ЖУРНАЛ ФИЗИЧЕСКОЙ ХИМИИ, 2009, том 83, № 9, с. 1633-1636
^^^^^^^^^^^^^^^ CHEMICAL KINETICS ^^^^^^^^^^^^^
AND CATALYSIS
УДК 541.124
INFLUENCE OF DIFFERENT DEGRADATION MEDIUM ON RELEASE OF ASCORBIC ACID FROM Poly (D,L-LACTIDE-CO-GLYCOLIDE)NANO
AND MICROSPHERES © 2009 M. Stevanovic*, D. Uskokovic*
*Institute of Technical Sciences of the Serbian Academy of Sciences and Arts, Belgrade; Serbia E-mail:Magdalena@int.sanu.ac.rs; magdalena.stevanovic@gmail.com
Abstract — The major goals of the present study were to examine the effects of the type of release medium on the resulting drug release kinetics and to get further insight into the underlying drug release mechanisms. Spherical micro and nanoparticles were prepared by a physicochemical solvent/non-solvent method with polyvinyl pyrrolidone as a surfactant and characterized with ultraviolet spectroscopy and scanning electron microscopy before and upon exposure to various release media.
INTRODUCTION
Controlled drug delivery occurs when a polymer, whether natural or synthetic, is judiciously combined with a drug or other active agent in such a way that the active agent is released from the material in a predesigned manner. The release of the active agent may be constant over a long period, it may be cyclic over a long period, or it may be triggered by the environment or other external events. In any case, the purpose behind controlling the drug delivery is to achieve more effective therapies while eliminating the potential for both under- and overdosing [1—5]. Polymers have been used in the pharmaceutical and biomedical fields as delivery vehicles, (such as, microspheres and nanoparticles) and as scaffold materials as a consequence of their biodegradability and relative biocompatibility [6]. Biodegradable nano/microparticles of poly(D,L-lactide-co-glycolide) (PLGA) and PLGA-based polymers are widely explored as carriers for controlled delivery of therapeutics such as proteins, peptides, vaccines, genes, antigens, growth factors, vitamins, etc [7-10].
Vitamins are crucial for normal physiologic functioning of the organism, and vitamin deficiencies are relatively often associated with modern life style including inappropriate dietary habits, increased vitamin requirements or different diseases. Ascorbic acid (vitamin C) is known to be very unstable and easily destroyed by temperature, pH, oxygen, etc. [11-14]. In order to overcome some of these shortcomings of ascorbic acid, the microencapsulation technique may be suitable for ascorbic acid. System for the controlled delivery PLGA/vitamin can bring to the more balanced and efficient concentration of the vitamin throughout the extended period of time.
Polymer degradation plays a key role in medicament release from sustained release polyester systems, therefore in order to elucidate the mechanism govern-
ing release, it appears essential to analyze the in vitro degradation behavior of these devices [14-18]. Release from PLGA microspheres occurs via diffusion, polymer erosion or a combination thereof [17, 19]. PLGA erosion occurs via hydrolysis of the ester bonds in the polymer backbone. It is widely established that PLGA degradation starts with water uptake, and that hydrolysis leads to the production of acidic oligomers [17].
The selection of an appropriate release medium for in vitro tests simulating in vivo conditions can be very important for getting rapid feedback on the release characteristics of a specific batch.
The aim of the present study was to examine the effects of the type of degradation medium on the resulting drug release kinetics and to get further insight into the underlying drug release mechanisms.
EXPERIMENTAL
Materials
In the experiment we used poly(DL-lactide-co-gly-colide) (PLGA) which was obtained from Durect, Lac-tel and had a lactide-to-glycolide ratio of 50:50. Molecular weight of the polymer was 40000-50000 g/mol. Molecular weight of ascorbic acid is 176.13 g/mol (Microvit™, Adisseo). Polyvinyl pyrrolidone (povi-done, PVP) was obtained from Merck Chemicals Ltd (k-25, Merck, Germany). All other chemicals and solvents were of reagent grade.
Methods
Poly(DL-lactide-co-glycolide) micro and nano-particles without and with encapsulated ascorbic acid were prepared using a physicochemical solvent/nonsolvent method as we reported in our previously work [9, 10]. Ascorbic acid was encapsulated into the poly-
1634 STEVANOVIC,
Fig. 1. Release of the ascorbic acid (a) over the period of time of the degradation in case of (a) physiological solution and (b) phosphate buffered saline as a degradation medium (relative review) from PLGA/ascorbic acid 85/15% nanoparticles.
mer matrix by means of homogenization of aqueous and organic phases. Polyvinyl pyrrolidone (povidone, PVP) was used as a stabilizer of the particles. Ascorbic acid-loaded PLGA nanoparticles were separated from the suspension by centrifugation, decantation followed by drying.
