PLGA Microcapsules Research Articles

Optimum formulation for sustained-release insulin

Our aim was to prepare an optimum formulation for a sustained-release preparation of insulin using biodegradable polymer composed of co-poly( d, l-lactic/glycolic) acids (PLGA)(L/G ratio: 50/50). Various kinds of PLGA microcapsules containing 3% insulin were administered subcutaneously (250 U/kg) as a single dose to rats with streptozotocin-induced diabetes, and plasma insulin levels were monitored. The following results were obtained. (1) Glycerin and water were suitable additives to prepare a reproducible injectable formulation. (2) Addition of zinc compounds was essential to diminish rapid insulin release and six-fold molar excess of ZnO to insulin was desirable. (3) The size of insulin particles showed the following order: human insulin>lyophilized human insulin>Zn-free human insulin. Zn-free insulin was similar to lyophilized insulin with respect to control of rapid release, so a smaller particle size was essential. (4) The size of the microcapsules also affected the release of insulin. With larger microcapsules (∼30 μm), there was gradual release and a significant second phase of insulin release, while smaller microcapsules did not allow sustained release. Some variation in microcapsule size contributed to more constant and sustained release. (5) Based on the insulin release profile in vivo, a suitable molecular weight for PLGA was around 6000. The biological activity of insulin extracted from the formulation was similar to that of normal insulin. These experiments allowed us to prepare a desirable sustained-release insulin formulation.

Ultrasound degradation of novel polymer contrast agents.

This report describes an investigation into factors affecting the degradation of novel poly(lactic-co-glycolic acid) (PLGA) contrast agents. Contrast agents fabricated by two different methods and varying in acoustic properties were compared. The effect of ultrasound frequency (5 and 10 MHz) on degradation of the microcapsules was also studied. High-performance liquid chromatography was used to quantify the production of lactic and glycolic acid to monitor agent degradation. The degradation pattern from the microcapsules was found to be closely related to capsule morphology; the more acoustically efficient capsules (maximum enhancement of 25 dB at 5 MHz with 0.004 mg/mL) degraded at a faster rate than those with lower acoustical efficiency (maximum enhancement of 25 dB at 5 MHz only achieved with 0.6 mg/mL). The capsules also degraded fastest when insonated at the frequency at which they gave highest backscatter. In addition, despite the use of a 50:50 PLGA copolymer, more glycolic than lactic acid was released at early time points, which reflects the greater hydrophilicity of the glycolic acid residues, and greater degradation rate of glycolic acid repeat units. The results from this study provided unique insight into the degradation behavior of hollow PLGA microcapsules, and their potential in ultrasound diagnosis and therapy.

Insulin-loaded biodegradable PLGA microcapsules: initial burst release controlled by hydrophilic additives

We investigated the controlled release of human insulin at an initial stage from poly( dl-lactic-co-glycolic acid) (PLGA, M w 6600) spherical matrices. PLGA microcapsules were prepared by the novel solvent evaporation multiple emulsion process. When the crystalline insulin was dispersed in dichloromethane as solid-in-oil (S/O) dispersion, it was found that most of insulin molecules were inlaid on the surface of PLGA microcapsules. Consequently, insulin-loaded PLGA microcapsules exhibited marked rapid release of insulin within several hours in both in vivo and in vitro experiments. On the other hand, the addition of glycerol or water in the primary dichloromethane dispersion results in drastically suppressed initial release. It was found by SEM observation that water- or glycerol-in-oil (W/O or G/O) type mini-emulsion droplets with a mean diameter of 300–500 nm were formed in this primary solution. This phenomenon can be theoretically presumed to occur because insulin and PLGA molecules, having amphiphilic properties, converge on the interface between the hydrophilic additive and dichloromethane. Hence, insulin molecules heterogeneously located in the inside of PLGA microcapsules, not on the surface, would be gradually released with PLGA hydrolytic decomposition. As an additional effect of glycerol, the initial burst was further suppressed due to the decrease of the glass transition temperature of PLGA from 42.5 to 36.7 °C. Since the annealing of PLGA molecules took place at around 37 °C, the porous structure of microspheres immediately disappeared after immersion in PBS or subcutaneous administration. The insulin diffusion through the water-filled pores would be effectively prevented. The strict controlled initial release of insulin from the PLGA microsphere suggested the possibility of utilization in insulin therapy for type I diabetic patients who need construction of a basal insulin profile.

