A new technique to efficiently entrap leuprolide acetate into microcapsules of polylactic acid or copoly(lactic/glycolic) acid

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).

Reversible suppression of pituitary-testicular function by a sustained-release formulation of a gnrh agonist (Leuprolide acetate) in dogs

Pituitary-testicular function was studied in 15 dogs following treatment with a sustained-release formulation of a GnRH agonist, leuprolide acetate (LA). Adult male dogs were treated with a single subcutaneous injection of microencapsulated LA (0.1 or 1 mg/kg). Treatment with LA at a dose of 1 mg/kg resulted in decreased (P<0.001) ejaculatory volume and disappearance of morphologically normal spermatozoa within 8 wk and the effect persisted for 6 wk, while the 0.1 mg/kg dose was not adequate to effect suppression of spermatogenesis. The larger dose treatment (1 mg/kg) caused a transient rise in plasma levels of LH and testosterone followed by a marked decline to below the normal level by 2 wk, the low levels being maintained for at least 5 wk, indicating a prolonged effect of LA treatment on pituitary-gonadal axis. Twenty weeks after treatment with LA, a complete return to normal spermatogenesis was observed. The full reversibility of spermatogenesis in the dog after LA treatment suggests that this peptide could be used as a reversible method of male contraception.

A month-long effect from a single injection of microencapsulated human growth hormone

An injectable sustained-release form of human growth hormone (hGH) was developed by stabilizing and encapsulating the protein, without altering its integrity, into biodegradable microspheres using a novel cryogenic process. A single injection of microspheres in monkeys resulted in elevated serum levels of recombinant hGH (rhGH) for more than one month. Insulin-like growth factor-I (IGF-I) and its binding protein IGFBP-3, both of which are induced by hGH, were also elevated for four weeks by the rhGH containing microspheres to a level greater than that induced by the same amount of rhGH administered by daily injections. These results show that, by using appropriate methods of stabilization and encapsulation, the advantages of sustained-release formulations previously demonstrated for low-molecular-weight drugs can now be extended to protein therapeutics.

Preparation and characterization of polylactic acid microspheres containing water-soluble dyes using a novel w/o/w emulsion solvent evaporation method

Polylactic acid (PLA) microspheres containing soluble dyes as water-soluble model compounds were prepared using the water-in-oil-in-water (w/o/w) emulsion solvent evaporation method. Addition of electrolytes such as NaCl or CaCl2 into the external aqueous phase significantly improved brilliant blue (BB) entrapment efficiency compared to the case of no additives. NaCl was the most effective for obtaining high entrapment efficiency (80-90% of theoretical BB content). The average diameter of the obtained microspheres was in the region of 10-20 microns in all cases. PLA microspheres containing 5 and 10% (w/w) BB exhibited the so-called burst release. The release rate decreased with decrease in the internal aqueous droplet volume in the preparation process. In particular, with PLA microspheres containing 5% (w/w) BB, those prepared with the smallest internal droplet volume (63 microliter), the initial burst release was reduced significantly, and 50% (w/w) of the loaded BB remained in the microspheres for 7 days.

The microencapsulation of protein using a novel ternary blend based on poly(epsilon-caprolactone)

Microspheres with an entrapped protein were prepared from poly(epsilon-caprolactone) (PCL), and a novel ternary blend, comprising of high and low molecular weight PCL in combination with poloxamer 181, a triblock copolymer of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide). The inclusion of low molecular weight PCL served to enhance phase mixing by a reduction in the molecular weight of the polymeric components. Encapsulation of the protein, bovine serum albumin, was possible using a water-in-oil-in-water multiple emulsion solvent evaporation technique. Microspheres prepared from unblended PCL were irregular and porous in nature. The presence of surface imperfections and macroscopic pores was attributable to the high rate of crystallization of the PCL polymer from solution. The inclusion of poloxamer 181 into the matrix retarded the rate of crystallization of the PCL, thereby enhancing particulate sphericity and regularity. Manipulation of the process parameters of blended microspheres provided a means of controlling the particle size and the entrapment efficiency of the protein. The influence of variables such as protein to polymer ratio, internal phase volume and emulsifier concentration in both the internal and external aqueous phases, on the properties of the microspheres was investigated. A mean particle size ranging from 10 to 42 microns could be achieved by altering the internal phase volume of the primary emulsion, whilst a high protein entrapment (11% w/w) was possible with a protein to polymer ratio of 1:4. Native-PAGE analysis of the entrapped protein indicated a maintenance of bulk structural integrity.

