Study On Acyclovir-loaded Bioadhesive Microspheres

Acyclovir [9-（2-hydroxyethoxymethyl） guanine] [ACV], a synthetic purine nucleoside analog derived from guanine, is one of the most effective and selective antiviral drugs. ACV shows an antiviral effect on Herpes simplex virus HSV-1 and HSV-2 virus through interfering with DNA synthesis and inhibiting viral replication. Unfortunately, its absolute oral bioavailability is considerably poor （about 15-30%） because of its low solubility and short half-life （about 2.5 h）. Therefore, Acyclovir must be taken in an oral dose of 200 mg five times daily, which could cause compliance problems.The purpose of this study was to design bioadhesive microspheres of acyclovir in order to improve its oral bioavailability. The study includes four main parts: formulation and preparation techniques; in vitro and in vivo properties; bioadhesive evaluation techniques; and industrial manufacture possibility.Acyclovir-loaded bioadhesive microspheres （Acv-ad-ms） with ethylcellulose as matrix and Carbopol 974P NF as bioadhesive polymer were prepared by emulsification- evaporation techniques. The methods of acyclovir determination and in vitro acyclovir release were established. And the formulation and conditions for preparation were optimized by single-factor investigation. The final optimal formula was as follows: 25% as the amount of drug, 1:3 as the proportion of Carbopol/Ethylcellulose, 1:7.5 as the proportion of disperses/continuous phase, 2.5% （w/v） of Span 80 as emulsifier. The yield of microspheres was above 85% and the size of the microspheres ranges between 200-800μm.The physico-chemical characteristics and in vitro/in vivo properties of Acv-ad-ms were investigated. Acv-ad-ms were well-rounded, smooth spheres with uniform size distribution. The drug loading rates of the microspheres were among 16-17% and the drug release profile in pH 3.6 PBS was as follows: 15%<Q_0.5h<20%, 45%<Q_2h<55%, Q_6h>80%, which suggested a combination of diffusion and erosion mechanisms. The results of in vitro and in vivo bioadheisve evaluation showed Acv-ad-ms had a good bioadhesion property that they could retaine in the gastrointestinal tract for an extended time. In Pharmacokinetics study, relatively steady plasma drug concentrations were observed within 8 h after oral administration of Acv-ad-ms to rats. The AUC and mean residence time （MRT） of ACV-ad-ms were significantly higher than that of ACV suspension, which indicated that the bioavailability of acyclovir was greatly improved due to the prolonged retention of Acv-ad-ms in gastrointestinal tract.The techniques of bioadhesive evaluation were studied. The results of interaction between mucin and bioadhesive microspheres showed that the interaction equilibrium was reached at about 90 min, and the interaction was stronger in acidic medium than in neutral medium, however, there were no significant difference with different Carbopol content of microspheres. A novel natural but non-animal substrate, the eggshell membrane, was employed substituting the animal stomach mucosa in the in vitro mucoadhsion evaluation of microspheres. The good correlationship between the in vitro data from the eggshell membrane and the in vivo bioadhesion studies in rats demonstrated the potential of the eggshell membrane for bioadhesive evaluation.The feasibilities of preparing Acyclovir-loaded bioadhesive microspheres using several kinds of industrial equipments were also investigated. A promising two-step technique was selected using extrusion-spheronization method for immediate release acyclovir microsphere preparation followed by coating with ethylcellulose and Carbopol to form an outer layer for sustained drug release and bioadhesiveness.