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Test cell lines (T-lymphoblastic leukemia CEM, chronic myelogenous leukemia K562, breast carcinoma MCF7, cervical carcinoma HeLa, malignant melanoma G-361, and human fibroblasts BJ) were obtained from the American Type Culture Collection. Cell line suspensions (ca. 1.0  10^5 cells/mL) were placed in 96-well microtiter plates and after 24 h stabilization (time zero), test compounds which were serially diluted in DMSO were added (4 x 20 µL aliquots). Control cultures were treated with DMSO alone, such that final DMSO concentration in the incubation mixtures never exceeded 0.6%. Narwedine was evaluated at six 3-fold dilutions and the highest final concentration was generally 50 µM. After 72 h incubation, 100 µl Calcein AM solution (Molecular Probes, Invitrogen, CA, USA) was added, and incubation was continued for a further hour. The fluorescence of viable cells was then quantified using a Fluoroskan Ascent instrument (Labsystems, Finland). The percentage of surviving cells in each well was calculated by dividing the intensity of the fluorescence signals from the exposed wells by the intensity of signals from control wells and multiplying by 100.