Pharmaceutical excipients for topical use may contain impurities, which are often neglected from a toxicity qualification viewpoint. The possible impurities in the most frequently used topical excipients were evaluated in-silico for their toxicity hazard. Acetol, an impurity likely present in different topical pharmaceutical excipients such as propylene glycol and glycerol, was withheld for the evaluation of its health risk after dermal exposure.
An ex-vivo in-vitro permeation study using human skin in a Franz Diffusion Cell set-up and GC as quantification methodology showed a significant skin penetration with an overall Kp value of 1.82 ? 10 ? 3 cm/h. Using these data, limit specifications after application of a dermal pharmaceutical product were estimated. Based on the TTC approach of Cramer class I substances, i.e. 1800 mg/(day?person), the toxicity-qualified specification limits of acetol in topical excipients were calculated to be 90 mg/mL and 180 mg/mL for propylene glycol and glycerol, respectively.

As part of the method development, the injection volume as a critical quality attribute in fast fused-core chromatography was evaluated. Spilanthol, a pharmaceutically interesting N-alkylamide currently under investigation in our laboratory, was chosen as the model compound. Spilanthol was dissolved in both PBS and MeOH/H2O (70/30, v/v) and subsequently analyzed using a fused-core system hereby selecting five chromatographic characteristics (retention time, area, height, theoretical plates and symmetry factor) as responses. We demonstrated that the injection volume significantly influenced both the qualitative and quantitative performance of fused-core chromatography, a phenomenon which is confounded with the nature of the used sample solvent. From 2 mL up to 100 mL injection volume with PBS as solvent, the symmetry factor decreased favorably by 20%. Moreover, the theoretical plates and the quantitative parameters (area and height) increased up to 30%. On the contrary, in this injection volume range, the theoretical plates for the methanol-based samples decreased by more than 60%, while the symmetry factor increased and the height decreased, both by 30%. The injection volume is thus a critical and often overlooked parameter in fused-core method description and validation.