The rationale of indoleamine 2,3-dioxygenase inhibition for cancer therapy
Lieve Brochez a,b,d,e,*, Ines Chevolet a,b,d, Vibeke Kruse c,d
Abstract
Indoleamine 2,3-dioxygenase (IDO, also referred to as IDO1) has been demonstrated to be a normal endogenous mechanism of acquired peripheral immune tolerance in vivo. In the field of oncology, IDO expression and/or activity has been observed in several cancer types and has usually been associated with negative prognostic factors and worse outcome measures. This manuscript reviews current available data on the role of IDO in cancer and the current results obtained with IDO inhibition, both in animal models and in phase 1 and 2 clinical trials in humans. Preliminary results with IDO inhibitors, usually combined with other anti-cancer drugs, seem encouraging. Further studies are needed to clarify the conditions in which IDO inhibitors can be of value as an anti-cancer strategy. In addition, further research should address whether the expression of IDO in tissue or blood can be a marker to select patients who can benefit most from IDO inhibition.
KEYWORDS
Indoleamine
2-3-dioxygenase
(IDO);
Immune tolerance;
Cancer;
Melanoma;
IDO inhibition;
Epacadostat;
Indoximod
1. Introduction
The domain of immunotherapy is gaining interest in different cancer types, and it is now clear that this is becoming a major anti-cancer strategy in the management of solid and haematological tumours. Given its high immunogenicity, several immunotherapeutic strategies have been explored and used in melanoma for decades going from interleukin 2, interferons, peptide vaccinations, dendritic cell vaccinations and adoptive T-cell transfer. The major breakthrough of immunotherapy in melanoma came with the checkpoint inhibitor ipilimumab (Yervoy, BMS; an anti-CTLA4 antibody) leading to a continuous switch on of the immune system and being the first immunotherapy that demonstrated a significant survival benefit in metastatic melanoma compared with classical chemotherapy [1]. This immunotherapy was also the first to demonstrate long-lasting remissions (>3 years) in 10e15% responders rising the assumption that a subset of these patients with metastatic melanoma might actually have been cured. In the meantime, PD1inhibition (nivolumab (Opdivo, BMS); pembrolizumab (Keytruda, MSD)) has become the new first line immunotherapy in melanoma, with an overall response rate (ORR) of 43.7% and a 2-year OS of 57.7% for nivolumab (dosing 3 mg/kg q2 weeks) and an ORR of 32.9% and a 2-year OS of 55% for pembrolizumab (dosing 10 mg/kg every 3 weeks) [2e4]. Anti-PD1 agents have also proven clinical benefit in renal cell carcinoma and non-small cell lung cancer, with response rates of approximately 25% in patients with previously treated advanced renal cell cancer and approximately 19% in patients with previously treated NSLCL [5,6].
Despite these very encouraging results, responses in immunotherapy remain limited to specific subgroups of patients, and there are currently no solid markers available to pre-identify responders. The detection of signatures that can predict response to immunotherapy or the detection of other immune strategies that can further increase responses would be of great value to further optimise the clinical results in cancer. In this domain IDO might be an ideal candidate.
2. Indoleamine 2,3-dioxygenase and its function
IDO, also referred to as IDO1, is a 403 amino acid cytosolic haem enzyme encoded by the INDO gene on human chromosome 8p22. It is expressed ubiquitously in various tissues and cell types throughout the body, but the small intestine, epididymis, lung, female genital tract and placenta have been reported as its main sites of expression [7,8]. The control of IDO transcription is complex and cell-type specific. IFN-stimulated response elements and gamma activation sequences (GASs) were identified in the promoter region of the INDO gene.
These response elements are necessary for IFN responsiveness and interferon-g (IFN-g) is widely considered to be the major inducer of IDO in most cells [9]. Apart from IFN-g, other cytokines such as IFN-a, IFN-b, IL10, IL-6, IL-4, IL-1 and TGF-b can modulate IDO expression [10e12]. The principal effect of IDO is the catabolism of tryptophan to kynurenine and its downstream metabolites. Tryptophan depletion is a mechanism of immunoregulation (Fig. 1), but tryptophan metabolites such as kynurenine, kynurenic acid, 3-hydroxy-kynurenine and 3-hydroxy anthralinic acid can also actively suppress T-cell function [13]. Tryptophan is an essential amino acid that is necessary for vital cellular functions and its degradation by IDO can cause a tryptophan deficiency. This deficiency can lead to ‘death by starvation’ by inducing an accumulation of uncharged tryptophan-tRNA. Uncharged tRNAs are sensed by the stress-response kinase GCN2, which then prevents Tcell activation. T-cells in which GCN2 is genetically disrupted are not susceptible to IDO-mediated proliferation suppression in vivo, and these T-cells are not anergised by IDO-expressing dendritic cells (DCs) [14]. Another effect of IDO-mediated tryptophan depletion is the production of kynurenine, which can bind the aryl hydrocarbon receptor. The biological effects of aryl hydrocarbon receptor activation include inhibition of Tcell activation, induction of Treg activation and IDO expression by DCs [15e17].
