The aim of this present study is to investigate the possibility of obtaining prolonged, relatively constant effective levels of lamivudine from microspheres using Eudragit polymers. The polymers used are Eudragit RLPO & Eudragit RSPO, which give pH independent release from the formulations. Microspheres were prepared by solvent evaporation technique. Microspheres were discrete, spherical, and free flowing with smooth surfaces and completely covered with polymer coat. Drug release was controlled by diffusion from the microspheres that was slow and spreads over an extended period of time depending upon drug polymer ratio. Concentration 1:3 of both polymers was found to control the release with good dissolution up to 12 hours exhibiting 86 per cent drug entrapment efficiency. Drug release from microspheres was dependent up on the wall thickness and size of microspheres and as the proportion coat increased, the drug release rate was delayed.

Microspheres are spherical, free flowing with a wide range of sphericity, having smooth surface and completely covered with polymer coat. Microspheres improve effectiveness of drug therapy over conventional therapy. This improvement can take the form of increased therapeutic activity compared to intensity of side effects; reducing number of drug administration required for drug treatment or eliminating the need of specialized drug action (like repeated injections).

Micro encapsulation (MEC) has been the subject of massive research efforts since its inception around 1950's of all sustained release systems. The technique popularly can be attributed mainly to its wide variety of applications. This includes chemotherapeutic drug, immunosuppressant, anti-inflammatory, antibiotic, antagonist, steroids and hormones. The advantages of MEC include longer duration of action, control of content release, increased therapeutic efficiency, and protection of drug from biological environment, reduction of toxicity, bio compatibility, sterilizability, release stability, water solubility and targeting. MEC provides more efficient drug delivery because it increases the ability of drug to interact with the body. The active ingredient of a drug is encapsulated into particle that may be as small as one micron. Within a normal tablet, there may be millions of these microcapsules and each able to release the drug in the body. Compared to normal tablet, the microcapsules have much greater surface area, which increases the solubility and effectiveness. Perhaps the greater future of microsphere is the control provided by the choice of coating.

Lamivudine is an antiviral drug called as nucleoside reverse transcriptase inhibitor (or) NRTI. Lamivudine has been used for treatment of chronic hepatitis B at a lower dose than for treatment of HIV. It improves the sero-conversion of e-antigen positive hepatitis B.

Materials
Lamivudine was obtained as a gift sample from Hetero Drugs, Hyderabad, Magnesium stereate (Lab Chem.), Liquid paraffin (Paxmy, Chennai), n-hexane (Paxmy, Chennai), Eudrigit (Hetero Drugs, Hyderabad) acetone (Paxmy, Chennai). All ingredients were of pharmaceutical grade and were used as received.

Methods
The microspheres were prepared by solvent evaporation technique. Different amount of polymer (1, 2, 3 gm of Eudragit RSPO & Eudragit RLPO) was dissolved in 27 ml of acetone by using a magnetic stirrer. Powdered lamivudine and magnesium stereate were dispersed in the polymer solution. The resulting dispersion was then poured into a vessel of 100 ml containing the mixture of 270 ml liquid paraffin and 30 ml n-hexane while stirring. A cylindrical vessel and a mechanical stirrer with a blade is used for stirring. The stirring was continued for 1 hour until acetone evaporated completely. Drug: polymer ratio (1:1,1:2,1:3) was changed. After evaporation of acetone, the microspheres formed were collected by filtration in vacuum washed 4-5 times with 50 ml n-hexane each and dried at room temperature for 24 hours and preserved in air tight desiccators until further evaluation.

Drug loading capacity
Weighed 100 mg of microspheres and triturated with 0.1M HCl and transferred into 100 ml volumetric flask and made up to with 0.1M HCl. The solution was transferred to a beaker and stirred for 12 hours, then filtered and finally diluted with 0.1M HCl to 100 ml. The drug loading capacity was determined by UV at 271 nm.

Particle size distribution
The size distribution is carried out by optical microscopy and average of about 200 particles is carried out and the average particle size is determined.

Entrapment efficiency
Percentage of drug loaded in the microspheres (Qp) is divided by the quantity of drug added for loading (gms)(Qt) that is equal to per cent of entrapment efficiency (E).
E = Qp/Qt x100

Scanning electron microscope
The microspheres were observed under a scanning electron microscope (SEM) operating at 20 kV. The samples were maintained on a metal stub with double adhesive tape and coated with platinum/palladium alloy under vacuum.

Invitro drug release
Dissolution study was conducted in USP XXIII dissolution apparatus using paddle containing 900 ml of pH 1.2 and phosphate buffer pH 7.4 and the temperature was maintained at 370C. The paddle was rotated at 50 rpm, 5 ml of sample was taken periodically at 1,2,3,4,5,6,7 and 8 hours and estimated for Lamivudine spectrophotometrically at 271 nm using buffer as a blank.

Results and discussion
The drug content in various formulations found that there was not much difference in drug loading capacity. All formulations showed smooth surface and were spherical shaped with not much difference in average particle diameter. This was confirmed by determining the particle size by optical microscope and compared with SEM photograph for its smooth surface phenomena. It was observed that the concentration of the polymer increased against the drug.

Conclusion
The method was adopted to prepare various formulations of microspheres loaded with Lamivudine using two different formulations. It was concluded from the release pattern that polymer concentration increased as drug release rate decreased. The particle size falls within micron sizes and also smooth surface of the particles was confirmed. Microspheres with RSPO have been proved to be successful sustained release formulation without the addition of ethylcellulose. The mean particle size results showed that the particle size increases with increase in polymer concentration.

(The authors are with KVSR Siddhartha College of Pharmaceutical Sciences, Vijayawada, Andhra Pradesh).