KIT Inhibition Affects Heat Shock Protein (HSP) Activity: A Potential Rescue Mechanism towards KIT Tyrosine Kinase Inhibition and a Rationale for Dual KIT/HSP Inhibition.

Abstract 2753

Poster Board II-729

Activating mutations of the KIT class III receptor tyrosine kinases are associated with core binding factor leukemias (CBF AML), systemic mastocytosis (SM), gastrointestinal stromal tumors (GIST), melanomas, seminoma/dysgerminoma and sinonasal natural killer/T-cell non-Hodgkin lymphoma. Despite the encouraging therapeutic potential of KIT-tyrosine kinase inhibitors (TKI), resistance leading to disease progression occurs in many patients, specifically after TKI monotherapy. We hypothesized that resistance to therapy is promoted by activation of alternative signaling pathways which override TKI inhibition.

To explore the downstream signaling pathways of class III receptor tyrosine kinases, we performed unbiased phoshoproteomic analyses of mutant FLT3 or KIT leukemia and mastocytosis cell lines before and after TKI treatment. Tantalizingly, immunoaffinity purification of phosphopeptides followed by tandem mass spectrometry following KIT-inhibition with Imatinib at IC90 (100nM) revealed a significant upregulation of phosphorylation levels of peptides identified as members of the heat shock protein (HSP) family. Of interest, mRNA GeneChip® Array analysis of hematopoietic Ba/F3 cells transfected with either a mutant KIT isoform (D816V) or a mutant FLT3 isoform (ITD) and treated with TKI revealed significant downregulation of HSP family members in the FLT3 model – but stable mRNA levels in the KIT model. Taken together, our phosphoproteome and mRNA data suggest a protective function of HSP in mutant-KIT tumors treated with TKI.

Next we studied the antiproliferative and proapoptotic effects of the HSP90 inhibitor IPI-504, a 17-AGG-derivative, in mutant-KIT cell models. IPI-504 potently inhibited proliferation and induced apoptosis with an IC50 of 0.5 up to 5μM depending on the KIT isoform. Importantly, combination of IPI-504 with TKIs resulted in potentiation of the antiproliferative and proapoptotic effects achieved by either drug alone. Antitumor efficacy in combination therapy was observed even at HSP90 inhibitor concentrations that did not display antitumor activity if administered alone.

In conclusion, our model suggests that inhibition of KIT affects heat shock protein activity serving to stabilize the functionality of targeted autoactivated receptor tyrosine kinases, which provides a potential mechanism for resistance to TKI therapy. Importantly, we provide a rationale to combine TKI with (low-dose) HSP-inhibitors such as IPI-504 to optimize TKI therapy.