17-AAG (KOS-953), an inhibitor of Hsp90 function, significantly enhances the cytotoxicity of anticancer drugs: An effective approach for combination therapy

1992

Hsp90 is an essential protein for maintaining the stability and function of a multitude of key oncogenic client proteins, including protein kinases and transcription factors. Geldanamycin, a benzoquinone ansamycin antibiotic, binds to the N-terminal ATP binding site of Hsp90, leading to disruption of the Hsp90-client protein complexes and degradation of the client proteins via the proteosome pathway. A geldanamycin analog, 17-allylamino-17-demethoxygeldanamycin (17-AAG; KOS-953), is currently being evaluated in multiple Phase 1b and Phase 2 clinical trials. Based on the unique activity of 17-AAG that simultaneously decreases a number of oncogenic proteins involved in many oncogenic signal transduction pathways and the importance of combination chemotherapy in cancer therapeutics, we evaluated whether 17-AAG could synergize with other anticancer drugs. In this study, we evaluated the antiproliferative effects of combining 17-AAG and over twenty different anticancer agents in the human colon cancer cell line DLD-1 and breast cancer cell line SKBr3. The anticancer agents selected represent a broad spectrum of mechanisms including alkylating agents, antimetabolites, anticancer antibiotics, microtubule-interacting agents, and protein kinase inhibitors. The dose-response curve and IC₅₀ value of each drug in these two cell lines were determined using a MTS assay. For the drug combination studies, cells were treated with increasing concentrations of these drugs either alone or in combination at their equipotent molar ratio with two different treatment schedules. In one schedule, the cells were exposed to 24 h of 17-AAG; the other drug was then added to the culture and incubated for 48 h. In another schedule, cells were exposed to a drug for 24 h followed by addition of 17-AAG for 48 h. Synergism, additivity, or antagonism of the combinations was determined by the median effect analysis using the combination index. Our results show that, in DLD-1 cells, approximately 80% of the drugs demonstrated synergistic effects with 17-AAG, whereas, in SKBr-3 cells, approximately 50% of the drugs showed synergistic effects. Importantly, more than 25% of the drugs including the microtubule-interacting agents docetaxel, vinblastine, vincristine and KOS-862 (Epothilone D), were synergistic with 17-AAG in both DLD-1 and SKBr3 cells regardless of treatment schedules. The observation that 17-AAG had synergistic effects with a broad range of anticancer agents in different tumor types suggests that 17-AAG is an effective agent for improving drug therapies by sensitizing tumor cells or potentiating their anticancer activity. The findings have important implications for combination strategies with 17-AAG in clinical development.