Background and importance Tetracaine hydrochloride is a local anaesthetic agent commonly used for surface anaesthesia, typically used in concentrations of 2%–4% for anaesthesia of the nose and throat. The substance is an ester and slowly degrades over time. Over the same time the solution, in our experience, also discolours. For this reason tetracaine solutions have been made extemporaneously in our pharmacy, with a limited shelf life.

Aim and objectives The aim of the study was to investigate the stability of tetracaine solutions as a function of pH, and from this determine the optimum stability as regards drug content and appearance.

Material and methods Tetracaine hydrochloride (Sigma-Aldrich, St Louis, Missouri, USA). Instrument: ultra-high performance liquid chromatography (UHPLC) system (Shimadzu Corp., Kyoto, Japan) with a Nexera diode array detector (DAD) detector. Analytical column: Ace Excel 2 C8, 2 μm 2.1 × 100 mm (Advanced Chromatography Technologies Ltd, Aberdeen, GB). The analytical method was validated for linearity, precision and specificity. pH stability study: samples were prepared containing tetracaine hydrochloride (20.0 mg/mL), methyl parahydroxybenzoate (1 mg/mL) and sodium chloride (5.5 mg/mL) with pH values spanning 2–6. The samples were stored until visible discolouration was observed in all solutions: First for 3 days at 70°C, then at 3 days at 25°C at ambient humidity, and protected from light.

Results Upon heat stress, the drug content remained highest at pH 5: 20.5±0.1 mg/mL (97.2%) (n=3); the appearance, however, changed to yellowish brown, and the solution was unclear. The content decreased the most at pH 2: 18.2±0.0 (86.8%) (n=3); in appearance, this solution remained clear, but turned yellow.

Conclusion and relevance Using a validated UHPLC method the optimum stability for tetracaine hydrochloride is found at pH 4–5 as regards assay value. Paradoxically, however, this is not the pH at which the appearance is the most acceptable.

Conflict of interest No conflict of interest