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Synchronization of cells in the S phase of the cell cycle by 3′-azido-3′-deoxythymidine: implications for cell cytotoxicity


The mechanism of synergy between 3′-azido-3′-deoxythymidine (AZT) and anticancer agents was inverstigated with emphasis on cell-cycle events. Exposure of exponentially growing WiDr human colon carcinoma cells to AZT resulted in synchronization of cells in the S phase of the cell cycle. Forllowing treatment with AZT at 50 or 200 μM, 62%±3% or 82%±4% of the cells were in the S phase as compared with 36%±2% in the control. Bromodeoxyuridine uptake studies revealed that the synchronized cells actively synthesized DNA. At concentrations of up to 200 μM, AZT produced a cytostatic rather than cytotoxic effect as indicated by viability and cell growth measurements. at 200 μM, AZT-induced synchronization was significant (P=<0.001) after 12 h of drug exposure, reached a maximum at 24 h, and reversed to baseline levels by 72 h even in the continued presence of the drug. This indicates that AZT-induced cytostasis is a transient and reversible effect. The cell-cycle events seen with AZT in WiDr cells were also observed in eight of nine human tumor cell lines tested. Isobologram analysis of WiDr cells preexposed to AZT for 24 h and then exposed to either AZT-5-fluorouracil or AZT-methotrexate for a further 72 h revealed synergy between AZT and the anticancer agents, indicating that AZT-induced synchronization may have therapeutic benefits.

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5-FU :




BrdUrd :



3-(4-5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide


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Correspondence to David S. Duch.

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Chandrasekaran, B., Kute, T.E. & Duch, D.S. Synchronization of cells in the S phase of the cell cycle by 3′-azido-3′-deoxythymidine: implications for cell cytotoxicity. Cancer Chemother. Pharmacol. 35, 489–495 (1995).

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Key words

  • AZT
  • S-phase cytostasis
  • cytotoxicity
  • DNA histograms
  • 5-FU
  • MTX
  • flow cytometry