DEPLETION OF P185(ERBB2), RAF-1 AND MUTANT P53 PROTEINS BY GELDANAMYCIN DERIVATIVES CORRELATES WITH ANTIPROLIFERATIVE ACTIVITY

Purpose: Recently, it has been shown that geldanamycin (GA), a benzoqu inone ansamycin, is able to deplete mutant p53, p185(erbB2) and Raf-1 proteins in cancer cells. However, the relationship between these acti vities of GA and its antiproliferative activity is not clear. Here we investigated the effects of 28 GA derivatives in SKBr3, a human breast cancer cell line. Methods: We performed Western blot analysis of Raf- 1, p185(erbB2) and mutant p53 proteins following drug treatment and co rrelated these findings with the cytotoxicity of the various GA deriva tives. Results: We found that downregulation of Raf-1, p185(erbB2) and mutant p53 proteins was correlated. Thus, a drug that was active agai nst one oncoprotein was equally active against the two others. Inactiv e derivatives were identified by their inability to downregulate these oncoproteins, even at a high dose (2 mu M). These inactive drugs also had no or minimal antiproliferative activity (IC50 > 3 mu M). All oth er analogs (at a concentration of 2 mu M) downregulated p53, p185(erbB 2), and Raf-1, and also displayed cytotoxicity (IC50 in the range 6-60 0 nM). This category of drugs was further divided into more- and less- active agents by testing at lower doses (40 nM). The drugs that remain ed active against their molecular targets had an IC50 for antiprolifer ative activity of less than 40 nM. Maximal effects on mutant p53, p185 (erbB2) and Raf-1 were observed at doses that were 4-5 times greater t han the cytotoxic IC50. Conclusions: These findings suggest that GA an d its derivatives are cytostatic/cytotoxic at concentrations that also downregulate Raf-1, p185(erbB2) and mutant p53, and raise the possibi lity that depletion of these proteins and the antiproliferative activi ties of GA have a common mechanism.