In Vitro Evaluation of the Inhibitory Effect of Topical Ophthalmic Agents on Acanthamoeba Viability

Purpose: To compare the antimicrobial effect of topical anaesthetics, antivirals, antibiotics, and biocides on the viability of Acanthamoeba cysts and trophozoites in vitro.
Methods: Amoebicidal and cysticidal assays were performed against both trophozoites and cysts of A. castellanii (ATCC 50370) and A. polyphaga (ATCC 30461). Test agents included topical ophthalmic preparations of commonly anaesthetics, antivirals, antibiotics, and biocides. Organisms were exposed to serial two-fold dilutions of the test compounds in the wells of a microtitre plate to examine the effect on Acanthamoeba spp. In addition, the toxicity of each of the test compounds was determined against a mammalian cell line.
Results: Proxymetacaine, oxybuprocaine and especially tetracaine were all toxic to the trophozoites and cysts of Acanthamoeba but lidocaine was well tolerated. The presence of benzalkonium chloride (BAC) preservative in levofloxacin caused a high level of toxicity to trophozoites and cysts. With the diamidines the presence of BAC in the propamidine drops was responsible for the activity against Acanthamoeba. Hexamidine drops without BAC showed good activity against trophozoites and the biguanides PHMB, chlorhexidine, alexidine and octenidine all showed excellent activity against trophozoites and cysts of both species.
Conclusions: The anti-amoebic effect of BAC, povidone iodine and tetracaine are superior to the current diamidines and slightly inferior to the biguanides used in the treatment for Acanthamoeba keratitis. Translational Relevance: Ophthalmologists should be aware that certain topical anaesthetics and ophthalmic preparations containing BAC, prior to specimen sampling may affect the viability of Acanthamoeba in vivo, resulting in false negative results in diagnostic tests.