Apart from IDO1 (further designated as IDO), there are two other enzymes that catabolise tryptophan along the same pathway; indoleamine 2,3-dioxygenase 2 (IDO2) and tryptophan dioxygenase (TDO). IDO2 is encoded by the INDOL1 gene, which lies centromeric to the INDO gene, suggesting that they arose by gene duplication [18,19]. It is structurally very similar to IDO1 with 42% similarity at the amino acid level. It is mainly expressed in the kidney, the liver and reproductive organs, which is a more limited expression pattern than IDO1. Important genetic polymorphisms exist for IDO2, abolishing the enzyme’s function in up to 50% of Caucasians [20]. IDO2 can also be expressed by DCs, albeit without exerting enzymatic activity [21]. Taken together, these data may indicate that IDO2 may be of less importance in cancer immunology. TDO (encoded by the gene TDO2) is expressed in high levels in the liver. Although functionally related to IDO1 and IDO2, structural similarities at the amino acid level are only 10% [22]. TDO was long thought to be the one and only responsible in tryptophan homoeostasis in the body by metabolising dietary tryptophan intake. More recently, TDO expression was also found in human tumour cell linesdincluding melanomadand was also capable of suppressing antitumour immune responses [23]. No TDO expression in host immune cells has been documented so far. IDO has been demonstrated to be a normal endogenous mechanism of acquired peripheral tolerance in vivo, in a variety of settings. Functionally active IDO is expressed in the placenta, the ideal challenge to immune tolerance. Pregnant mice treated with the IDO inhibitor 1-methyl-trypophan developed T-cell responses against paternal allo-antigens, leading to the rejection of their concepti [24]. Many other examples of IDO-mediated acquired tolerance exist, e.g. at mucosal interfaces in gastrointestinal and pulmonary tissues [25e27]. One of the best illustrations is that murine tissue allografts designed to overexpress IDO are not rejected, even in cases with full MHC haplotype mismatches, and this without the need for any additional immunosuppression [28e30]. As powerful as IDO may be in inducing acquired systemic tolerance to foreign antigens, it does not seem to play a role in the constitutive maintenance of tolerance to self-antigens. This can be deduced from the fact that mice genetically modified to lack IDO (IDO/ mice) do not develop spontaneous autoimmune or lymphoproliferative disorders [31]. It is clear that this acquired immune tolerance related to IDO, if occurring in cancer, can act as a mechanism that facilitates immune escape/evasion by the tumour.
3. IDO in cancer
IDO expression and/or activity has been reported in several human cancers and has usually been associated with a worse prognosis. Table 1 tries to summarise all publications up to date (Table 1). IDO expression in tumoral cells has been observed in the primary tumour and/or metastatic tissue of different tumour types. It has been correlated with factors reflecting a worse outcome such as decreased survival, increased disease stage, presence of metastatic disease, decreased tumour-infiltrating lymphocytes and increased FoxP3þ Tregs [32e35]. IDO expression by tumour cells can be part of genetic changes involved in malignant transformation such as loss of Bin1 [36]. Alternatively, IDO in tumour cells could also be induced by IFN-g or other inflammatory mediators.
IDOcanalsobeexpressedbythehostcells(endothelial and immune cells) in the peri-tumoural stroma and/or the tumour-draining lymph nodes. The tumour-draining lymph node is crucial in the orientation of the antitumour immune response [37]. In melanoma, a strong correlation of IDO expression in the peritumoral stroma of the primary and IDO expression in the sentinel has been reported. This IDO expression was associated with decreased tumour-infiltrating lymphocytes (TILs) at the primary, which are known to be of prognostic relevance [38]. Which cell type is responsible for host IDO expression remains to be clarified, but plasmacytoid dendritic cells seem most plausible [39]. This IDO-competent subtype of DCs in TDLNs has been shown to suppress T-cell responses and induce antigen-specific immune tolerance [14,40]. Moreover, the effect of IDO þ DCs is mediated by bystander suppression, meaning that IDO þ DCs can suppress T-cell responses to antigens presented by neighbouring IDO- DCs [40]. This explains how even small populations of IDO þ DCs could have a powerful effect in vivo.
IDO activity in the blood has usually been measured indirectlybyanincreasedratioofkynurenine/tryptophan as a reflection of tryptophan degradation by enzymatically active IDO. This has been associated with worse outcome in AML, breast cancer, cervical cancer, glioblastoma multiforme, non-small cell lung cancer and melanoma(Table1).Inmelanoma,IDOexpressionatthe sentinel node was correlated with IDO expression by peripheral blood mononuclear cells in samples taken during follow-up of the patients after their initial surgery. Thepredominant cell typesexpressing IDO in bloodwere plasmacytoid dendritic cells (pDCs) and monocytic myeloidederived suppressor cells (mMDSCs), and these were correlated with increased Kyn/Trp ratio. IDOexpressing PBMCs were strongly correlated with PD-L1 expressing cytotoxic Tcells and CTLA4 expressing Tregs indicating that these immune regulatory molecules are closely intertwined [41,42].
Although in most studies IDO expression has been associated with a worse outcome in several cancer types, some studies also reported a positive prognostic effect.
There are two hypotheses that can explain this putative contradiction. First, local tryptophan deprivation has been reported to also decrease tumour cell proliferation [43,44]. Second, the most powerful cytokine to induce IDO expression is IFN-g [44]. During an anti-tumour immune response, large quantities of pro-inflammatory cytokines (including IFN-g) are secreted. IDO expression could therefore also be a marker of an ongoing antitumoural immune response. Finally, part of the inconsistencies in reported data on IDO expression in human tissues, both normal and cancerous, may be due to technical differences, such as the variety of used antibodies (monoclonal versus polyclonal, mouse versus rabbit versus goat,.)
