Figure 5: TDP1 and TOP1 levels modulate the cellular response of SCLC to topotecan. (A) The indicated SCLC cell lines were seeded in triplicate at 10,000 cells per well in a 96-well plate in 100 μl media containing the indicated concentration of topotecan (TPT) and incubated at 37°C for 4 days. Viability was assessed by adding 20 μL of the CellTiter-Blue viability assay reagent to wells on day 4, followed by 2 hour incubation at 37°C. Fluorescence was determined using a GloMax Multi Detection System (Promega, Southamption, UK) at excitation and emission wavelengths of λex 560 nm and λem 590 nm, respectively. For each cell line, percentage viability (%) was calculated as [(Fluorescence readtreatment/Fluorescence readuntreated)*100%] and the graphical representation shows the results obtained from three independent experiments ± STD. Graphical representations depicting Pearson’s correlation coefficient (r) between viability at the indicated TPT concentrations and TDP1 (B), TOP1 (C), or TDP1/TOP1 levels (D). The solid line represents the correlation coefficient for all six SCLC cell lines whereas the dotted line represents the correlation coefficient for the five SCLC cell lines HCC33, N417, H2171, H209 and H510, excluding H69 (triangle), which exhibits high levels of TDP1 and an unexpected hypersensitivity to topotecan.