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A new approach combining different MRI methods to provide detailed view on 2 swelling dynamics of xanthan tablets influencing drug release at different pH and 3 ionic strength
Mikac, Urška (Author), Sepe, Ana (Author), Kristl, Julijana (Author), Baumgartner, Saša (Author)

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Abstract
The key element in drug release from hydrophilic matrix tablets is the gel layer that regulates the penetration of water and controls drug dissolution and diffusion. We have selected magnetic resonance imaging (MRI) as the method of choice for visualizing the dynamic processes occurring during the swelling of xanthan tablets in a variety of media. The aims were (i) to develop a new method using MRI for accurate determination of penetration, swelling and erosion fronts, (ii) to investigate the effects of pH and ionic strength on swelling, and (iii) to study the influence of structural changes in xanthan gel on drug release. Two dimensional (2D) MRI and one dimensional single point imaging (SPI) of swollen xanthan tablets were recorded, together with T2 mapping. The border between dry and hydrated glassy xanthan-the penetration front-was determined from 1D SPI signal intensity profiles. The erosion front was obtained from signal intensity profiles of 2D MR images. The swelling front, where xanthan is transformed from a glassy to a rubbery state (gel formation), was determined from T2 profiles. Further, the new combination of MRI methods for swelling front determination enables to explain the appearance of the unusual Ćbright frontĆ observed on 2D MR images in tablets swollen in HCl pH 1.2 media, which represents the position of swelling front. All six media studied, differing in pH and ionic strength, penetrate through the whole tablet in 4 h 0.3 h, but formation of the gel layer is significantly delayed. Unexpectedly, the position of the swelling front was the same, independently of the different xanthan gel structures formed under different conditions of pH and ionic strength. The position of the erosion front, on the other hand, is strongly dependent on pH and ionic strength, as reflected in different thicknesses of the gel layers. The latter are seen to be the consequence of the different hydrodynamic radii of the xanthan molecules, which affect the drug release kinetics. The slowest release of pentoxifylline was observed in water where the thickest gel was formed, whereas the fastest release was observed in HCl pH 1.2, in which the gel layer was thinnest. Moreover, experiments simulating physiological conditions showed that changes of pH and ionic strength influence the xanthan gel structure relatively quickly, and consequently the drug release kinetics. It is therefore concluded that drug release is greatly influenced by changes in the xanthan molecular conformation, as reflected in changed thickness of the gel layer. A new method utilizing combination of SPI, multi-echo MRI and T2 mapping eliminates the limitations of standard methods used in previous studies for determining moving fronts and improves current understanding of the dynamic processes involved in polymer swelling.

Language:English
Keywords:magnetna resonanca, matrične tablete, ksantan, sproščanje tablet
Work type:Not categorized (r6)
Tipology:1.01 - Original Scientific Article
Organization:FFA - Faculty of Pharmacy
Year:2010
Number of pages:str. 247-256
Numbering:vol. 145, iss. 3
UDC:66.02
ISSN on article:0168-3659
DOI:10.1016/j.jconrel.2010.04.018 Link is opened in a new window
COBISS.SI-ID:2780273 Link is opened in a new window
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Downloads:200
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Record is a part of a journal

Title:Journal of controlled release
Shortened title:J. control. release
Publisher:Elsevier
ISSN:0168-3659
COBISS.SI-ID:27318784 This link opens in a new window

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