4. IDO inhibition in animal models with cancer
Some first attempts to target IDO as an immunotherapeutic strategy have been done in animal models. The competitive IDO inhibitor 1-methyltryptophan had antitumour activity in a mouse model subcutaneously injected with an IDOþ plasmacytoma-derived tumour cell line [45]. Muller et al. [36] demonstrated that combining cytotoxic agents with small-molecule IDO inhibition could induce responses in tumours refractory to single-agent therapy in an MMTV-Neu mouse, an established breast cancer model. They suggested that IDO inhibition may improve response to cancer chemotherapy.
In a murine subcutaneous bladder and colon tumour model, the induction of an antitumoural cytotoxic T-cell response was demonstrated after IDO small interfering RNA skin delivery with a gene gun (Yen 2009) [46]. The same research group demonstrated anti-tumour activity and inhibition from development of metastatic disease after skin delivery with IDO short hairpin RNA in a mouse model with orthotopic and metastatic liver cancer [47]. In a B16 mouse melanoma model, Holmgaard et al. [48] demonstrated that host IDO expression had an inhibitory role in both anti-CTLA-4 and anti-PD1/PDL1 therapy. Pharmacological inhibition of IDO combined with CTLA-4 blockade gave superior responses.
5. IDO inhibition in human cancers
Some IDO inhibitor compounds have been/are currently being tested in humans in phase 1 and 2 trials. Epacadostat (Incyte) has gone to phase 3 trial in combination with pembrolizumab in melanoma.
5.1. Epacadostat (Incyte)
Epacadostat (INCB024360, Incyte) is an orally available IDO1 inhibitor. In a phase 1 dose escalation study (NCT01195311), including 52 patients with advanced malignancies (56% colorectal carcinoma, 12% melanoma and 33% other malignancies) daily doses in 28d cycles in 8 cohorts were evaluated (50 mg once daily; 50-mg, 100-mg, 300-mg, 400-mg, 500-mg, 600mg or 700-mg BID). The most common grade 3 or 4 adverse events were abdominal pain, hypokalemia and fatigue (9.6% each). Fifteen patients showed stable disease at 56 d, lasting over 112 d in 7 patients. No objective responses were seen. Significant dosedependent reductions in plasma kynurenine/tryptophan (Kyn/Tryp) ratios and Kyn levels were detected at all doses and in all pts. Maximal effects were observed at doses 300-mg BID. Overall, doses 300-mg BID achieved greater than 90% inhibition of IDO1 throughout the dosing period. Based on these results, the authors concluded that INCB024360 was generally well tolerated with a recommended dose of 600 mg as monotherapy [49].
The combination of epacadostat (2 cohorts; 300-mg BID and 25-mg BID) and ipilimumab (3-mg/kg IV q 3 wks 4) has been evaluated in patients with stage IV melanoma (NCT01604889). Enrolment in the 300 mg BID cohort (n Z 7) was prematurely stopped due to significant ALT elevations after 30e76 d in five patients. ALT elevations were reversible with corticosteroids and treatment discontinuation. Six of the 7 patients had evaluable on-study scans before treatment discontinuation and all showed stable disease according to the immune-related response criteria. Time to subsequent therapy was >90 d in all 7 patients and >180 d in 4 of 7 patients. Eight patients were re-enrolled at 25-mg BID, of which 6 of 8 patients had tumour reduction by the 1st evaluation after 9 weeks. Confirmed disease control rate was 75%. Three patients had confirmed partial response according to the immune-related response criteria (irRC; 2 occurred by the 1st and the 2nd scan). Based on these results, the authors concluded that epacadostat 25 mg in combination with ipilimumab is generally well tolerated with immune-related adverse effects that were generally manageable and reversible [50]. In the updated results of this study, presented in 2015, responses were obtained in 31.3% and disease control in 62.5% of immunotherapy-naı¨ve patients according to irRC. Six patients were progression-free at 13 months of followup. Median PFS by irRC was 8.2 months in immunotherapy-naı¨ve patients and 2.5 months in patients who had received prior immunotherapy. The authors concluded that epacadostat can safely be combined with ipilimumab and may enhance clinical activity of ipilimumab among patients with advanced or metastatic melanoma. (Gibney et al., updated results, poster from ECCO 2015).
Epacadostat has also been evaluated in combination withpembrolizumabinaphase1e2studyinpatientswith advanced cancer (Keynote 037/NCT02178722). In the dose escalation study, patients were treated in 5 cohorts (epacadostat 25 mg BID þ pembrolizumab 2 mg/kg q3w, epacadostat 50-mg BID þ pembrolizumab 2 mg/kg q3w, epacadostat 100 mg BID þ pembrolizumab 2 mg/kg q3w and epacadostat 300 mg BID þ pembolizumab 200 mg q3w). In the second dose expansion part of the study, the latest 3 cohorts moved on. Patients should have received no previous treatment with checkpoint inhibitors.
Response assessment was planned every 9 weeks per RECIST. In total, 56 patients were treated in this study, with melanoma (n Z 36), renal cell cancer (n Z 20) and NSCLC (n Z 18) being the most frequent pathologies. Generally, the combination was well tolerated with very few patients experiencing DLT’s or grade 3 treatmentrelated AE’s. No grade 4 treatment-related AE’s or deaths were observed. Six patients experienced a grade 3 adverseevent.Theefficacyresultswerepromisingwithan ORR of 53% and DCR of 74% in patients with advanced melanoma (Hamid et al., abstract SMR 2016). Based on these encouraging results, a phase 3 trial comparing pembrolizumab (fixed dose 200 mg q3w) þ epacadostat (100 mg BID) versus pembrolizumab þ placebo has recently been initiated (NCT02752074).