A novel sustained-release formulation of insulin with dramatic reduction in initial rapid release

To ensure a strictly controlled release of insulin, a preparation method for insulin-loaded microcapsules was designed. Microcapsules were prepared with an injectable, biodegradable polymer composed of co-poly( d, l-lactic/glycolic) acids (PLGA) (mean molecular weight 6600, LA/GA ratio 50:50). Morphological examination using scanning electron microphotography demonstrated spherical particles with a main diameter of 15–30 μm. When 3% insulin-loaded PLGA microcapsules were administered subcutaneously as a single dose (250 U/kg) to streptozotocin-induced hyperglycemic rats, plasma insulin levels increased and were sustained at levels showing hypoglycemic effects. When glycerin, ethanol, or distilled water was used throughout the preparation procedure, the resultant microcapsules dramatically reduced the initial burst. The formulation in which glycerin was added to an oil phase containing PLGA, insulin, and ZnO increased plasma insulin levels to 86.7, 108.4, and 84.9 μU/ml at 1, 2, and 6 h, respectively. The levels remained at 36.2–140.7 μU/ml from day 1 to day 9. The AUC 0–24 h/AUC 0–336 h ratio was calculated to be 9.7%. The formulation prepared without additives gave such a rapid insulin release that animals receiving it became transiently hypoglycemic.

Optimization of spray drying by factorial design for production of hollow microspheres for ultrasound imaging.

A process for producing hollow microcapsules as ultrasound contrast agents was optimized using a 2(3) factorial experimental design method with two replicates. Spray drying, a conveniently scalable encapsulation technique, was used to encapsulate a volatile core material, such as ammonium carbonate, using biodegradable 50-50 poly(D,L-lactide-co-glycolide). Various effects due to changes in processing variables and their interactions were studied using the factorial grid. The high- and low-incremented variables examined included the temperature difference between the inlet and outlet of the spray dryer (5 degrees and 15 degrees C), air atomization pressure (80 and 100 psi), and polymer concentration in solvent (0.005 and 0.025 g/mL). Responses analyzed for computing the main effects and interactions were microcapsule morphology, yield, mean size, and zeta potential. Experimental results showed that polymer concentration was most important for determining microcapsule morphology. The temperature difference for drying prominently affected mean size, and atomization pressure was the main effect for microcapsule yield. Interactions among variables were not present in this case. The best conditions for producing PLGA microcapsules was a temperature difference of 5 degrees C, an initial polymer concentration of 0.005 g/mL, and an atomization pressure of 80 psi. The microcapsule zeta potentials were unaffected by spray-drying conditions.

New strategies for the microencapsulation of tetanus vaccine

The progress toward the development of a single dose tetanus vaccine has been limited by the poor stability of the protein antigen, tetanus toxoid (TT), during its encapsulation in, and release from, biodegradable polymer microspheres. To investigatc alternative microencapsulation approaches that may improve the stability of TT under these conditions, a two-step microencapsulation method has been devised to form microcapsules which consist of (a) forming micro-cores of TT in a hydrophilic support matrix by spray-congealing, followed by (b) coating the microcores with poly(lactide-co-glycolide) (PLGA) by an oil-in-oil solvent extraction method. Several protein stabilizers including gelatin (with or without poloxamer 188), dextran, sodium glutamate, and polyethylene glycol were examined as potential core-materials. Among them, gelatin was superior in its ability to impart stability to TT against heat and moisture-induced inactivation. Microcores of this latter stabilizer and TT were encapsulated in PLGA using the foregoing technique, which exposed the dry antigen to minimal water in order to prevent its irreversible inactivation during exposure to the organic solvent. The microencapsulation method resulted in minimal loss of antigenically active TT (∼10–20%). Microscopic analysis of the micro-capsules following preparation showed the microcores to be fully encapsulated. However, microcapsules containing TT and gelatin released the active antigen nearly completely within one day. Fluorescence confocal microscopy revealed that the swelling of the hydrophilic core-material was responsible for the burst-release behaviour. Manipulation of the polymer coating could not slow down this 'explosion' of the microcapsules. TT-containing PLGA microcapsules have been prepared using a novel microencapsulation method, which retains an extremely high fraction of antigenically active TT. Hence, these mechanistic approaches may be useful in the development of effective single-dose vaccines.

