Comparison of in vitro assays to study the effectiveness of antiparasitics against Acanthamoeba castellani trophozoites and cysts

Novel Drug Screening Assay for Acanthamoeba castellanii and the Anti-Amoebic Effect of Carbonic Anhydrase Inhibitors

Acanthamoeba castellanii is an amoeba that inhabits soil and water in every part of the world. Acanthamoeba infection of the eye causes keratitis and can lead to a loss of vision. Current treatment options are only moderately effective, have multiple harmful side effects, and are tedious. In our study, we developed a novel drug screening method to define the inhibitory properties of potential new drugs against A. castellanii in vitro. We found that the clinically used carbonic anhydrase inhibitors, acetazolamide, ethoxzolamide, and dorzolamide, have promising antiamoebic properties.

Efficacy of Off-Label Anti-Amoebic Agents to Suppress Trophozoite Formation of Acanthamoeba spp. on Non-Nutrient Agar Escherichia Coli Plates

Acanthamoeba keratitis (AK) is a dangerous infectious disease, which is associated with a high risk of blindness for the infected patient, and for which no standard therapy exists thus far. Patients suffering from AK are thus treated, out of necessity, with an off-label therapy, using drugs designed and indicated for other diseases/purposes. Here, we tested the capability of the off-label anti-amoebic drugs chlorhexidine (CH; 0.1%), dibromopropamidine diisethionate (DD; 0.1%), hexamidine diisethionate (HD; 0.1%), miltefosine (MF; 0.0065%), natamycin (NM; 5%), polyhexamethylene biguanide (PHMB; 0.02%), povidone iodine (PVPI; 1%), and propamidine isethionate (PD; 0.1%) to suppress trophozoite formation of Acantamoeba castellanii and Acanthamoeba hatchetti cysts on non-nutrient agar Escherichia coli plates. Of the eight off-label anti-amoebic drugs tested, only PVPI allowed for a complete suppression of trophozoite formation by drug-challenged cysts for all four Acanthamoeba isolates in all five biological replicates. Drugs such as NM, PD, and PHMB repeatedly suppressed trophozoite formation with some, but not all, tested Acanthamoeba isolates, while other drugs such as CH, DD, and MF failed to exert a relevant effect on the excystation capacities of the tested Acanthamoeba isolates in most, if not all, of our repetitions. Our findings suggest that pre-testing of the AK isolate with the non-nutrient agar E. coli plate assay against the anti-amoebic drug intended for treatment should be performed to confirm that the selected drug is cysticidal for the Acanthamoeba isolate.

The Effect of Anti-Amoebic Agents and Ce6-PDT on Acanthamoeba castellanii Trophozoites and Cysts, In Vitro

Purpose

The purpose of this study was to analyze the concentration-dependent effects of biguanides (polyhexamethylene biguanide [PHMB], chlorhexidine [CH]); diamidines (hexamidine-diisethionate [HD], propamidine-isethionate [PD], dibromopropamidine-diisethionate [DD]); natamycin (NM); miltefosine (MF); povidone iodine (PVPI), and chlorin e6 PDT on Acanthamoeba trophozoites and cysts, in vitro.

Methods

Strain 1BU was cultured in peptone-yeast extract-glucose medium. Trophozoites or cysts were cultured in PYG medium containing each agent at 100%, 50%, and 25% of maximum concentration for 2 hours. The percentage of dead trophozoites was determined using a non-radioactive cytotoxicity assay and trypan blue staining. Treated cysts were also maintained on non-nutrient agar Escherichia coli (E. coli) plates and observed for 3 weeks.

Results

All tested drugs displayed significant cytotoxic effects on 1BU cells based on the biochemical and staining-based viability assays tested. On non-nutrient agar E. coli plates, neither trophozoites nor freshly formed cysts were observed after PHMB, PD, NM, and PVPI treatment, respectively, within 3 weeks. However, CH-, HD-, DD-, and MF-treated cysts could excyst, multiply, and encyst again.

Conclusions

The off-label drugs PHMB, PD, NM, and PVPI are under in vitro conditions more effective against strain 1BU than CH, HD, DD, and MF. Our findings also suggest that the non-nutrient agar E. coli plate assay should be considered as method of choice for the in vitro analysis of the treatment efficacy of anti-amoebic agents.

Translational Relevance

Ophthalmologists may optimize the treatment regime against Acanthamoeba keratitis by pre-testing the in vitro susceptibilities of the Acanthamoeba strain against drugs of interest with the non-nutrient E. coli agar plate assay.