Epacadostat has also been investigated in a phase 2 clinical trial in patients with WHO-defined myelodysplastic syndrome and AML with a myeloblast percentage between 20 and 30% (RAEB-t by F [51] AB). Fifteen patients were included in the trial and all patients were treated with 600 mg BID for 16 weeks until clear evidence of disease progression of toxicity. The best response was stable disease in 12 (80%); 3 (20%) patients experienced disease progression and no haematological improvement was observed. The median duration of follow-up was 10 months, median duration on study treatment 3.9 months and median overall survival was not reached. The authors concluded that epacadostat was relatively well tolerated in MDS patients, significant clinical activity was observed [51].
Table 2 lists the ongoing phase 1 and 2 trials involving epacadostat.
5.2. Indoximod (NewLinkGenetics)
Indoximod (D-1-methyl-tryptophan, NLG-8189) is an orally available IDO inhibitor. Data on the first-in-men trial on Indoximod were presented in 2009 (NLG-8189). In a phase I dose escalation study, oral indoximod was well tolerated up to a dose of 2000 mg twice daily. Among the seven evaluable patients who received 200mg indoximod per day, 5 experienced objective responses or disease stabilisation. (NCT00567931) [52]. In another phase 1 trial, indoximod (300- to 1200-mg BID) was investigated in combination with docetaxel (60e75 mg/m2 IV every 3 weeks) in patients with solid tumours. In total, 27 patients were included, of which 22 were evaluable for response. Four patients showed partial response (2 breast cancer, 1 NSLCL, 1 thymic tumour), with duration of response ranging from 5.9 months up to 15.4 months. The most frequent adverse events were fatigue (58.6%), anaemia (51.7%), hyperglycemia (48.3%), infection (44.8%) and nausea (41.4%). (NCT01191216) [53] Following this phase 1 trial, a phase 2 double-blinded, randomised, placebo-controlled study of indoximod (1200 mg BID) in combination with docetaxel chemotherapy (75 mg/m2 IV every 3 weeks) was initiated in metastatic breast cancer patients. The results of this trial are still awaited. (NCT01792050).
Recently, data have been presented on a phase 1b dose escalation study of oral indoximod twice daily in combination with ipilimumab (3 mg/kg q3 weeks 4 doses) in metastatic melanoma patients. In case of progression, therapy could be changed from ipilimumab to anti-PD1 while continuing indoximod. To date, 45 patients of a planned 105 have been enrolled of which 9 patients comprised the phase 1b cohort without DLT.
One patient in phase 1 demonstrated an ongoing CR currently at 11 months. Six out of 9 patients are still alive 9e14 month from enrolment and receiving additional treatment after coming off study (Zakharia et al., abstract 3075, ASCO 2016). In the absence of significant toxicities in the phase 1-study, a phase 2 clinical trial was initiated in which indoximod is given with another immune checkpoint inhibitor (ipilimumab, pembrolizumab or nivolumab) according to the provider’s choice (NCT02073123).
Data have also been presented from a phase II trial, evaluating the combination of indoximod (1200 mg BID continuous dosing) with gemcitabine/nab-paclitaxel (1000 mg/m2/125 mg/m2 q week 3 per 4 week cycle) in patients with treatment naı¨ve metastatic pancreatic cancer or 1st line salvage therapy after previous resection and adjuvant therapy. Target enrolment was 80 patients in phase 2. At interim analysis, 30 of the 80 expected patients had been enrolled and completed protocol treatment at least through imaging at the end of cycle 2. Of these, 11 (37%) demonstrated an objective response by RECIST criteria including one patient with a confirmed complete response. One adverse event of immunological significance (colitis) was observed and required study withdrawal (NCT02077881). (Baharay et al. abstract 3020, ASCO 2016). Currently, 6 phase 1 or 2 trials are recruiting, combining indoximod with different chemotherapeutic or targeted agents in patients with a variety of solid tumours (Table 3).
5.3. Other IDO inhibitors
For the moment, a phase 1 trial with an IDO1 inhibitor NLG919 (ApexBt) is recruiting among patients with different solid tumours who have progressed following standard therapy excluding prior ipilimumab or other CTLA4 therapy. Different doses are being evaluated (NCT02048709). Another IDO inhibitor currently being evaluated in patients with different solid tumours is GDC-0919 (Genentech). GDC-0919 is being evaluated in monotherapy in different doses (NCT02048709) or in combination with atezolizumab (NCT02471846). An IDO1 inhibitor by iTeos Therapeutics/Pfizer is in the preclinical phase. The company is also working on a TDO2 inhibitor.
5.4. IDO peptide vaccination
A synthetic HLA-A2erestricted IDO peptide vaccine has been administered to 15 patients with metastatic NSCLC (NCT01219348). The vaccine was safe and easy to administer with only modest side effects. One patient achieved a significant regression of liver metastases and continued the vaccination for more than 2 years.
Another 6 of the 15 patients demonstrated prolonged disease stabilisation. When comparing the vaccinated HLA-A2þ patients to the HLA-A2 negative patients (who could not be included due to HLA type) in the intention-to-treat population, the median OS was 25.9 months and 7.7 months, respectively [54]. Based on these results, a phase 1/2 trial combining the IDO peptide vaccination with ipilimumab in metastatic melanoma was initiated. (NCT2077114) [54,55]. In this trial 10 melanoma patients were treated with a combination of an IDO peptide vaccination with standard ipilimumab therapy (3-mg per kq every 3 weeks for 4 cycles). The vaccination was initiated concomitantly with the first dose op ipilimumab, and a total of 7 vaccinations were delivered every 1e2 weeks, the first starting with the first dose of ipilimumab. Treatment was associated with mild to moderate toxicity in most patients and the vaccine was generally safe and well tolerated.
