The cytotoxicity and metabolism of 1-beta-D-arabinofuranosylcytosine (AraC) and its effects on DNA synthesis and integrity were studied in HCT 116 and NCI-H630 human colon cancer cells. In 116 cells, 0.1 microM AraC decreased colony formation by approximately 50%, whereas 1 microM was required in H630 cells. AraCTP levels after a 24-hr AraC exposure were 2.3- to 3.5-fold lower in H630 cells due to increased ability to deaminate AraCMP. AraC DNA levels increased in proportion to AraCTP pools (r = 0.99) and were 2-fold higher in 116 cells after a 24-hr exposure to 0.1 and 1 microM AraC. Although the half-life of AraCTP was < 1 hr in both lines, > 80% of AraC DNA was retained at 24 hr after drug removal. Clonogenic capacity was inversely related to the extent of AraC DNA incorporation. Interference with nascent DNA chain elongation increased with increasing AraC concentration x time. A 24-hr AraC exposure produced a dramatic shift in the elution profile of nascent DNA during a 15-hr elution at pH 12.1; these effects were greater in 116 cells (DNA retained on filter [percentage of control]): 78%, 23%, and 9% with 0.1, 1, and 10 microM AraC versus 84%, 42%, and 18% in H630 cells, respectively. The extent of nascent DNA damage was proportional to AraC DNA content. Net DNA synthesis was potently inhibited during AraC exposure in both lines. H630 cells had partial recovery of DNA synthesis at 24 hr after drug removal, whereas persistent inhibition was noted in 116 cells. A slight excess of double-strand breaks in parental DNA was detected after a 24-hr exposure to 10 microM AraC in 116 cells. The extent of DNA fragmentation was more pronounced 16 hr after drug removal and was greater in 116 cells: 8.5%, 19%, and 21% with 0.1, 1, and 10 microM AraC DNA content, magnitude of nascent DNA damage, duration of DNA synthetic inhibition, and induction of double-stranded DNA fragmentation appeared to be the crucial determinants of lethality.