Solvent selection in the preparation of poly(dl-lactide) microspheres prepared by the solvent evaporation method

Incorporation of protein in PLG-microspheres with retention of bioactivity

The enzyme urease was incorporated into poly(lactide-co-glycolide) microspheres using a double emulsion solvent removal technique. Ethyl acetate was used as organic solvent since it is less toxic than the more commonly used methylene chloride. The effect of the two solvents on urease was compared. Although this preparation technique is well established, it is often associated with reduced bioactivity and low entrapment efficiency of proteins. In order to retain a high degree of bioactivity, the well known protein stabilisers: sucrose, trehalose and poloxamer 407, were added to the urease in the preparation. The bioactivity of the entrapped urease was reduced more by methylene chloride than by ethyl acetate. The gelled form of poloxamer was shown to highly favour the retention of bioactivity, demonstrated by an increase of 41% compared to preparations without poloxamer. Moreover, the presence of poloxamer strongly increased the in vitro release rate of urease from the microspheres. The entrapment efficiency was increased by 44% using the sugars in the preparation. These results clearly show the great potential of small quantities of additive in the formulation to control the properties of the microspheres. The amount and type of additive could be adjusted according to the therapeutic application of the preparation.

Poly(ethylene carbonate) microspheres: manufacturing process and internal structure characterization

The granulocyte-macrophage colony stimulating factor (GM-CSF), a water-soluble cytokine, was encapsulated in poly(ethylene carbonate) microspheres (MS) by a double emulsion w(1)/o/w(2) solvent evaporation method. Poly(ethylene carbonate) is a new polymer of high molecular weight (MW) and forms polymer matrices that are exclusively surface bioerodible. In the frame of this study, the influence of the polymer molecular weight and the polymer concentration in the organic phase on the physico-chemical characteristics of the microspheres were investigated. Ninety percent of the microspheres had a diameter ranging between 4 and 136 microm, with a mean value of 30 microm. The encapsulation ratios ranged from 2.22 to 2.51% (w/w) depending on the molecular weight of the polymer corresponding to an encapsulation efficiency of 70 to 100%, respectively. Independent of the polymer molecular weight used, the in vitro drug release was very low, ranging from 5.61 to less than 1% of the total encapsulated GM-CSF amount. Scanning electron microscopy (SEM) analysis showed microparticles with spherical shapes and smooth surfaces containing a few small globules. The inner structure of the microspheres appeared to consist of a polymeric matrix surrounding numerous globules. These globules have different sizes, shape and distribution in the polymeric matrix, depending on the concentration of the polymer solution and on the polymer molecular weight. In addition, it was demonstrated that the GM-CSF lowered the interfacial tension between the GM-CSF aqueous solution and the methylene chloride organic phase. The active critical concentration was as low as 0.008 mg/ml. It was therefore suggested that this particular behavior contributed to the stabilization of the primary emulsion during the formation of the microspheres, leading to rather high encapsulation efficiency.

The preparation and evaluation of poly(epsilon-caprolactone) microparticles containing both a lipophilic and a hydrophilic drug

An original dosage form for oral delivery based on the encapsulation of both, lipophilic and hydrophilic drugs, in poly(epsilon-caprolactone) (PCL) microparticles prepared either by the oil-in-water (o/w) or the water-in-oil-in-water (w/o/w) solvent evaporation method was developed. Microparticles were characterized in terms of morphology, size, encapsulation efficiency and drug release. The physical state of the drugs and the polymer was determined by scanning electron microscopy (SEM), X-ray powder diffractometry, and differential scanning calorimetry (DSC). Nifedipine (calcium antagonist) and propranolol HCl (beta-blocker), used for the treatment of hypertension, were chosen as lipophilic and hydrophilic drugs. The microparticles were spherical with diameters in the range of 191-351 microm by the o/w-method, and in the range of 302-477 microm by the w/o/w-method. The encapsulation efficiency (EE) was 91% for nifedipine and 37% for propranolol HCl with the o/w-method, and 83% for nifedipine and 57% for propranolol HCl with the w/o/w-method. DSC and X-ray diffraction studies showed that PCL maintained its semi-crystalline structure, while the drugs were either dispersed or dissolved in the polymer. In vitro release studies revealed a controlled release of nifedipine and propranolol HCl from microparticles prepared by the o/w-method; a burst release of propranolol HCl was observed from microparticles prepared by the w/o/w-method. In conclusion, microparticles containing both a hydrophilic and a lipophilic drug were successfully prepared.