Regarding clinical efficacy, one patient had a partial remission at the first evaluation after 12 weeks on treatment, with a 44% reduction of target lesion diameter, and four patients were within the limits of stables disease. Five patients progressed and were referred to other treatments. Of the five patients with stable disease by the first evaluation, two were confirmed and two progressed by the second evaluation (8e12 weeks later) and one died between the first and the second evaluation [115].
6. Conclusion
This review focuses on the role of IDO, also referred to as IDO1, in oncology. However, IDO has been implicated in physiological conditions such as pregnancy and in a variety of other pathophysiological processes, including infection, chronic inflammation, allergic and autoimmune disorders, transplantation, neuropathological disorders and depression [56]. Of note, overactivation of IDO in response to IFN-a treatment in melanoma patients is considered a key event in the pathogenesis of IFN-aerelated depression. In patients with melanoma and renal cell carcinoma receiving IFNa therapy, serum tryptophan concentrations are negatively correlated with depressive symptoms, further supporting a role of IDO in IFN-induced depression [57,58].
IDO is expressed in several cancers and is usually associated with worse prognosis. IDO may be a general principle of acquired immune tolerance in cancer [59], suggesting that IDO checkpoint inhibition might prove beneficial in several tumour types. There are some encouraging results coming out from clinical phase 1 and 2 trials with IDO inhibition in human cancers. In most of these trials IDO inhibition is combined with either chemotherapy or other immunotherapeutic strategies.
One reason for combining IDO inhibition with other anti-cancer agents is that IDO expression might be spontaneously upregulated in any host immune response, including an antitumoural response induced by an established therapy. Such mechanism could dampen the effectiveness of treatment, as (part of) the antitumoural immune response is attenuated. As Muller et al. [36] demonstrated in a mouse breast cancer model, IDO inhibition can improve response to chemotherapeutic agents as the immunological reaction that may be induced by chemotherapy can be attenuated by IDO expression and will be stimulated by IDO inhibition. In melanoma patients treated with interferon a-2b in an adjuvant setting increased levels of IDO þ pDCs with an increase in kynurenine/tryptophan ratio pointing to systemic IDO activity have been reported [60]. This was also associated with increased Tregs and increased PDL1þ cytotoxic T cells. These data suggest that even an immunotherapeutic strategy that is expected to increase antitumoural immune responses may be accompanied by negative feedback mechanisms (IDO induction), thereby attenuating the overall antitumoural response.
Since data in metastatic tissue [61] and blood [41] have suggested that the expression of IDO and other immune regulatory molecules such as CTLA4 and PDL1 may be significantly interconnected, this may be another reason why combination therapies blocking several of these molecules simultaneously may be particularly attractive. In this respect, the combination of anti-CTLA4 and anti-PD1 blockade has proven to produce superior response rates in metastatic melanoma over either of them as single-agent therapy [62]. The work of Holmgaard et al. [48] in a mouse melanoma model suggests that IDO expression decreases response in both anti-CTLA4 and anti-PD1/PDL1 therapy and that IDO inhibition increases response rates during antiCTLA4 therapy. The preliminary results in humans combining IDO inhibition with established immunotherapeutic strategies in stage IV melanoma seem promising, and the combination epacadostat with the anti-PD1 agent pembrolizumab went into phase 3 trial. Of notice administration of IDO inhibitors seems to be well tolerated and does not seem to be accompanied by important immune-related adverse events. This illustrates that IDO inhibition can break an installed climate of immune tolerance, but in itself has little immunestimulating effects.
Our own data in melanoma demonstrate that IDO expression is present very early in disease, has independent prognostic effect and is consistently expressed in blood and metastatic tissue of patients years after their initial surgery. This IDO expression impacts the expression of IDO in PBMCs, mainly pDCs and monocytic myeloidederived suppressor cells, and other immune regulatory molecules such as PD-L1 and CTLA4 in the blood suggesting altered immunity in these patients. Such patterns of early and consistent IDO expression could also be present in other cancer types, and there is a need for further study on this. We confirmed similar consistent patterns in colorectal cancer, with strong correlations between intratumoural IDO expression in the primary, the tumour-draining lymph nodes and metastatic tissue of the same patient (data to be published). This is in line with previous publications where consistency of IDO expression in different tissues of the same patients with colorectal cancer has been reported [33,63,64] and where IDO expression was correlated with worse prognosis [63,65]. The detection of such early signal of immune resistance in cancer could be important as a prognostic marker and a response marker for immunotherapy. It could also lead to new immunotherapeutic strategies approaches where one tries to reverse this climate of immune tolerance in an early phase of cancer. In addition, the data on blood may open the way for immunomonitoring during cancer (immune) therapy. Both the data on tissue and blood might well fit into the interest in the introduction of an immunoscore for the classification of cancer [66] and personalised therapeutic approaches.
In conclusion, available data suggest that IDO inhibition strategies might become a new additional anticancer strategy in a variety of cancers. As IDO can be demonstrated both in tissue and in blood, it might become a good marker to stratify patients who could benefit (most) from IDO inhibition. Further research about the mechanisms that regulate IDO expression can increase our insights how this is used best for the benefit of the patient.
References
[1] Hodi FS, O’Day SJ, McDermott DF, Weber RW, Sosman JA, Haanen JB, et al. Improved survival with ipilimumab in patients with metastatic melanoma. N Engl J Med 2010;363:711e23.
