Exposure-time dependent cell-kill by triapine (A), the interval necessary to restore the sensitivity to the second challenge of triapine (B), and the proposed mechanisms for the slow (C) and rapid (D) phases of cell-kill by triapine and Dp44mT. Panels A, MCF-7 cells were exposed to various concentrations of triapine for 8, 16 or 24 h, and clonogenic survival was determined. Panel B, MCF-7 cells were first challenged with 5 µM triapine for 1 h and washed. At 0, 1, 2 and 4 h thereafter, cells were again challenged with 5 µM triapine for 1 h and washed, and clonogenic survival was determined. Panel C, the sequences of the slow phase of cell-kill by triapine and Dp44mT are summarized. RPA denotes replication protein A. Panel D, the sequences of the rapid phase of cell-kill by triapine and Dp44mT are summarized.