Molecular and Biological Characteristics of Acquired Vorinostat-Resistant Cutaneous T-Cell Lymphoma Cells.

Abstract 1732

Poster Board I-758

Vorinostat (suberoylanilide hydroxamic acid, SAHA), a pan-histone deacetylase (HDAC) inhibitor, has an overall response rate of 24-30% in two phase II studies of patients with cutaneous T cell lymphoma (CTCL). Since a considerable proportion of CTCL patients did not reach a partial response and loss of response could occur after only a few months, development of resistance became an important clinical problem. Although we have shown that constitutive activation of STAT signaling may be involved in resistance to vorinostat, the other mechanisms of resistance to vorinostat in CTCL are largely unknown. To further investigate mechanisms of vorinostat resistance, we have generated a vorinostat-resistant CTCL cell line (HH/VOR) from a parent vorinostat-sensitive HH CTCL cell line by long term exposure to stepwise increasing concentrations of vorinostat. The HH parental cells were cultured in increasing concentrations of vorinostat starting at 10 nM. Viable cells were then passaged into a higher concentration of vorinostat in 10 nM increments until a concentration of 1 μM vorinostat was reached. The HH/VOR cells were then maintained in complete RPMI1640 medium containing 1 μM vorinostat. We studied the molecular and biological characteristics of the vorinostat-resistant CTCL cells. Compared with the parental vorinostat-sensitive HH cells, the vorinostat-resistant HH/VOR cells were highly resistant to vorinostat-mediated growth inhibition and apoptosis. Of interest, the HH/VOR also exhibited cross resistance to another pan-HDAC inhibitor panobinostat and a class I HDAC inhibitor SNDX-275. The vorinostat-resistant HH/VOR cells had reduced accumulation of acetylated histones (H3 & H4) and decreased expression of class I and class II HDACs (1-11), and no expression of multi-drug resistant (MDR) efflux transporters. Moreover, the vorinostat-resistant HH/VOR cells had elevated DNA binding of NF-κB and increased expression of phospho-NF-κB p65 and phospho-STAT-1 compared with the parent sensitive HH cells. Co-treatment with the RXR-selective retinoid bexarotene selectively enhanced vorinostat-induced apoptosis in the vorinostat-sensitive HH and -resistant HH/VOR CTCL cell lines as well as in patients' Sézary cells compared to normal CD4+ T-cells. Taken together, our findings provide further evidence for the potential of vorinostat to cause acquisition of HDAC inhibitor resistance in CTCL. This acquired HDAC inhibitor resistance neither correlates with over-expression of HDACs nor with expression of MDR but correlates with abnormal activation of the NF-κB and STAT-1. Bexarotene may override this acquired resistance of CTCL cells to vorinostat and other HDAC inhibitors.