A potential soft tissue filling material: chloramphenicol loaded poly(D,L-lactide) sponges

Poly(D,L-lactide) (PDLLA) homopolymers were produced by the ring opening polymerization of a D,L-lactide dimer by using stannous chloride as the catalyst. Chloramphenicol loaded PDLLA sponges were pre- pared by a solvent evaporation procedure by using the PDLLA homopolymers with three different molecular weights (i.e., 11,000, 20,000 and 35,000 daltons). Chloramphenicol loading was changed by using three different solvents (i.e., acetone, ethyl acetate, and acetonitrile) and by changing the initial polymer concentration and its molecular weight and the initial concentration of the drug. Higher degradation rates of the chloramphenicol loaded PDLLA sponges in alkaline pH 9.0 and at 37 degrees C were observed. Chloramphenicol release rates were also high at these conditions. It was concluded that chloramphenicol release was both degradation and diffusion controlled.

Particle size studies for subcutaneous delivery of poly(lactide-co-glycolide) microspheres containing ovalbumin as vaccine formulation

The primary objectives of the present study were to produce poly(lactide-co-glycolide) (PLGA) microspheres with different diameters, to characterize these microspheres which were loaded with a model antigen, ovalbumin and to evaluate the effect of microsphere particle size on the serum antibody levels following administration to mice. Four kinds of ovalbumin-loaded PLGA microspheres with different diameters (1.2, 3.5, 7.0 and 14.3 microns as mean volume diameter) were manufactured by a w/o/w emulsion/solvent evaporation method. Low loading percent (0.08%-0.25% w/w) and efficiencies (8-25% w/w) were observed. Examination using scanning electron photomicrographs showed smooth spherical particles. The in-vitro release of ovalbumin from microspheres showed an expected burst release with all batches and the extent of the burst release increased with decreasing diameters of spheres; PLGA microspheres with the smallest diameter (1.2 microns) showed an 80% burst release within one day. Approximately 10-60% of ovalbumin remained unreleased 30 days later. The single subcutaneous administrations of ovalbumin-loaded PLGA microspheres with different diameters to mice induced good antibody responses above ovalbumin saline negative controls at 3, 6, 9, and 12 weeks after inoculation. Especially, 0.16% ovalbumin-loaded PLGA microspheres having mean volume diameter of 3.5 microns exhibited the best immune responses with values greater than those obtained after inoculation with adjuvants such as complete Freund's adjuvant or alum as positive control. The strong adjuvant activity of PLGA microspheres as vaccine formulation was suggested.

1994 Whitaker Lecture: polymers for drug delivery and tissue engineering

This paper reviews three areas of the author's research. The first area concerns the development of technologies to release macromolecules continuously from solid polymers. By embedding solid protein (or other macromolecule) powders at the correct concentration in hydrophobic polymers, prolonged release for over 100 days can be achieved. The second area involves the synthesis of new biodegradable polymers specifically designed for drug delivery. A novel family of polymers, polyanhydrides, now being explored in a number of medical applications is examined. The use of these polymers to deliver chemotherapeutic agents locally may provide a new approach to treat brain cancer. The final research topic is in the area of tissue engineering. By placing mammalian cells on biodegradable polymer scaffolds, a variety of tissues have been created in animal models. Cartilage is discussed as a model tissue.

