The introduction of temozolomide (TMZ) has advanced chemotherapy for malignant gliomas. A considerable number of glioblastoma cases are refractory to TMZ, however, and the development of novel chemotherapeutic regimens is needed. The authors of previous studies have revealed that hsp90 is expressed at higher levels in human neoplastic tissues, including gliomas, than in normal tissues. Heat shock protein 90 is involved in a cytoprotective mechanism against cellular stressors such as DNA damage, and the authors hypothesized that hsp90 inhibitors might act as antitumor agents against gliomas and potentiate the cytotoxicity of DNA-damaging agents.
The authors examined the cytotoxicity of an hsp90 inhibitor, 17-(allylamino)-17-demethoxygeldanamycin (17-AAG), both alone and in combination with 1 of 3 DNA-damaging agents (cisplatin, 1,3-bis(2-chloroethyl)-1-nitrosourea, and TMZ) in human glioma cell lines. The cytotoxicity of these agents to glioma cells was measured using a colony formation assay. The cell cycle phase distribution, protein expression, and number of apoptotic cells were measured using a fluorescence-activated cell sorting assay, immunoblot assays, and double staining with annexin V and propidium iodide. In an in vivo experiment, 17-AAG, cisplatin, or 17-AAG and cisplatin were administered intraperitoneally to mice with xenografted U87MG cells, and the resulting tumor volumes were measured.
The authors found that 17-AAG reduced the clonogenicity of U87MG cells, and at a low concentration (< 100 nM) potentiated the cytotoxicity of the DNA-crosslinking agents cisplatin and 1,3-bis(2-chloroethyl)-1-nitrosourea, but not that of the DNA-methylating agent TMZ. This 17-AAG–induced potentiation of DNA crosslinking agent–induced cytotoxicity was a consequence of prolonged G2-M arrest accompanied by the suppression of cdc2 and cdc25C and of increased apoptotic cell death accompanied by the degradation of the antiapoptosis proteins Akt and survivin. Similar effects were observed when cells were treated with radicicol, another hsp90 inhibitor. The 17-AAG–induced enhancement of DNA crosslinking agent–induced cytotoxicity was also observed in other cell lines. In addition, 17-AAG sensitized xenografted U87MG cells to cisplatin in nude mice.
Heat shock protein 90–targeted therapy may be an effective strategy for potentiating chemotherapy using DNA-crosslinking agents for TMZ-refractory gliomas.