PLA-Starch Microparticles Containing Clays Focusing Controlled Release of Rifampicin

Polymer nanocomposites have been successfully used as excipients in pharmaceutical technology because of the convenient features that make them suitable for many applications, especially in controlled drug delivery therapies. The objective of this work was to prepare and characterize PLA and starch microparticles containing Viscogel B8 organophilic clay as a matrix for controlled release of rifampicin, one of the most spread drugs for tuberculosis treatment. The systems obtained by the spray drying technique were charac-terized by DRX, FTIR, SEM and low-field NMR. In addition, dissolution tests were performed on simulated gastric fluid. The micrographs indicate that the presence of the drug caused a small change in the shape and size of the particles. In the presence of the drug, the particles were spherical and presented wider size distribution. The XRD assays didn’t show crystalline peaks of rifampicin in the microparticles, which suggests that rifampicin exists in an amorphous solid solution inside the starch-PLA particles. The mathematical model of Baker-Lonsdale was chosen for the treatment of the dissolution data, describing the controlled release of the drug from the matrix by a spherical diffusion process. The results suggest that the proposed release system may be used for the delivery of orally administered drugs, such as rifampicin. treatment, reducing the required doses and systemic side effects.


Introduction
Due to its various applications and functionality, especially in therapies for con- Starch, a polysaccharide of vegetable origin, is one of the most frequently used excipients in the manufacture of solid dosage forms, and it may be formulated as a filler and disintegrating agent or a coating agent. The starch is of great interest for the controlled release formulation due to its water swelling and gel barrier formation [4] [5] [6] [7].
Poly (lactic acid) (PLA) belongs to a type of polymer having hydrolysable groups in its chains, which are susceptible to biodegradation. It is a material of great interest in science as a biodegradable and bioabsorbable polymer with extensive use in biomedicine [8] [9] [10].
Starch and PLA nanocomposites with different nanoparticles are promising materials in the area of pharmaceutical technology, acting as a matrix in the modified drug release. The modified drug release has advantages such as: reducing the frequency of administration, with consequent improvement in patient adherence to therapy, reducing fluctuations of the drug in blood levels, reducing the adverse effects and reduction of treatment price [11] Current treatment of tuberculosis involves oral administration and high long-term doses of various drugs. This treatment can lead to serious side effects such as liver damage. Moreover, it has a low patient acceptance [12].
Rifampicin (RIF) is the first-line drug for use in tuberculosis therapy and is included in the list of treatments recommended for the treatment of latent Mycobacterium tuberculosis infection in adults. Mainly due to the low solubility, rifampicin presents problems as its bioavailability [13].
The objective of this study was to develop and characterize a polymeric nanocomposite for controlled release of rifampicin to improve adherence to tuberculosis treatment, reducing the required doses and systemic side effects.

Drying Conditions
The process yield in percentages was calculated for each sample. The operating conditions maintained during the assays are summarized in Table 1.

Scanning Electron Microscopy
The SEM analysis was performed in order to identify the morphology (surface and shape) of particles obtained by spray drying using SEM Jeol Microscope JSM-5610 LV. The photomicrographs were obtained using a 20 Kv potential and zoom from 1000× to 8000×.

Preparation of Tablets
Tablets were prepared using a 300 psi manual press-molding compressor. Tablets with mixtures of starch and PLA have been prepared with Viscogel B8 containing 10 and 100 mg of rifampicin.

Dissolution in Simulated Gastric Fluid
The simulated gastric fluid (SGF) was prepared according to the specifications of U.S. Pharmacopeia 30 (USP, 2007). NaCl (2.0 g) was solubilized in a sufficient amount of distilled water. 7.0 ml of HCl were added to the NaCl solution and the volume was adjusted to 1.0 liter with distilled water. The pH of the solution was monitored during the addition of calibrated potentiometer acid and adjusted to 1.2 ± 0.1. For the test, simulated gastric fluid in each vat was added to one tablet; 10 ml aliquots were taken every 1 h for each of the vats, replenishing the volume of gastric medium removed at a temperature of 37˚C. Assays were performed in a total time of 24 h [14].

Evaluation of Release Kinetics
The release kinetics of a formulation is an important parameter and should be assessed during the development phase, because from this analysis it is possible to evaluate the influence of certain parameters, such as crystallinity and solubility of active [15]. The data obtained from the release profile were placed on mathematical models according to different equations that describe drug release kinetics: Zero-order, First order, Higuchi, Hixon-Crowell, Baker-Lonsdale.

X-Ray Diffraction
X-ray analyses were carried out in a Rigaku D/Max 2400 diffractometer, with nickel-filtered CuKα radiation of wavelength 1.54 Å, at room temperature. The 2θ scanning range was varied from 2˚ to 30˚, with 0.02˚ steps, operated at 40 KV and 30 mA.

