Texture characterization of sodium thiosulfate hydrogels

J. Couturas1, N. Pecout2, J. Jost1, V. Guigonis3, M. Viana2, V. Ratsimbazafy1
1 Pharmacy Department, CHU Dupuytren, Limoges, France
2 Pharmaceutical Sciences Department, Faculty of Pharmacy, Limoges, France
3 University Hospital, Pediatry, Limoges, France


Introduction
Sodium thiosulfate (STS) is used locally in the treatment of ectopic osteo-calcifications. The lesser efficacy of initial STS 25% in an ointment-based formulation on deepest subcutaneous lesions has led to test of formulations which improve the penetration. From this work emerged a hydrogel form, at 25% of STS, with xanthan gum (2%) and different alcohol concentrations (3%, 5%, 8% and 10%). The objective of this work was to compare the rheology of the different formulations to study the possible impact of the ethanol concentration on the gel’s characteristics, and as a galenic stability predictor. These results will complete the data of skin permeation and stability of the active ingredient obtained in previous works.

Materials and methods
Controls: Measurement of viscosity and spreading capacity.
Equipment: Rotational coaxial cylindrical viscosimeter RHEOMAT RM 200® coupled to a water bath. The spreadability (represented by the diameter evolution of a 1 g test sample as a function of 7 cumulative weights - from 284 g to 2334 g - applied for 30 s each) was measured on the 4 gels at 3 different times: Day 0 (D0), D14 and D28. Concerning the viscosity, the measurements were also made on 3 times in order to evaluate the stability of the gels at D0, D14 and D28. At D0, viscosities were measured at room temperature and after heating between 30 and 32°C.

Results-discussion
Viscosity and spread: no significant difference between the different formulations was observed whatever the time of the measurement. The variation of alcohol concentration does not seem to have an impact on these parameters. The viscosity of the gels does not change with temperature (average room temperature of 21.78°C ± 0,41, and average increased temperature of 30.62°C ± 0,33). The rheological behavior of hydrogels was non-Newtonian. The shear stress values revealed a minimal rheology anomaly: a thixotropy. The slight decrease in viscosity under stress of the gels may allow a comfort of use when spreading on the injured areas. In total, the measurements at D0, D14 and D28 show a stability of the viscosity meaning a stability of the galenic form over a period of one month.

Conclusion
The choice of the formulation to be retained will not be made on the criteria of rheology but on the parameters of the previous work. This work allows to validate the galenic stability of the hydrogel on a period of one month, period corresponding to the duration of a dispensation.

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