ONC201 Induces Apoptosis in Cutaneous T-Cell Lymphoma Cells through a Mechanism That Involves the Integrated Stress Response and Inactivation of Jak/Stat Signaling

Background: ONC201 (also called TIC10) is an orally active, first-in-class small molecule that is in phase II clinical trials for advanced cancers based on its ability to activate apoptosis in tumor cells, but not normal cells, in a p53-independent manner. Recent studies have implicated the integrated stress response as an early stage mechanism of ONC201 that may lead to the previously observed downstream anti-cancer effects. This study was conducted to evaluate the anti-tumor activity of ONC201 on Cutaneous T-Cell Lymphoma (CTCL) cell lines and Sézary cells.

Methods: We treated 8 CTCL cell lines (H9, HH, Hut78, Mac2A, MJ, MyLA, Pb2b and SeAx), peripheral blood mononuclear cells (PBMC) from 5 Sézary syndrome (SS) patients and 6 healthy donors with ONC201. MTS viability Assay was used to assess the anti-proliferation effect.  Apoptosis was assessed using Annexin V FITC/PI staining and sub-G1 analysis by flow cytometry.  Protein expression by western blotting was analyzed in 3 CTCL cell lines (HH, Hut78, MJ) and in PBMCs from 2 SS patients treated for 72 hours with ONC201including Activating Transcription Factor 4 (ATF4), interferon regulatory factor 7 (IRF7/pIRF7), Signal transducer and activator of transcription 3 (STAT3), Janus Kinase 3 (JAK3/pJAK3) and NF-κB family members (p65, RelB, c-Rel and p105).

Results: MTS viability assay showed pronounced cell growth inhibition, significantly increasing in a time-dependent manner in H9, HH, Hut78, Mac2A, MJ, MyLa, Pb2b, and SeAx lines after a 96-hour incubation within a very narrow efficacy threshold ranging from 1.25 to 10µM (n = 8, p < 0.05). To determine whether growth inhibition of ONC201 is due to cell-cycle arrest and/or to apoptosis in CTCL cell lines, sub-G1 analysis by flow cytometry was measured in 3 CTCL cell lines (HH, Hut78 and MJ). The percentages of HH, Hut78 and MJ cells with sub-G1 population increased, suggesting that cells are undergoing apoptosis (n = 3, p < 0.05). Induction of apoptosis was further confirmed by Annexin V FITC/PI staining, revealing dose and time dependent apoptosis induction in all 8 cell lines (Figure 1). To confirm cell line results in refractory ex vivo samples, we tested the pro-apoptotic effects of ONC201 on PBMCs from 5 SS patients who had high circulating CD4+CD26- malignant T-cells compared with PBMCs from 6 healthy donors. ONC201 induced significant levels of apoptosis in PBMCs from SS patients, but not in normal PBMCs (Figure 1, p < 0.001). Western blot analysis revealed that ONC201 increased the expression of ATF4, a hallmark of the integrated stress response and a negative regulator of IRF7. ONC201 decreased expression of IRF7/pIRF7 and down-regulated JAK3/pJAK3 and STAT3 expression in CTCL cell lines and PBMCs from SS patients. ONC201 also decreased the protein expression of NF-κB family members ( p65, RelB, c-Rel and p105) that can cause resistance to apoptosis in CTCL cells.

Conclusions: ONC201 appears to be active as a single oral agent. It may impact key signaling pathways in CTCL preclinical models by inhibiting proliferation and inducing apoptosis through a mechanism that involves the integrated stress response, leading to inactivation of JAK/STAT signaling and down-regulation of the NFκB pathway.

Figure 1. ONC201 induces apoptosis in CTCL cell lines and PBMCs from SS patients