Upregulated JAK/STAT Signaling Represents a Major Mode of Resistance to HDAC Inhibition In Lymphoma and Provides a Rationale for Novel Combination Therapy
Authors: Smith Jason , Walsh Katherine J. , Jacobs Cassandra L , Liu Qingquan , Fan Siyao , Patel Amee , Dave Sandeep 
Published: October 2019

Abstract
Abstract 434

Background
Histone deacetylase inhibitors (HDACis) have demonstrated significant clinical activity in hematologic malignancies; however, single agent response rates have ranged between 20–50% with the duration of response often measured in months, suggesting that drug resistance is a major mode of failure. The pathways through which these agents work and the means by which tumors develop resistance to them are poorly understood. Combination therapy targeting multiple oncogenic pathways holds the promise to improve upon both the depth and durability of these responses.
We investigated the mechanisms of inherent and acquired resistance to HDACis in a broad range of lymphomas. By detailing the molecular pathways implicated in resistance to HDACi, we sought to identify novel combinations of compounds that could overcome potential mechanisms that confer resistance.

Methods and Results
We tested two separate HDACis, LBH589 and SAHA in 51 cell lines representing a wide range of lymphomas including Burkitt lymphoma, diffuse large B cell lymphoma (DLBCL), mantle cell lymphoma, and Hodgkin lymphoma. Gene expression array data was generated for all these cell lines. We then identified genes that were significantly associated with resistance to both LBH589 and SAHA (p<.01) and applied hierarchical clustering to identify the functional significance of these genes. Histology was not predictive of sensitivity to either HDACi. These data were then analyzed using gene set enrichment to identify known molecular pathways associated with resistance. Activation of JAK/STAT signaling was found to be a major determinant of resistance among the cell lines that were relatively resistant to HDACi. (P<0.001, FDR <.25).
To determine whether these genes that we found to be associated with resistance reflected potential mechanisms of acquired resistance to HDACi therapy, we separately engineered resistance to LBH589 and SAHA in three DLBCL cell lines (LY3, BJAB, Farage) through incremental dose escalation over a period of up to 6 months. Each of these three cell lines demonstrated sustained growth at drug concentrations that were at or above their original IC50. Each of these cell lines were then exposed to the other HDACi and tested for cross resistance. In each case, the cell lines demonstrated complete cross-resistance to the other drug. We then profiled the gene expression of these cell lines that had acquired resistance. Similar to our previous results, these cell lines demonstrated increased signaling through the JAK/STAT pathway, suggesting that mechanisms of inherent and acquired resistance are similar.
We therefore reasoned that combining HDAC and JAK inhibition may overcome both inherent and acquired resistance. To investigate this hypothesis, we tested LBH589 and INCB018424, a JAK1/2 inhibitor, alone and in combination in the LY3, TMD-8, U2932, and BJAB cell lines. While INCB018424 demonstrated no single agent cytotoxicity, it yielded a high degree of synergy when combined with LBH589 with the combination index computed by the Chou-Talalay method ranging from .19 to .9.

Conclusion
HDACis show single agent activity in the treatment of a number of hematologic malignancies, however most patients develop resistance to these drugs after relatively short-lived remissions. Thus, the greatest promise of these drugs may lie in combination with other agents that target molecular pathways that underlie resistance to these drugs. Using gene expression profiling of a broad range of tumor types and sensitivity to HDACis we were able to identify activation of the JAK/STAT pathway as a common feature of inherent and acquired resistance to HDACis. We combined the JAK1/2 inhibitor INCB018424 with LBH589 and demonstrated a high degree of synergy. As the number of small molecule inhibitors with clinical activity increases, the need to identify rational preclinical combinations becomes greater. Pairing gene expression profiling and resistant cell lines is a promising approach to the selection of combinations likely to maximize clinical benefit while limiting toxicity.

Disclosures:
No relevant conflicts of interest to declare.