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

Purpose To compare the antimicrobial effect of topical anesthetics, 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 Acanthamoeba castellanii (ATCC 50370) and Acanthamoeba polyphaga (ATCC 30461). Test agents included topical ophthalmic preparations of common anesthetics, antivirals, antibiotics, and biocides. Organisms were exposed to serial two-fold dilutions of the test compounds in the wells of a microtiter 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 spp., but lidocaine was well tolerated. The presence of the 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 spp. Hexamidine drops without BAC showed good activity against trophozoites, and the biguanides polyhexamethylene biguanide, chlorhexidine, alexidine, and octenidine all showed excellent activity against trophozoites and cysts of both species. Conclusions The antiamoebic effects 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 anesthetics and ophthalmic preparations containing BAC prior to specimen sampling may affect the viability of Acanthamoeba spp. in vivo, resulting in false-negative results in diagnostic tests.