FTIR
The infrared spectra were recorded with a Varian 3100 FTIR spectrometer at room temperature. A zinc selenide (ZnSe) internal reflection element (IRE) with a fixed incidence angle of 45˚ was used for attenuated total reflection (ATR) measurements.

Results
After spray drying, the yields were calculated and the particles were characterized by scanning electron microscopy. Table 2 shows the yield of the material obtained after drying.
The main advantage of spray drying is that small droplets provide a high surface area which promotes mass and heat transfer, facilitating rapid evaporation. The drying time of a droplet is only a fraction of a second and the total time inside the dryer is only a few seconds. The resulting powder has a uniform particle size, which can control production costs; spray drying is a process that provides a post-dry and free-flowing, with spherical particles that are especially appropriate for preparing tablets due to their excellent flow properties and compression.
However, there are drawbacks to this technique, such as the size of the equipment and low thermal efficiency, since the air exiting the dryer should be hot enough to prevent moisture from condensing; similarly, large volumes of hot air, pass through the chamber without contact with the particles and thus, this hot air does not contribute directly to the drying process [16].
Another disadvantage that should be mentioned is the loss of material, due to the adhesion in the drying tower and the increased capillary adhesion. Nevertheless, the disadvantages can be minimized by adding a mannitol solution or by adding silica particles before atomization of the material or by proper adjustment of the drying parameters for the type of material studied [17] [18]. However, according to the literature, it can be seen that the yields are in agreement with results obtained by other authors.   It is well known that rifampicin has variable solubility, mainly due to its particle size, purity level of crystals, electrostatic interactions and pH of the medium. At pH 6.8 the amorphous form has higher intrinsic dissolution of the isoforms I and II, at pH 2.0 isoform II contains less than the intrinsic dissolution and this amorphous form is slightly smaller than isoform I [20].
The reduction of particle size leads to an increase in the specific surface of the powder. The dissolution and absorption of the drug, content uniformity, and the stability of the pharmaceutical form are dependent. In many cases, it is necessary to reduce the particle size of both the drug and the adjuvants, trying to obtain the desired physical and chemical characteristics.
Poorly water-soluble drugs, whose absorption stage is limited by the efficiency of the dissolution process, will present better bioavailability when administered as finely divided particles, because of their improved contact area. This permits to overcome the issue of low solubility, increasing thus the drug release efficiency of poorly water-soluble drugs.    In general, it was observed that the particles agglomerated and the presence of clays did not cause a significant difference in the physical appearance of the particles when observed at higher magnification.
The spray dried products are most often uniform. The particles have a characteristic form of hollow spheres, sometimes with a small orifice that results from the drying process [21].

X-ray diffraction
Regarding the mixture, it was observed the appearance of the characteristic peak of PLA at approximately 17˚ and the appearance of an amorphous halo related to the structure of the starch. teristic peaks can be identified in Figure 5.
No crystalline rifampicin was detected according to Figure 6. This suggests that rifampicin exists as amorphous solid solution in the starch-PLA particles.

FTIR
Since the starch-PLA particles were obtained by a spray-drying method using chloroform and water as solvents, solvent elimination was accompanied through the FTIR technique by monitoring chloroform band [22]. The peaks at about 1756 cm −1 and 1180 cm −1 , belonging to the CO stretching and C-O-C stretching of PLA, were visible in all the IR spectra [9] [15]. Drying process was stopped when the solvent peak was not detected in the FTIR spectrum (Figure 7).

Dissolution tests
The dissolution of a drug is the phenomenon by which a solid drug is released in its pharmaceutical form and then solubilized in the medium, which consequently causes its absorption.     Because of this fact, and following the developments with regard to formulations, it is necessary to develop dissolution studies, which reproduce in vivo the results obtained in vitro.

Conclusion
The present study obtained promising results in relation to the formulation obtained; the micrographs showed the crystallinity and irregularity of rifampicin, which was not observed in the starch and PLA particles, suggesting a possible encapsulation of the drug. The XRD diffractograms suggest that rifampicin exists as an amorphous solid solution in the starch-PLA particles. To explain the dissolution test results, the model chosen was a Zero-order for the system where the physical mixture of the microparticles without rifampicin and the Baker and Lonsdale model for the systems with 10 and 100 mg of rifampicin, respectively.
The results suggest that the proposed release system (tablets containing the drug and incorporated into microspheres) may be used for the delivery of orally administered drugs, such as rifampicin. However, the formulation proposed in this work needs to be optimized to obtain a dosage suitable for the oral administration of rifampicin.