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Abstract Background Epacadostat, an oral, selective inhibitor of IDO1, has shown activity when administered with pembrolizumab. We evaluated the addition of chemotherapy to epacadostat and pembrolizumab in patients with advanced or metastatic solid tumors. One proposed mechanism of resistance to PD-1 checkpoint inhibition is through immunosuppression mediated by L-kynurenine. IDO1, indoleamine-2,3-dioxygenase 1 is the rate-limiting enzyme catalyzing the conversion of L-tryptophan to L-kynurenine. If IDO1 is a mechanism of tumor escape from checkpoint inhibition, then addition of an IDO1 inhibitor with a PD-1 checkpoint inhibitor could enable tumor response to immunotherapy. Methods Patients received one of 7 tumor-appropriate chemotherapy regimens. Pembrolizumab 200 mg was infused intravenously every 3 weeks. Epacadostat 100 mg was administered orally twice daily. The primary objectives of phase I were determining safety/tolerability and defining the maximum tolerated or pharmacologically active dose of epacadostat. Phase II of the study was designed to enroll efficacy-expansion cohorts and to assess changes in the tumor and tumor microenvironment via mandatory-biopsy cohorts. Results A total of 70 patients were enrolled. Twelve patients were enrolled in the phase II mandatory-biopsy cohorts. Due to early study closure, efficacy expansion did not enroll. Grades 3 and 4 treatment-emergent adverse events (TEAEs) occurred in 78.6% of patients. Neutropenia and disease progression were the only grades 3 and 4 TEAEs reported in ≥10.0% of patients. One treatment-related death was reported. The ORR was 31.4% across all treatment groups. Conclusion The combination of epacadostat 100 mg bid with pembrolizumab and chemotherapy had an acceptable safety profile. This regimen showed antitumor activity across multiple types of advanced or metastatic solid tumors (ClinicalTrials.gov Identifier: NCT03085914). The phase I/II ECHO-207/KEYNOTE-723 Study evaluated the addition of chemotherapy to epacadostat and pembrolizumab in 70 patients with advanced or metastatic solid tumors.

Indoleamine 2,3-dioxygenase 1 (IDO1), a monomeric heme-containing enzyme, catalyzes the first and rate-limiting step in the kynurenine pathway of tryptophan metabolism, which plays an important role in immunity and neuronal function. Its implication in different pathophysiologic processes including cancer and neurodegenerative diseases has inspired the development of IDO1 inhibitors in the past decades. However, the negative results of the phase III clinical trial of the would-be first-in-class IDO1 inhibitor (epacadostat) in combination with an anti-PD1 antibody (pembrolizumab) in patients with advanced malignant melanoma call for a better understanding of the role of IDO1 inhibition. In this review, the current status of the clinical development of IDO1 inhibitors will be introduced and the key pre-clinical and clinical data of epacadostat will be summarized. Moreover, based on the cautionary notes obtained from the clinical readout of epacadostat, strategies for the identification of reliable predictive biomarkers and pharmacodynamic markers as well as for the selection of the tumor types to be treated with IDO1inhibitors will be discussed.

Indoleamine 2,3-dioxygenase 1 (IDO1) is a tryptophan metabolizing enzyme chronically activated in many cancer patients and its expression and activity correlate with a poor prognosis. In fact, it acts as an immune regulator and contributes to tumor-induced immunosuppression by determining tryptophan deprivation and producing immunosuppressive metabolites named kynurenines. These findings made IDO1 an attractive target for cancer immunotherapy and small-molecule inhibitors, such as epacadostat, have been developed to block its enzymatic activity. Although epacadostat was effective in preclinical models and in early phase trials, it gave negative results in a metastatic melanoma randomized phase III study to test the benefit of adding epacadostat to the reference pembrolizumab therapy. However, the reason for the epacadostat failure in this clinical trial has never been understood. Our data suggest that a possible explanation of epacadostat ineffectiveness may rely on the ability of this drug to enhance the other IDO1 immunoregulatory mechanism, involving intracellular signaling function. These findings open up a new perspective for IDO1 inhibitors developed as new anticancer drugs, which should be carefully evaluated for their ability to block not only the catalytic but also the signaling activity of IDO1.

