highly variable size. VSP objective is to maximize the efficiency of layers 1 and 2 through a joint optimization of access and transport mechanisms mainly for satellite systems addressing residential or corporate terminals market through a multiple-access scheme. It is particularly attractive for the return link of next-generation satellite systems to be IP-based, as a possible evolution of the digital video broadcast-return channel via satellite (DVB-RCS) standard. The VSP solution relies on the definition of multiple encapsulation sizes, and is associated to a waveform and an access adapted to variable size packets' transport. It uses the traffic's variable size nature to increase encapsulation efficiency, coding, and spectral efficiency performances, and system flexibility. It is well adapted to systems using an adaptive waveform leading to system capacity's increase and better performances. This paper describes the VSP principle, advantages and possible high-level system tradeoffs. It then focuses on one critical issue, the coding, and presents a suitable coding scheme's choice with regards to performances and implementation complexity.

Polymerized ion pair amphiphile that shows remarkable enhancement in encapsulation efficiency and very slow release of fluorescent markers

N,N-dimethyl- N,N-dihexadecylammonium 12-(lipoyloxy)dodecanoate and three other analogues were synthesized as polymerizable ion pair amphiphiles (PIPA) to increase the stability of ion pair amphiphile (IPA) liposome and control the release of encapsulated materials. Among the four PIPA candidates made, only N,N-dimethyl- N,N-dihexadecylammonium 12-(lipoyloxy)dodecanoate showed reliable and stable liposome formation with remarkable increase in encapsulation efficiency, and the polymerized IPA liposome thus formed was stable enough to withstand the lysis from SDS or Triton X-100. In addition, the polymerization method was as easy and convenient as just shaking the solution at room temperature without any need for the addition of initiator and input of energy such as light. Physical properties were also checked in the point of phase transition, release rate of marker, and ion permeation through the bilayer. Reasons for such high encapsulation and future applications are briefly discussed.

Reduction of the environmental impact of pesticides: waxy microspheres encapsulating the insecticide carbaryl.

A controlled-release system with reduced environmental impact was produced by encapsulating the pesticide carbaryl in the waxy lipophilic material Gelucire 54/02. The microspheres were prepared by a modified hydrophobic congealable disperse-phase method. The influence of experimental parameters, such as the reciprocal ratio between the amounts of pesticide and wax employed, on size, morphology, loading efficiency, and release behavior of the particles was evaluated. Microspheres were free-flowing and showed a nonporous scaly surface at SEM analysis. The mean particle size ranged from 15.8 to 19.8 microm and was independent of the amount of Gelucire used to prepare the microspheres. At a fixed Gelucire content, the increase in theoretical carbaryl content yielded up to 72% loading efficiency, whereas at a fixed carbaryl content the increase in Gelucire amount produced a 64% increase in encapsulation efficiency. These data were accounted for by the carbaryl leakage from molten Gelucire toward the dispersing aqueous phase. The release profiles of carbaryl from microspheres showed that the use of increasing amounts of waxy material decreased the carbaryl release rate, whereas at a fixed Gelucire content, the release was the slowest when carbaryl was not completely dissolved within the matrix. The possibility to achieve different burst effects by simply varying the formulation parameters offers an efficient tool to ensure the fast release of an active dose of insecticide. The lower vertical mobility of carbaryl encapsulated in waxy microspheres compared to the vertical mobility of the technical-grade product showed that the controlled-release system has a lower potential risk for groundwater contamination.

