Cloretazine (VNP40101M; 101M; 1,2-bis(methylsulfonyl)-1-(2-chloroethyl)-2-[(methylamino)carbonyl]hydrazine) is a sulfonylhydrazine prodrug that generates both chloroethylating and carbamoylating species on activation. To explore the molecular mechanisms underlying the broad anticancer activity observed in preclinical studies, cloretazine and chloroethylating-only [i.e., 1,2-bis(methylsulfonyl)-1-(2-chloroethyl)hydrazine] and carbamoylating-only (i.e., 1,2-bis(methylsulfonyl)-1-[(methylamino)carbonyl]hydrazine) analogues were evaluated in five murine hematopoietic cell lines. These cell lines were separable into two groups by virtue of their sensitivity to 1,2-bis(methylsulfonyl)-1-(2-chloroethyl)hydrazine; the sensitive group included L1210, P388, and F-MEL leukemias (IC50s, 6-8 micromol/L) and the resistant group consisted of Ba/F3 bone marrow and WEHI-3B leukemia cells (IC50s, 50-70 micromol/L). Resistant cells expressed O6-alkylguanine-DNA alkyltransferase (AGT), whereas sensitive cells did not. A correlation existed between AGT expression and the functional status of p53; AGT- cells possessed defective p53, whereas AGT+ cells contained wild-type p53. Based on recent findings on regulation of AGT gene expression by others, we suspect that silencing of the AGT gene by promoter hypermethylation frequently occurs during tumor progression involving p53 inactivation. O6-Chloroethylguanine is the initial DNA lesion that progresses to lethal interstrand DNA cross-links. Cloretazine exhibited a much higher preference toward the O6-chloroethylation of guanine, as measured by the difference in IC50s to wild-type and AGT-transfected L1210 cells, than 1,3-bis(2-chloroethyl)-1-nitrosourea, which targets the same site in DNA. Preferential toxicity of cloretazine against AGT- tumor cells coupled with decreased toxicity to AGT+ cells in host tissues constitute the therapeutic basis for cloretazine.