The degradation of the PLGA/ascorbic acid nano-particles and release rate of the ascorbic acid were
USKOKOVIC
studied for more than fifty days in a physiological solution (0.9% sodium chloride in water) or in phosphate buffered saline (PBS, one tablet dissolved in 200 ml of deionized water yields 0.137 M sodium chloride, 0.01 M phosphate buffer and 0.0027 M potassium chloride) with sodium azide (0.1M solution natriumazid NaN3) as a degradation medium. In the PBS was added 110 ^l sodium azide because sodium azide acts as a bacteriostatic.
The UV measurements were performed on Perkin-Elmer Lambda 35 UV-VIS Spectrophotometer in the frequency interval of 200—400 nm. The pH of the physiological solution or PBS has been measured using pH indicator strips obtained from Merck (KGaA, Germany) at various time periods to follow the acidity of the degrading medium with time. The morphology of PLGA/ascorbic acid 85/15% nanoparticles has been examined, after two, 24 and 39 days of the degradation in physiological solution and after 17 and 28 days of the degradation in phosphate buffered saline, by scanning electron microscope JEOL JSM-649OLV.
RESULTS AND DISCUSSION
The degradation of the PLGA and release of the ascorbic acid have been tracked based on the intensity of the absorbance maximum which is in the correlation with the concentration of the PLGA and ascorbic acid within the solution. PLGA degrades via backbone hydrolysis (bulk erosion) and the final degradation products are the monomers, lactic acid and glycolic acid. PLGA completely degrades within period of 8 weeks in physiological solution (Fig. 1a) as a degradation medium as well as in phosphate buffered saline (Fig. 1b), fully releasing all the encapsulated ascorbic acid.
Morphological changes of degraded PLGA/ascorbic acid 85/15% nanoparticles including particle size, shape and surface were monitored throughout the 39 days incubation in physiological solution and during the 28 days in PBS. At a beginning of the degradation, PLGA nanoparticles displayed a relatively smooth and nonporous surface. From the Figs. 2 and 3 we can see that during the degradation the particles were first agglomerated, then forming the film. The particles are in a very close contact during the degradation process which brings to higher agglomeration of the particles and creation of the porous film. By the end of the experiment the particles have fully degraded and there were no more traces of them in the solution.
Polymer degradation and drug release kinetics of ascorbic acid from PLGA microspheres were investigated under, initial, neutral pH conditions in different degradation medium. The pH of the solution began to decrease after two days of immersion. From the Fig. 4 it can be noted that during the time of the degradation pH of the solution decreases as a result of the accumulation of PLGA degradation products and ascorbic
INFLUENCE OF DIFFERENT DEGRADATION MEDIUM
1635
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(b).. i^Ê
(c)
Fig. 2. SEM images of PLGA/ascorbic acid 85/15% nano-spheres after (a) two,(b) 24 and (c) 39 days of the degradation in physiological solution.
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(c)
Fig. 3. SEM images of PLGA/ascorbic acid 85/15% nano-spheres (a) before and after (b) 17 and 28 (c) 28 days of the degradation in phosphate buffered saline.
acid. It could be expected that the faster degradation of the lower molar mass fraction, present in copoly-mer, increase the local acidity, thereby, accelerating the hydrolysis of higher molar mass species. In other words, when acid accumulation creates a local pH drop, catalytic degradation of the polymer itself occurs. pH of the physiological solution had dropped from pH 7.0 to 2.6, as well as pH of the PBS with sodium azide, in the case of the PLGA/ascorbic acid 85/15% nanoparticles.
CONCLUSION
The release dynamics of the ascorbic acid from the polymer matrix is different when PLGA particles degrade in PBS and when they degrade in physiological solution, as a degradation medium. The ascorbic acid is released slower from the PLGA particles at the beginning, when PBS is used. This is explained with the
pH
\
20
40 60
Time, days
Fig. 4. Changes in the pH of the degradation medium with immersion time for the PLGA/accorbic acid 85/15% nanoparticles in the case of (1) physiological solution and (2) phosphate buffered saline.
7
5
1
2
3
0
1636
STEVANOVIC, USKOKOVIC
slower change of pH solution as well as with the presence of sodium azide.
ACKNOWLEDGMENTS
Authors would like to thank Milos Bokorov for his help in SEM analysis. The Ministry of Science and Technological Development of Republic of Serbia supports this work through the project No. 142006.
REFERENCES
1. L. Brannon-Peppas, J. O. Blanchette, Advanced Drug Delivery Reviews 56, 1649 (2004).
2. R. K. Kulkarni, E. G. Moore, A. F. Hegyeli, F. Leonard, Journal of Biomedical Material Research 5, 169 (1971).
3. J. H. Park, S. Lee, J. H. Kim, K. Park, K. Kim, I. C. Kwon, Progress in Polymer Science 33 (1), 113 (2008).
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