Oncolytic Reovirus Is an Effective Treatment for Histone Deacetylase Inhibitor Resistant T-Cell Lymphoma

Aberrant gene expression plays a pivotal role during tumorigenesis and cancer progression. The acetylation status of histones is an important determinant of gene expression and is controlled by two opposing classes of enzymes: histones acetyl transferases (HATs) and histone deacetylases (HDACs). The deacetylation of histones is associated with repression of key tumor suppressor genes and has been linked to HDAC overexpression in multiple forms of cancer including lymphomas. Several HDAC inhibitors have been FDA approved for T-cell lymphoma (TCL) therapy including belinostat, vorinostat, and romidepsin. Despite the promising anti-lymphoma activity of HDAC inhibitors as a drug class, resistance is a significant clinical issue. Identification of new strategies that are more effective in the drug resistant patient population is a high priority, but the mechanisms underlying HDAC inhibitor-induced cell death and the development of drug resistance are not completely understood. Elucidating the molecular mechanisms driving HDAC inhibitor resistance and/or the specific targets that are altered in drug-resistant cells may facilitate the development of strategies that overcome drug resistance and are effective for refractory patients. To pursue this goal, we generated novel TCL cell line models of acquired HDAC inhibitor resistance through repeated exposure to belinostat. The sensitivity of parental and resistant TCL cells to belinostat and other clinically relevant HDAC inhibitors was initially characterized using cell viability and flow cytometric assays. Notably, belinostat-resistant cells displayed significant cross-resistance to other HDAC inhibitors including vorinostat, romidepsin, panobinostat, and ricolinostat. This indicates that TCL patients that fail one HDAC inhibitor regimen may not benefit significantly from subsequent treatment with other drugs of this class. Consistent with a lack of sensitivity to HDAC inhibitors, the resistant cells failed to induce increased acetylated histones, tubulin acetylation (an HDAC6 target), and exhibited reduced upregulation of the CDK inhibitor p21 following belinostat treatment. In agreement with the absence of these hallmark characteristics of HDAC inhibition, belinostat also failed to cleave caspase-3 in the belinostat-resistant cells. Comprehensive transcriptome analysis was conducted to further characterize these new drug-resistant models and identify potential mechanisms of resistance and targets for second-line treatment. Drug resistant cells featured significantly increased basal levels of the reovirus receptor junctional adhesion molecule-A (JAM-A) as well as decreased JAK/STAT activity, a key antiviral response pathway. Based on these findings, we investigated the efficacy of the proprietary clinical formulation of reovirus (Reolysin) in parental and drug-resistant models. Our investigation revealed that belinostat-resistant cells displayed enhanced sensitivity to oncolytic reovirus-induced cell death compared to their parental counterparts. The increased benefit of Reolysin in resistant models was linked to elevated viral loads and more efficient endoplasmic reticular stress-mediated apoptosis. The heightened sensitivity of HDAC-resistant TCL cells to Reolysin was further validated in parental and belinostat-resistant T-cell lymphoma xenograft models. Collectively, these data demonstrate that oncolytic reovirus may be a novel therapeutic approach to treat T-cell lymphoma patients that are relapsed/refractory to HDAC inhibitors. We are currently planning an early phase clinical trial to further test the safety and benefit of this new approach.