The effect of
oryzalin on excystation and metacystic development of Entamoeba invadens strain IP-1 was examined by transfer of
cysts to a growth medium containing the
drug. Excystation, which was assessed by counting the number of metacystic amoebae after induction of excystation, was inhibited by
oryzalin in a concentration-dependent manner. Metacystic development, which was determined by the number of nuclei in metacystic amoebae, was also inhibited by
oryzalin because the percentage of 4-nucleate amoebae at day 1 remained unchanged at day 3. The addition of
oryzalin after the induction of excystation decreased the number of metacystic amoebae, compared with control cultures. When
cysts were incubated for 1 day in growth medium plus
oryzalin, little increase in the number of metacystic amoebae was observed after removal of the
drug. Excystation and metacystic development were further inhibited when the
cysts were incubated for 30 min in encystation medium containing
oryzalin before transfer to growth medium with the
drug. When
cysts were incubated for 30 min in encystation medium before transfer to growth medium without the
drug, metacystic amoebae decreased in number. Pretreatment of
cysts with
oryzalin for 30 min in
phosphate-buffered saline markedly reduced viability and prevented excystation in growth medium with or without the
drug. The results indicate that
oryzalin inhibits excystation and metacystic development of E. invadens, suggesting that it may be an inhibitor of
Entamoeba infection.