Synthesis of Novel Podophyllotoxin–Benzothiazole Congeners and Their Biological Evaluation as Anticancer Agents

A series of novel podophyllotoxin derivatives containing benzothiazole scaffolds were synthesized and evaluated for their in vitro cytotoxic activity against five cancer cell lines (MCF-7, SKOV-3, B16F10, LOVO, and HeLa). Two compounds, <b>7</b> and <b>11</b>, which are different only by the absence or presence of the ester group, showed the strongest cytotoxic effect towards all tested cancer cell lines with the IC₅₀ 0.68-2.88 µM. In addition, it was demonstrated that these compounds inhibit cancer cell proliferation by inducing G2/M phase arrest in HeLa cells. The structure-activity relationship was analyzed and it confirmed the importance of the core structural features like a dioxolane ring and free-rotating trimethoxyphenyl group for cytotoxicity. Moreover, the <i>R</i> configuration of the ester group at the C-8' position proved to be substantial since its epimer was inactive. The molecular docking studies revealed that the most potent compounds have a different binding mode to β-tubulin than podophyllotoxin; however, the benzothiazole fragment docked in a similar location as the trimethoxyphenyl group of podophyllotoxin, exhibiting similar hydrophobic interactions. These findings clearly indicate that podophyllotoxin-benzothiazole derivatives could be addressed for further pharmacological studies in anticancer research.