Abstract
1991
Hsp90 is a molecular chaperone protein involved in the folding and function of a number of client proteins, including those important in promoting tumour proliferation, survival and chemoresistance e.g. EGFR, ErbB2, c-Raf-1 and p-Akt. Inhibition of Hsp90 by 17AAG leads to depletion of client proteins by targeting them for proteasomal degradation. This pleiotrophic effect is believed to be an important factor in the activity of 17AAG as an antitumour agent. 17AAG has single agent activity in clinical trials but studies have suggested that it may be combined usefully with conventional chemotherapy. For example, Munster et al demonstrated the sensitisation of ErbB2 overexpressing breast cancer cell lines to taxol by 17AAG (Clin. Cancer Res; 2001) and Solit et al implicated suppression of Akt activity in the mechanism (Cancer Res; 2003). Ovarian tumours frequently display elevated levels of Akt-2 and has been linked with chemoresistance. SKOV-3 cells very highly overexpress ErbB2 and EGFR, lack PTEN and consequently have constitutively activated Akt. When 17AAG was combined with taxol in these cells at equitoxic concentrations (ratios of their IC50s) the combination index (CI) was <1 indicating greater than additivity (synergy; CI at ED50 = 0.62 ± 0.19). Individually, IC50 concentrations of 17AAG and taxol (80nM and 50nM respectively) did not induce a significant level of cell detachment until 48h to 72h, at which time apoptosis was confirmed by nuclear morphology. However, when the drugs were combined, ∼50% of the cells were detached and apoptotic at 24h (drugs alone ∼9%). This concentration of 17AAG led to increased Hsp70 (a marker of Hsp90 inhibition) and decreased ErbB2 at 8h, and decreased p-Erk and p-Akt at 12h. A non-growth inhibitory concentration of 17AAG (6nM) also sensitised SKOV-3 cells to taxol (IC50 decreased from 70nM to 20nM). Furthermore, cells exposed to an IC50 concentration of taxol alone did not display a significant level of detachment at 24h (∼8% of total) but the co-addition of 6nM 17AAG led to a marked increase at this time (∼35% were apoptotic). This concentration of 17AAG reduced the level of ErbB-2 and p-Akt at 24h but did not reduce c-Raf-1 or p-Erk. These data suggest that cell lines that overexpress ErbB2 may be sensitised to taxol by the effects of 17AAG probably via the downregulation of the PI3K/Akt pathway. This is supported, in part by the results of this combination of drugs in 3 other cell lines: A431 (squamous; high ErbB2; moderate p-Akt); CH1 (ovarian; low ErbB2 but moderate p-Akt); and HX62 (ovarian; low ErbB2 and low p-Akt). Synergy was observed between 17AAG and taxol in A431 cells (CI = 0.72 ± 0.13). Less than additivity was observed in CH1 and HX62 cells (2.7 ± 0.97 and 3.6 [mean of 2] respectively). Studies are underway to deconvolute further the molecular determinants of synergistic interaction between 17AAG and taxol with the aim of identifying molecular predictors of drug sensitisation in ovarian cancer.
[Proc Amer Assoc Cancer Res, Volume 45, 2004]