Effects of cytotoxicity of 2-chloro-2'-deoxyadenosine and 2-bromo-2'-deoxyadenosine on cell growth, clonogenicity, DNA synthesis, and cell cycle kinetics.

The cytotoxic effects of the adenosine deaminase resistant analogues 2-bromo-2'-deoxyadenosine (2-BrdAdo) and 2-chloro-2'-deoxyadenosine (2-CldAdo) have been compared with those of deoxyadenosine (dAdo). Like 2-CldAdo, 2-BrdAdo is highly effective in inhibiting the growth of many T-lymphoblastoid, B-lymphoblastoid, and myeloid cell lines in culture. Concentrations required to inhibit growth of CCRF-CEM human T-lymphoblastoid cells by 50% (IC50) are: 2-CldAdo, 0.045 microM; 2-BrdAdo, 0.068 microM; dAdo, 0.9 microM in the presence of 5 microM erythro-9-(2-hydroxy-3-nonyl)adenine. Like dAdo, 2-BrdAdo causes a much greater decrease in DNA synthesis than in RNA and protein synthesis. For each of the nucleosides the concentration required to cause 50% inhibition of DNA synthesis (as measured by thymidine incorporation) in an 18-h exposure is very similar to the IC50 for growth and to the concentration required to decrease viability (clonogenicity) over 18 h by 50% (EC50). A fraction of CCRF-CEM cells (approximately equal to 30%) is resistant to killing by exposure to 2-BrdAdo or 2-CldAdo for 4 h at concentrations 100 times the EC50, but 3% of cells are resistant to exposure for 4 h to a concentration of dAdo 3 times the EC50. Each of the three nucleosides causes accumulation of cells in S phase, the accumulation becoming more marked with longer periods of exposure and with higher concentrations of nucleoside. During exposures for 18-24 h at a concentration of nucleoside near the EC50 most cells accumulate in S, with most in early S, whereas exposure to concentrations greater than EC95 accumulates cells at the G1/S border. This suggests that loss of viability is associated with a blockade of some process specifically occurring at the initiation of S phase. At an optimum dosage schedule, 2-BrdAdo and 2-CldAdo have similar therapeutic effects against L1210 in vivo, both producing over 99% cell kill, but the optimum dosage of 2-CldAdo is lower.