We further tested the effects of compound 3c on the proliferation of A549 cells, using flow cytometry-based cell cycle assay (Fig. ). Quantification of the percentage of mitotic cells demonstrated a clear concentration-dependent effect of compounds 2a, 3a, 3c (Fig. ). The IC50 value for 3c was equal to 6.6 ± 0.3 μM, considerably lower compared to the apparent IC50 for 3a (28 ± 2 μM). The IC50 for 2a could not be reliably measured.