The vinca alkaloids and taxanes are successful antitumor agents. However, drug resistance limits their clinical utility in cancer chemotherapy. Overexpression of P-glycoprotein (Pgp) and expression of the β-tubulin subtype, β-III tubulin, are two clinically established drug resistance mechanisms that reduce the effectiveness of these agents. In 2010, we reported that the 6-CH3 cyclopenta[d]pyrimidine scaffold affords water soluble antimitotic agents targeting the colchicine site on tubulin. Molecular modeling and docking studies of the parent compound in the X-ray crystal structure of tubulin (PDB: 1SA0) suggested that the N4-CH3 moiety involved in hydrophobic interactions with the side chain C of Lysβ254, Alaβ250 and Leuβ248. However, this binding pocket in tubulin is relatively large. We designed a variety of N4-alkyl substitutions to determine optimal size of the substituent at the N4-position for these hydrophobic interactions. Among the different substitutions at the N4 were n-propyl, i-propyl and cyclopropyl. These compounds inhibited the growth of human breast cancer cell line MDA-MB-435 in culture, with IC50’s 130, 410, and 300nM, respectively. The previously reported methyl substituent at the N4-position is the most potent analog against these cancer cells, with IC50 of 10.6 nM. Thus, increasing the size of the N4-alkyl substituent caused a decrease in the potency of the analogs indicating that groups larger than methyl may be progressively detrimental to potency. This could be due to conformational restriction of various rotatable bonds in the molecule, lack of appropriate hydrophobic interactions with the colchicine site, and/or due to other factors such as transport into the tumor cells. The experiments to distinguish among these possibilities are undergoing.

Citation Format: Aleem Gangjee, Weiguo Xiang, Susan L. Mooberry, Ernest Hamel. Structure-activity relationship study anti-microtubule agents targeting the colchicine site on tubulin that circumvent drug resistance. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 4492. doi:10.1158/1538-7445.AM2013-4492