DOES DNA TARGETING AFFECT THE CYTOTOXICITY AND CELL UPTAKE OF BASIC NITROQUINOLINE BIOREDUCTIVE DRUGS

Purpose: A series of 4-(N,N-dimethylaminopropylamino)-5-nitroquinoline bioreductive drugs was studied to determine whether DNA binding influ ences cytotoxic potency, hypoxic selectivity or cellular uptake in cel l culture. Methods and Materials: Cytotoxicity was assessed by clonoge nic assay of stirred suspension cultures of aerobic (20% O-2) or hypox ic (< 10 ppm O-2) late log-phase AA8 cells. Drug uptake was measured b y high performance liquid chromatography of acetonitrile-extracted cel l pellets and extracellular medium, or by using radiolabelled drug. Dr ug binding to calf-thymus DNA was measured by equilibrium dialysis. In tracellular pH was determined using the [C-14]-5,5-dimethyl-2,4-oxazol idinedione method and intralysosomal pH using the fluoroscein isothioc yanate-labelled dextran method. Result: The compounds were weak DNA bi nders under physiological conditions, with association constants in th e range 25-480 M(-1). There was no correlation between DNA binding aff inity and hypoxic or aerobic cytotoxic potency, or hypoxic selectivity . These compounds were accumulated by cells to high concentrations (25 -60 fold higher than extracellular), but cell uptake also showed no re lationship to DNA binding affinity. Ammonium chloride selectively rais ed intralysosomal pH and inhibited the cellular accumulation of these drugs. Conclusion: These results indicate that DNA binding is not the major determinant of cytotoxic potency, hypoxic selectivity, or cellul ar uptake of the 5-nitroquinolines. Instead, the variable contribution of a nonbioreductive mechanism of toxicity appears to underlie the di fferences in cytotoxic potency and hypoxic selectivity within this ser ies. The high intracellular drug concentrations of these diprotic base s appear to be due primarily to lysosomal uptake rather than DNA bindi ng. Lysosomal uptake might restrict diffusion of basic bioreductive dr ugs to the target hypoxic regions of solid tumors.