The heat shock protein 90 inhibitor IPI-504 induces KIT degradation, tumor shrinkage, and cell proliferation arrest in xenograft models of gastrointestinal stromal tumors

The activity of the receptor tyrosine kinase KIT is crucial for gastrointestinal stromal tumor (GIST) growth and survival. Imatinib and sunitinib are very effective in advanced GIST, but have no curative potential. The observation that heat shock protein 90 (HSP90) inhibition results in KIT degradation prompted us to assess the efficacy of the HSP90 inhibitor retaspimycin hydrochloride (IPI-504) alone or in combination with imatinib or sunitinib in two GIST xenografts with distinctive KIT mutations. Nude mice were grafted with human GIST carrying KIT exon 13 (GIST-882; n = 59) or exon 11 (GIST-PSW; n = 44) mutations and dosed with imatinib (50 mg/kg twice daily), sunitinib (40 mg/kg once daily), IPI-504 (100 mg/kg 3 times per week), IPI-504 + imatinib, or IPI-504 + sunitinib. We evaluated tumor volume, proliferation and apoptosis, KIT expression and activation, as well as adverse events during treatment. Treatment with IPI-504 alone resulted in tumor regression, proliferation arrest, and induction of tumor necrosis. We documented downregulation of KIT and its signaling cascade in IPI-504-treated animals. Treatment effects were enhanced by combining IPI-504 with imatinib or sunitinib. On histologic examination, liver damage was frequently observed in animals exposed to combination treatments. In conclusion, IPI-504 shows consistent antitumor activity and induces KIT downregulation in GIST, as a single agent, and is more potent in combination with imatinib or sunitinib. The sequence of drug administration in the combination arms warrants further studies.

The Novel HSP90 inhibitor, IPI-493, is highly effective in human gastrostrointestinal stromal tumor xenografts carrying heterogeneous KIT mutations

PURPOSE

KIT activity is crucial for gastrointestinal stromal tumors (GIST). Imatinib (IMA) and sunitinib (SUN) are very effective KIT-inhibitors in patients with advanced GIST but have no curative potential. We evaluated the efficacy of the novel HSP90 inhibitor IPI-493 alone, or in combination with IMA or SUN in GIST xenografts with KIT mutations.

EXPERIMENTAL DESIGN

Nude mice (n = 98) were grafted bilaterally with human GIST carrying KIT exon 11 (GIST-PSW), KIT exon 9 (GIST-BOE), or double, KIT imatinib-sensitive exon 11 and imatinib-resistant exon 17 mutations (GIST-48). Mice were divided into six treatment groups and dosed orally for 15 days as follows: (i) control group, sterile water; (ii) IMA alone; (iii) SUN alone; (iv) IPI-493 alone; (v) IPI-493+IMA; and (vi) IPI-493+SUN.

RESULTS

Treatment with IPI-493 resulted in tumor growth stabilization, variable proliferation arrest, induction of apoptosis and necrosis, and downregulation of KIT and its signaling cascade, especially in the GIST-BOE model. Significant reduction of vessel density was observed with IPI-493 treatment, and was equal to SUN treatment in GIST-PSW and GIST-BOE xenografts. IPI-493 treatment effects were enhanced in combination with TKIs, especially with IPI-493+SUN. In our hands, IPI-493 showed dose-dependent liver damages.

CONCLUSIONS

When administered as a single agent in a xenograft model, the HSP90 inhibitor IPI-493 has consistent antitumor activity and induces KIT downregulation in GISTs with heterogeneous KIT mutations. IPI-493 synergizes with TKIs that are commonly used for the treatment of advanced or IMA-resistant GIST. The antitumor response of IPI-493 is particularly enhanced in combination with SUN.

Combined treatment strategies in gastrointestinal stromal tumors (GISTs) after imatinib and sunitinib therapy

Resistance to tyrosine-kinase inhibitors remains an open issue in the treatment of patients with gastrointestinal stromal tumors. The complex biology of disease in the multi-resistant setting has led a progressively growing urgency and interest in development combined or integrated therapies. This mini-review outlines the rationale for developing new combined therapeutic approaches, and describes the state of the art of the various potential strategies and the promising research perspectives.