alvocidib and bryostatin-1

Chronic lymphocytic leukemia (CLL) is the most common adult leukemia for which therapeutic options remain unsatisfying as cure has remained elusive. Recent laboratory discoveries and promising results from completed phase III studies with fludarabine provide reason to reassess therapeutic goals in the treatment of patients with symptomatic CLL. Early enrollment of patients on protocols using combination therapies with fludarabine and new agents such as flavopiridol, IDEC-C2B8, Campath-1H, UCN-01, bryostatin, FR 901228, and melarsoprol will hopefully represent the next advance to improved overall survival and ultimately the cure of CLL.

Topics: Alkaloids; Antineoplastic Agents, Alkylating; Bryostatins; Flavonoids; Humans; Lactones; Leukemia, Lymphocytic, Chronic, B-Cell; Macrolides; Piperidines; Staurosporine; Topoisomerase I Inhibitors

Interactions between the protein kinase C activator bryostatin 1 and the cyclin-dependent kinase (CDK) inhibitor flavopiridol (FP) have been examined in human myeloid leukemia cells (U937 and HL-60). Previous studies have demonstrated synergistic induction of apoptosis in leukemic cells exposed to the potent differentiation-inducer phorbol 12-myristate 13-acetate (PMA) in conjunction with FP [L. Cartee et al., Cancer Res., 61: 2583-2591, 2001]. Although bryostatin 1 (10 nM) is a very weak inducer of differentiation compared with PMA in these cells, coadministration of a minimally toxic concentration of FP (100 nM) did not promote bryostatin 1-related maturation but instead caused a marked increase in mitochondrial damage (e.g., cytochrome c release; loss of Deltapsi(m)), caspase activation, poly(ADP-ribose) polymerase cleavage, and apoptosis. Bryostatin 1/FP-induced apoptosis was significantly diminished in cells ectopically expressing dominant-negative Fas-associated death domain or by coadministration of tumor necrosis factor (TNF)-alpha soluble receptors, implicating the extrinsic pathway in bryostatin 1/FP actions. Enhanced apoptosis in bryostatin 1/FP-treated cells was accompanied by down-regulation of Mcl-1 and a sustained increase in TNF-alpha release. The selective protein kinase C inhibitor GFX blocked TNF-alpha and cytochrome c release in bryostatin 1/FP-treated cells and attenuated apoptosis. Finally, coadministration of bryostatin 1 (or PMA) with FP induced a marked increase in apoptosis in U937 cells ectopically expressing an NH(2)-terminal phosphorylation loop-deleted Bcl-2 protein, which are otherwise highly resistant to FP-mediated lethality. Taken together, these findings suggest that synergistic induction of apoptosis by bryostatin 1 and FP does not stem from disruption of the leukemic cell maturation process but instead results from enhanced release of TNF-alpha and activation of the extrinsic apoptotic cascade, culminating in cell death.

Topics: Antineoplastic Agents; Apoptosis; Bryostatins; Cell Adhesion; Cell Death; Drug Synergism; Flavonoids; HL-60 Cells; Humans; Intracellular Membranes; Kinetics; Lactones; Macrolides; Membrane Potentials; Mitochondria; Piperidines; Protein Kinase C; Receptors, Tumor Necrosis Factor; U937 Cells