Secnidazole gels (G1-G10) were prepared byhydroxypropyl methylcellulose at different concentrations (2.5-7.5%) and glycerin and propylene glycol were used as humectants which may affect the rheological properties of SEC gels. The viscosities of SEC gels were 3100-5300 centipoise. Different formulations of SEC gels were packed and subjected to accelerated stability studies at 40±2°C/75%±5% relative humidity for 6 months. Due to the stability of SEC against dry heat and hydrolytic degradation, the degradation rate of SEC gels in different formulation of glycerin and propylene glycol was 6.29-8.11% and 7.52-9.57%, respectively.
The kinetics of SEC photolysis was also investigated using UV-visible spectrophotometer. The absorption spectra of SEC and its photolyzed product showed its peak (λmax) at 318 and 209nm, respectively, with an isosbestic point at ~295 nm. The values of first-order rate constants (kobs) of SEC in gels were 3.92-6.03×10–2 min–1, compared to 11.81×10–2 min–1 in aqueous solution. A linear relation was observed between the kobs of SEC mainly with an increase in the concentration of HPMC, GL, and PG in gels with a negative slope. The shelf-life of photolyzed SEC in the gels increased to ~3.0 fold.
Since there is no chemical interaction between SEC and excipients used in formulations, the stability of SEC gels is mainly due to the increased viscosity by decreasing oxygen penetration. Thus, the chances of oxidation and photo-oxidation degradation were minimized. It is suggested that such an approach may be useful in the stabilization of photo-labile drugs in viscous formulations