Effect of the antitubulin drug oryzalin on the encystation of Entamoeba invadens

We have recently demonstrated that the antimicrotubule drug oryzalin inhibi ts the growth of Entamoeba invadens as well as E. histolytica, the former b eing more resistant to the drug than the latter, and that effective doses o f oryzalin are higher for Entamoeba than for the other parasitic protozoa e xamined thus far. The aim of the present study was to examine the effect of oryzalin on the encystation of E. invadens using an axenic encystation sys tem in vitro. Oryzalin inhibited the encystation of E. invadens strain IP-1 in a dose-dependent manner. The addition of oryzalin after the induction o f encystation was also inhibitory for encystation and cyst maturation. Trop hozoites incubated for 1 day in encystation medium with oryzalin did not en cyst after removal of the drug. Although trophozoites grown in the presence of 300 mu M oryzalin for 2 days did not encyst after their transfer to enc ystation medium containing the same concentration of drug, a number of trop hozoites survived for at least 3 days. In contrast, trophozoites grown in t he absence of oryzalin neither survived nor encysted after their transfer t o encystation medium supplemented with the drug, which suggests that pretre atment of trophozoites with oryzalin contributes to their continued surviva l as trophozoites, i.e., without their transforming into cysts, in encystat ion medium. Trophozoites grown with oryzalin did encyst after their transfe r to encystation medium without the drug. Accumulation of trophozoites in t he mitotic phase was observed after culture with oryzalin. When cysts prepa red at day 1 of encystation, most of which were mononucleate, were reincuba ted in the presence of oryzalin for an additional 2 days, inhibition of the ir maturation was observed. Thus, oryzalin is a potent mitotic-phase inhibi tor of E. invadens and may become a useful tool for studies on the relation ship between the cell cycle and encystation of this parasite.