Specific DNA repair mechanisms may protect some human tumor cells from DNA interstrand crosslinking by chloroethylnitrosoureas but not from crosslinking by other anti-tumor alkylating agents.

In two recent reports we have shown that pretreatment of MER+ cells [cells proficient at: reactivating N-methyl-N'-nitro-N-nitrosoguanidine (MNNG)-treated adenovirus; removing O-6 methylguanine from their DNA; and preventing DNA interstrand crosslinks produced by the chloroethylnitrosoureas (CNUs)] with MNNG apparently inhibits the repair process that these cells utilize to prevent CNU-induced DNA interstrand crosslinking. The MNNG pretreatment, accompanied by a subsequent CNU treatment, resulted in a synergistic increase in cell kill of 2-3 logs. In the present study we have examined whether or not conditions which inhibit the ability of a cell to prevent CNU-induced DNA interstrand crosslinking can also prevent DNA interstrand crosslinking induced by four clinically used alkylating anti-tumor agents. The agents used in the present study include cis-diamminedichloroplatinum(II) (cis-Pt), L-phenylalanine mustard (L-PAM), nitrogen mustard (HN-2) and 4-S-(propionic acid)-sulfidocyclophosphamide (C-2), a derivative of cyclophosphamide. Alkaline elution analysis was used to measure DNA interstrand crosslinking, and colony formation assays to estimate cell survival. Unlike the CNUs, all four agents produced DNA interstrand crosslinks in a Mer+ cell line in the absence of MNNG pretreatment. MNNG pretreatment did not alter the levels of DNA interstrand crosslinks formed. Similar results were seen with a Mer- cell line. In cytotoxicity studies, in contrast to the CNUs, MNNG pretreatment did not appreciably increase the cell kill produced by the four agents. Since all four agents studied are thought to react primarily at the N-7 position in guanine, these data suggest that: the DNA repair system which prevents CNU-induced crosslinking is specific for methyl, ethyl, and chloroethyl monoadducts; this DNA repair system is specific for adducts only at the O-6 position of guanine and does not recognize and remove adducts at other positions in DNA; or a combination of the two explanations.