Noncycling A549 Human Lung Carcinoma Cells Flavopiridol: A Cytotoxic Flavone That Induces Cell Death in

Flavopiridol (NSC @98%,L86-8275), a potent inhibitor of cyclin dependent kinase 1Jp34@ phosphorylation and kinase activity, is cur rently undergoing Phase I clinical testing as a potential antineoplastic agent. Previous studies have suggested that flavopiridol is cytostatic but not cytotoxic when applied to exponentially growing cells. In the present study, various human tumor cell lines were assayed for trypan blue exclusion and ability to form colonies after exposure to flavopiridol under a variety of growth conditions. When log phase A549 non-small cell lung cancer cells were examined 72 h after the start of a 24-h flavopiridol exposure, as many as 90% of the cells accumulated trypan blue. A 24-h exposure to 250—300ns@resulted in trypan blue uptake in 50% of A549 cells at 72 h and a 50% reduction in colony formation. Similar results were observed in HCT8 ileocecal adenocarcinoma, T98G glioblastoma, MCF-7 breast adenocarcinoma, and HL-60 leukemia cells. With A549 cells, iden tical results were obtained in actively growing logarithmic phase cells and growth-arrested confluent cells. Treatment with the DNA synthesis inhib itor aphidicolin only minimally affected the cytotoxicity of flavopiridoLIn contrast, the RNA synthesis inhibitor 5,6-dichloro-1-fl.D-ribofuranosyl benzimidazole or the protein synthesis inhibitor cycloheximide reduced the cytotoxicity of flavopiridol. These results suggest that: (a) flavopiridol is not only cytostatic, but also cytotoxic to a variety of human tumor cell lines; (b) flavopiridol is equally active against cycling and noncydlingA549 cells; and (c) RNA and protein synthesis appear to play a role in flavopiri dol-induced cytotoxicity.