The combi-targeting concept: Mechanism of action of the pleiotropic combi-molecule RB24 and discovery of a novel cell signaling-based combination principle

RB24 (NSC 741279), a 3-methyltriazene termed "combi-molecule" designed to possess mixed epidermal growth factor receptor (EGFR) targeting and DNA methylating properties showed over a 100-fold greater antiproliferative activity than Temodal (R) (TEM), a 4-fold greater potency than gefitinib and a 5-fold stronger activity than an equi-effective combination of gefitinib + TEM against the O-6-alkylguanine transferase (AGT)-proficient DU145 cell line that co-expresses EGFR. Investigation of the mechanisms underlying the unique potency of RB24 revealed that cell exposure to TEM was accompanied by activation of p38MAPK and concomitant elevation of the levels of X-ray repair cross-complementing group 1 (XRCC1) protein. Levels of phospho-p38MAPK and XRCC1 were increased by 2-fold in EGF-stimulated cells. In contrast, EGF-stimulation did not alter the status of these proteins in RB24-treated cells and this translated into a 2-fold lower level of XRCC1 when compared with those exposed to TEM + EGF. These effects correlated with significantly delayed DNA repair activity in combi-molecule-treated cells when compared with TEM-exposed ones. Further analysis demonstrated that in contrast to TEM, RB24 could block Bad phosphorylation at serine 136 in a dose-dependent manner and induced significantly higher levels of apoptosis than the former molecule. Tandem depletion of XRCC1 and Bad activation through alternative pathways using the MEK1 inhibitor, PD98059, led to substantial levels of apoptosis in RB24-treated cells. The results in toto indicate that the superior activity of the combi-molecule may be attributed to its ability to down-regulate DNA repair proteins such as XRCC1 and to alleviate anti-apoptotic signaling through blockade of EGFR-mediated signaling while inflicting high levels of DNA lesions to the cells. (C) 2010 Elsevier Inc. All rights reserved.