Protamine as a Potential Amoebicidal Agent for Contact Lens Disinfection

Abstract

To evaluate the amoebicidal efficacy of protamine with polyhexamethylene biguanide (PHMB) and ethylenediamine tetraacetic acid (EDTA). The International Organization for Standardization 14729:2001 procedure was modified to test amoebicidal activity. Acanthamoeba cells were inoculated into dilutions of protamine alone (57 to 228 μM) or in combination with PHMB/EDTA and incubated at 25°C for 6 hours. The number of survivors was determined after 7 days of incubation at 32°C on Escherichia coli-seeded agar plates. For encystment, Acanthamoeba trophozoites were incubated in protamine/PHMB/EDTA for 24 hours, and then the number of cysts was counted using a hemocytometer. Protamine showed significant (p < 0.01) activity against trophozoites of both Acanthamoeba strains, which reached 2 log reductions or more for 228 μM compared with that in phosphate buffered saline. The addition of PHMB to protamine significantly (p = 0.002) improved anti-Acanthamoeba effect (0.8 logs reduction) of Acanthamoeba castellanii only. The addition of EDTA to protamine/PHMB only slightly improved efficacy (0.1 logs). Protamine at 228 μM significantly (p < 0.0001) killed the cysts of either strain by between 0.6 and 0.9 logs. Protamine/PHMB significantly increased killing (p = 0.014) of cysts of A. castellanii only. Protamine/PHMB/EDTA did not show synergy against Acanthamoeba cysts. Protamine or protamine/PHMB with or without EDTA did not cause encystment. Protamine shows good activity against Acanthamoeba trophozoites and cysts and works more effectively in combination with PHMB against A. castellanii. Protamine may be a promising ingredient in contact lens-disinfecting solutions to control Acanthamoeba growth.