Potentiation of cytotoxicity by 3-aminobenzamide in DNA repair-deficient human tumor cell lines following exposure to methylating agents or anti-neoplastic drugs

We studied the potentiation by 3-aminobenzamide (3AB) of killing of nine human cell lines exposed to alkylating agents. Cell lines included normal, transformed and DNA repairproficient and -deficient pbenotypes. 3AB potentiated cell killing by the methylating agents methyhnethanesulfonate (MMS) and N-methyl-N′-nitro-N-nitrosoguanidine (MNNG) in all lines tested. The degree of potentiation ranged from 1.7- to 3.8-fold, based on the LD99. The average potentiation observed with MMS (2.7-fold) was greater than with MNNG (2.2-fold). On average the potentiation of MMS and MNNG killing of repair-deficient Mer− lines (2.4-fold) was similar to that of repair-proficient Mer+ lines. The degree of 3AB potentiation of MNNG killing (2.0-fold) was similar in Mer+ Rem− lines and in Mer+ Rem+ lines. Mer+ Rem+, Mer+ Rem−, Mer− Rem+, and Mer− Rem− strains all appeared proficient in a 3AB-sensitive DNA repair pathway. Within experimental error, 20 mM 3AB did not inhibit the removal of the MNNG-induced methylpurines 7-methylguanine, O6-methylguanine and 3-methyladenine from the DNA of repair-proficient Mer+ Rem+ HT29 cells, consistent with evidence that 3AB inhibits the ligation step of excision repair. 3AB potentiated cell killing by the bifunctional alkylating agents 1-(2-chlorethyl)-1-nitrosourea or busulfan, two antineoplastic drugs, by only 0.9- to 1.5-fold. These drugs therefore produce DNA damage which is not efficiently repaired by the pathways that repair methylated bases.