Surface drug removal from ibuprofen-loaded PLA microspheres

The preparation, characterisation and drug release behaviour of ibuprofen loaded poly(D,L-lactic acid) (PLA) microspheres are described. Depending on the gelatin concentration in the aqueous external solution (1, 0.5, 0.1% w/v), microspheres with three different sizes (2.2, 4.1, 7.5 micrometer) were obtained. The properties of microspheres washed with water (Untreated microspheres) (Un-Ms) were compared to those of the microspheres washed with a sodium carbonate solution in order to remove the surface drug (treated microspheres) (T-Ms). The results indicate that the removal of the surface drug did not induce any change in the size of the microspheres whereas the morphology of the smallest T-Ms appeared to be modified. The release profiles of both Un-Ms and T-Ms resulted in biphasic patterns. The initial burst effect (first release phase) of the T-Ms was lower than that of the Un-Ms. The rate of the second release phase did not change for the microspheres with the biggest size but increased for the smallest microspheres probably owing to the modification of the matrix porosity.

Pseudolatex preparation using a novel emulsion-diffusion process involving direct displacement of partially water-miscible solvents by distillation

Pseudolatexes were obtained by a new process based on an emulsification-diffusion technique involving partially water-miscible solvents. The preparation method consisted of emulsifying an organic solution of polymer (saturated with water) in an aqueous solution of a stabilizing agent (saturated with solvent) using conventional stirrers, followed by direct solvent distillation. The technique relies on the rapid displacement of the solvent from the internal into the external phase which thereby provokes polymer aggregation. Nanoparticle formation is believed to occur because rapid solvent diffusion produces regions of local supersaturation near the interface, and nanoparticles are formed due to the ensuing interfacial phase transformations and polymer aggregation that occur in these interfacial domains. Using this method, it was possible to prepare pseudolatexes of biodegradable and non-biodegradable polymers such as poly(D,L-lactic acid) and poly(epsilon-caprolactone), Eudragit E, cellulose acetate phthalate, cellulose acetate trimellitate using ethyl acetate or 2-butanone as partially water-miscible solvents and poly(vinyl alcohol) or poloxamer 407 as stabilizing agent. A transition from nano- to microparticles was observed at high polymer concentrations. At concentrations above 30% w/v of Eudragit E in ethyl acetate or cellulose acetate phthalate in 2-butanone only microparticles were obtained. This behaviour was attributed to decreased transport of polymer molecules into the aqueous phase.

Biodegradable monodispersed nanoparticles prepared by pressure homogenization-emulsification

The aim of the present work was to investigate the preparation of nanoparticles (NP) as potential drug carriers for proteins. The hydrophilic protein bovine serum albumin (BSA) was chosen as the model drug to be incorporated within NP. Owing to the high solubility of the protein in water, the double emulsion technique has been chosen as one of the most appropriate method. In order to reach submicron size we used a microfluidizer as a homogenization device with a view to obtaining NP with a very high grade of monodispersity. Two different biodegradable polymers, poly[D, L-lactic-co-glycolic acid] 50/50 (PLGA) and poly[epsilon-caprolactone] (PCL) has been used for the preparation of the NP. The drug loading has been optimized by varying the concentration of the protein in the inner aqueous phase, the polymer in the organic phase, the surfactant in the external aqueous phase, as well as the volume of the external aqueous phase. The BSA encapsulation efficiency was high (>80%) and release profiles were characterized by a substantial initial burst release for both PLGA and PCL NP. A higher release was obtained at the end of the dissolution study for PLGA NP (92%) compared with PCL NP (72%).

Solvent influence on spray-dried biodegradable microspheres

Microspheres of fixed poly(D,L-lactic acid) (PDLLA) composition--Resomer R104:R202H (30:70)--containing 20% w/w rifampicin have been spray-dried from a range of acetonic, halogenated, and solvent mixtures thereof under constant process conditions to examine the influence of solvent selection on microsphere characteristics. Solubility of the polymer composite in the studied solvents determined the kinetics of polymer deposition during drying. Viscosity studies provided an indirect index of solvent power in ascending order: acetone (ACT) < dichloromethane (DCM) < chloroform (CFM) < halothane (HAL). Accordingly, poorer acetonic solvents produced a more open, porous matrix of increased mean diameter, whereas DCM, CFM and HAL generated more coherent matrices of greater density and elevated glass transition temperature, which significantly retarded drug release. Yield generally increased in parallel with solvent strength and microsphere density consistent with the proposed generalized particle formation mechanism. Residual solvent also increased with particle density, both parameters being interrelated and dictated by the inherent affinity of the polymer composite for individual solvents. In turn, the position of glass transition temperature (Tg) and the quantity of associated polymer stress-relaxation were a direct function of amount and persistence of organic residue. The magnitude of these changes determined the relative rates and extents of microsphere ageing, as measured by drug release studies. In general, rate of drug release increased with Tg, after corrections were made for specific surface area (r2 = 0.963). Overall, solvent choice for spray-drying has a remarkable influence on microsphere characteristics and, accordingly, technological as well as toxicological considerations should be paid during selection of same.

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.

