CDK inhibition is effective in treating xenografted triple-negative tumors in mice. (A) Tumor growth in mouse xenograft models of triple-negative breast cancer. Representative photos of tumors after 2 wk of treatment with either vehicle alone or with dinaciclib (50 mg/kg i.p. twice weekly) are shown. (B) Growth of triple-negative tumors in nude mice treated with the CDK inhibitor dinaciclib (50 mg/kg i.p. twice weekly) for 2 wk. Each treatment group per tumor cell type contained the indicated number of mice. The error bars represent means ± SEM. P-values were calculated by two-tailed Student’s t test. (C) PARP cleavage and serine phosphorylation of presumptive CDK substrates occurs in tumors within 24 h of dinaciclib administration. This antibody recognizes amino acid sequences that contain phosphorylated CDK consensus epitope, which is (K/R)(phosphorylated-S)(PX)(K/R) where X can be any amino acid. Two independent tumor samples for each treatment (vehicle or dinaciclib) are shown per cell line.