IDO family: the metabolic crossroads connecting immunity, nerves and tumors

Abstract

Background

Tryptophan metabolism is essential for immune homeostasis, neurological function regulation, and tumor microenvironment modulation. The indoleamine 2,3-dioxygenase (IDO) family, including IDO1, IDO2, and tryptophan 2,3-dioxygenase (TDO2), serves as the rate-limiting enzymes in the kynurenine pathway of tryptophan catabolism. These enzymes act as a critical molecular hub linking immune metabolism, neural regulation, and tumorigenesis, and their aberrant activity is closely associated with the pathogenesis of various diseases such as tumors, autoimmune disorders, infectious diseases, and neurological conditions. Although IDO family inhibitors have shown potential in cancer immunotherapy, clinical trial results remain controversial, highlighting the complexity of their mechanisms and the need for systematic summarization of relevant research progress.

Main body

This review first elaborates on the structural characteristics, tissue distribution, catalytic efficiency, and core biological functions of IDO1, IDO2, and TDO2, emphasizing their distinct and complementary roles in tryptophan metabolism and immune regulation. It then systematically summarizes the regulatory mechanisms of the IDO family at transcriptional, translational, and post-translational levels. Subsequently, the review details the roles of each family member in different disease contexts: IDO1 predominantly mediates local immunosuppression and tumor immune escape; IDO2 drives B cell-related inflammation and autoimmune responses; TDO2 maintains systemic tryptophan homeostasis and links neurometabolism to immunity. Additionally, the article comprehensively discusses current therapeutic strategies targeting the IDO family, including small-molecule inhibitors (single-target, dual-target, and multi-target), peptide vaccines, and nano-delivery systems, while analyzing the challenges faced in clinical translation, such as pathway compensation, insufficient patient stratification, and off-target effects.

Conclusions

The IDO family plays a multifaceted and context-dependent role in various diseases through the kynurenine pathway, making it a promising target for diagnostic biomarkers and therapeutic intervention. Future research should focus on optimizing multi-target inhibitors, developing innovative delivery systems, establishing biomarker-guided precision medicine strategies, and exploring non-enzymatic functions and downstream signaling networks of the IDO family. These efforts will help overcome the limitations of current therapies and provide new treatment paradigms for refractory diseases related to immune metabolic disorders.

Graphical Abstract

Introduction

Tryptophan metabolism is pivotal for maintaining immune homeostasis, regulating neurological functions, and modulating the tumor microenvironment. The indoleamine 2,3-dioxygenase (IDO) family, which comprises IDO1, IDO2, and tryptophan 2,3-dioxygenase (TDO2), functions as the rate-limiting enzymes driving tryptophan catabolism through the kynurenine pathway [1–3]. As a critical molecular hub bridging immune metabolism, neural regulation, and tumorigenesis, the IDO family exerts its biological effects primarily by regulating immune responses—specifically, by depleting tryptophan and generating kynurenine metabolites [4, 5].

This tryptophan-kynurenine metabolic pathway mediated by the IDO family contributes to the pathogenesis of various diseases, including tumor immune evasion, autoimmune tolerance, and neurological disorders. Notably, despite the well-documented immunosuppressive function of IDO1, clinical trials focusing on single-target inhibition of IDO1 have largely failed. This outcome highlights the inadequacy of targeting IDO1 alone to address the sophisticated regulatory mechanisms of the entire tryptophan catabolic pathway [6].

The functional landscape of the IDO family is characterized by the distinct roles of IDO1, IDO2, and TDO2, varying significantly regarding their expression patterns, enzymatic activity, and downstream biological functions [7]. Crucially, these enzymes elicit opposing immunological outcomes depending on the specific pathological context. This functional heterogeneity provides critical insights into the intricate nature of immune metabolic regulation and highlights the shortcomings of conventional single-targeted therapeutic approaches.

The present review systematically summarizes the structural characteristics and functional discrepancies of the IDO family. It highlights the underlying mechanisms governing their roles in tumors, autoimmune diseases, infectious diseases, and neurological disorders. Furthermore, we discuss the challenges and future perspectives of current therapies targeting the IDO pathway, thereby establishing a theoretical foundation for the precise intervention of immunometabolism-associated diseases (Fig. 1).