[2] Robert C, Long GV, Brady B, Dutriaux C, Maio M, Mortier L, et al. Nivolumab in previously untreated melanoma without BRAF mutation. N Engl J Med 2015;372:320e30.
[3] Larkin J, Hodi FS, Wolchok JD. Combined nivolumab and ipilimumab or monotherapy in untreated melanoma. N Engl J Med 2015;373:1270e1.
[4] Robert C, Schachter J, Long GV, Arance A, Grob JJ, Mortier L, et al. Pembrolizumab versus ipilimumab in advanced melanoma. N Engl J Med 2015;372:2521e32.
[5] MotzerRJ, Escudier B, McDermott DF, GeorgeS, HammersHJ, Srinivas S, et al. Nivolumab versus everolimus in advanced renalcell carcinoma. N Engl J Med 2015;373:1803e13.
[6] Borghaei H, Paz-Ares L, Horn L, Spigel DR, Steins M, Ready NE, et al. Nivolumab versus docetaxel in advanced nonsquamous non-small-cell lung cancer. N Engl J Med 2015; 373:1627e39.
[7] Theate I, van Baren N, Pilotte L, Moulin P, Larrieu P, Renauld JC, et al. Extensive profiling of the expression of the indoleamine 2,3-dioxygenase 1 protein in normal and tumoral human tissues. Cancer Immunol Res 2015;3:161e72.
[8] Dai X, Zhu BT. Indoleamine 2,3-dioxygenase tissue distribution and cellular localization in mice: implications for its biological functions. J Histochem Cytochem 2010;58:17e28.
[9] Chon SY, Hassanain HH, Gupta SL. Cooperative role of interferon regulatory factor 1 and p91 (STAT1) response elements in interferon-gamma-inducible expression of human indoleamine 2,3-dioxygenase gene. J Biol Chem 1996;271: 17247e52.
[10] Grohmann U, Orabona C, Fallarino F, Vacca C, Calcinaro F, Falorni A, et al. CTLA-4-Ig regulates tryptophan catabolism in vivo. Nat Immunol 2002;3:1097e101.
[11] Babcock TA, Carlin JM. Transcriptional activation of indoleamine dioxygenase by interleukin 1 and tumor necrosis factor alpha in interferon-treated epithelial cells. Cytokine 2000;12: 588e94.
[12] MacKenzie CR, Hadding U, Daubener W. Interferon-gammainduced activation of indoleamine 2,3-dioxygenase in cord blood monocyte-derived macrophages inhibits the growth of group B streptococci. J Infect Dis 1998;178:875e8.
[13] Berthon C, Fontenay M, Corm S, Briche I, Allorge D, Hennart B, et al. Metabolites of tryptophan catabolism are elevated in sera of patients with myelodysplastic syndromes and inhibit hematopoietic progenitor amplification. Leuk Res 2013; 37:573e9.
[14] Munn DH, Sharma MD, Baban B, Harding HP, Zhang Y, Ron D, et al. GCN2 kinase in T cells mediates proliferative arrest and anergy induction in response to indoleamine 2,3dioxygenase. Immunity 2005;22:633e42.
[15] Stevens EA, Mezrich JD, Bradfield CA. The aryl hydrocarbon receptor: a perspective on potential roles in the immune system. Immunology 2009;127:299e311.
[16] Nguyen NT, Kimura A, Nakahama T, Chinen I, Masuda K, Nohara K, et al. Aryl hydrocarbon receptor negatively regulates dendritic cell immunogenicity via a kynurenine-dependent mechanism. Proc Natl Acad Sci U S A 2010;107:19961e6.
[17] Mezrich JD, Fechner JH, Zhang X, Johnson BP, Burlingham WJ, Bradfield CA. An interaction between kynurenine and the aryl hydrocarbon receptor can generate regulatory T cells. J Immunol 2010;185:3190e8.
[18] Murray MF. The human indoleamine 2,3-dioxygenase gene and related human genes. Curr Drug Metab 2007;8:197e200.
[19] Ball HJ, Sanchez-Perez A, Weiser S, Austin CJ, Astelbauer F, Miu J, et al. Characterization of an indoleamine 2,3dioxygenase-like protein found in humans and mice. Gene 2007;396:203e13.
[20] Metz R, Duhadaway JB, Kamasani U, Laury-Kleintop L, Muller AJ, Prendergast GC. Novel tryptophan catabolic enzyme IDO2 is the preferred biochemical target of the antitumor indoleamine 2,3-dioxygenase inhibitory compound D-1-methyltryptophan. Cancer Res 2007;67:7082e7.
[21] Lob S, Konigsrainer A, Schafer R, Rammensee HG, Opelz G, Terness P. Levo- but not dextro-1-methyl tryptophan abrogates the IDO activity of human dendritic cells. Blood 2008;111: 2152e4.
[22] Thackray SJ, Mowat CG, Chapman SK. Exploring the mechanism of tryptophan 2,3-dioxygenase. Biochem Soc Trans 2008; 36:1120e3.
[23] Pilotte L, Larrieu P, Stroobant V, Colau D, Dolusic E, Frederick R, et al. Reversal of tumoral immune resistance by inhibition of tryptophan 2,3-dioxygenase. Proc Natl Acad Sci U S A 2012;109:2497e502.
[24] Munn DH, Zhou M, Attwood JT, Bondarev I, Conway SJ, Marshall B, et al. Prevention of allogeneic fetal rejection by tryptophan catabolism. Science 1998;281:1191e3.
