Abstract
The aim of our study was to identify factors affecting the in vitro release of diclofenac sodium (DS) from hypromellose-based gels (HPMC).
 Materials and methods. Gels with HPMC and liquids without HPMC were studied by viscosity-rotating viscometer method and spin probe electron paramagnetic resonance spectroscopy. Rheograms were used to determine the flow behavior and the apparent viscosity, and the EPR spectra were used to determine the rotational correlation time (τ–1) of the dissolved spin probes. The in vitro release tests were performed using vertical diffusion cells according to a validated procedure. The assay of DS and isopropyl alcohol (IPA) in the receptor medium was performed by high performance liquid chromatography (HPLC) and gas chromatography (GC) according to validated procedures, and the water content was determined using semi-micro method.
 Results. The apparent viscosity of the gels increased with increasing HPMC content and depended on the HPMC grade. The high apparent viscosity of the gels did not affect the values of τ–1 of the dissolved spin probes. In viscous gels and Newtonian fluids, the composition of which corresponded to the dispersion medium of gels, the values of τ–1 were identical and were in the range of rapid rotation, which is a prerequisite for similar and rapid release of the dissolved substances from gels and liquids. It was shown that the HPMC-based gel and Newtonian liquid without HPMC in terms of in vitro release parameters DS and IPA were equivalent. During in vitro testing the release of dissolved DS increased with increasing its concentration in the gel and depended on the dispersed state of DS. When the content of IPA was changed from 45.0 % to 22.5 %, the water absorption by the gel and the release of IPA decreased, and the release of DS increased, which was due to the decrease in the solubility of DS in the gel.
 Conclusions. HPMC, which provided high apparent viscosity of the gels, did not affect the value of τ–1 of the dissolved spin probes and the in vitro release of DS from the gels. The gel and Newtonian liquid were equivalent in terms of in vitro release of DS and IPA. The release of DS altered proportionally with the concentration of DS and depended on its dispersed state. As the content of IPA decreased, the release of IPA decreased, but the release of DS increased because of the decrease in the solubility of the DS in the gel
Highlights
Gels are semi-solid preparations for cutaneous application; they consist of liquids gelled by means of suitable gelling agents [1]
It should be noted that almost identical rotational correlation times of each of the spin probes with a certain functional group in water and gel indicated the absence of significant dipole-dipole or ion-dipole interactions with HPMC (Table 3), which could affect the release of active substances with same functional groups in their molecules
It was shown that high apparent viscosity of the gels did not affect the values of τ–1 of the dissolved spin probes
Summary
Gels are semi-solid preparations for cutaneous application; they consist of liquids gelled by means of suitable gelling agents [1] Various polymers such as carbomers and cellulose derivatives are used to form hydrogels [2]. It was shown that the interaction of the spin probe 4-amino-TEMPO and carboxylate anions of carbomers caused an increase in the rotational correlation time (τ–1) of this spin probe in the gels within a certain pH range. This interaction can affect the release of active substances with amino groups in the molecules, or prevent their introduction into formulations of gels based on carbomers [3]. That article does not contain any results of studies with other polymers, such as hypromellose (HPMC), which molecules do not possess reactive carboxylate anions
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