Biodegradable microcapsules prepared by a w/o/w technique: effects of shear force to make a primary w/o emulsion on their morphology and protein release

A water-in-oil-in-water (w/o/w) technique, sometimes known as in-water drying method, was used to prepare microcapsules consisting of polylactic acid and poly(lactide-co-glycolide). The influence of shear force to produce an initial water-in-oil (w/o) emulsion on the characteristics of microcapsules and protein release was investigated. Bovine serum albumin (BSA) was used as the model protein drug for encapsulation. The initial w/o emulsion was prepared by a Polytron homogenizer. The shear rate was varied from 11 to 23 krpm to produce w/o emulsions with different shear forces. This study revealed pronounced effects of shear force on the characteristics of microcapsules and release profiles of BSA. Depending on the degree of the shear applied, the inner structure of microcapsules showed very different morphology, which was responsible for different release patterns. A low shear produced microcapsules with a high initial burst release of BSA, whereas microcapsules using a high shear exhibited a controlled release of protein without any initial burst release. Also, at a given shear force, a variation in polymer composition of microcapsules was found to be effective in controlling the release characteristics of protein. Thus, the homogenization technique should be carefully considered in designing microcapsules with desirable release profiles of proteins and an adequate period of protein delivery.

Quality improvement of spray-dried, protein-loaded D,L-PLA microspheres by appropriate polymer solvent selection

The aim was to study the effect of the type of polymer solvent on characteristics of microspheres produced by spray drying. The water-soluble model protein, bovine serum albumin (BSA) was microencapsulated into biodegradable poly(D,L-lactic acid) using the following 10 different polymer solvents: acetaldehyde dimethyl acetal, acetone, dichloromethane, dioxane, ethyl acetate, ethyl vinyl ether, nitromethane, tetrahydrofuran, 1,1,1-trichloroethane, and 1,1,2-trichloroethylene. These solvents having similar toxicity levels differ greatly in their physico-chemical characteristics such as boiling point, vapour pressure, miscibility and interfacial tension with an aqueous phase, and solubility parameter. The effect of these solvents on microsphere morphology was studied by SEM-micrographs. Regular particle morphology was obtained when dichloromethane, ethyl acetate, or nitromethane was used as the polymer solvent, whereas the trichlorinated solvents, tetrahydrofuran, and dioxane produced a substantial number of coalesced particles. The results are interpreted in terms of boiling point, vapour pressure, and polymer-solvent affinity. Further, BSA-loading and -integrity in the microspheres, and burst release were analysed. The theoretical loading of 2.9% was attained with dichloromethane, ethyl acetate and nitromethane, in agreement with observations of particle morphology. HPLC- and SDS-PAGE analysis of the microencapsulated BSA did not show any protein degradation or dimerization, whereas solid-phase ELISA clearly revealed that the in vitro protein antigenicity was substantially reduced (50%), particularly by water miscible solvents. Dichloromethane and ethyl acetate did not show any detrimental effect on protein antigenicity. Finally, burst release could be related again to particle morphology, with dichloromethane and nitromethane giving a burst release of only 5%. In conclusion, dichloromethane, ethyl acetate and nitromethane proved to be the most suitable solvents for the polymer-protein system studied.

Preparation and characterization of ethylcellulose microspheres containing 5-fluorouracil

Ethylcellulose microspheres containing 5-fluorouracil (5-FU) were prepared by a solvent evaporation technique using light mineral oil as the continuous phase. The drug was suspended in the acetone solution of the polymer. Three drug/polymer ratios (1/1, 1/2 and 1/3) were utilized. The microspheres were studied with respect to size, drug content and surface characteristics; the higher the polymer content, the smoother the microspheres. The drug was suspended in the polymer and the drug loading was important (more than 90%) with the three types of microspheres. In vitro dissolution studies in phosphate buffer showed that the 5-FU release was dependent on the drug/polymer ratio for the 400-500 microns granulometric fraction.

Preparation of three-month depot injectable microspheres of leuprorelin acetate using biodegradable polymers

To obtain a three-month release injection of leuprorelin acetate, microspheres were prepared with copoly(DL-lactic/glycolic acid) or poly(DL-lactic acid) (PLA) using an in-water drying method, and drug release was evaluated. The content of water-soluble oligomers in the polymers was found to strongly affect the initial burst, and reducing the content to less than 0.1% was necessary to keep the first-day release below 10%. Drug loading of more than 15% also increased the initial drug release; the acceptable maximum loading was 12%. Elevation of the glass transition temperature of the microspheres was observed with an increase in drug loading. This suggests formation of a rigid structure, possibly with arrangement of the polymer around the drug cores like in a micelle. This structure provides a hydrophobic barrier against diffusion of the hydrophilic peptide, resulting in high trapping efficiency and long-term sustained release dependent on polymer erosion. The microspheres prepared with PLA having a m.w. of 12,000 to 18,000 provided linear sustained release and persistent serum levels of the drug in rats for over 3 months.