Effect of solvent on the characteristics of pentamidine loaded microcapsule

Biodegradable microcapsules of pentamidine/poly(L-lactide-co-D,L-lactide) were prepared by solvent evaporation technique using a mixture of organic solvents. The control batch of microcapsules was prepared using dichloromethane. The effect of solvent on the characteristics of pentamidine loaded microcapsules was examined by substituting up to 30% of dichloromethane with acetone, methanol, DMSO, ethyl acetate, and ethanol, respectively. No significant change in the surface morphology was observed when dichloromethane was substituted with 20% or less amount of other solvents. These microcapsules were all porous and spherical. However, the use of 30% DMSO or ethanol, along with dichloromethane, resulted in a mixture of elongated and spherical microcapsules. The efficiency of encapsulation of these two batches was also significantly higher than the other batches of microcapsules. The average particle size of the microcapsules prepared with 30% DMSO (165 microm) was significantly higher than the other batches (< 80 microm). A substitution of 10-30% dichloromethane with other listed organic solvents also showed a significant difference in the initial drug release. The drug release within the first twenty-four hours varied from 4 to 16%. The use of a second organic solvent, except ethanol, resulted in a significantly higher drug release during the second half of the dissolution study. The drug release continued more than 60 days.

Preparation of non-porous microspheres with high entrapment efficiency of proteins by a (water-in-oil)-in-oil emulsion technique

Emulsification-solvent removal methods have been widely used for encapsulating bioactive macromolecules like proteins and polypeptides in biodegradable polymers. We report, a (water-in-oil)-in-oil emulsion technique wherein proteins and polypeptides differing in molecular weight and shape were encapsulated in polymers of current biomedical interest. When an oil was used as the processing medium in combination with a carefully selected mixed solvent system such that a stable (w/o1/o2 emulsion is formed and solvents are removed by a combination of extraction and evaporation, the entrapment efficiency was high and the product nonporous. The entrapment efficiency of globular proteins exceeded 90% while that of fibrous proteins was around 70%. Fracture studies revealed that the polymer matrix was dense. The mechanism of entrapment involved solvent-induced precipitation of the protein as the microspheres were being formed. The principle of the method will find use in preparation of non-porous polymer microparticles with reduced burst effect.

Biodegradable poly(DL-lactic-co-glycolic acid) microspheres containing tetracaine hydrochloride. In-vitro release profile

Tetracaine does not result in effective treatment of intractable pain caused by trigeminal neuralgia because of its short duration of effect. In a sustained release system a controlled delivery of the drug at the site of administration, would avoid successive administrations. Tetracaine hydrochloride (HCl) has been encapsulated using a technique based on the evaporation of solvent from an O/O emulsion, using poly(DL-lactic-co-glycolic acid) (PLGA) 50:50. Microspheres were separated into three fractions: 106-212, 212-300 and 300-425 microns. The effects of two variables of the manufacturing method (volume of the inner phase of the emulsion and volume of surfactant added to the external phase) on the drug loading into microspheres, dissolution profiles and SEM characterization of the microspheres were evaluated. Microspheres containing tetracaine hydrochloride (up to 94% referred to the theoretical) released the drug, in-vitro, over 35 days. Tetracaine HCl was delivered according to zero order kinetics from day 5 until the end of the release assay. The rate of drug release depended mainly on the viscosity of the discontinuous phase and on the size of microparticles. Microsphere size resulted more homogeneous when using the highest volume of the surfactant, being almost 80% of microparticles within the range 212-300 microns.

Preparation techniques and mechanisms of formation of biodegradable nanoparticles from preformed polymers

The techniques available to prepare biodegradable nanoparticles (nanospheres and nanocapsules) from preformed polymers are reviewed. Although there is abundant literature on this topic, only a few focus on the thorough analysis of preparative procedures. In particular, four techniques are discussed in terms of their technological advantages and drawbacks: emulsification evaporation, solvent displacement, salting-out, and emulsification diffusion. The proposed mechanism of nanoparticle formation for each technique is described from a physicochemical perspective. The effects of preparative variables on nanoparticle size and drug-entrapment efficiency are also discussed.

Bioabsorbable polymers for implantable therapeutic systems

For a long time, subcutaneous implantable drug pellets using nondegradable polymers have been used for long-term, continuous drug administration. The procedure requires surgical implantation and removal of the drug-containing devices or polymeric matrices, which has a significant negative impact on the acceptability of the product candidate. In addition, the release profile from such devices is neither constant nor readily controlled in terms of precision of rate of release and duration of action. These facts have led to the research and development of novel, controllable, nonirritating, noncarcinogenic, biocompatible, and bioabsorbable drug delivery systems for overcoming the drawbacks of nondegradable implantable pellets for prolonged continuous release. Biodegradable implantable systems release the drug over a long period of time with simultaneous or subsequent degradation in the tissue of the polymer to harmless constituents, thus avoiding removal once the therapy is complete. This approach has considerably improved patient acceptability and patient compliance. Various bioabsorbable polymers have been evaluated for controlled implantable drug delivery, including hydrogels, copolymers of polylactic and polyglycolic acids, polylactic acid, poly(orthoesters), polyanhydrides, poly(E-caprolactone), and polyurethanes. Their characteristics have been studied using a variety of drugs, like anticancer agents, hormone agonists and antagonists, nonsteroidal anti-inflammatory agents, neuroleptics, contraceptives, and others. The present paper describes the current research on implantable therapeutic systems, the bioabsorbable polymers, and the biologically active agents being used in this approach.