Primary or acquired mutations in RAS/RAF genes resulting in cetuximab resistance have limited its clinical application in colorectal cancer (CRC) patients. The mechanism of this resistance remains unclear. RNA sequencing from cetuximab‐sensitive and ‐resistant specimens revealed an activation of the tryptophan pathway and elevation of IDO1 and IDO2 in cetuximab‐resistant CRC patients. In vitro, in vivo, and clinical specimens confirmed the upregulation of IDO1and IDO2 and the Kyn/Trp after cetuximab treatment. Additionally, the IDO inhibitor, epacadostat, could effectively inhibit the migration and proliferation of cetuximab‐resistant CRC cells while promoting apoptosis. Compared to epacadostat monotherapy, the combination of cetuximab and epacadostat showed a stronger synergistic anti‐tumor effect. Furthermore, in vivo experiments confirmed that combination therapy effectively suppressed tumor growth. Mechanistically, KEGG pathway analysis revealed the activation of the IFN‐γ pathway in cetuximab‐resistant CRC tissues. Luciferase reporter assays confirmed the transcriptional activity of IDO1 following cetuximab treatment. Silencing IFN‐γ then suppressed the upregulation induced by cetuximab. Moreover, we observed that the combination reduced the concentration of the tryptophan metabolite kynurenine, promoted the infiltration of CD8+ T lymphocytes, and enhanced the polarization of M1 macrophages within the tumor microenvironment, thereby exerting potent anti‐tumor immune effects. Overall, our results confirm the remarkable therapeutic efficacy of combining cetuximab with epacadostat in cetuximab‐resistant CRC. Our findings may provide a novel target for overcoming cetuximab resistance in CRC. The effect of cetuximab and epacadostat combinative therapy on CRC cells.

It was previously reported that the activation of antitumor immune response by photodynamic therapy (PDT) is crucial for its therapeutic outcome. Excessive PDT-mediated inflammation is accompanied by immunosuppressive mechanisms that protect tissues from destruction. Thus, the final effect of PDT strongly depends on the balance between the activation of an adoptive arm of immune response and a range of activated immunosuppressive mechanisms. Here, with flow cytometry and functional tests, we evaluate the immunosuppressive activity of tumor-associated myeloid cells after PDT. We investigate the antitumor potential of PDT combined with indoleamine 2,3-dioxygenase 1 (IDO) inhibitor in the murine 4T1 and E0771 orthotopic breast cancer models. We found that the expression of IDO, elevated after PDT, affects the polarization of T regulatory cells and influences the innate immune response. Our results indicate that, depending on a therapeutic scheme, overcoming IDO-induced immunosuppressive mechanisms after PDT can be beneficial or can lead to a systemic toxic reaction. The inhibition of IDO, shortly after PDT, activates IL-6-dependent toxic reactions that can be diminished by the use of anti-IL-6 antibodies. Our results emphasize that deeper investigation of the physiological role of IDO, an attractive target for immunotherapies of cancer, is of great importance.

Keywords: Epacadostat, IDO-1 inhibitor, PD-1 inhibitor, Clinical trial

The tryptophan-degrading enzyme indoleamine 2,3-dioxygenase 1 (IDO1) is a plastic immune checkpoint molecule that potently orchestrates immune responses within the tumor microenvironment (TME). As a heme-containing protein, IDO1 catalyzes the conversion of the essential amino acid tryptophan into immunoactive metabolites, called kynurenines. By depleting tryptophan and enriching the TME with kynurenines, IDO1 catalytic activity shapes an immunosuppressive TME. Accordingly, the inducible or constitutive IDO1 expression in cancer correlates with a negative prognosis for patients, representing one of the critical tumor-escape mechanisms. However, clinically trialed IDO1 catalytic inhibitors disappointed the expected anti-tumor efficacy. Interestingly, the non-enzymatic apo-form of IDO1 is still active as a transducing protein, capable of promoting an immunoregulatory phenotype in dendritic cells (DCs) as well as a pro-tumorigenic behavior in murine melanoma. Moreover, the IDO1 catalytic inhibitor epacadostat can induce a tolerogenic phenotype in plasmacytoid DCs, overcoming the catalytic inhibition of IDO1. Based on this recent evidence, IDO1 plasticity was investigated in the human ovarian cancer cell line, SKOV-3, that constitutively expresses IDO1 in a dynamic balance between the holo- and apo-protein, and thus potentially endowed with a dual function (i.e., enzymatic and non-enzymatic). Besides inhibiting the catalytic activity, epacadostat persistently stabilizes the apo-form of IDO1 protein, favoring its tyrosine-phosphorylation and promoting its association with the phosphatase SHP-2. In SKOV-3 cells, both these early molecular events activate a signaling pathway transduced by IDO1 apo-protein, which is independent of its catalytic activity and contributes to the tumorigenic phenotype of SKOV-3 cells. Overall, our findings unveiled a new mechanism of action of epacadostat on IDO1 target, repositioning the catalytic inhibitor as a stabilizer of the apo-form of IDO1, still capable of transducing a pro-tumorigenic pathway in SKOV-3 tumor. This mechanism could contribute to clarify the lack of effectiveness of epacadostat in clinical trials and shed light on innovative immunotherapeutic strategies to tackle IDO1 target.