Steam sterilisation of vesicular phospholipid gels

Vesicular phospholipid gels (VPGs), highly concentrated phospholipid dispersions of semisolid consistency and vesicular morphology are under investigation as potential implantable depots for sustained release of drugs and as intermediates for subsequent dilution into ‘conventional’ liposome dispersions. It was investigated here if VPGs can be steam sterilised. VPGs prepared from 400 mg/g egg-phosphatidylcholine by high-pressure homogenisation retained their vesicular structure but showed a slight increase in vesicle size (freeze-fracture electron microscopy). However, autoclaving slowed down both, the in vitro release of the hydrophilic marker carboxyfluorescein and vesicles from VPGs. This was assumed to be due to bigger vesicle sizes and corresponding increase in packing density of the vesicular matrix. Upon dilution into a liposome dispersion both negative staining electron microscopy and dynamic laser light scattering analysis confirmed a distinct increase in liposome size, mainly due to fusion of small (20 nm) vesicles with unfavourable curvature. This was consistent with the observed increase in encapsulation efficiency of carboxyfluorescein. Phospholipid hydrolysis during autoclaving was negligible with lysophosphatidylcholine formation of less than 2% (thin layer chromatography). Despite significant change of their morphological and functional properties during autoclaving VPGs retained their main characteristics, such as vesicular structure, sustained release and dilutability to liposome dispersions, and are, therefore, considered as autoclavable.

Controlled release of vancomycin from biodegradable microcapsules

Poly D,L-lactic acid (PLA) and its copolymers with glycolide PLGA 90:10 and 70:30 were polymerized under various conditions to yield polymers in the molecular weight range 12000-40000 daltons, as determined by gel permeation chromatography. Vancomycin hydrochloride was the hydrophilic drug of choice for the treatment of methicillin resistant Staphyloccoccal infections. It was microencapsulated in the synthesized polymers using water-oil-water (w/o/w) double emulsion and solvent evaporation. The influence of microcapsule preparation medium on product properties was investigated. An increase in polymer-to-drug ratio from 1:1 to 3:1 caused an increase in the encapsulation efficiency (i.e. from 44-97% with PLGA). An increase in the emulsifier (PVA) molecular weight from 14-72 kD caused an increase in encapsulation efficiency and microcapsule size. The in vitro release of vancomycin from microcapsules in phosphate buffer saline (pH 7.4) was found to be dependent on molecular weight and copolymer type. The kinetic behaviour was controlled by both diffusion and degradation. Sterilization with 60Co (2.5 Mrad) also affected the degradation rate and release profiles. Degradation of microcapsules could be seen by scanning electron microscopy, by the increase in the release rate from PLA and by the decrease in the Tg values of microcapsules. In vitro bactericidal effects of the microcapsule formulations on S.aureus were determined with a special diffusion cell after the preparations had been sterilized, and were found to have bactericidal effects lasting for 4 days.

Controlled Release Tacrine Delivery System for the Treatment of Alzheimer's Disease

Alzheimer's disease is a neurodegenerative condition that affects ~5 million people and is the fourth leading cause of death in America. Tacrine is one of the three drugs approved by the FDA for the treatment of Alzheimer's disease. However, the drug has a short biologic half-life of 2?3 hr and gastrointestinal, cholinergic, and hepatic adverse reactions that are associated with high doses of the drug. The aim of our study was to formulate a controlled release delivery system of tacrine that could be used to minimize the side effects associated with the drug. Microparticles of tacrine were formulated using poly(D,L-lactide-co-glycolide) (PLG). PLG and tacrine were dissolved in mixed organic solvents and added to a polyvinyl alcohol solution that was stirred at a constant rate. The organic solvent was evaporated overnight and the formed microparticles were collected by filtration, dried, and sieve-sized. The effects of such formulation variables, as molecular weight of polymer, stir speed during preparation, and drug loading on encapsulation efficiency (EEF), and in vitro release profiles of tacrine were investigated. An increase in the molecular weight of polymer from 8,000 to 59,000 and 155,000 resulted in ~10-fold increase in EEF, but the rate of release decreased with increasing molecular weight. Stir speed during preparation had an effect on the EEF but not on the rate of release. Drug loading did not have a significant effect on the EEF but had an effect on the rate of tacrine release. The results suggest that tacrine could be delivered at controlled levels for weeks for the treatment of Alzheimer's disease.