[25] Ferdinande L, Demetter P, Perez-Novo C, Waeytens A, Taildeman J, Rottiers I, et al. Inflamed intestinal mucosa features a specific epithelial expression pattern of indoleamine 2,3dioxygenase. Int J Immunopathol Pharmacol 2008;21:289e95.
[26] Matteoli G, Mazzini E, Iliev ID, Mileti E, Fallarino F, Puccetti P, et al. Gut CD103þ dendritic cells express indoleamine 2,3-dioxygenase which influences T regulatory/T effector cell balance and oral tolerance induction. Gut 2010;59:595e604.
[27] Odemuyiwa SO, Ebeling C, Duta V, Abel M, Puttagunta L, Cravetchi O, et al. Tryptophan catabolites regulate mucosal Epacadostat sensitization to ovalbumin in respiratory airways. Allergy 2009; 64:488e92.
[28] Liu H, Liu L, Fletcher BS, Visner GA. Novel action of indoleamine 2,3-dioxygenase attenuating acute lung allograft injury. Am J Respir Crit Care Med 2006;173:566e72.
[29] Guillonneau C, Hill M, Hubert FX, Chiffoleau E, Herve C, Li XL, et al. CD40Ig treatment results in allograft acceptance mediated by CD8CD45RC T cells, IFN-gamma, and indoleamine 2,3-dioxygenase. J Clin Invest 2007;117:1096e106.
[30] Swanson KA, Zheng Y, Heidler KM, Mizobuchi T, Wilkes DS. CDllcþ cells modulate pulmonary immune responses by production of indoleamine 2,3-dioxygenase. Am J Respir Cell Mol Biol 2004;30:311e8.
[31] Mellor AL, Baban B, Chandler P, Marshall B, Jhaver K, Hansen A, et al. Cutting edge: induced indoleamine 2,3 dioxygenase expression in dendritic cell subsets suppresses T cell clonal expansion. J Immunol 2003;171:1652e5.
[32] Yu J, Sun J, Wang SE, Li H, Cao S, Cong Y, et al. Upregulated expression of indoleamine 2, 3-dioxygenase in primary breast cancer correlates with increase of infiltrated regulatory T cells in situ and lymph node metastasis. Clin Dev Immunol 2011;2011: 469135.
[33] Brandacher G, Perathoner A, Ladurner R, Schneeberger S, Obrist P, Winkler C, et al. Prognostic value of indoleamine 2,3dioxygenase expression in colorectal cancer: effect on tumorinfiltrating T cells. Clin Cancer Res 2006;12:1144e51.
[34] Ino K, Yoshida N, Kajiyama H, Shibata K, Yamamoto E, Kidokoro K, et al. Indoleamine 2,3-dioxygenase is a novel prognostic indicator for endometrial cancer. Br J Cancer 2006; 95:1555e61.
[35] Ino K, Yamamoto E, Shibata K, Kajiyama H, Yoshida N, Terauchi M, et al. Inverse correlation between tumoral indoleamine 2,3-dioxygenase expression and tumor-infiltrating lymphocytes in endometrial cancer: its association with disease progression and survival. Clin Cancer Res 2008;14:2310e7.
[36] Muller AJ, DuHadaway JB, Donover PS, Sutanto-Ward E, Prendergast GC. Inhibition of indoleamine 2,3-dioxygenase, an immunoregulatory target of the cancer suppression gene Bin1, potentiates cancer chemotherapy. Nat Med 2005;11:312e9.
[37] Cochran AJ, Huang RR, Lee J, Itakura E, Leong SP, Essner R. Tumour-induced immune modulation of sentinel lymph nodes. Nat Rev Immunol 2006;6:659e70.
[38] Chevolet I, Speeckaert R, Haspeslagh M, Neyns B, Kruse V, Schreuer M, et al. Peri-tumoral indoleamine 2,3-dioxygenase expression in melanoma: an early marker of resistance to immune control? Br J Dermatol 2014;171(5):987e95.
[39] Munn DH, Sharma MD, Lee JR, Jhaver KG, Johnson TS, Keskin DB, et al. Potential regulatory function of human dendritic cells expressing indoleamine 2,3-dioxygenase. Science 2002; 297:1867e70.
[40] Munn DH, Sharma MD, Hou D, Baban B, Lee JR, Antonia SJ, et al. Expression of indoleamine 2,3-dioxygenase by plasmacytoid dendritic cells in tumor-draining lymph nodes. J Clin Invest 2004;114:280e90.
[41] Chevolet I, Speeckaert R, Schreuer M, Neyns B, Krysko O, Bachert C, et al. Characterization of the immune network of IDO, tryptophan metabolism, PD-L1, and in circulating immune cells in melanoma. Oncoimmunology 2015;4:e982382.
[42] Chevolet I, Speeckaert R, Schreuer M, Neyns B, Krysko O, Bachert C, et al. Clinical significance of plasmacytoid dendritic cells and myeloid-derived suppressor cells in melanoma. J Transl Med 2015;13:9.
[43] Ozaki Y, Edelstein MP, Duch DS. Induction of indoleamine 2,3dioxygenaseda mechanism of the antitumor-activity of interferon-gamma. P Natl Acad Sci USA 1988;85:1242e6.
[44] Takikawa O, Kuroiwa T, Yamazaki F, Kido R. Mechanism of interferon-gamma actiondcharacterization of indoleamine 2,3dioxygenase in cultured human-cells induced by interferongamma and evaluation of the enzyme-mediated tryptophan degradation in its anticellular activity. J Biol Chem 1988;263: 2041e8.