A poly(D,L-lactide-co-glycolide) microsphere depot system for delivery of haloperidol

Haloperidol-loaded biodegradable poly(d,l-lactide-co-glycolide) (PLG) microspheres, with theoretical mean particle sizes of about 0. 8, 2 and 8 micrometer, have been successfully prepared by using an emulsification-solvent evaporation method. The effect of various processing parameters such as the content of polymer in dichloromethane, the content of polyvinyl alcohol in the aqueous phase, and the stirring speed of emulsification on the particle size and size distribution of microspheres has been investigated and three optimal procedures suggested. The influence of particle size on drug content and incorporation efficiency of haloperidol-loaded PLG microspheres also has been evaluated. In vitro a linear release of haloperidol from PLG microspheres over an extended period of time without a significant burst effect has been achieved; the time of 50% drug release (T50%) being around 55 days. The effect of drug content and particle size on the cumulative release of haloperidol from PLG microspheres was also studied together with the reproducibility of drug content and release profile from batch-to-batch with different particle sizes.

Preparation and characterization of oil-in-water type poly (D,L-lactic acid) microspheres containing testosterone enanthate

Poly (D,L-lactic acid) (PLA) microspheres containing testosterone enanthate (ET) were prepared by using an oil-in-water (O/W) emulsion technique. The size distribution of the microspheres obtained could be explained by a log-normal distribution, and as a result, it was found that ET fully incorporates into microspheres even when the drug is loaded at up to 50%. On the other hand, the dissolution behavior of ET from microspheres was strongly dependent on particle size, suggesting that dissolution of the drug from microspheres can be easily controlled by controlling the preparative conditions.

Controlled drug delivery by biodegradable poly(ester) devices: different preparative approaches

There has been extensive research on drug delivery by biodegradable polymeric devices since bioresorbable surgical sutures entered the market two decades ago. Among the different classes of biodegradable polymers, the thermoplastic aliphatic poly(esters) such as poly(lactide) (PLA), poly(glycolide) (PGA), and especially the copolymer of lactide and glycolide referred to as poly(lactide-co-glycolide) (PLGA) have generated tremendous interest because of their excellent biocompatibility, biodegradability, and mechanical strength. They are easy to formulate into various devices for carrying a variety of drug classes such as vaccines, peptides, proteins, and micromolecules. Most importantly, they have been approved by the United States Food and Drug Administration (FDA) for drug delivery. This review presents different preparation techniques of various drug-loaded PLGA devices, with special emphasis on preparing microparticles. Certain issues about other related biodegradable polyesters are discussed.

Functionality of protective colloids affecting the formation, size uniformity and morphology of drug-free polylactic acid microspheres

Drug-free polylactic acid (PLA) microspheres were prepared by an emulsification-solvent evaporation technique using different types of protective colloids. The influence of five types of hydrophilic prolymers (polysaccharides, proteins, synthetic cellulose derivatives, synthetic nonionic polymers and surfactants) on the formation, size uniformity and morphology of PLA microspheres was investigated. Four characteristic functions (surface activity, viscosity, electric charge and interfacial film formation) of the hydrophilic polymer aqueous solutions were used to evaluate the efficacy of these protective colloids used. The results indicate that these four functions were the key parameters to achieve the formation of PLA microspheres. The best protective colloid should have high surface activity, optimum viscosity, adequate electric charge, and form an interfacial film to give a higher recovery, better size uniformity and smoother topography of the PLA microspheres.

The control of protein release from poly(DL-lactide co-glycolide) microparticles by variation of the external aqueous phase surfactant in the water-in oil-in water method

Poly(DL-lactide co-glycolide) microparticles below 5 microm in size and containing ovalbumin (OVA), were prepared using the water-in oil-in water (w/o/w) technique with either polyvinyl alcohol (PVA) or polyvinylpyrrolidone (PVP) as stabilisers in the external aqueous phase. PVP-stabilised microparticles exhibited higher protein loading (8.2%, w/w relative to 4.0% for PVA stabilised microparticles) and increased core loading (encapsulation) of protein (70% vs. 30% for the PVA system). The use of PVP instead of PVA to prepare microparticles also resulted in reduction in the initial burst release of OVA, together with sustained protein release over 28 days and an increase in the protein delivery capacity from 35 to 45 microg/mg particles. The changes in protein loading and delivery characteristics are considered to arise in part from an increase in the viscosity of the droplets of polymer solution, constituting the primary water-in oil emulsion, by diffusion of PVP from the external aqueous phase. Variation of the external aqueous phase surfactant provides a promising approach for improving the loading of therapeutic proteins and vaccine antigens within biodegradable microparticles and for modulating their release pattern.

