Background Regulatory T cell (Tregs) inhibit activity of anti-tumor T cells, and have been shown to limit checkpoint inhibitor effectiveness. Depletion of Tregs seems desirable during immunotherapy, but chronic Treg depletion with antibody therapies can lead to serious autoimmune adverse events. Compared to antibodies, the fusion protein E7777 (IL-2/diphtheria toxin) has a relatively short half-life in circulation, which allows for transient and selective Treg depletion. The potential therapeutic benefit of combining E7777 with anti-PD-1 was tested in syngeneic solid tumor models.
Methods CT26 colon and H22 liver cancer tumors were implanted subcutaneously in immunocompetent BALB/c mice. E7777 (2.5 mcg/mouse, i.v.) was given on a Q7Dx3 schedule. Anti-murine PD-1 was given (100 mcg/mouse, i.v.) Q4Dx5. Groups of 16 mice received each agent as monotherapy or in combinations. Sequencing of combination administration was also varied: Group 4 started treatment on the same day; Group 5 received E7777 2 days prior to start of anti-PD-1; Group 6 received anti-PD-1 first. Tumor growth was compared across all groups. In survival studies, mice were treated for 3 weeks and observed with twice weekly tumor measurements. In other experiments, tumors, tumor-draining lymph nodes, and spleens were examined by IHC and by flow cytometry of immune cells from dissociated tissues at defined points, for immune biomarkers.
Results Figure 1 shows additive benefit from the E7777 + anti-PD-1 combinations over either monotherapy. Most importantly, figure 2 and table 1 show significantly enhanced overall survival from a 3 week course of combinations compared to either agent alone (p<0.005) or to vehicle controls (p<0.000001). There was no clear distinction among different sequencing regimens. Benefit correlated with enhanced CD8:Treg ratios in tumors.
Conclusions Depletion of Tregs by E7777 significantly increased anti-tumor activity and durably extended overall survival compared to treatment with anti-PD-1 alone in syngeneic solid tumor models. Clinical studies of a combination of the two agents are planned.
Ethics Approval All studies were conducted at Crown Bio, and were approved by the Crown Bio IACUC.
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