Fig. 1.

Historical milestones and future directions of IDO family research. Key discoveries and therapeutic developments of IDO enzymes from 1963 to present, highlighting emerging multi-target and nano-based strategies

Structural and functional overview of the IDO family

As heme-dependent enzymes catalyzing the rate-limiting step of the kynurenine pathway, IDO1, IDO2, and TDO2 display architectural divergence in genetic origin, tissue distribution, and catalytic kinetics. Functionally, IDO1 orchestrates immune tolerance, IDO2 specifically regulates B cell–mediated inflammatory responses, and TDO2 sustains systemic tryptophan homeostasis while exerting indirect modulation on immune and neural pathways (Fig. 2).

Fig. 2.

Regulation of T Cell differentiation and function by tryptophan metabolism and its metabolites. This schematic depicts how tryptophan depletion and its downstream metabolites (e.g., Kyn, QA, KYNA) modulate T cell fate. Key enzymes (IDO1, IDO2, TDO2) initiate the kynurenine pathway, whose metabolites influence critical signaling nodes (AhR, GCN2, mTOR) and transcription factors (STAT3, Foxp3), thereby steering Th1/Th2/Th17/Treg differentiation, cytokine production, and cell survival

Genetic structure, protein composition and enzyme active center

IDO family members (IDO1, IDO2 and TDO2) share conserved heme-dependent catalytic features but differ markedly in genetic origin, structural properties, tissue distribution and biological functions, forming a non-redundant regulatory network in tryptophan metabolism and immune modulation (Table 1).

Table 1.

Structural and functional comparison of IDO family member

	IDO1	IDO2	TDO2
Genetic origin	It is highly homologous to IDO2 and exists in most vertebrates and absent in some lower vertebrates [8]	Homologous to IDO1, the gene is closely linked [9]	Independently encoding genes, non-homologous to IDO1/IDO2 [10]
Characteristics of evolution	Formed early in vertebrate evolution, the function is more efficient [11]	Evolutionary conservation but low enzyme efficiency [12]	Evolutionarily independent and functionally biased towards systemic metabolic regulation [13]
Mainly expressed tissues/cells	Immune cells (DC, macrophages), tumor cells [14]	B cells and some immune cells [15]	It is mainly expressed in the liver, but can also be expressed in the nervous system and some tumors [15]
Protein Structural characteristics	It has strong conformational plasticity and contains multiple regulatory sites. It has both enzymatic and non-enzymatic immunoregulatory functions	The structure is similar to IDO1, but the key residues are different	Typical heme enzyme, structurally distinct from IDO1/IDO2
Efficiency of catalysis	High [12]	Low [12]	Moderate-high (heme load dependent) [16]
Dependence of heme	Yes [17]	Yes [17]	Yes [17]
Key residues of the active center	Ser167 (human), which raises L-Trp [18]	Thr replaces Ser and reduces substrate affinity [12]	Dependence on heme and its stabilizing insertion affinity [19]
Post-translation modification	It is susceptible to tyrosine nitrification (e.g., Y345, Y353) and affects its activity [18]	Less reported	It is mainly regulated by heme loading and molecular chaperones [19]
Essential biological functions	Local immunosuppression, regulation of T cell function, and tumor immune escape [15]	B cell-associated inflammation regulated, autoimmune-driven [15]	Systemic tryptophan homeostasis, neurometabolism, and tumor immune regulation [15]
Summary of Functional characteristics	It is highly effective and mainly immunosuppressive	Inefficient but non-redundant, pro-inflammatory	Systemic metabolic regulation, linking nerves and immunity

The IDO family catalyzes tryptophan metabolism along the kynurenine pathway

Tryptophan (Trp) is predominantly metabolized through the kynurenine (KP) pathway, accounting for approximately 95% of dietary tryptophan degradation [7, 20]. Trp is initially oxidized to N-formylkynurenine (NFK) by IDO family enzymes, representing the first and rate-limiting step of the KP pathway [7, 20–22]. NFK is hydrolyzed by formylase to produce kynurenine, which is further metabolized to a variety of bioactive metabolites, including 3-hydroxykynurenine, quinolinic acid, kynurenic acid and niacin [7, 20, 23, 24].

