Current microtubule inhibitory agents used in the clinic to treat cancer have severe side effects, and development of resistance is frequent. We have evaluated the antitumor effect of a novel 30-compound library of phenoxy pyridine and phenyl sulfanyl pyridine derivatives. MTT assays revealed that, of all 30 compounds tested, compounds 2 and 3 showed the largest decrease in proliferation (low muM range) against Panc1 and HS766T human pancreatic cancer cells. Flow cytometry experiments with MCF7 breast cancer cells showed a G2/M arrest comparable to that of colcemid. Immunofluorescence staining demonstrated complete disappearance of intracellular microtubules. Tubulin assembly assays, however, showed a dose-dependent decrease in tubulin assembly with compound 3 that seemed limited to about 50% of the control reaction. With compound 2 treatment, there was only a delay in the onset of assembly, with no effect on the extent of the reaction. Taken together, our results show that these novel microtubule inhibitors have promising anticancer activity and can be potentially used to overcome paclitaxel resistance in the clinical setting.