[45] Uyttenhove C, Pilotte L, Theate I, Stroobant V, Colau D, Parmentier N, et al. Evidence for a tumoral immune resistance mechanism based on tryptophan degradation by indoleamine 2,3-dioxygenase. Nat Med 2003;9:1269e74.
[46] Yen MC, Lin CC, Chen YL, Huang SS, Yang HJ, Chang CP, et al. A novel cancer therapy by skin delivery of indoleamine 2,3dioxygenase siRNA. Clin Cancer Res 2009;15:641e9.
[47] Huang TT, Yen MC, Lin CC, Weng TY, Chen YL, Lin CM, et al. Skin delivery of short hairpin RNA of indoleamine 2,3 dioxygenase induces antitumor immunity against orthotopic and metastatic liver cancer. Cancer Sci 2011;102:2214e20.
[48] Holmgaard RB, Zamarin D, Munn DH, Wolchok JD, Allison JP. Indoleamine 2,3-dioxygenase is a critical resistance mechanism in antitumor T cell immunotherapy targeting CTLA4. J Exp Med 2013;210:1389e402.
[49] Beatty GL, O’Dwyer PJ, Clark J, Shi JG, Newton RC, Schaub R, et al. Phase I study of the safety, pharmacokinetics (PK), and pharmacodynamics (PD) of the oral inhibitor of indoleamine 2,3-dioxygenase (IDO1) INCB024360 in patients (pts) with advanced malignancies. J Clin Oncol 2013;31.
[50] Gibney GT, Hamid O, Gangadhar TC, Lutzky J, Olszanski AJ, Gajewski T, et al. Preliminary results from a phase 1/2 study of INCB024360 combined with ipilimumab (ipi) in patients (pts) with melanoma. J Clin Oncol 2014;32.
[51] Komrokji RS, Wei S, Mailloux AW, Zhang L, Padron E, Lancet JE, et al. A phase II study to determine the safety and efficacy of the oral inhibitor of indoleamine 2,3-dioxygenase (IDO) enzyme INCB024360 in patients with myelodysplastic syndromes. Blood 2014;124.
[52] Soliman HH, Antonia S, Sullivan D, Vanahanian N, Link C. Overcoming tumor antigen anergy in human malignancies using the novel indeolamine 2,3-dioxygenase (IDO) enzyme inhibitor, 1-methyl-D-tryptophan (1MT). J Clin Oncol 2009;27.
[53] Soliman HH, Jackson E, Neuger T, Dees EC, Harvey RD, Han H, et al. A first in man phase I trial of the oral immunomodulator, indoximod, combined with docetaxel in patients with metastatic solid tumors. Oncotarget 2014;5:8136e46.
[54] Iversen TZ, Engell-Noerregaard L, Ellebaek E, Andersen R, Larsen SK, Bjoern J, et al. Long-lasting disease stabilization in the absence of toxicity in metastatic lung cancer patients vaccinated with an epitope derived from indoleamine 2,3 dioxygenase. Clin Cancer Res 2014;20:221e32.
[55] Andersen MH, Svane IM. Indoleamine 2,3-dioxygenase vaccination. Oncoimmunology 2015;4:e983770.
[56] Yeung AW, Terentis AC, King NJ, Thomas SR. Role of indoleamine 2,3-dioxygenase in health and disease. Clin Sci (Lond) 2015;129:601e72.
[57] Hoyo-Becerra C, Schlaak JF, Hermann DM. Insights from interferon-alpha-related depression for the pathogenesis of depression associated with inflammation. Brain Behav Immun 2014;42:222e31.
[58] Capuron L, Ravaud A, Neveu PJ, Miller AH, Maes M, Dantzer R. Association between decreased serum tryptophan concentrations and depressive symptoms in cancer patients undergoing cytokine therapy. Mol Psychiatr 2002;7:468e73.
[59] Prendergast GC. Immune escape as a fundamental trait of cancer: focus on IDO. Oncogene 2008;27:3889e900.
[60] Chevolet I, Schreuer M, Speeckaert R, Neyns B, Hoorens I, van Geel N, et al. Systemic immune changes associated with adjuvant interferon-alpha2b-therapy in stage III melanoma patients: failure at the effector phase? Melanoma Res 2015;25:357e61.
[61] Spranger S, Spaapen RM, Zha Y, Williams J, Meng Y, Ha TT, et al. Up-regulation of PD-L1, IDO, and T(regs) in the melanoma tumor microenvironment is driven by CD8(þ) T cells. Sci Transl Med 2013;5. 200ra116.
[62] Larkin J, Chiarion-Sileni V, Gonzalez R, Grob JJ, Cowey CL, Lao CD, et al. Combined nivolumab and ipilimumab or monotherapy in untreated melanoma. N Engl J Med 2015;373: 23e34.
[63] Gao YF, Peng RQ, Li J, Ding Y, Zhang X, Wu XJ, et al. The paradoxical patterns of expression of indoleamine 2,3dioxygenase in colon cancer. J Transl Med 2009;7:71.
[64] Engin A, Gonul II, Engin AB, Karamercan A, Dincel AS, Dursun A. Relationship between indoleamine 2,3-dioxygenase activity and lymphatic invasion propensity of colorectal carcinoma. World J Gastroenterol 2016;22:3592e601.
[65] Ferdinande L, Decaestecker C, Verset L, Mathieu A, Moles Lopez X, Negulescu AM, et al. Clinicopathological significance of indoleamine 2,3-dioxygenase 1 expression in colorectal cancer. Br J Cancer 2012;106:141e7.