The Novel HDAC Inhibitor Chidamide Synergizes with Rituximab to Inhibit DLBCL Tumor Growth in Vitro and In Vivo By up-Regulating CD20 Expression
Authors: Guan Xu-Wen , Hua-Qing Wang , Jia Li , Liu Feng-Ting 
Published: October 2019

Abstract
Background: Histone deacetylases (HDACs) are crucial proteins for supporting tumorigenesis. HDACs reverse chromatin acetylation and alter transcription of oncogenes and tumor suppressor genes by removing acetyl groups from histones. HDAC inhibitors are considered as promising anti-cancer drugs, particularly in combination with other standard treatment regimens. Chidamide is the world first oral HDAC inhibitor which selectively inhibits class I HDAC1, HDAC2, and HDAC3 as well as class IIb HDAC10. Chidamide has been approved by China FDA in 2015 for the treatment of relapsed or refractory peripheral T-cell lymphoma.
Diffuse large B-cell lymphoma (DLBCL) is the most aggressive form of B-cell lymphoma. Treatment with R-CHOP i.e. Rituximab (the anti-CD20 monoclonal antibody) plus CHOP (Cyclophosphamide, doxorubicin, vincristine, and prednisone) has significantly improved clinical outcome for DLBCL patients. However, treatment-induced deacetylation of CD20 gene and consequently down-regulation of CD20 protein expression causes an acquired resistance to further treatment with R-CHOP.
We hypothesize that inhibition of HDACs by Chidamide could overcome Rituximab-mediated down-regulation of CD20 and facilitate Rituximab-induced DLBCL tumor growth inhibition. The aim of this study is to determine the synergistic effect of Chidamide and Rituximab in the treatment of DLBCL in vitro and in vivo.
Methods: The levels of CD20 (MS4A1) mRNA expression and clinical outcomes in patients with DLBCL treated either with R-CHOP or CHOP were obtained from the Gene Expression Omnibus (GEO) repository (NCBI GSE 10846). The association of CD20 expression with overall survival (OS) was analyzed by Cox regression analysis and the cut-off point was calculated by the X-tile software. CD20 protein surface expression and Rituximab-induced cell death were analyzed by flow cytometry. The IC50s of Chidamide and the synergisms with Rituximab (10 µg/ml) on five DLBCB cell lines (OCI-LY3, OCI-LY7, Su-DHL6, Su-DHL8, and Su-DLH10) were determined by MTT test after cells were treated with a range of concentrations of Chidamide with or without Rituximab for 24 hours. The synergism was calculated using ComboSyn software to obtain the combination index (CI). For in vivo experiments, the human DLBCL cell line OCI-LY7 were injected to 6 weeks BALB/C nude mice to develop xenograft DLBCL mice models. After tumors were palpable, mice were divided into four groups and injected with NaCl (control), Rituximab, Chidamide and Rituximab plus Chidamide daily for three weeks. The tumor volumes were monitored frequently during the treatment.
Results: In R-CHOP treated cohort (n=233), higher expression of CD20 expression (n=137) is significantly associated with superior clinical outcomes compared with lower CD20 expression (n=96) with P=0.0038, HR=0.4753, 95% CI=0.274-0.779. However, the levels of CD20 have no effect on clinical outcome in DLBCL patients treated with CHOP (n=183). The levels of CD20 protein surface expression on five DLBCL cell lines were significantly and positively correlated with the sensitivities of cells to Rituximab-induced cell death (P=0.0018, R=0.88). HDAC1, HDCA2 and HDCA3 proteins were detected in these DLBCL cell lines. Treatment with Rituximab significantly reduced CD20 surface expression but treatment with Chidamide significantly increased CD20 surface expression in DLBCL cells. The CI numbers for combined treatment with Chidamide and Rituximab were either <0.01 (very strong synergism) or <0.3 (strong synergism), indicating that Chidamide significantly synergized Rituximab-induced cell death. For in vivo assay, treatment with either Rituximab or Chidamide alone slightly but not significantly reduced tumor volume. Combination with Chidamide and Rituximab significantly inhibited tumor growth in DLBCL xenograft mice (P<0.0001). Mice with combined treatment showed significantly prolonged survival compared with other groups.
Conclusions: our data demonstrate for the first time that inhibition of HDACs by Chidamide significantly synergized Rituximab-induced tumor growth inhibition in vitro and in vivo. We propose that CD20 surface expression should be used clinically to evaluate treatment response in patients with DLBCL. Chidamide is a promising sensitizer for the treatment of DLBCL with R-CHOP.

Disclosures
No relevant conflicts of interest to declare.