In vitro drug release behavior of D,L-lactide/glycolide copolymer (PLGA) nanospheres with nafarelin acetate prepared by a novel spontaneous emulsification solvent diffusion method

Nanospheres with D,L-lactide/glycolide copolymer (PLGA) were prepared as a biodegradable and biocompatible polymeric carrier for peptide drugs by a novel spontaneous emulsification solvent diffusion method. Nafarelin acetate (NA), a luteinizing hormone-releasing hormone analogue, was employed as a model peptide drug to investigate the encapsulation efficiency. The drug and PLGA, dissolved in an acetone-dichloromethane-water mixture, were poured into an aqueous solution of polyvinyl alcohol under moderate stirring at room temperature. Spontaneous emulsification arising from a rapid diffusion of acetone from the organic to the aqueous phase enables preparation of PLGA submicron spheres 200-300 nm in size. The entrapment of NA in nanospheres was improved by blending low molecular weight (Mw = 4500) PLGA with higher molecular weight PLGA due to the synergistic effect of the rapid deposition of PLGA and the ionic interaction between NA and PLGA. By coadmixing a small amount of negatively charged phospholipids such as dipalmitoyl phosphatidylglycerol or dicetyl phosphate, the leakage of water-soluble NA was further prevented. The NA encapsulated in PLGA nanospheres was more stable than native NA in acidic medium (pH = 1.2). The drug-release behavior from nanospheres suspended in the disintegration test solution no. 1 (Japanese Pharmacopeia XII) exhibited a biphasic pattern. It was found tht the initial burst of release might be due to the degradation of the PLGA chain, as monitored by gel permeation chromatography. At a later stage, the drug was released more slowly, the rate of which was determined by the diffusion of the drug in the porous matrix structure.(ABSTRACT TRUNCATED AT 250 WORDS)

Biodegradable microspheres as controlled-release tetanus toxoid delivery systems

Purified tetanus toxoid, a high-molecular-weight protein, was entrapped within poly(L-lactic acid) (PLA) and poly(D,L-lactic/glycolic acid) (PLGA) microspheres prepared by either a solvent extraction or a solvent evaporation method carried out in a multiple emulsion system (water-in-oil-in-water). The physical integrity and antigenicity of the protein treated under different processing conditions were investigated. A reduction of antigenicity that was related to the percentage of aggregated protein was noticed under some experimental conditions. This partial loss of antigenicity was associated with the lyophilization process and affected by the nature of the organic solvent. All types of microspheres prepared with different molecular weight PLA and PLGA displayed a high protein-loading efficiency (> 80%) but their size was strongly influenced by polymer molecular weight (3000 versus 100,000). Protein release pattern was influenced by both polymer molecular weight and composition (PLA versus PLGA). A constant release pattern after an induction period of 10 days was observed for microspheres composed of high-molecular-weight polymers (PLA and PLGA). The release rate was lower from PLA microspheres than from PLGA microspheres. In contrast, a continuously increasing release rate preceded by a burst was observed for low-molecular-weight (3000) PLGA microspheres. Microencapsulated tetanus toxoid was significantly more immunogenic in mice than fluid toxoid as determined by IgG anti-tetanus antibody levels and neutralizing antibodies. However, the magnitude and duration of the antibody response did not differ significantly from a similar dose of aluminium phosphate-adsorbed toxoid. We conclude that microencapsulated tetanus toxoid shows significant adjuvant activity.(ABSTRACT TRUNCATED AT 250 WORDS)

Controlled release of polypeptides from polyanhydrides

The effects of both polymer hydrophobicity and addition of stabilizers on the release and integrity of polymer-encapsulated proteins were studied. By using very hydrophobic poly[1,3-bis(p-carboxyhydroxy)hexane anhydride] with sucrose as an excipient, both recombinant bovine somatotropin and zinc insulin were released intact over 3 weeks. The released proteins appeared to maintain their integrity as judged by acidic reverse-phase HPLC, size-exclusion HPLC, radioimmunoassay, and conformation-sensitive immunoassays. Our results also suggest how polymer hydrophobicity can be used to enhance protein stability.

