Depletion of p185erbB2, Raf-1 and mutant p53 proteins by geldanamycin derivatives correlates with antiproliferative activity

Purpose: Recently, it has been shown that geldanamycin (GA), a benzoquinone ansamycin, is able to deplete mutant p53, p185^erbB2 and Raf-1 proteins in cancer cells. However, the relationship between these activities 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 correlated these findings with the cytotoxicity of the various GA derivatives. Results: We found that downregulation of Raf-1, p185^erbB2 and mutant p53 proteins was correlated. Thus, a drug that was active against one oncoprotein was equally active against the two others. Inactive derivatives were identified by their inability to downregulate these oncoproteins, even at a high dose (2 μM). These inactive drugs also had no or minimal antiproliferative activity (IC₅₀ > 3 μM). All other analogs (at a concentration of 2 μM) downregulated p53, p185^erbB2, and Raf-1, and also displayed cytotoxicity (IC₅₀ in the range 6–600␣nM). This category of drugs was further divided into more- and less-active agents by testing at lower doses (40 nM). The drugs that remained active against their molecular targets had an IC₅₀ for antiproliferative 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 than the cytotoxic IC₅₀. Conclusions: These findings suggest that GA and its derivatives are cytostatic/cytotoxic at concentrations that also downregulate Raf-1, p185^erbB2 and mutant p53, and raise the possibility that depletion of these proteins and the antiproliferative activities of GA have a common mechanism. Received: 6 June 1996 / Accepted: 28 September 1996