Preparation and characterization of lamivudine microcapsules using various cellulose polymers

Purpose: The objective of the present study was to prepare and evaluate microcapsules for the controlled release of lamivudine using various cellulose polymers Methods: The microcapsules were prepared by the solvent evaporation method. The prepared microcapsules were characterized for the percent drug content, entrapment efficiency, FTIR, DSC, scanning electron microscopy (SEM) and in vitro dissolution studies. Accelerated stability studies were also carried out. Results: The microcapsules were spherical and free flowing. The entrapment efficiency was 76-86%. The release of drug from the microcapsules extended up to 8 to 12 hours. FTIR and DSC thermograms showed the stable character of lamivudine in the microcapsules. SEM revealed that the microcapsules were porous in nature. The release kinetics study revealed that the prepared microcapsules were best fitted to the zero order for F-2, F-4 and F-5 formulations and Higuchi model, for F-1 and F-3 microcepsules Conclusion: The release kinetics data and characteristion studies indicate that drug release from microcapsules was diffusion – controlled and that the micrapsules were stable..


INTRODUCTION
Acquired Immunodeficiency Syndrome (AIDS), which is now a plague in several countries, was first identified in California in 1981.It is a disease in which the body's immune system breaks down and is unable to fight off infections caused by human immunodeficiency virus (HIV).HIV infects human cells and uses the energy and nutrients provided by those cells to grow and reproduce and it is often necessary to take several medicines for prolonged periods.This problem is compounded especially for drugs having a shorter biological half life which have to be administered frequently.It is crucial for the success of AIDS therapy to maintain systemic drug levels consistently above its target antiretroviral concentration throughout the course of the treatment 1, 2 .This can be achieved prolonging release.
Lamivudine is a synthetic nucleoside analog that is being increasingly used as the core of an antiretroviral regimen for the treatment of HIV infection. 3,4In vivo, nucleoside analogs are phosphorylated intracellularly by endogenous kinases to putatively active 5′-triphosphate (3TC-TP) derivatives that prevent HIV replication by competitively inhibiting viral reverse transcriptase and terminating proviral DNA chain extension. 5-7Lamivudine is rapidly absorbed after oral administration with an absolute bioavailability of 86% ± 16%, peak serum concentration of lamivudine (C max ) of 1.5 ± 0.5 mcg/mL and mean elimination half-life (t ½ ) of 5 to 7 hours, thus necessitating frequent administration to maintain constant therapeutic drug levels. 8Therefore, the objective of the present work is to provide a long acting pharmaceutical composition containing lamivudine in a modified release matrix formulation, to maintain the blood levels of the active ingredient for a prolonged period of time.
Only a limited study lamivudine extended release formulations has been carried out using ethyl cellulose 9 , Matrix tablets of anti-HIV drug didanosine 10 have also been formulated.

Materials
Lamivudine was obtained as a gift from Alkem laboratories Ltd (Mumbai, India).Cellulose acetate phthalate (CAP) was obtained from GM Chemicals, India, cellulose acetate butyrate (CAB) was obtained from Eastman chemical company, USA, ethyl cellulose (EC) was obtained from The DOW chemical company, USA and hydroxy propyl methyl cellulose acetate phthalate (HPMCP) was obtained from SHIN-Etsu, Japan.All other chemicals and reagents used in the study were of analytical grade.

Particle size distribution
Particle size analysis 12 of the microcapsules was done by sieving method using Indian Standard Sieves # 16, #20, #30, #40, #60 and #80.The results of particle size distribution are given in the Table1.

Fourier Transforms Infrared Spectroscopy (FT-IR) 13 and Differential scanning calorimeter (DSC)
The FT-IR spectra acquired were taken from dried samples.An FT-IR (Thermo Nicolet 670) spectrometer was used for the analysis in the frequency range between 4000 and 400 cm-1, and 4 cm-1 resolution.The results are the means of 16 determinations.A quantity equivalent to 2 mg of pure drug and drug loaded microcapsules were used.Differential scanning calorimetry (DSC) study of drug loaded microcapsules was performed using a Diamond DSC (Mettler Star SW 8.10) to determine the drug excepient compatibility study.

Scanning electron microscopy (SEM)
Morphological characterization of the microcapsules was carried out scanning electron microscopy (JEOL JSM -5200).The samples were coated to 200A° thickness with goldpalladium using prior to microscopy.

In Vitro Drug Release Studies
In vitro dissolution studies were performed using USP type I dissolution apparatus (LABINDIA, DISSO-2000, Mumbai, India) at 75 rpm.The micro capsules were weighed and filled in the empty capsule shells and placed in the basket.The dissolution medium (900ml) consisted of 0.1M hydrochloric acid for the first 2 hours and then changed to phosphate buffer pH 7.4 from 3rd to 12th hour, Temperature was maintained at 37°C ± 5°C.An aliquot (5 mL) was withdrawn at specific time intervals and rephenished with an equivalent volume of dissolution fluid.Drug content was determined by UV -visible spectrophotometer (Schimadzu, UV-1700 E 23) at 271 nm.The release studies were conducted in triplicate and the results are showed in Fig1.

Determination of stability of the microcapsules
The microcapsules prepared in the present study were filled in the hard gelatin capsules ands stored in HDPE containers at 40°C/75% RH for 3 months as per ICH guidelines.The samples were then characterized for % drug content.The results are summarized in the Table 2.

RESULTS
Maximum release of lamivudine from the various formulations was achieved in 12 hrs or longer, Figure1.The release mechanism of the lamivudine formulations was determined by comparing their respective correlation coefficients given in Table 1.Drug release from formulations F-1 and F-3 followed the Higuchi model but for F-2, F-4 and F-5 formulations, the release was best fitted to the zero order model.FTIR studies indicate four bands present in the lamivudine spectrum, namely; N-H, O-H, C=O, C=N linkages respectively.The same bands were also found in the spectra of the formulations, showing that no drug-polymer interaction occurred (see Figure2).DSC thermogram of pure lamivudine showed a sharp endothermic peak at 180°C.The thermograms of the formulations F-1 to F-5 also showed a similar the same endothermic peak at 180 0 C.This further confirms that there was no drug polymer interaction.Scanning electron microscopy (SEM) results (Figure4) show that the microcapsules were spherical and that the microcapsules prepared with cellulose acetate phthalate (CAP) had a formed smooth surface.The results of all the formulations were found good.High entrapment efficiency was observed in the microcapsules prepared with ethyl cellulose and combination of cellulose acetate phthalate and cellulose acetate butyrate.The results of accelerated stability study on the microcapsules (shown in Table 1) revealed a good correlation between the original and the aged samples.

DISCUSSION
The formulations either followed zero order release or the Higuchi release model.Thus drug release was diffusion controlled.The release mainly deepened on the type of polymer and viscosity.
The results indicate that F-5, formulations showed the slowest release rate while FTIR indicated that there was no drugpolymer interaction.This was further confirmed by the DSC results.The results of accelerated stability study showed the stable nature of the drug.A good entrapment efficiency was observed.SEM demonstrated the spherical nature of the microcapsules and the presence of drug particles on their surface.

CONCLUSION
The lamivudine microcapsules prolonged drug release for 12 hours or longer.Would be capable of reducing the frequency of administration and the dose-dependent side effects associated with the repeated administration of conventional lamivudine tablets.No drug polymer interaction was found and lamivudine remained stable over a long period of time.
Indian Institute of Chemical Technology, Hyderabad, India for help in performing the characterization studies.

Table 1 :
Physical characteristics of microcapsules