Dynamics of Bromodeoxyuridine Incorporation into DNA of Squamous Carcinoma Cells During Mid and Late Logarithmic Growth

Head and neck squamous carcinoma cell lines, UM-SCC 1, 5, 9, 11B, and 14B, were exposed in vitro to bromodeoxyuridine (BUdR) during logarithmic growth to determine the effects of drug concentration (0.01 to 10 µM) and duration of exposure (3, 7, and 10 days) on cell growth and on incorporation of BUdR into DNA. Concentrations of less than 1.0 µM were not growth inhibitory except with UM-SCC-11B. After 10 days of exposure to 5 µM BUdR, survival fractions for all lines ranged from 2 to 65% of controls. Replacement of thymidine by BUdR in DNA was assessed by gas chromatography/mass spectroscopy. Percentage replacement (% R) was described by the equation % R = 100 (C/t)^s/[(C/t)₅₀ ^s + (C/t)^s], where C is the concentration of BUdR (µM), t is the time in days, s is a constant, and (C/t)₅₀ is a constant corresponding to % R = 50%. BUdR incorporation reached a time-and concentration-dependent maximum that, after 3 to 7 days of culture in 10 µM BUdR, ranged from 30 to 60% R. Subsequently, % R declined with time even though the cells were fed daily with fresh BUdR-containing medium.