Targeting regulatory T cells by E7777 enhances CD8 T-cell-mediated anti-tumor activity and extends survival benefit of anti-PD-1 in solid tumor models

Front Immunol. 2023 Oct 27:14:1268979. doi: 10.3389/fimmu.2023.1268979. eCollection 2023.


Introduction: Regulatory T cell (Treg)-targeting cancer immunotherapy aims to transiently deplete Treg cells in the tumor microenvironment, without affecting effector T cells (Teff), thus both enhancing anti-tumor activity and avoiding autoimmunity. This study evaluated whether adding E7777 (a new formulation of denileukin diftitox [DD]) improved the efficacy of anti-PD-1 antibody therapy. DD is a recombinant protein containing the hydrophobic and catalytic portions of diphtheria toxin fused to full-length human IL-2. E7777 has the same amino acid sequence and brief circulatory half-life as DD, but with greater purity and potency.

Methods: Subcutaneous syngeneic murine solid tumor models (colon cancer CT-26 and liver cancer H22) were used to evaluate safety, efficacy, and overall survival with E7777 and anti-PD-1 antibodies, each administered as monotherapy or in concurrent or sequential combination. In Experiment 1, treatments were compared to assess anti-tumor activity at various time points, with tumors excised and dissociated and tumor leukocytes characterized. In Experiment 2, tumor growth, response, and overall survival were characterized for 100 days following a 3-week treatment.

Results: E7777 administered in combination with anti-PD-1 led to significantly increased anti-tumor activity and durable, extended overall survival compared to either treatment alone. In both tumor models, the Treg cell infiltration induced by anti-PD-1 treatment was counterbalanced by co-treatment with E7777, suggesting potential synergistic activity. Combination therapy showed the most favorable results. Treatment with E7777 was safe and well-tolerated.

Discussion: Combined E7777 and anti-PD-1 therapy was well tolerated and more effective than monotherapy with either drug.

Trial registration: NCT04855253 NCT05200559.

Keywords: Denileukin diftitox; cancer; immune checkpoint; immunotherapy; preclinical.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Animals
  • CD8-Positive T-Lymphocytes
  • Colonic Neoplasms* / drug therapy
  • Diphtheria Toxin
  • Humans
  • Immunotoxins* / pharmacology
  • Mice
  • T-Lymphocytes, Regulatory
  • Tumor Microenvironment


  • Immunotoxins
  • Diphtheria Toxin

Associated data


Grants and funding

The author(s) declare financial support was received for the research, authorship, and/or publication of this article. This study was funded by Dr. Reddy’s Laboratories. In September 2021, Citius Pharmaceutical Inc. acquired E7777, along with all commercialization and intellectual property rights. Clinical trials NCT04855253 and NCT05200559 are being supported by Citius Pharmaceutical Inc. Citius Pharmaceutical Inc had no role on the influence on the study as a funder.