The antitumor action of
HO-221, a novel benzoylphenylurea derivative, was studied. The in vitro cytotoxic strength of
HO-221 was investigated, as measured by IC50 values, compared with those of other drugs with different action mechanisms, using Chinese hamster lung (CHL) cells, mouse
leukemia L1210 cells and human promyelocytic
leukemia HL-60 cells. Morphological alterations following treatment were observed under a phase contrast microscope, and the mitotic index was determined at regular intervals to check for accumulation of metaphase cells.
HO-221 was found to have a very strong toxic effect on all cell types, equal to that of the spindle
poisons used as controls.
HO-221 also produced the same specific morphological changes as the spindle
poisons, with a significant accumulation of metaphase cells. A chromosome analysis of treated cells showed that
HO-221 frequently induced
polyploid and
aneuploid cells, but without accompanying chromosome-breaking activity. An in vivo mouse bone marrow micronucleus assay was also carried out. The assay allowed the in vivo identification of a chromosome breaker or a spindle
poison through the measurement of the relative sizes of micronuclei produced and erythrocytes.
HO-221 was found frequently to induce relatively large micronuclei, an action regarded as specific to spindle
poisons. It was thus demonstrated that
HO-221 acts as a spindle
poison both in vitro and in vivo. In order to investigate the mechanism of this action, a study of
tubulin assembly using purified calf brain
tubulin was carried out, which demonstrated clearly that
HO-221 inhibits microtubule assembly. A detailed investigation of the action mechanism of
HO-221 as a spindle
poison is now called for.