We have recently demonstrated that the antimicrotubule
drug oryzalin inhibits the growth of Entamoeba invadens as well as E. histolytica, the former being more resistant to the
drug than the latter, and that effective doses of
oryzalin are higher for Entamoeba than for the other parasitic protozoa examined 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 system in vitro.
Oryzalin inhibited the encystation of E. invadens strain IP-1 in a dose-dependent manner. The addition of
oryzalin after the induction of encystation was also inhibitory for encystation and
cyst maturation. Trophozoites incubated for 1 day in encystation medium with
oryzalin did not encyst after removal of the
drug. Although trophozoites grown in the presence of 300 microM
oryzalin for 2 days did not encyst after their transfer to encystation medium containing the same concentration of
drug, a number of trophozoites survived for at least 3 days. In contrast, trophozoites grown in the absence of
oryzalin neither survived nor encysted after their transfer to encystation medium supplemented with the
drug, which suggests that pretreatment of trophozoites with
oryzalin contributes to their continued survival as trophozoites, i.e., without their transforming into
cysts, in encystation medium. Trophozoites grown with
oryzalin did encyst after their transfer to encystation medium without the
drug. Accumulation of trophozoites in the mitotic phase was observed after culture with
oryzalin. When
cysts prepared at day 1 of encystation, most of which were mononucleate, were reincubated in the presence of
oryzalin for an additional 2 days, inhibition of their maturation was observed. Thus,
oryzalin is a potent mitotic-phase inhibitor of E. invadens and may become a useful tool for studies on the relationship between the cell cycle and encystation of this parasite.