ONC-212 (I-39), a Novel Inhibitor of the UPR, Is Cytotoxic and Cytostatic Against CLL Cells Under in Vitro Conditions That Mimic the Tumor Microenvironment

Introduction

Despite the revolution in the treatment of chronic lymphocytic leukemia (CLL) over the past decade with the introduction of novel inhibitors targeting the B-cell receptor (BCR) signaling pathway and the Bcl-2 family of proteins, relapse is still common.

Recent studies suggest that imipridones, a novel class of small molecule agents that attenuate mitochondrial respiration and modulate an immune response against cancer cells, may be an effective treatment option for several difficult to treat cancers.

We investigated the effects of the imipridone, ONC-212 (I-39, first published by Nanjing Gator Meditech), as a potential therapeutic strategy for CLL using the OSU-CLL cell line and a modified OSU-CLL line in which TP53 was stably knocked out and primary CLL cells cultured under conditions that mimic the tumour microenvironment (TME).

Methodology

Primary CLL cells were co-cultured with CD40L-expressing fibroblasts to mimic aspects of the TME. The cytotoxicity of ONC-212 was assessed using the mitochondrial dye DiIC₁(5), propidium iodide and flow cytometry. The effects of the drug on the adhesive and migratory capacity of primary CLL cells were evaluated using antibodies against CD49d, CXCR4 and an in vitro migration assay using stroma-derived factor 1a (SDF1-α). Changes in protein expression were assessed by immuno-blotting. The effects of ONC-212 on the cell cycle and proliferation were assessed using the OSU-CLL cell line. OSU-CLL cells were modified using the CRISPr-Cas9 technology to be TP53 deficient (OSU-TP53ko). The proportion of cells in each cycle phase was determined using propidium iodide and flow cytometry. Cell proliferation rates were determined using carboxyfluorescein succinimidyl ester (CFSE) and flow cytometry.

Results

ONC-212 induced apoptosis in a dose-dependent manner in primary CLL cells cultured in medium alone or in contact with CD40L-fibroblasts (Figure 1); the IC50 values were 72.97 nm +/- 1.45 nM and 472 +/- 2.04 nM, respectively. OSU-CLL and OSU-TP53ko cells were also sensitive to ONC-212, although the TP53 deficient line was less sensitive than OSU-CLL(Figure 1). IC50 values for the cell lines were 22 +/- 1.37 nM (OSU-CLL) and 48 +/- 3.25 nM (OSU-TP53ko).

ONC-212 induced cell cycle arrest of the OSU-CLL and OSU-TP53ko lines at the G1/S phase transition. This effect was concomitant with a significant reduction in the proliferation of both lines.

ONC-212 significantly down-regulated expression of the adhesion molecule CD49d and the G-coupled protein receptor CXCR4 on primary CLL cells. Down-regulation of CXCR4 translated into a decrease in the migratory capacity of CLL cells along an SDF1-α gradient.

Immunoblotting suggested the mechanisms of action of ONC-212 include inhibition of ERK1/2-MAPK, a decrease in the Bcl-2/Bax ratio and upregulation of the pro-apoptotic Puma and Bak proteins.

Conclusions

ONC-212 is highly effective against CLL cells at nanomolar concentrations, against cells cultured under conditions that mimic aspects of the TME and against TP53-deficient cells. ONC-212 has cytotoxic effects, induces cell cycle arrest, slows proliferation and inhibits the mechanisms by which CLL cells migrate to and are retained within the TME. ONC-212 inhibited signaling downstream of the BCR and induced a pro-apoptotic 'tipping' of the balance in expression of BCl-2 family proteins. These data suggest ONC-212 may represent an effective treatment for CLL, particularly for patients who have high risk, relapsed/refractory disease associated with loss or mutation of TP53.

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