A Raman spectroscopic investigation of bioadhesive tetracaine local anaesthetic formulations

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Raman spectroscopy at 785 nm has been employed to characterise the properties of tetracaine in bioadhesive gel and patch formulations. In the first study, interactions between the drug and excipients in novel bioadhesive patch systems were characterised. It was determined that the drug did not interact with any of its formulation components, and that this was an important factor in its clinical performance, particularly the rapid onset of anaesthesia. Investigations of drug uptake in the stratum corneum from a gel formulation suggested that tetracaine rapidly undergoes a phase-change upon application to the skin. The intensity of the tetracaine Raman bands at approximately 1600 cm−1 suggests that the local anaesthetic is rapidly absorbed into the skin. Decreases in Raman tetracaine band intensities, along with an absence in the concomitant alteration in the internal standard spectra, indicates an decrease in the tetracaine concentration present in the gel. Further, a baseline indicating complete tetracaine absorption appears to be reached after approximately 40 min of exposure. After this time little further absorption was observed, suggesting that the stratum corneum “reservoir” was saturated with tetracaine at this time. This is consistent with the optimum application time required for tetracaine gels to attain maximum clinical efficacy. This study has indicated the effectiveness of Raman spectroscopy in the analysis of gel-based pharmaceutical preparations, showing it to be a simple, rapid, virtually non-invasive technique for determination of tetracaine.

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