Drug microencapsulation by PLA/PLGA coacervation in the light of thermodynamics. 1. Overview and theoretical considerations

Phase separation of poly(lactide) (PLA) and poly(lactide-co-glycolide) (PLGA), often called "coacervation" in the pharmaceutical field, is one of the classical methods for peptide drug microencapsulation in biodegradable polyesters. Although numerous studies have used this technique, the underlying physicochemical mechanisms of polyester coacervation under conditions of microsphere production have not been well-described yet. Moreover, the quality of microencapsulation in terms of drug loading efficiency and residual organic solvents is often not entirely satisfactory and depends greatly on the specific drug and polymer used. The first part of this contribution reviews briefly the scientific and patent literature on PLA/PLGA coacervation. Then, the underlying physicochemical principles of polyester coacervation are discussed and relevant thermodynamic models presented. More specifically, attempts were made to clarify the necessary characteristics of polymers, solvents, and coacervating and hardening agents for successful phase separation and microsphere formation. These basic considerations may contribute to a better understanding of the boundary conditions crucial for efficient drug microencapsulation by polyester coacervation.

Influence of processing on the stability and release properties of biodegradable microspheres containing thioridazine hydrochloride

Biodegradable microspheres of poly(DL-lactic-co-glycolic acid) (PLGA) containing thioridazine HCl were produced by four emulsion-solvent evaporation methods including an O/W emulsion method, an O/O emulsion method, a W/O/W multiple emulsion method, and a W/O/O/O multiple emulsion method. Gel permeation chromatography was used to determine the molecular weight of the polymer before and after processing. Resultant microspheres were either incubated in an oven at 40 degrees C, or stored in a desiccated chamber at 20 degrees C. Change in the molecular weight of the polymer was monitored as a function of time. Premature degradation of the polymer was evident in microspheres produced by the O/W conventional solvent evaporation method. Thioridazine HCl catalyzed hydrolysis of PLGA was evident in normalized molecular weight distribution plots of the O/W microspheres. The in vitro release of thioridazine HCl from multiphase microspheres produced by potentiometric dispersion was compared with the release of drug from conventional microspheres prepared from the same polymer. Release of thioridazine HCl from multiphase microspheres of the W/O/O/O type occurred by diffusion during initial stages of drug release.

Selective in vitro removal of anti-A antibodies by adsorption on encapsulated erythrocyte-ghosts

Large volume plasma exchanges are used for the removal of anti-A or anti-B antibodies from the plasma of patients undergoing transplantation from donors with major ABO incompatibility. Previous works suggest that solid-phase immunoadsorption can be substituted for plasma exchange in situations where antigens can be purified and immobilized on columns through which plasma is percolated. However, the preparation of purified antigens of the ABO system is large quantities is laborious and requires the use of considerable blood volumes. Studies were therefore undertaken to determine the feasibility of an original immunoadsorbent based on porous microparticles prepared by a water/oil/water emulsification-solvent evaporation method, within which erythrocytes-ghosts carrying blood group antigens were entrapped. The decrease of the antibody hemagglutinating titre after adsorption onto encapsulated ghosts suggests that antibodies can cross the polymeric membrane and bind to the antigens. This original approach of using encapsulated antigens for the batchwise removal of antibodies could be extended to affinity chromatography, and immunoadsorption therapy with a chromatographic column linked to an extracorporeal circulation could be considered.

Multiple emulsion technology for the design of microspheres containing peptides and oligopeptides

This paper reviews the preparation and characterization of small poly(lactic-co-glycolic acid) microspheres (mean size lower than 10 µm) containing small peptides and prepared by a water-in-oil-in-water emulsion solvent evaporation technique. These microspheres were shown to encapsulate very efficiently a 33 amino acid peptide (V3 BRU) and in vitro release kinetics studies showed that such microspheres could be employed for both oral vaccination and controlled release. The encapsulation of a seven amino acid peptide (pBC 264) led on the contrary to a very low encapsulation efficiency. In order to increase the encapsulation of pBC 264, two strategies were adopted: (i) taking into account the solubility of pBC 264 at different pH, the inner aqueous phase was maintained at a basic pH where the peptide was soluble, while the external aqueous phase was acidic; (ii) ovalbumin was added to stabilize the inner emulsion. These two strategies allowed us to increase significantly the encapsulation rate of pBC 264. Nevertheless, the in vitro release kinetics of the peptide were strongly influenced by the presence of ovalbumin which seems to form pores in the microsphere structure. By contrast, when ovalbumin was replaced by Pluronic(R) F68, microspheres did not have pores, thus the release profile and the extent of the burst were much smaller. When microspheres were stereotactically implanted in the rat brain, in vivo release profiles were in good agreement with the release observed in vitro. In conclusion, these microspheres are well suited for the slow delivery of neuropeptides in the brain, a feature expected to facilitate the study of long term effects of these compounds.

