Rhabdoid Tumor Growth is Inhibited by Flavopiridol

Purpose: Rhabdoid tumors are aggressive and incurable pediatric malignancies. INI1/hSNF5, a tumor suppressor biallelically deleted/inactivated in rhabdoid tumors, directly represses cyclin D1. Rhabdoid tumors and cells are exquisitely dependent on cyclin D1 for genesis and survival, suggesting that targeting the cyclin/cyclin-dependent kinase (cdk) axis may be an effective therapeutic strategy for these tumors. Because cdk inhibitors have not been used for preclinical or clinical testing on rhabdoid tumors, we investigated the effect of flavopiridol, a pan-cdk inhibitor with promising clinical activity, on rhabdoid tumors. Experimental Design: The effect of flavopiridol on rhabdoid cells was tested in vitro using survival, cell cycle, and apoptosis assays. Its effect was assessed in vivo using xenografted rhabdoid tumor models. Immunoblot and immunohistochemical analysis was used to assess the effect of flavopiridol on cyclin D1 and p21 expression in vitro and in vivo , respectively. Results: Nanomolar concentrations of flavopiridol inhibited rhabdoid cell growth (IC 50 ∼200 nmol/L), induced G 1 and G 2 arrest, and apoptosis in vitro in a concentration-dependent manner. These effects were correlated with the down-modulation of cyclin D1, up-regulation of p21, and induction of caspase 3/7 activities. Flavopiridol (at 7.5 mg/kg) significantly inhibited the growth of xenografted rhabdoid tumors, and its effect was correlated with the induction of p21 and down-modulation of cyclin D1. Conclusions: Flavopiridol is effective in inducing cell cycle arrest and cytotoxicity in rhabdoid tumors. Its effects are correlated with the down-regulation of cyclin D1 and the up-regulation of p21. Flavopiridol is potentially a novel chemotherapeutic agent for rhabdoid tumors.