The Novel Sequence-Specific DNA Cross-Linking Agent SJG-136 (NSC 694501) Has Potent and Selective In vitro Cytotoxicity in Human B-Cell Chronic Lymphocytic Leukemia Cells with Evidence of a p53-Independent Mechanism of Cell Kill

Abstract

SJG-136 (NSC 694501) is a novel DNA cross-linking agent that binds in a sequence-selective manner in the minor groove of the DNA helix. It is structurally novel compared with other clinically used DNA cross-linking agents and has exhibited a unique multilog differential pattern of activity in the NCI 60-cell line screen (i.e., is COMPARE negative to other cross-linking agents). Given this profile, we undertook a preclinical evaluation of SJG-136 in primary tumor cells derived from 34 B-cell chronic lymphocytic leukemia (B-CLL) patients. SJG-136 induced apoptosis in all of the B-CLL samples tested with a mean LD₅₀ value (the concentration of drug required to kill 50% of the cells) of 9.06 nmol/L. Its cytotoxicity was undiminished in B-CLL cells derived from patients treated previously, those with unmutated V_H genes, and those with p53 mutations (P = 0.17; P = 0.63; P = 0.42, respectively). SJG-136-induced apoptosis was associated with the activation of caspase-3 that could be partially abrogated by the caspase-9 inhibitor Z-LEHD-FMK. Furthermore, SJG-136 did not trigger the phosphorylation of p53 or the up-regulation of GADD45 expression in B-CLL cells whereas the cross-linking agent chlorambucil elicited both of these effects. This suggests that SJG-136 cross-linking adducts are not subject to p53-mediated DNA excision repair mechanisms in B-CLL cells. Taken together, these data demonstrate a novel mechanism of action for SJG-136 that appears to circumvent the effects of poor prognostic markers. This unique cytotoxicity profile warrants further investigation and supports the evaluation of this agent in Phase I clinical trials for patients with B-CLL.