Inhibition of Chk1 inhibits cell proliferation in human triple-negative breast and ovarian cancer cell lines. A. Human tumor cell lines were treated with the Chk1 inhibitor V158411 for 72 hours and the cell number determined using the SRB assay. Values are the average of at least four independent determinations ± SD. B. Breast and ovarian cell lines were exposed to the Chk1 inhibitors V158411, PF-477736 and AZD7762 for 72 hours. Cell number was determined by SRB assay. Values are the mean ± SD for n ≥ 4. C. Comparison of cell line sensitivity to V158411 and PF-477736. D. Sensitivity to V158411 was independent of p53 mutational status. The p53 mutation status was determined from the Cancer Genome Project. E. Comparison of the sensitivity of breast (square), ovarian (triangle), colon (diamond) and lung (circle) cell lines to growth inhibition by V158411 and cisplatin. F. ER-positive breast cancer cells were exposed to V158411 in the presence of 0 or 10 μM 4-hyrdroxytamoxifen (4-OHT) for 72 hours and the cell number (GI50) determined by SRB assay. The values are the average of 4 determinations ± SD and the fold difference between 0 and 10 μM 4-OHT shown.