MECHANISM OF CYTOTOXICITY OF 2-CHLORO AND 2-BROMO DEOXYADENOSINE FOR A HUMAN LYMPHOBLASTIC CELL LINE, CCRF-CEM: 15

Abstract

Cytotoxicity as measured by inhibition of growth and DNA synthesis (IC₅₀) and by decrease in clonogenicity (EC₅₀) has been compared with that of dAdo. IC₅₀ for growth and for DNA synthesis is similar in all 3 cases. IC₅₀ for 2-CldAdo (0.045 μM) is lower than for 2-BrdAdo (0.068 μM) and dAdo (0.9 μM). For 2-CldAdo and 2-BrdAdo, EC₅₀ is somewhat higher than IC₅₀ and much greater in the case of dAdo. About 20% of cells are resistant to concentrations of 2-BrdAdo >>EC₅₀ but viability is decreased to approximately 2% by 18 hr exposure to 2-CldAdo at 20 × EC₅₀ or to dAdo at 3 × EC₅₀. Treatment with 2-CldAdo or dAdo sufficient to cause large losses in viability result in accumulation of cells at the G₁/S border. The nucleosides are converted over 24 hr to increasing levels of the triphosphates. The latter inhibit reduction of CDP and of ADP by L1210 ribonucleotide reductase with IC₅₀ in the order: 2-BrdATP < 2-CldATP < dATP << ara-ATP. In intact CCRF-CEM cells dAdo specifically decreases dGTP with other dNTPs increasing, whereas 2-CldAdo and 2-BrdAdo decrease dCTP and dATP with a transitory decrease in dGTP but dGTP and dTTP are elevated above controls by 24 hr. Data indicate that inhibition of ribonucleotide reductase possibly contributes to inhibition of DNA synthesis but is probably not the lesion critical for cell death. Supported by American Lebanese Syrian Associated Charities.

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