The Combination of Histone Deacetylase Inhibitors and Hypomethylating Agents Exhibits Marked Synergy In Preclinical Models of T-Cell Lymphoma

Abstract 3937

CHOP and CHOP-like chemotherapy programs remain the most commonly used regimens for the treatment of peripheral T-cell lymphomas (PTCLs) despite often sub-optimal results. Histone deacetylase inhibitors (HDACIs) are epigenetic agents known to be active in T-cell lymphoma. Recently romidepsin (R) was approved for patients with relapsed or refractory CTCL. Both R and belinostat (B) are being investigated in patients with relapsed or refractory PTCL. We have previously shown that hypomethylating agents as decitabine (D) produce synergistic interactions with HDACIs in B-cell lymphomas. We investigated the in vitro and in vivo activity of D, R and B alone or in combination in different T-cell lymphoma and leukemia cell lines including CTCL (H9, HH), and T- acute lymphoblastic leukemia (T-ALL) lines resistant to gamma-secretase inhibitors (GSI) (P12, PF-382). For all cytotoxicity assays, luminescent cell viability was performed using CellTiter-Glo™ followed by acquisition on a Biotek Synergy HT. The IC₅₀s for D, B and R were calculated using the Calcusyn software (Biosoft). Drug: drug interactions were analyzed using the calculation of the relative risk ratios (RRR) based on the GraphPad software (RRR<1 are defining synergism). Apoptosis was assessed by staining with Yo-Pro-1 and propidium iodine followed by FACSCalibur acquisition. Whole cell lysate proteins were extracted and quantified according to Bradford assay. After electrophoresis on a gradient 4–20% SDS-PAGE gels the proteins were transferred to nitrocellulose membrane. After blocking and incubation with the primary and the secondary antibodies, the chemiluminescent agent was added and the x-ray films were exposed to the membranes. The IC₅₀s for belinostat alone at 24, 48 and 72 hours were generally in the nanomolar range: H9: 108.1nM – 35.7nM – 29.1nM; HH: 240.1nM - 67.6nM – 39.01nM; P12: 386.9nM – 99.9nM – 99.8nM; PF 382: 267.1nM – 135nM – 118.3nM. The IC₅₀s for romidepsin alone at 24, 48 and 72 hours were generally in the low nanomolar range: H9: 5nM – 2.1nM – 2.2nM; HH: 14nM – 2.6nM - 2.5nM; P12: 6.2nM – 2.4nM – 2.1nM; PF382: 6.1nM – 1.7nM – 1.5nM. The IC₅₀s for D alone at 72 and 96 hours were in the micromolar range: H9: 7.4uM – 3.7uM; HH: > 20 uM. In the cytotoxicity assays, the combination of D and B or R at 72 hours showed synergism in all the cell lines studied. The most representative RRRs are showed in table 1.

When H9, HH, P12 and PF382 cell lines were treated with D and B or R for 72 hours, all the combination groups showed significantly more apoptosis than the single drug exposures and controls. Table 2 displays the range of apoptosis induction for B, R and D or for them used in combination and the RRR value after the analysis for the most significant data.

Increased acetylation of H3 was observed when H9 cells were treated with R alone and synergistically increased after exposing cells to the combination of D + B and D + R. The expression of phosphorylated Stat3 was decreased after exposure of H9 cells to the combination of D and R. Additional interrogation of the effects of this epigenetic therapy on the JAK-STAT signaling pathway are now underway. An in vivo xenograft study in six to eight weeks old female SCID beige mice injected subcutaneously with 2 × 10⁷ HH cells has also begun and will be reported. Mice were separated into different cohorts and treated with intraperitoneal injections of D or B or their combination according to the following schedules: D alone at 1.5 mg/kg on days 1, 5; B alone at 35 mg/Kg/day for 7 days. Collectively, the data suggest that the combination of a hypomethylating agent like D and a HDACI (B and R) are synergistic in in vitro models of human T-cell lymphoma, and may lead to a new platform for the treatment of these diseases.