Photostabilization of Dermal Preparations

Because semisolid drugs require a different handling than solid drugs, it is important to consider the experimental conditions for in vitro photostability testing. Due to the limited penetration depth of the incident radiation, it is important to provide samples of constant thickness. As noted previously, the volume effect can affect the result obtained. Furthermore, the sample area must also be kept constant if repeatable results are to be obtained. 

Control sample wrapped with aluminiurn - foil 

The experimental set-up used by Thoma et al. (Fig. 17) can be used as a reference. Samples are spread, as a film, 200 to 300 pm in depth, on a glass slide of defined area. A second sample, similarly prepared, is covered with aluminium foil and exposed along side of the test sample, as a control (45). 

The problem of temperature control needs to be addressed because even below the melting point of semisolid preparations, their structure may be affected by the temperature and thus the photostability of the formulation will change. 

It can be assumed that the hydrocortisone s solubility will vary in different formulations. This was shown by the following experiment. The solubility and photostability of triamcinolone acetonid in water were tested using three concentrations of the surfactant sorbitan monostearate. The drug solubility was found to increase with the concentration of the surfactant. Therefore, the extent 

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