Formulation and release characteristics of poly(lactic-co-glycolic acid) microspheres containing chemically modified protein

Chemical modification of proteins may influence their formulation into and release from polymeric microspheres. Three chemical modifications of rat serum albumin (RSA) were effected on the amine groups of this protein: conjugation with a polyanion using 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide, intermolecular cross-linking using glutaraldehyde, and reductive alkylation using propyl aldehyde. The modified proteins had different physicochemical properties as well as improved encapsulation efficiencies compared with native RSA microspheres. The microspheres were incubated at 37 degrees C for over one month to investigate the influence of protein modification on the release profiles. Microsphere degradation accelerated from the ninth day of the release studies and this coincided with an increase in the release rates. The degradation rates of poly(lactic-co-glycolic acid) microspheres containing either native or cross-linked RSA were more rapid than those containing either heparin conjugated or propylated RSA. This was in agreement with the release data, since the release of the native and cross-linked RSA were more rapid than those of the other modified proteins. The release profiles of the RSA-heparin conjugates and the propylated RSA were approximately zero rather than first order between the tenth and thirtieth day of study. Chemical modification of protein may be a useful method to increase encapsulation efficiency and to decrease release rates of proteins that are to be used in microsphere formulations of potent therapeutic proteins.

Effect of solvent composition during preparations on the characteristics of enoxacin microparticles.

We have studied the effect of the solvent system during preparation on the morphology, encapsulation efficiency, and release characteristics of enoxacin microparticles intended for localized delivery to the bone for the treatment of bone infections. Microparticles of enoxacin were formulated using poly(glycolic acid-co-DL-lactic acid) (PGLA) of different viscosity grades by the solvent-evaporation technique. Microparticles prepared with pure dichloromethane had smoother surfaces and less tendency to aggregate than microparticles prepared with dichloromethane-acetone solvent mixtures, which had porous surfaces. Approximately 65% of the microparticles prepared with pure dichloromethane were < 125 microm in diameter compared with 16% (approx.) of microparticles prepared with dichloromethane-acetone mixtures. Increasing the proportion of acetone from dichloromethane-acetone, 10:0, to dichloromethane-acetone, 1:1, resulted in an increase in encapsulation efficiency from 25 to 37%, and an increase in the yield of microparticles harvested from 39 to 51%. Although a further increase in the amount of acetone to dichloromethane-acetone, 1:9, had no significant effect on the yield, aggregation, or fraction of microparticles below 125 microm in diameter, the encapsulation efficiency increased to 56%. Approximately 55% of enoxacin was released in 24 h for microparticles prepared with dichloromethane-acetone, 1:9, compared with 100% release in 10h and 2h for microparticles of the same size range prepared with dichloromethane-acetone, 1:1, and dichloromethane-acetone, 10:0, respectively. The results suggest that the composition of the dichloromethane-acetone solvent system significantly influences the encapsulation efficiency and the rate of release of enoxacin from microparticles. This is important for the formulation of sustained-release enoxacin microparticles for the localized treatment of osteomyelitis.

Antibiotic release from biodegradable PHBV microparticles

For the treatment of periodontal diseases, design of a controlled release system seemed very appropriate for an effective, long term result. In this study a novel, biodegradable microbial polyester, poly(3-hydroxybutyrate-co-3-hydroxyvalerate), PHBV of various valerate contents containing a well established antibiotic, tetracycline, known to be effective against many of the periodontal disease related microorganisms, was used in the construction of a controlled release system. Tetracycline was loaded in the PHBV microspheres and microcapsules both in its acidic (TC) and in neutral form (TCN). Microcapsules of PHBV were prepared under different conditions using w/o/w double emulsion and their properties such as encapsulation efficiency, loading, release characteristics, and morphological properties were investigated. It was found that concentration of emulsifiers polyvinyl alcohol (PVA) and gelatin (varied between 0–4%) influenced the encapsulation efficiency appreciably. In order to increase encapsulation efficiency (from the obtained range of 18.1–30.1%) and slow down the release of the highly soluble tetracycline.HCl, it was neutralized with NaOH. Encapsulation efficiency of neutralized tetracycline was much higher (51.9–65.3%) due to the insoluble form of the drug used during encapsulation. The release behaviour of neither of the drugs was found to be of zero order. Rather the trends fitted reasonably well to Higuchi's approach for release from spherical micropheres. Biodegradability was not an appreciable parameter in the release from microcapsules because release was complete before any signs of degradation were observed.

