Radiosensitization and hypoxic cell toxicity of NLA-1 and NLA-2, two new bioreductive compounds

Synthesis of novel 2-nitroimidazole-tethered tricyclic quinolines, bearing a second heteroatom, and their in vitro evaluation as hypoxia-selective cytotoxins and radiosensitizers

Two novel nitroimidazole-based bioreductive compounds, 10-[3-(2-nitroimidazolyl)-propylamino]-3,4-dihydro-1H-thiopyrano[4,3-b]quinoline hydrochloride (8a) and 10-[3-(2-nitroimidazolyl)propylamino]-2-methyl-1,2,3,4-tetrahydro-benzo[b]-1,6-naphthyridine hydrochloride (8b) have been synthesized and evaluated in V79 cells as hypoxia-selective cytotoxins and radiosensitizers that target DNA through weak intercalation. Both compounds were relatively good radiosensitizers (C(1.6) values of 40.0+/-0.8 and 59.0+/-0.4 microM for 8a and 8b, respectively) but neither of the compounds was superior to 2 which does not carry a second heteroatom in the DNA-intercalating chromophore.

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 influences cytotoxic potency, hypoxic selectivity or cellular uptake in cell culture.

METHODS AND MATERIALS

Cytotoxicity was assessed by clonogenic assay of stirred suspension cultures of aerobic (20% O2) or hypoxic (< 10 ppm O2) late log-phase AA8 cells. Drug uptake was measured by high performance liquid chromatography of acetonitrile-extracted cell pellets and extracellular medium, or by using radiolabelled drug. Drug binding to calf-thymus DNA was measured by equilibrium dialysis. Intracellular pH was determined using the [14C]-5,5-dimethyl-2,4-oxazolidinedione method and intralysosomal pH using the fluorescein isothiocyanate-labelled dextran method.

RESULT

The compounds were weak DNA binders under physiological conditions, with association constants in the range 25-480 M-1. There was no correlation between DNA binding affinity 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 relationship to DNA binding affinity. Ammonium chloride selectively raised 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 cellular uptake of the 5-nitroquinolines. Instead, the variable contribution of a nonbioreductive mechanism of toxicity appears to underlie the differences in cytotoxic potency and hypoxic selectivity within this series. The high intracellular drug concentrations of these diprotic bases appear to be due primarily to lysosomal uptake rather than DNA binding. Lysosomal uptake might restrict diffusion of basic bioreductive drugs to the target hypoxic regions of solid tumors.

Potentiation of 1-(2-chloroethyl)-3-cyclohexyl-1-nitrosourea's toxicity in vitro by two new bioreductive agents

Two new bioreductive compounds, 9-[3-(2-nitro-1-imidazolyl)propylamino]acridine hydrochloride (NLA-1) and 9-[2-(2-nitro-1-imidazolyl)ethylamino]acridine hydrochloride (NLA-2), which behave as hypoxic cytotoxins and radiosensitizers, have been investigated for potentiation of 1-(2-chloroethyl)-3-cyclohexyl-1-nitrosourea's (CCNU) cytotoxic activity in vitro using V-79 cells. The preincubation effect as well as conditions of coadministration of CCNU with each sensitizer have been examined. In this latter case, the median-effect analysis was applied to evaluate whether the phenomenon was additive or synergistic. A clonogenic assay was used to score survival. Both bioreductive compounds, even at very low concentrations, significantly enhance the cytotoxic activity of CCNU under conditions of hypoxic preincubation. The enhancement of CCNU cytotoxicity is dependent upon preincubation time and the concentrations of both CCNU and the specific bioreductive agent. Coincubation of cells under hypoxia with CCNU and each bioreductive agent led to some potentiation, but only at lower survival levels. No chemosensitization was observed under aerobic conditions with either sensitizer.