Flavopiridol displays preclinical activity in acute lymphoblastic leukemia

Background New agents are needed for treatment of children with relapsed acute lymphoblastic leukemia (ALL). Based on altered expression of cell cycle regulatory proteins, including frequent p16 (INK4A) and p] 5 (INK4B) deletions, flavopiridol (FP; Alvocidib) is an attractive agent for relapsed ALL. Procedure. We evaluated the efficacy of FP in ALL cell lines using cell proliferation assays, determined the effects of FP treatment on cell growth and viability in cell lines and patient samples, examined cell cycle kinetics, and evaluated the effect of FP on endogenous cyclin-dependent kinase (CDK) activity, Mcl-1 expression, and RNA polymerase 11 expression and phosphorylation. Results. ALL cell lines are sensitive to FP. At lower concentrations, FP induces transient G(1)-S cell cycle arrest and modest levels of apoptosis in cell lines. In contrast, a sustained G(1)-S and G(2)-M arrest and substantial apoptosis are observed following exposure to higher FP concentrations. After treatment with FP, ALL cell lines have decreased expression of retinoblastoma protein phosphorylated at serines 795 and 807/811, indicating reduced CDK activity. We also show that ALL cell lines are sensitive to clinically achievable concentrations of FP in medium supplemented with human serum and that FP reduces the expression of Mcl-1 and phosphorylated forms of the C-terminal domain of RNA polymerase II. FP also increases cell death by approximately twofold over baseline in primary ALL blasts. Conclusions. These data provide a biological rationale for testing FP in relapsed ALL.