Adding Indoximod to Hypofractionated Radiotherapy with Anti-PD-1 Checkpoint Blockade Enhances Early NK and CD8+ T-Cell–Dependent Tumor Activity

Abstract

Purpose: There is growing interest in combinations of immunogenic radiotherapy (RT) and immune checkpoint blockade, but clinical responses are still limited. Therefore, we tested the triple therapy with an inhibitor of the indoleamine 2,3-dioxygenase pathway, which like immune checkpoints, downregulates the antitumor immune response.Experimental Design: Triple treatment with hypofractionated RT (hRT) þ anti-PD-1 antibody (aPD1) þ indoximod was compared with the respective mono-and dual therapies in two syngeneic mouse models.Results: The tumors did not regress following treatment with hRT þ aPD1. The aPD1/indoximod combination was not effective at all. In contrast, triple treatment induced rapid, marked tumor regression, even in mice with a large tumor. The effects strongly depended on CD8 þ T cells and partly on natural killer (NK) cells.Numbers and functionality of tumor-specific CD8 þ T cells and NK cells were increased, particularly early during treatment. However, after 2.5-3 weeks, all large tumors relapsed, which was accompanied by increased apoptosis of tumor-infiltrating lymphocytes associated with a non-reprogrammable state of exhaustion, terminal differentiation, and increased activation-induced cell death, which could not be prevented by indoximod in these aggressive tumor models. Some mice with a smaller tumor were cured. Reirradiation during late regression (day 12), but not after relapse, cured almost all mice with a large B16-CD133 tumor, and strongly delayed relapse in the less immunogenic 4T1 model, depending on CD8 þ T cells.Conclusions: Our findings may serve as a rationale for the clinical evaluation of this triple-combination therapy in patients with solitary or oligometastatic tumors in the neoadjuvant or the definitive setting.

Combinations of Radiotherapy with Vaccination and Immune Checkpoint Inhibition Differently Affect Primary and Abscopal Tumor Growth and the Tumor Microenvironment

“…In preclinical mouse models, the local irradiation with a dose per fraction of 8-10 Gy delivered in two to three fractions has been shown to mimic a hypofractionated irradiation schedule in humans [24,33,34]. Recent research further supports the hypothesis that combined strategies are superior to induce efficient systemic anti-tumor responses [35,36].…”

Safety and interim survival data after intracranial administration of M032, a genetically engineered oncolytic HSV-1 expressing IL-12, in pet dogs with sporadic gliomas

“…[32][33][34] Indoximod is commercially available and has shown promising immunomodulatory effects in canine metastatic melanoma and sarcoma when used in combination therapies. 35,36 Prior studies have demonstrated elevated numbers of CD4+ Foxp3+ regulatory T cells intratumorally and in the peripheral blood of dogs with gliomas, as well as an increased proportion of plasmacytoid dendritic cells. 29,37 In humans, the IDO pathway has been shown to play an important role in plasmacytoid dendritic cell-induced Foxp3+ regulatory T-cell differentiation.…”

Natural killer cells in cancer biology and therapy

“…A triple-combination therapy which inhibits PD-1 and MER proto-oncogene tyrosine kinase plus radiotherapy, increases NK cells infiltration in abscopal TME [ 264 ]. Adding indoximod, an inhibitor of IDO pathway, to radiotherapy and PD-1 blockade also enhances NK cell activity and shows great clinical response [ 265 ]. Another representative drug of triple-combination is selenium-containing nanoparticles,which delivers doxorubicin to the tumor site and releases high energy rays.…”