Preparation of microspheres by the solvent evaporation technique

The microencapsulation process in which the removal of the hydrophobic polymer solvent is achieved by evaporation has been widely reported in recent years for the preparation of microspheres and microcapsules based on biodegradable polymers and copolymers of hydroxy acids. The properties of biodegradable microspheres of poly(lactic acid) (PLA) and poly(lactic-co-glycolic acid) (PLGA) have been extensively investigated. The encapsulation of highly water soluble compounds including proteins and peptides presents formidable challenges to the researcher. The successful encapsulation of such entities requires high drug loading in the microspheres, prevention of protein degradation by the encapsulation method, and predictable release of the drug compound from the microspheres. To achieve these goals, multiple emulsion techniques and other innovative modifications have been made to the conventional solvent evaporation process.

Optimization of the preparation of loperamide-loaded poly (L-lactide) nanoparticles by high pressure emulsification-solvent evaporation

The entrapment of loperamide hydrochloride (LPM) in biodegradable polymeric drug carriers such as nanoparticles might enable its passage across the blood-brain barrier. The optimization of the preparation of the LPM-loaded PLA nanoparticles was performed employing high pressure emulsification-solvent evaporation. The resulting nanoparticles were characterized by particle size, distribution, thermal analysis, and drug release profiles. The partition of LPM into the organic phase increased with an increase in pH of the aqueous phase and with addition of lipophilic surfactants such as sorbitan fatty acid esters, resulting in an increase in the drug entrapment in the nanoparticles. Evaporation of the organic phase under reduced pressure and the addition of ethanol in the organic phase yielded a high drug entrapment due to the rapid polymer precipitation. The addition of the sorbitan fatty acid esters further increased the drug entrapment even at higher LPM concentrations. The results of thermal analysis suggest that LPM was homogeneously dispersed in the amorphous polymer matrix. The in vitro release of the drug from nanoparticles was biphasic, with a fast initial phase, followed by a second slower phase. Different drug release profiles from nanoparticles can be achieved by addition of sorbitan fatty acid esters, or the employment of different solvents as the organic phase.

Chitosan microcapsules as controlled release systems for insulin

This study analyses the use of chitosan for the production of surface crosslinked microparticulate systems containing insulin. The microcapsules were prepared by an innovative technique based on interfacial crosslinkage of different amounts of chitosan solubilized in the inner phase of a water/oil emulsion by means of ascorbyl palmitate. The correlation between the main formulation parameters and the functional properties of the microcapsules were analysed. Insulin is incorporated with high efficiency. The peptide release is constant for appreciable periods of time. The content of chitosan modulates the main physico-chemical properties of the matrix.

The effect of ethylcellulose molecular weight on the properties of theophylline microspheres

Microspheres of theophylline, with both ethylcellulose of high and low molecular weight and also their mixtures as a coating material, were prepared using the solvent evaporation technique. No permeability through intact isolated polymer films was found. Therefore, this study investigated the drug release dependence of structure and mechanical properties of the polymer matrix. In vitro dissolution studies exhibited the square-root of time (Higuchi model) release characteristics. The size distribution of microspheres was dependent on the ratio of ethylcellulose mixtures with high and low molecular weights.

In vitro ciprofloxacin release from poly(lactide-co-glycolide) microspheres

Biodegradable microspheres of PLGA (75:25 and 50:50) containing Ciprofloxacin (CIPRO) were prepared by two different procedures based on: (i) solvent-evaporation, and (ii) evaporation-extraction of the organic phase. The encapsulation rates from the different formulations were quite variable, as well the release patterns of the drug from the microspheres. The evaporation-extraction method seems to be more efficient for the CIPRO encapsulation compared with the solvent evaporation method. The 50:50 PLGA composition released the drug faster and showed degradation signs after incubation in aqueous medium in both manufacturing methodologies.