The preparation and characterization of poly(lactide-co-glycolide) microparticles. II. The entrapment of a model protein using a (water-in-oil)-in-water emulsion solvent evaporation technique

Poly(lactide-co-glycolide) (PLG) microparticles with entrapped antigens have recently been investigated as controlled-release vaccines. This paper describes the preparation of PLG microparticles with an entrapped model antigen, ovalbumin (OVA), using a (water-in-oil)-in-water emulsion solvent evaporation technique. In a series of experiments, the effects of process parameters on particle size and OVA entrapment were investigated. It was found that smooth, spherical microparticles 1-2 microns in diameter containing up to 10% (w/w) OVA could be produced using a small volume of external aqueous phase containing a high concentration of emulsion stabilizer and a 1:5 antigen:polymer ratio. PAGE analysis, isoelectric focusing, and Western blotting of OVA released from the microparticles in vitro confirmed that the molecular weight and antigenicity of the protein remained largely unaltered by the entrapment procedure.

Use of polylactic acid for the preparation of microparticulate drug delivery systems

This report reviews the development of particulate drug delivery systems technology based on polylactic acid (PLA). The following methods used to produce PLA microparticles are considered: emulsion solvent evaporation, emulsion solvent extraction, interfacial phase deposition, spray coating, and the melting method. The different preparation processes and parameters used, the characteristics of microparticles obtained and those parameters affecting the characteristics of these systems, are discussed.

Sustained release of nerve growth factor from biodegradable polymer microspheres

Although grafted adrenal medullary tissue to the striatum has been used both experimentally and clinically in parkinsonism, there is a definite need to augment long-term survival. Infusion of nerve growth factor (NGF) or implantation of NGF-rich tissue into the area of the graft prolongs survival and induces differentiation into neural-like cells. To provide for prolonged, site-specific delivery of this growth factor to the grafted tissue in a convenient manner, we fabricated biodegradable polymer microspheres of poly(L-lactide)co-glycolide (70:30) containing NGF. Biologically active NGF was released from the microspheres, as assayed by neurite outgrowth in a dorsal root ganglion tissue culture system. Anti-NGF could block this outgrowth. An enzyme-linked immunosorbent assay detected NGF still being released in vitro for longer than 5 weeks. In vivo immunohistochemical studies showed release over a 4.5-week period. This technique should prove useful for incorporating NGF and other growth factors into polymers and delivering proteins and other macromolecules intracerebrally over a prolonged time period. These growth factor-containing polymer microspheres can be used in work aimed at prolonging graft survival, treating experimental Alzheimer's disease, and augmenting peripheral nerve regeneration.

Polymers for biodegradable medical devices. IX: Microencapsulation studies; effects of polymer composition and process parameters on poly-hydroxybutyrate-hydroxyvalerate microcapsule morphology

Reservoir-type microcapsules were prepared using a double emulsion solvent evaporation technique from a series of nine different poly-beta-hydroxybutyrate (PHB)-based polymers in which both molecular weight and hydroxyvalerate content were varied. Particles prepared from a low molecular weight (MW 43,000) homopolymer had a shrivelled appearance, but were not porous. When the molecular weight of the fabricating homopolymer was increased to 540,000, however, these features disappeared and non-wrinkled particles with microporous surfaces were observed. Microcapsules prepared from a high molecular weight copolymer of PHB with 10.8 per cent hydroxyvalerate (HV) had a similar appearance, but particles prepared from a high molecular weight 20.1 per cent HV copolymer had much smoother and less porous surfaces. Lowering the molecular weight of the copolymer had the effect of producing particles that were generally distorted in shape and had highly irregular, macroporous, surface morphologies. Increasing the double emulsion temperature to 40 degrees C at 2 min after phase combination typically generated smoother and/or less porous particles and improved the batch yield. The numerous factors potentially responsible for the development of morphological characteristics of the products are discussed in the light of these observations.