Time-dependent cytotoxicity induced by SJG-136 (NSC 694501): influence of the rate of interstrand cross-link formation on DNA damage signaling.

SJG-136 is a new pyrrolobenzodiazepine dimer inducing time-dependent cytotoxicity. HCT 116 cells were exposed to 50 nmol/L of SJG-136 for 1 hour or 1 nmol/L of SJG-136 for 24 hours to achieve similar levels of interstrand cross-links (ICL). The short exposure led to a rapid formation of ICLs (1 hour), early H2AX foci formation (4 hours), prominent S phase arrest, and greater phosphorylation of Nbs1 (on serine 343) and Chk1 (on serine 317) than a 24-hour exposure. The prolonged exposure at low concentrations of SJG-136 induced a gradual formation of ICLs (up to 24 hours) which was associated with a limited S phase arrest and delayed Nbs1 phosphorylation. Prolonged exposure was also associated with a reduced phosphorylation of p53 on serines 15 and 20, a limited and delayed phosphorylation on serine 392, and a less prominent increase in p21 levels. These data suggest that the 24-hour exposure to a low concentration of SJG-136 led to delayed and reduced DNA damage signaling compared with a higher concentration of SJG-136 for 1 hour, resulting in greater cytotoxicity and contributing to the time-dependent cytotoxic effect of SJG-136.