Preparation and characterization of ofloxacin microspheres for the eradication of bone associated bacterial biofilm

Biodegradable polymers for localized delivery of antibiotics have emerged as an important approach to treating orthopaedic infections. In chronic forms of osteomyelitis which arethought tobeassociated with bacterial biofilm, localized delivery of a suitable antibiotic is desirable. This paper describes the formulation and in vitro evaluation of biodegradable ofloxacin microspheres for the eradication of bone associated bacterial biofilm infections. Ofloxacin microspheres were formulated using poly(glycolic acid-co-dl-lactic acid) (PGLA) by the emulsion solvent evaporation technique. The effects of process parameters such as phase volume, poly(vinyl alcohol) (PVA) concentration, and viscosity grade of the polymer during preparation on encapsulation efficiency (EEF) and in vitro release profiles were investigated. An increase in the phase volume or volume fraction from 21 to 35% at a constant internal phase volume resulted in an increase in EEF from 34 to 74%. Increasing PVA concentration from 0.25 to 2.5% w/v at a constant phase volume or volume fraction did not have an effect on the EEF. Ofloxacin release from the microsopheres was biphasic with an initial burst release followed by a slow release phase. An optimum slowing down of release was observed when the phase volume was 29%. Above and below this phase volume, release of ofloxacin was higher. The higher the viscosity grade of the polymer used for the preparation of microspheres, the higher the PVA concentration needed to prepare microspheres with slower release. The study indicates that various rates of ofloxacin release is possible by varying formulation conditions. This should provide a means for formulating sustained release microspheres of antibiotics for the treatment of biofilm infections associated with the bone.

Long circulating liposomes of 2′,3′-dideoxyinosine: Formulation and stability

Abstract2′,3′-Dideoxyinosine (ddI), an anti–human immunodeficiency virus (HIV) agent, was encapsulated in liposomes. The influence of the phospholipid/cholesterol ratio, concentration of phospholipid (PL), and chain length of PL on the encapsulation of ddI in multilamellar vesicles (MLVs), frozen and thawed multilamellar vesicles (FAT MLVs), and large unilamellar vesicles (LUVs) was studied. An optimum formulation was then selected to prepare long circulating liposomes. Stability studies at 4, 25, and 37°C and under certain stress conditions were performed. Release characteristics in phosphate buffer (pH 7.4) at 37°C were studied. Results show an increase in encapsulation efficiency (EE) with increasing amounts of cholesterol, a decrease in EE and increase in encapsulation yield (EY) with increasing concentrations of PL, and an increase in EE with increases in PL chain length, in both MLVs and LUVs. Freezing and thawing of MLVs had no influence on EE at a PL concentration of 10 mg/mL but increased EE at h...

Influence of membrane components on the stability and drug release properties of reverse phase evaporation vesicles (REVs): light sensitive all-trans retinal, negatively charged phospholipid dicetylphosphate and cholesterol.

Incorporation of a negatively charged phospholipid, dicetylphosphate, initially increased encapsulation efficiency (from 12 to 24%) but beyond 5% (molar) a detrimental effect was observed. Rate of drug release from REVs was, for most cases, found to be bi-phasic implying partitioning between the lipid bilayer and the aqueous compartment. It was not possible to prepare liposomes with more than 1% (molar) all-trans retinal (ATR) as a membrane component. When ATR was reduced to 0.5% (molar), encapsulation efficiency increased to 7.76%. Upon exposure to long wave UV (365 nm), release from ATR containing REVs was increased and this was attributed to the formation of 13-cis isomer as indicated by HPLC and UV spectroscopy data.

