Epi-C3-cryptophycin-24, epi-C3-m-chlorobenzyl-cryptophycin-24, and the corresponding styreneswere synthesized and tested in vitro against the MCF-7 and multidrug-resistant MCF-7/ADRbreast cancer cell lines and in an in vitro tubulin assembly assay. The results demonstratethat the S configuration at the C3 stereocenter is not required to induce potent cytotoxicityand the m-Cl substituent present on the C10 side chain did not induce any large change inactivity.