UCN-01 is a hydroxylated derivative of staurosporine and a potent protein kinase C (PKC) inhibitor. Interest in the potential usefulness of this compound as an anticancer drug stems mainly from its unique anti-signaling, growth-arresting properties on tumor cells. This include activation of CDC2 kinase (CDK1) which interacts with either cyclin A or cyclin B1 at the G1 or G2/M border, suggeting that this event is one of the major consequences of the drug action on eukaryotic cells. Nonetheless, the antiproliferative activity of UCN-01 on normal rapidly dividing cells (intestinal epithelial and bone marrow cells) is not well documented. Thus, the main objective of this study was to investigate the in vivo antiproliferative activity of UCN-01 on these normal hyperproliferative cells and evaluate whether cellular response to UCN-01 could be modulated in the presence of DNA damage. Mice were injected i.m. with a single dose of UCN-01 (2.5 mg/kg-20 mg/kg) followed 3 and 24 h later by in vivo BrdU labeling for 1 h. At autopsy, bone marrow cells were collected and fixed for dual parameter BrdU/DNA flow cytometry. Different regions of the gut were also fixed for immunoperoxidase BrdU assays. Newly replicated cells were mainly located in the lower compartments of the crypt columns and were scored for BrdU stained nuclei using an image analysis system. A comparison between groups showed that 5 mg/kg UCN-01 induced inhibition in BrdU incorporation at 3 and 24 h, as compared to the other groups injected with various doses of UCN-01. Flow cytometric analysis of bone marrow cells stained with fluorescein tagged anti-BrdU (FITC) along with propidium iodide (PI) also showed inhibition in BrdU incorporation of S phase fraction cells in mice treated with 5 mg/kg UCN-01. These bone marrow cells were arrested primarily in the G1 phase of the cell cycle. The colony-forming unit (CFU) assay of the bone marrow cells was then used to determine the level of drug interaction of UCN-01 and, topotecan, a topoisomerase I inhibitor, at a fixed dose ratio. An antagonistic drug interaction (CI > 1) was observed as determined by the median-effect analysis. However, an additive interaction (CI = 1) was obtained with the use of camptothecin or 10,11-methylenedioxycamptothecin and UCN-01. The results of the in vitro drug interaction with UCN-01 may predict protection from topotecan-induced bone marrow toxicity.

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