In-vitro studies of enteric coated diclofenac sodium-carboxymethylcellulose microspheres

MIcrospheres containing diclofenac sodium (DS) were prepared using carboxymethylcellulose (CMC) as the main support material (1.0, 2.0, 3.0% (w/v)) and aluminum chloride as the crosslinker. Drug to polymer ratios of 1:1, 1:2 and 1:4 were used to obtain a range of microspheres. The microspheres were then coated with an enteric coating material, Eudragit S-100, efficiency, % yield value, particle sizes an in-vitro dissolution behaviour were investigated. The surface of the enteric coated microspheres seemed to be all covered with Eudragit S-100 from scanning electron microscopy observation. It was also observed that increasing the CMC concentration led to an increase in the encapsulation efficiency, % yield value and particle size and decreased the release rate. Eudragit S-100 coating did not significantly alter the size but the release rate was significantly lower even when the lower concentration solution was used.

Investigation of in-vitro release characteristics of NSAID-loaded polylactic acid microspheres

The intra-pulmonary delivery of non-steroidal anti-inflammatory drug-loaded PLA microspheres has potential utility in the treatment of inflammatory lung conditions such as asthma. Drug encapsulation efficiency and release kinetics depend upon a variety of parameters in the production process, all of which affect the properties of the microspheres produced. The release of piroxicam from PLA microspheres followed an apparently biphasic release profile. PLA microspheres containing indomethacin, however, exhibited kinetics which approached more closely to zero order release. The effect of microsphere production parameters upon these release profiles has been investigated. Results indicate that factors affecting the nature of the microsphere matrix have the greatest influence on release profiles. The use of halothane as organic solvent in the microsphere production increases the burst release effect. Residual halothane is known to be present in the microspheres, producing a less stable matrix, thus allowing much faster release of the drug. The nature of drug incorporated also appears to affect the nature of the microspheres matrix. Piroxicam-loaded microspheres possess a much more porous matrix than indomethacin-loaded microspheres, as evidenced by washing procedures. This difference could explain the difference in release profiles between the two types of microspheres.

Microencapsulation of ovalbumin in poly(lactide-co-glycolide) by an oil-in-oil (o/o) solvent evaporation method

The objective of this study was to produce biodegradable poly(lactide-co-glycolide) (PLGA; 50/50) microspheres by an oil-in-oil (o/o) solvent evaporation method to prolong the in vitro release of ovalbumin (OVA) as a model protein. The effects, on loading efficiency, microsphere yield, morphology and drug release, of two dispersing agents, aluminium tristearate and Span 80, in mineral oil were examined. PLGA 50/50 microspheres containing OVA powder (sieved through a 53 microns mesh) were prepared using an o/o solvent evaporation method. When aluminum tristearate was employed as a dispersing agent, the loading efficiency and yield of OVA had maximum values of 89 and 72% at 0.15% (w/v) aluminum tristearate, respectively. Morphology studies suggested that the obtained microspheres were spherical, and had a smooth surface. The diameters of the microspheres ranged between 100 and 200 microns. The loading efficiency, or yield, for microspheres decreased significantly above or below 0.15% (w/v) aluminum tristearate, and microspheres with irregular shapes were observed. The minimum sedimentation volume ratio (F) was obtained at a dispersity of carbon black particles in ethanol containing 0.15% (w/v) aluminum tristearate by a sedimentation study, and the cloudy supernatant suggested a deflocculated suspension. However, on the contrary, when Span 80 was added into the mineral oil as a dispersing agent, the concentration of Span 80 had little or no effect on the characteristics of the prepared microspheres. Drug loadings (60-70%) were obtained within the Span 80 concentrations employed in the present study (0.05-1.0% (w/v)). The yields were also in the same levels. The microspheres prepared in mineral oil containing Span 80 had an average diameter less than 50 microns in all cases. Sustained-release characteristics were demonstrated for PLGA microspheres prepared in mineral oil containing aluminum tristearate as a dispersing agent, even though a burst release at the initial phase was observed. This initial burst release from PLGA microspheres was reduced to some extent by micronization of the OVA powder using a planetary-type ball mill. However, PLGA microspheres prepared in mineral oil containing Span 80 as a dispersing agent, exhibited a large initial burst release. This burst release seems to be due to the smaller size of microspheres and the OVA powder adhering to the surface of PLGA microspheres (confirmed by scanning electron microscope (SEM) study).

Cellulose acetate butyrate microparticles for controlled release of carbamazepine

Cellulose acetate butyrate microparticles loaded in carbamazepine were prepared by a solvent evaporation technique. A decrease of the amount of organic solvent (from 80 to 40 ml of methylene chloride) increased the microparticle average diameter (73-111 and 207 microns) and decreased the carbamazepine release rate (T50% increased from 3.3 to 16.8 and 166.4 min). The microparticle area under the curve at 120 min was similar to that obtained with Tegretol LP 200 tablets.