Background Immunotherapy-based combinations have emerged as standard therapies for patients with metastatic renal cell carcinoma (mRCC). Pembrolizumab, a PD-1 inhibitor, combined with epacadostat, an indoleamine 2,3-deoxygenase 1 selective inhibitor, demonstrated promising antitumor activity in a phase 1 study in advanced solid tumors, including mRCC. Methods KEYNOTE-679/ECHO-302 was a randomized, open-label, parallel-group, multicenter, phase 3 study (NCT03260894) that compared pembrolizumab plus epacadostat with sunitinib or pazopanib as first-line treatment for mRCC. Eligible patients had histologically confirmed locally advanced or metastatic clear cell RCC and had not received systemic therapy. Patients were randomly assigned 1:1 to pembrolizumab 200 mg IV every 3 weeks plus epacadostat 100 mg orally twice daily versus sunitinib 50 mg orally once daily (4 weeks on treatment followed by 2 weeks off treatment) or pazopanib 800 mg orally once daily. Original dual primary end points were progression-free survival and overall survival. Enrollment was stopped when a phase 3 study in melanoma of pembrolizumab plus epacadostat compared with pembrolizumab monotherapy did not meet its primary end point. This protocol was amended, and primary end point was changed to investigator-assessed objective response rate (ORR) per RECIST 1.1. Results One-hundred-twenty-nine patients were randomly assigned to receive pembrolizumab plus epacadostat ( n  = 64) or sunitinib/pazopanib ( n  = 65). Median (range) follow-up, defined as time from randomization to data cutoff, was 10.3 months (2.2–14.3) and 10.3 months (2.7–13.8) in the pembrolizumab plus epacadostat and sunitinib/pazopanib arms, respectively. ORRs were similar between pembrolizumab plus epacadostat (31.3% [95% CI 20.2–44.1] and sunitinib/pazopanib (29.2% [18.6–41.8]). Grade 3–5 treatment-related adverse events occurred in 34.4% and 42.9% of patients in the pembrolizumab plus epacadostat and sunitinib/pazopanib arms, respectively. One patient in the sunitinib/pazopanib arm died of septic shock (not treatment-related). Circulating kynurenine levels decreased in the pembrolizumab plus epacadostat arm, but not to levels observed in healthy subjects. Conclusions ORRs were similar between pembrolizumab plus epacadostat and sunitinib/pazopanib as first-line treatment in patients with mRCC. Safety and tolerability appeared similar between treatment arms; no new safety concerns were identified. Antitumor responses observed in patients with RCC receiving pembrolizumab plus epacadostat may be driven primarily by pembrolizumab. Clinical trial registration ClinicalTrials.gov; NCT03260894 .