Incorporation of alphaxalone into different types of liposomes.

The poor solubility of steroid anaesthetics in water has been a serious drawback in the development of clinically acceptable intravenous formulations. The use of Cremophor EL to solubilize steroids such as alphaxalone led to unacceptable hypersensitivity reactions and consequent withdrawal of this anaesthetic. In principle, liposomes can act as a safe solvent for the intravenous administration of alphaxalone. We report the incorporation of [14C]acetylated alphaxalone in both multilamellar vesicles and stable plurilamellar vesicles prepared from a range of amphiphiles including synthetic polyhydroxyl lipids. For both types of preparations, addition of cholesterol to phosphatidylcholine-based lipids caused an increase in encapsulation efficiency. Maximum encapsulation was achieved with the stable plurilamellar vesicle preparation of 1-stearyl-2-myristylglycerate-3, N-methylglucamine:cholesterol:egg phosphatidylcholine (78%). The rate of efflux of this anaesthetic from a range of liposomes was measured in serum. The highest rate (85% after 30 min) was observed with an equimolar egg phosphatidylcholine:cholesterol stable plurilamellar vesicle preparation. From these studies it can be concluded that liposomes offer a suitable alternative for intravenous delivery of steroidal anaesthetics.

Fusogenic properties of Sendai virosome envelopes in rat brain preparations

Sendai virosomes were characterized with respect to their ability to bind to, fuse with, and introduce substances into several rat brain preparations. Encapsulation efficiency for Sendai virosomes was enhanced but binding to cerebral cortical P2 preparations was attenuated by addition of bovine brain phosphatidylcholine during reconstitution. A higher percentage of Sendai virosomes than phosphatidylcholine liposomes appeared to bind to, fuse with and subsequently deliver [14C]sucrose into osmotically labile pools of the P2 preparation. Fusogenic activity was estimated by measuring dequenching of fluorescently labelled N-NBD-phosphatidylethanolamine. More virosomally encapsulated [14C]sucrose was bound to the P2 fraction than introduced into osmotically labile organelles, and the fraction of vesicles undergoing fusion was intermediate between these two values. Non-encapsulated [14C]sucrose did not bind to and was not taken up by the P2 fraction in a quantifiable manner. Virosomal envelopes also bound to primary cultures of rat brain neurons and glia in an apparently saturable manner. Addition of increasing amounts of the adenoassociated virus-derived vector pJDT95 increased encapsulation efficiency, and virosomes reconstituted in the presence of 60 micrograms DNA retained most of their binding activity (5.4% of total label) compared to those containing [14C]sucrose alone (8.4%). These data indicate that Sendai virosomes may be useful in the delivery of substances into brain-derived tissues, potentially for the modulation of gene expression and neurotransmission.

LIPOSOMAL ENCAPSULATION OF ?-GALACTOSIDASE: EFFECT OF BUFFER MOLARITY, LIPID COMPOSITION AND STABILITY IN MILK

β-Galactosidases from E. coli and Aspergillus flavus were encapsulated in liposomes using the reverse-phase evaporation method. There was an inverse relationship between buffer molarity and encapsulation efficiency. The presence of cholesterol and stearylamine or dicetylphosphate along with phosphatidyl choline increased encapsulation efficiency significantly. Liposomes containing either bacterial or fungal β-galactosidase were stable in buffer as well as freshly pasteurized milk for up to 20 days at 4C. Lactose hydrolysis was negligible in milk containing liposomes with bacterial β-galactosidase, while there was significant (25%) hydrolysis of lactose in milk containing liposomes with fungal β-galactosidase during 20 days of storage at 4C. Results of this study suggest that liposomes containing β-galactosidase can be prepared and added to milk without affecting the lactose content for up to 20 days of refrigeration (4C).