Microtubule-targeted drugs are now indispensable for the therapy of various cancer types worldwide. In this article, we report MT119 [6-[2-(4-methoxyphenyl) -ethyl]-9-[(pyridine-3-ylmethyl)amino]pyrido[2 0 ,1 0 :2,3]imida-zo [4,5-c]isoquinolin-5(6H)-one] as a new microtubule-targeted agent. MT119 inhibited tubulin polymerization significantly both in tumor cells and in cell-free systems, which was followed by the disruption of mitotic spindle assembly. Surface plasmon resonance-based analyses showed that MT119 bound to purified tubulin directly, with the K D value of 10.6 lM. The binding of MT119 in turn caused tubulin conformational changes as evidenced by the quenched tryptophan fluorescence, the reduction of the bis-ANS reactivity and the decreased DTNB-sulfhydryl reaction rate. Competitive binding assays further revealed that MT119 bound to tubulin at its colchicine site. Consequently, by inhibiting tubulin polymerization, MT119 arrested different tumor cells at mitotic phase, which contributed to its potent antitumor activity in vitro. MT119 was also similarly cytotoxic to vincristine-, adriamycin-or mitoxantrone-resistant cancer cells and to their corresponding parental cells. Together, these data indicate that MT119 represents a new class of colchicine-site-targeted inhibitors against tubulin polymerization, which might be a promising starting point for future cancer therapeutics.Microtubule is composed of a-and b-tubulin heterodimers, whose dynamics is crucial for the proper function of spindles and guarantees the mitotic progression. 1 By interfering with microtubule dynamics, tubulin inhibitors cause mitosis arrest and ultimately lead to tumor cell death. Since the approval of vinca alkaloids in 1960s and then taxol in 1990s for cancer therapy, targeting microtubule has been recognized as the most effective strategy against different types of malignancies, including hematological, ovarian, mammary and lung cancers. 2 According to the differential impacts on microtubule polymer mass, tubulin inhibitors could be classified into microtubule destabilizers, such as vinca alkaloids, colchicine and combretastatins and microtubule stabilizers, such as taxol and epothilones. Regardless of their distinct structural types, these inhibitors bind to microtubules mainly at one of the three sites: the vinblastine site, the colchicine site or the taxol site. Despite of their proven therapeutic success, some drawbacks such as limited natural sources, high neurotoxicity and poor solubility have rendered serious questions onto the extensive use of current microtubule-targeted drugs in clinic. 3,4 Therefore, it is urgent to find more structurally-diverse compounds with tubulin inhibitory functions for future antitumor therapy.We previously reported a combinatorial library of 6H-Pyrido