Background Indoleamine 2,3-dioxygenase 1 (IDO1) levels correlate with poor outcomes in urothelial carcinoma (UC). IDO1 and programmed death-ligand 1 (PD-L1) are often co-expressed. Epacadostat is a potent and highly selective inhibitor of IDO1. In a subgroup analysis of patients with advanced UC participating in a phase I/II study, epacadostat-pembrolizumab treatment produced an objective response rate (ORR) of 35%. Methods ECHO-303/KEYNOTE-698 was a double-blinded, randomized phase III study of adults with metastatic or unresectable locally advanced UC with recurrence or progression following first-line platinum-based chemotherapy. Participants were randomized to epacadostat 100 mg twice daily (BID) plus pembrolizumab or placebo plus pembrolizumab until completion of 35 pembrolizumab infusions, disease progression, or unacceptable toxicity. The primary endpoint was investigator-assessed ORR per Response Evaluation Criteria in Solid Tumors version 1.1. Results Target enrollment was 648 patients; enrollment was halted early based on efficacy results from the phase III ECHO-301/KEYNOTE-252 study in metastatic melanoma. Forty-two patients were randomized to each treatment arm. Median duration of follow-up was 62 days in each arm. The investigator-assessed ORR (unconfirmed) was 26.2% (95% CI 16.35–48.11) for epacadostat plus pembrolizumab and 11.9% (95% CI 4.67–29.50) for placebo plus pembrolizumab. Two complete responses were reported, both in the placebo-plus-pembrolizumab arm. Circulating kynurenine levels increased from C1D1 to C2D1 in the placebo-plus-pembrolizumab arm and numerically decreased in the epacadostat-plus-pembrolizumab arm. The safety profile of epacadostat plus pembrolizumab was similar to that of pembrolizumab monotherapy, although a numerically greater proportion of patients in the combination vs. control arm experienced treatment-related grade ≥ 3 adverse events (16.7% vs. 7.3%). One patient in each arm died due to cardiovascular events, which were not deemed drug-related. No new safety concerns were identified for either agent. Conclusions Epacadostat plus pembrolizumab demonstrated anti-tumor activity and was generally tolerable as second-line treatment of patients with unresectable locally advanced or recurrent/progressive metastatic UC. Epacadostat 100 mg BID, when administered with pembrolizumab, did not normalize circulating kynurenine in most patients. Further study of combined IDO1/PD-L1 inhibition in this patient population, particularly with epacadostat doses that result in durable normalization of circulating kynurenine, may be warranted. Trial registration ClinicalTrials.gov, NCT03374488. Registered 12/15/2017.

Background The combination of the checkpoint inhibitor (CPI) pembrolizumab and platinum-based chemotherapy is effective frontline therapy for advanced non-small cell lung cancer (NSCLC) lacking targetable mutations. Indoleamine 2,3- dioxygenase 1 (IDO1), an enzyme involved in kynurenine production, inhibits immune responses. Inhibition of IDO1 may restore antitumor immunity and augment CPI activity. This trial evaluated addition of epacadostat, a potent and highly selective IDO1 inhibitor, to pembrolizumab and chemotherapy for metastatic NSCLC. Methods ECHO-306/KEYNOTE-715 was a partial double-blind, randomized phase II study of adults with treatment-naïve stage IV NSCLC not indicated for EGFR-, ALK-, or ROS1-directed therapy. Patients were randomized to one of three treatment arms: epacadostat-pembrolizumab-chemotherapy (E + P + C; blinded), epacadostat-pembrolizumab (E + P; open-label) or placebo-pembrolizumab-chemotherapy (PBO + P + C; blinded). Stratification was by PD-L1 tumor proportion score (< 50% vs. ≥ 50%) and tumor histology (non-squamous vs. squamous). A protocol amendment closed enrollment in the open-label E + P group, excluding it from efficacy analyses. Intravenous pembrolizumab (200 mg) was administered every 21 days and epacadostat 100 mg or matching placebo (oral) twice daily (BID) for ≤ 35 3-week cycles. The primary objective was objective response rate (ORR) for E + P + C vs. PBO + P + C. Results 178 patients were randomized to E + P + C ( n  = 91) or PBO + P + C ( n  = 87); 55 were enrolled in the E + P group. The E + P + C group had a lower confirmed ORR (26.4%; 95% CI 17.7–36.7) than the PBO + P + C group (44.8%; 95% CI 34.1–55.9), with a difference of − 18.5% (95% CI − 32.0 – (− 4.3); one-sided P  = 0.9948). The E + P + C group had a numerically higher percentage of confirmed responders with extended response ≥ 6 months (29.2% vs. 15.4%). Circulating kynurenine levels at C1D1 were similar to those at C2D1 in all treatment groups and were not reduced to normal levels with epacadostat 100 mg BID plus P + C. The safety profile of E + P + C was consistent with that for PBO + P + C. Conclusions Addition of epacadostat 100 mg BID to pembrolizumab and platinum-based chemotherapy was generally well tolerated but did not improve ORR in patients with treatment-naïve metastatic NSCLC. Evaluating epacadostat doses that normalize circulating kynurenine in combination with CPIs may help determine the clinical potential of this combination. Trial registration NCT03322566. Registered October 26, 2017.