Preparation and evaluation of sustained release AZT-loaded microspheres: optimization of the release characteristics using response surface methodology

The purpose of the study was to prepare and optimize a sustained release formulation of zidovudine (AZT). Ethylcellulose microspheres containing AZT were prepared using an emulsification/solvent evaporation technique. The critical formulation variables were emulsifier concentration, drug to polymer ratio, and ethyl acetate concentration in the internal phase of the emulsion. The time to release 85% of the contents of the microspheres (t85) was used as a measure for the release time. A second-year polynomial equation was fitted to the release data to systemically investigate the effect of the formulation variables on the release rate. This equation was then used to predict t85 in the optimum region. The t85 was found to be dependent on the three formulation variables, with strong interactions observed between these variables. The microspheres were characterized in terms of their particle size and surface morphology. The study indicated no overall correlation between the mean diameter of the microspheres and the t85.

Formulation of BCNU-loaded microspheres: influence of drug stability and solubility on the design of the microencapsulation procedure

The design of a microencapsulation procedure for the preparation of biodegradable BCNU-loaded microspheres used as intracerebral implants is the aim of this work. This approach will give sustained high local concentrations of the anti-tumour drug in the brain without the associated significant systemic toxicity. The microencapsulation technique used is a solvent-evaporation process based on the formation of an oil-in-water emulsion. The stability of BCNU in methylene chloride saturated with water and its high value of partition coefficient between methylene chloride and water justifies the selection of this organic solvent as the dispersed phase in the methodology. A spectrophotometric method for the quantification of BCNU in mixtures containing PLAGA is developed which allows the evaluation of drug photodecomposition. The volume of methylene chloride and the concentration of PVA in the external aqueous phase are the two variables that induce the largest variations of the microsphere size. The two main process parameters leading to the highest microencapsulation yield are the polymer concentration in the organic phase and the volume of the external aqueous phase; whereas the pH of the external aqueous phase and the use of co-solvents in the organic phase lead only to a small improvement in microsphere payload.

Study of a microencapsulation process of a virucide agent by a solvent evaporation technique

A highly water-soluble virucide agent was microencapsulated by a water/oil/water emulsification-solvent evaporation method. An aqueous drug solution was emulsified into a solution of polymer in methylene chloride, followed by emulsification of the primary emulsion in an external aqueous phase. Microcapsules were formed after solvent evaporation, the solidification of the microcapsule walls was followed by an optical method. The influence of stirring speed was analysed to find the optimal hydrodynamic conditions with respect to the process yield, corresponding to the weight of obtained microcapsules per litre of water/oil/water emulsion, the initial virucide agent content and the drug release kinetics. The optimal conditions were obtained for the complete suspension speed. The improvement of the microencapsulation process was attempted by increasing the concentration of the primary emulsion and by the reuse of the external aqueous phase after removal of the microcapsules.

Properties of multiphase microspheres of poly(D,L-lactic-co-glycolic acid) prepared by a potentiometric dispersion technique

Multiphase microspheres of poly(D,L-lactic-co-glycolic acid) (PLGA) were prepared by a multiple emulsion potentiometric dispersion technique. Water-soluble compounds were dissolved in the aqueous phase (W) and emulsified in soybean oil (O) to form a stable emulsion. This primary emulsion was dispersed in a solution of PLGA and acetonitrile (O) to form a W/O/O emulsion. The W/O/O emulsion was then dispersed in a hardening solution of light mineral oil (O) using a potentiometric dispersion technique to produce microspheres of the W/O/O/O type with a very narrow and selective size distribution. The size of the microspheres was controlled by varying the internal diameter of the conductive infusion tube or by the variation of voltage applied to the conductive tube. Particle size analysis revealed a particle size distribution of 1-50 microns for microspheres made by this method as compared to a distribution of 50-500 microns for microspheres made by conventional agitation methods. Chlorpheniramine maleate was encapsulated with a loading efficiency of 88.9% with the potentiometric method as compared with a loading efficiency of 74.3% for the agitation method.

Preparation and characterization of 5-fluorouracil-loaded microparticles as biodegradable anticancer drug carriers

To provide a device releasing 5-fluorouracil in a controlled manner and injectable into the brain by stereotaxy, biodegradable poly ((+/-)-lactide-co-glycolide) (PLAGA) microparticles were prepared by an emulsion-extraction process. Although the solubility profile of the drug was not suitable for its encapsulation by the aforementioned method, careful choice of process variables allowed significant drug loading, reaching 30%. Thus, the size of the 5-fluorouracil crystals, the organic phase/aqueous phase ratio, the theoretical drug loading and the microparticle size played a predominant role. The microsphere size was adjusted to 20-40 microns by selecting the appropriate PLAGA and polyvinylalcohol concentrations, and the stirring rate of the initial emulsion. It was shown that the microparticle structure depended directly on the experimental conditions governing the precipitation rate of the coating material: two types of microparticles, I and II, were characterized. The morphology of the particles influenced the 5-fluorouracil-release patterns, as did other process parameters, such as the 5-fluorouracil crystal size and the PLAGA concentration. It was possible to sustain the 5-fluorouracil release over 18 days.