Localisation and quantification of benzalkonium chloride in eye tissue by TOF-SIMS imaging and liquid chromatography mass spectrometry

Benzalkonium (BAK) chloride is the most commonly used preservative in eye drops. It is generally composed of benzyldimethyldodecylammonium C₁₂ and benzyldimethyltetradecylammonium C₁₄ and is supposed to increase penetration of active compounds. However, numerous studies have reported its toxic effect to ocular surface especially in long-term treatments like against glaucoma, a sight-threatening disease. Albino rabbits were treated with a hyperosmolar solution and a high concentration of BAK solution for 1 month. Enucleated eyes were cryo-sectioned and analysed by mass spectrometry. Mass spectrometry imaging using time-of-flight secondary ion mass spectrometry (TOF-SIMS) has been used to characterize the spatial distribution and to determine the relative quantity of BAK at the surface of rabbit eye sections. Liquid chromatography coupled with mass spectrometry (LC-MS) using a hybrid linear ion trap-Orbitrap® mass spectrometer was used to obtain relative quantification of BAK at the sample surface. TOF-SIMS images of BAK ions indicated a distribution at the ocular surface and in deeper structures. Didecyldimethylammonium (DDMAC), which is used in hospitals as a substitute for BAK, was also detected and showed an accumulation around the eyes. After extraction with acetonitrile and chromatographic separation using a Gemini C18 column and an original elution gradient, the relative quantities of BAK and DDMAC present in the whole eye section surface were determined. This LC-MS method was validated in terms of limits of quantification, linearity, repeatability and reproducibility and its feasibility was evaluated in surgically obtained human samples. Specimens of iris, lens capsule or trabecular meshwork were found with significant levels of BAK and DDMAC, thus confirming the penetration of BAK in deep ocular structures, with potential deleterious effects induced by this cytotoxic compound. The analytical method developed here could therefore be of primary interest in the field of pharmaco-toxicology in order to localise, identify and quantify drugs or xenobiotic compounds present at biological sample surfaces.