FLAVOPIRIDOL INDUCES APOPTOSIS IN CHRONIC LYMPHOCYTIC-LEUKEMIA CELLS VIA ACTIVATION OF CASPASE-3 WITHOUT EVIDENCE OF BCL-2 MODULATION OR DEPENDENCE ON FUNCTIONAL P53

Flavopiridol has been reported to induce apoptosis in lymphoid cell li nes via downregulation of bcl-2. The in vitro activity of flavopiridol against human chronic lymphocytic leukemia (CLL) cells and potential mechanisms of action for inducing cytotoxicity were studied. The in vi tro viability of mononuclear cells from CLL patients (n = Il)was reduc ed by 50% at 4 hours, 24 hours, and 4 days at a flavopiridol concentra tion of 1.15 mu mol/L (95% confidence interval [CI] +/-0.31). 0.18 mu mol/L (95% Cl +/-0.04), and 0.16 mu mol/L (95% CI +/-0.04), respective ly. Loss of viability in human CLL cells correlated with early inducti on of apoptosis. Exposure of CLL cells to 0.18 mu mol/L of flavopirido l resulted in both decreased expression of p53 protein and cleavage of the caspase-3 zymogen 32-kD protein with the appearance of its 20-kD subunit. Contrasting observations of others in tumor cell lines, flavo piridol cytotoxicity in CLL cells did not correlate with changes in bc l-2 protein expression alterations. We evaluated flavopiridol's depend ence on intact 953 by exposing splenocytes from wild-type (p53(+/+)) a nd p53 null (p53(-/-)) mice that demonstrated no preferential cytotoxi city as compared with a marked differential with F-ara-a and radiation . Incubation of CLL cells with antiapoptotic cytokine interleukin-4 (I L-4) did not alter the LC50 Of flavopiridol. as compared with a marked elevation noted with F-ara-a in the majority of patients tested. Thes e data demonstrate that flavopiridol has significant in vitro activity against human CLL cells through activation of caspase-3, which appear s to occur independently of bcl-2 modulation, the presence of IL-4, or 953 status. Such findings strongly support the early introduction of flavopiridol into clinical trials for patients with B-CLL. (C) 1998 by The American Society of Hematology.