Preparation of biodegradable microcapsules containing zidovudine (AZT) using solvent evaporation technique.

Sustained release biodegradable microcapsules of AZT were prepared using different concentrations of copolymer of poly(lactic/glycolic) acid (PLGA 50:50 and PLGA 90:10). Solid microcapsules were collected following the complete evaporation of the solvent. The yield of microcapsules was increased two fold with a two-fold increase of the polymer concentration. The efficiency of encapsulation of AZT was also increased with the increase of the polymer concentration. These microcapsules were characterized using scanning electron microscopy. The dissolution of AZT from the microcapsules of PLGA (50: 50) was higher than the microcapsules of PLGA (90:10); the PLGA (50:50) microcapsules containing 1:10 drug/polymer ratio showed higher dissolution than the microcapsules containing 1:20 drug/polymer ratio. The PLGA (90:10) microcapsules containing 1:6 drug/polymer ratio showed higher dissolution than the microcapsules containing 1:10 drug/polymer ratio. In conclusion, the dissolution of AZT was dependent on the type of the copolymer used and the relative concentrations of the drug and the copolymer.

A novel method of preparing PLGA microcapsules utilizing methylethyl ketone.

To substitute dichloromethane with a safer solvent, a solvent extraction process using methylethyl ketone (MEK) was developed to prepare poly(d,l-lactide-co-glycolide) microcapsules. The MEK dispersed phase containing PLGA and progesterone was emulsified in the MEK-saturated aqueous phase (W1) to make a transient oil-in-water (O/W1) emulsion. It was then transferred to a sufficient amount of water (W2) so that MEK residing in polymeric droplets could be extracted effectively into the continuous phase. This solvent extraction process provided the encapsulation efficiency for progesterone ranging from 77 to 60%. The amount of MEK predissolved in W1, as well as the degree of progesterone payload, influenced the encapsulation efficiency. The leaching profile of MEK analyzed by GC substantiated that, upon dispersion of O/W1, to W2, MEK quickly diffused into the continuous phase. Such a rapid diffusion of MEK from and the ingression of water into polymeric droplets produced hollow microcapsules, as evidenced by their SEM micrographs. When solvent extraction/evaporation techniques are employed for preparing PLGA microcapsules, water-immiscibility of a dispersed phase is not an absolute prerequisite to the successful microencapsulation. Adjustment of an initial extraction rate of MEK and formation of a primary transient O/W1 emulsion are found to be very crucial not only for the success of microencapsulation but also for the determination of microcapsule morphology.

In vivo release profiles of leuprolide acetate from microcapsules prepared with polylactic acids or copoly(lactic/glycolic) acids and in vivo degradation of these polymers.

To screen a suitable polymer for preparing a controlled-release dosage form effective for one month, square plates of polyactic acid (PLA) with average molecular weights of 6000 to 50000 and copoly(lactic/glycolic) acid (PLGA) with average molecular weights of 6000 to 13500 and a copolymer ratio of 90/10 to 55/45 were implanted in rats subcutaneously, and the weight losses were determined over 100 d. Typically the pattern of weight loss was biphasic with a lag time followed by a period when the weight fell exponentially. The lag times and the half lives of the degradation period increased with increase in the molecular weight or with decrease in glycolide content. Release of leuprolide acetate from PLA microcapsules at the injection site in rats (in vivo release) tended to be comparable to the bioerosion rate of the polymer used as the wall substance, and the in vivo release of the drug from PLGA microcapsules tended to be comparable to the biodegradation rate of the polymer. The in vivo release profile of the drug from microcapsules prepared with PLGA of average molecular weight 14000 and copolymer ratio 75/25 was ideal for one month's controlled release. The in vivo release profile from microcapsules injected subsutaneously in rats was comparable to the in vitro release.