Hedgehog Pathway Inhibition Is a Novel Therapeutic Approach in Lymphomas and Its Efficacy is Inversely Related to PI3 Kinase Inhibition

Abstract 1848

Background:

The hedgehog signaling pathway has been shown to be important in the development of malignancies, however, the role of this pathway in lymphomas is uncertain. GDC-0449 is a novel small molecule inhibitor that has been shown to block hedgehog activity by inhibiting SMO. The purpose of this study was to determine a potential role for hedgehog inhibition in the treatment strategies for lymphomas. We further sought to understand the determinants of sensitivity and potential resistance to therapeutic inhibition of this pathway.

Methods and Results:

We tested the small molecule inhibitor GDC-0449 in cell lines that span the spectrum of lymphoma including Burkitt lymphoma (N=4), ABC DLBCL (N=3), GCB DLBCL (N=7), mantle cell lymphoma (N=2), multiple myeloma (N=2), lymphoid leukemia (N=2), and T cell leukemia (N=2). Using the MTT assay, we identified the IC50 of drug response in these cell lines. We performed gene expression profiling in these cell lines and applied the Cox proportional hazards model using IC50 as the survival variable to identify genes that were associated with either sensitivity or resistance to the drug.

We found that the drug was lethal in these cell lines at physiologically achievable concentrations with IC50s ranging from 5 to 22μM. We applied hierarchical clustering to the genes associated with sensitivity and resistance and found that the method distinguished two groups of samples that differed in their average IC50s by over 2-fold (8μM vs 17μM). There was a suggestion of association with lymphoma cell type with Burkitt lymphoma lines being the dominant cell type in the resistant group and DLBCL being the dominant cell type in the sensitive group.

Resistance to hedgehog inhibition was found to be highly associated with the phosphatidylinositol 3′-kinase (PI3K) signaling pathway, suggesting that the hedgehog pathway and PI3K pathways act through independent, complementary mechanisms to promote tumor proliferation. In order to test the hypothesis that PI3K signaling is independent of hedgehog signaling we tested the lethality of PI3K inhibition using a dual PI3K/mTOR inhibitor (BEZ235) in the same cell lines.

We found an inverse relationship between the hedgehog inhibitor and PI3K inhibitor, i.e. the cell lines that were relatively sensitive to hedgehog inhibition (mean IC50 of 8μm) were resistant to PI3K inhibition(mean IC50 of 0.69μm) and the cell lines that were relatively resistant to hedgehog inhibition (17μm) were relatively sensitive to PI3K inhibition (0.32μm).

Conclusion:

Hedgehog pathway inhibition is a promising therapeutic approach in a carefully defined subset of patients with lymphoma. Hedgehog and PI3K inhibitors appear to act by distinct mechanisms and may be complementary in the treatment of lymphomas.