Herein, we describe the structure-activity relationship study of a new 3-aryl-8H-thieno[2,3-b]pyrrolizin-8-one series of antitubulin agents. The pharmacological results from the National Cancer Institute in vitro human disease oriented tumor cell line screening allowed us to identify compound 1d (NSC 676693) as a very efficient antitumoral drug in all cancer cell lines tested. This prompted us to define the structural requirements essential for this antiproliferative activity. Among all analogues synthesized in this study, compound 1o was the most promising, being 10-fold more potent than compound 1d. Its activity over a panel of nine tumoral cell lines was in the nanomolar range for all of the histological types tested, and surprisingly, the resistant KB-A1 cell line was also sensitive to this compound. Moreover, a flow cytometric study showed that L1210 cells treated by the most potent compounds were arrested in the G(2)/M phases of the cell cycle with a significant percentage of cells having reinitiated a cycle of DNA synthesis without cell division. This interesting pharmacological profile, resulting from inhibition of tubulin polymerization, encouraged us to perform preliminary in vivo studies that led to a new prodrug chemical approach.