Cimica et al. BMC Cancer 2010, 10:634 http://www.biomedcentral.com/1471-2407/10/634 R E S E A R C H A R T I C L E Open Access Potent inhibition of rhabdoid tumor cells by combination of flavopiridol and 4OH-tamoxifen Velasco Cimica1†, Melissa E Smith1†, Zhikai Zhang1, Deepti Mathur1, Sridhar Mani1,2,3, Ganjam V Kalpana1,3* Abstract Background: Rhabdoid Tumors (RTs) are highly aggressive pediatric malignancies with poor prognosis. There are currently no standard or effective treatments for RTs in part because treatments are not designed to specifically target these tumors. Our previous studies indicated that targeting the cyclin/cdk pathway is a novel therapeutic strategy for RTs and that a pan-cdk inhibitor, flavopiridol, inhibits RT growth. Since the toxicities and narrow window of activity associated with flavopiridol may limit its clinical use, we tested the effect of combining flavopiridol with 4-hydroxy-Tamoxifen (4OH-Tam) in order to reduce the concentration of flavopiridol needed for inhibition of RTs. Methods: The effects of flavopiridol, 4OH-Tam, and their combination on RT cell cycle regulation and apoptosis were assessed by: i) cell survival assays, ii) FACS analysis, iii) caspase activity assays, and iv) immunoblot analysis. Furthermore, the role of p53 in flavopiridol- and 4OH-Tam-mediated induction of cell cycle arrest and apoptosis was characterized using RNA interference (siRNA) analysis. The effect of p53 on flavopiridol-mediated induction of caspases 2, 3, 8 and 9 was also determined. Results: We found that the combination of flavopiridol and 4OH-Tam potently inhibited the growth of RT cells. Low nanomolar concentrations of flavopiridol induced G2 arrest, which was correlated to down-modulation of cyclin B1 and up-regulation of p53. Addition of 4OH-Tam did not affect flavopiridol-mediated G2 arrest, but enhanced caspase 3,7-mediated apoptosis induced by the drug. Abrogation of p53 by siRNA abolished flavopiridol- induced G2 arrest, but enhanced flavopiridol- (but not 4OH-Tam-) mediated apoptosis, by enhancing caspase 2 and 3 activities. Conclusions: Combining flavopiridol with 4OH-Tam potently inhibited the growth of RT cells by increasing the ability of either drug alone to induce caspases 2 and 3 thereby causing apoptosis. The potency of flavopiridol was enhanced by abrogation of p53. Our results warrant further studies investigating the combinatorial effects of flavopiridol and 4OH-Tam as a novel therapeutic strategy for RTs and other tumors that have been shown to respond to flavopiridol. Background RTs, including Malignant Rhabdoid Tumors (MRT), Atypical Teratoid and Rhabdoid Tumors (AT/RT), and extra renal rhabdoid tumors (ERRT) are rare, but highly aggressive pediatric solid tumors with poor prognosis [1]. Current therapy for RTs includes surgical resection, radiation therapy, and/or chemotherapy with empirically selected and highly toxic chemotherapeutics, which are * Correspondence: ganjam.kalpana@einstein.yu.edu † Contributed equally 1Department of Genetics, Albert Einstein College of Medicine, 1300 Morris Park Avenue, Bronx, New York 10461 USA Full list of author information is available at the end of the article largely ineffective [2,3]. Despite aggressive treatment, mean survival with surgical intervention alone is only 3 months and with adjuvant chemotherapy and radio- therapy is only 8 months [4]. Therefore, strategies based on understanding the genesis of RTs will aid in the development of novel therapies. RTs are characterized by biallelic deletions and/or mutations in INI1/hSNF5, a tumor suppressor and component of the chromatin remodeling SWI/SNF complex [5,6]. Reintroduction of INI1/hSNF5 into RT cells induces G1 cell cycle arrest and senescence. INI1/hSNF5 mediates these effects by directly activating p16Ink4a by recruiting the SWI/SNF complex and by directly repressing cyclin D1 by © 2010 Cimica et al; licensee BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.