In vitro evaluation of antimicrobial compounds for cysticidal activity against Acanthamoeba

Amebic keratitis presents a therapeutic dilemma because Acanthamoeba, unlike Naegleria, encysts in infected tissues. To date, the results of medical therapy have been disappointing, and the optimal medical regimen for acanthamoeba keratitis is yet to be defined. Clinical cure necessitates eradication of the encysted amebae with medications known to have ocular toxic effects in combination with surgical maneuvers. Antiinfective agents possessing in vitro cysticidal activity include paromomycin, neomycin, ketoconazole, natamycin, and ciclopirox olamine, but no agent has been shown to be uniformly effective against all isolates of Acanthamoeba. Treatment of amebic cysts with propamidine isethionate plus either paromomycin or neomycin has produced slightly additive cysticidal effects. Multiple factors, including the varied clinical presentation and the lack of standardized in vitro techniques for the assay of cysticidal activity, account for the lack of correlation between in vitro activity and in vivo efficacy.