IPI-504, a Novel, Orally Active HSP90 Inhibitor, Prolongs Survival of Mice with BCR-ABL T315I CML and B-ALL.

Development of mutations within the kinase domain is a major drug-resistance mechanism for tyrosine kinase inhibitors (TKIs) in cancer therapy. In CML (chronic myeloid leukemia), a disease driven by the constitutively active BCR-ABL oncoprotein, no available TKIs have been effective in treating patients with the BCR-ABL T315I mutation. Heat shock protein 90 (Hsp90) is a highly conserved, constitutively expressed molecular chaperone that facilitates folding of client proteins like BCR-ABL, and affects the stability of these proteins. Several labs have shown that Hsp90 inhibition in vitro results in the degradation of BCR-ABL T315I and induces potent killing of these cell lines. However, these results have not been demonstrated in animal models for BCR-ABL-induced CML and B-ALL (B-cell acute lymphoblastic leukemia, a disease that does not respond well to TKIs including imatinib and dasatinib). Thus, IPI-504, an orally administered Hsp90 inhibitor, was evaluated in murine models of CML and B-ALL. Treatment of mice with wild type (WT)- or T315I BCR-ABL-induced CML with IPI-504 resulted in BCR-ABL protein degradation and a decrease in circulating BCR-ABL positive cells. In response to treatment with vehicle the median survival time of WT and T315I CML mice is approximately 20 days. While the T315I CML mice were resistant to imatinib with a median survival of 21 days, IPI-504 (50 and 100 mg/kg, PO TIW) demonstrated dose-dependent prolonged survival of these mice by 30 and 70 days, respectively (p<0.001 for both doses). Both imatinib and IPI-504 similarly prolonged survival of mice with BCR-ABL-WT-induced CML. In the T315I CML mice prolonged survival of the IPI-504 treated cohort was associated with decreased peripheral blood BCR-ABL positive leukemia cells during treatment, less splenomegaly and improved pulmonary histopathlogy at necropsy. In CML mice receiving mixed BCR-ABL-WT- or T315I-transduced donor bone marrow cells, Hsp90 inhibition more potently suppressed T315I-expressing leukemia clones relative to the WT clones, consistent with in vitro studies where T315I BCR-ABL was more sensitive to IPI-504 induced degradation in cell lines than WT BCR-ABL. Combination treatment with IPI-504 and imatinib was more effective than either treatment alone in prolonging survival of mice bearing both WT and T315I leukemic cells. IPI-504 also significantly prolonged survival of B-ALL mice bearing the T315I mutation (p<0.001).

These results provide a rationale for use of an Hsp90 inhibitor as a novel approach to overcoming resistance to TKIs as well as the potential for first line combination treatment in CML patients. The potential for IPI-504 to eliminate mutant kinases via Hsp90 inhibition provides a new therapeutic strategy for treating BCR-ABL-induced CML, ALL as well as other cancers resistant to treatment with TKIs.