Regulation of immune escape by the IDO family

IDO exerts immunosuppressive effects by depleting tryptophan and generating kynurenine metabolites, thereby modulating immune cell activation and differentiation. Crucially, macrophage polarization serves as a pivotal hub for immune evasion. Exposure to tumors, viruses, or toxins, through cytokine signaling, metabolic reprogramming, and epigenetic modifications, induces an immunosuppressive M2/tumor-associated macrophage (TAM) phenotype. This shift effectively inhibits T/NK cell function and reshapes the tumor microenvironment toward immune exclusion [25–28].

IDO1 is predominantly expressed in immune cells such as dendritic cells and macrophages By modulating tryptophan metabolism, it inhibits the proliferation of effector T cells and promotes the differentiation of regulatory T cells, thereby inducing immune tolerance and extensively participating in pathological processes including tumor immune evasion [29–33]. Accumulating evidence has demonstrated that IDO1 is closely associated with macrophage polarization, and its regulatory effects exhibit distinct dependence on disease microenvironments and stimulatory cues, potentially exerting either pro-inflammatory or immunosuppressive roles under different pathological conditions [34–38].

IDO2 directly promotes inflammatory responses in B cells and drives the development of autoimmune diseases. Compared with IDO1, IDO2 plays a pro-inflammatory role in regulating B cell activation and autoantibody production, whereas it exhibits a synergistic and complementary relationship with IDO1 in Treg generation and T cell regulation [29, 30, 39].

Unlike IDO1, TDO2 contributes to immune evasion in non-lymphoid tissues by catalyzing tryptophan catabolism. It actively suppresses T cell effector functions through kynurenine accumulation, establishing a distinct pathway for tumor immune escape.

Collectively, the KP pathway represents a central immunometabolic axis through which IDO family enzymes exert context-dependent effects in cancer, autoimmunity and neurological disorders, providing the biological basis for therapeutic targeting discussed below.

Expression regulation of the IDO family

IDO1, IDO2, and TDO2 act as master regulators of tryptophan metabolism, orchestrating immune tolerance via their enzymatic activity. These enzymes are tightly regulated at multiple biological levels, including transcription, translation, and post-translational modification, to maintain precise immune homeostasis (Fig. 3).

Fig. 3.

IDO/TDO2 transcription activation and kynurenine-mediated immunosuppression pathway. External signals such as TNF-α and TGF-β enhance the synergistic induction of IDO/TDO2 gene transcription via JAK-STAT1 and IRF-1 pathways. Subsequent protein synthesis promotes kynurenine (Kyn) production, which activates the aryl hydrocarbon receptor (AhR), leading to immunosuppression and immune tolerance

Regulatory mechanisms at the transcriptional level

Transcriptional control of the tryptophan metabolism pathway involves intricate interactions between inflammatory signals and stress hormones. IFN-γ acts as the primary driver for IDO1 induction, significantly elevating its expression to facilitate immune tolerance and tumor evasion [31, 40]. Conversely, inflammatory cues predominantly target IDO2 splice variants rather than TDO2, highlighting a distinct regulatory divergence [40]. Notably, glucocorticoids demonstrate specific selectivity; they specifically upregulate the TDO2-FL transcript in hippocampal slices. Intriguingly, when combined with IFN-γ, glucocorticoids synergistically amplify IDO1-FL expression without significantly affecting IDO2 levels [40]. Collectively, these data indicate that TDO2 is primarily governed by the glucocorticoid/stress axis (highly relevant to liver and CNS tissues) [31, 40], while IDO1 operates under dual regulation by both the interferon and glucocorticoid pathways.

In the context of pro-inflammatory and tumor-associated microenvironments, the transcription of IDO1 is driven by a constellation of signaling molecules. IFN-γ, TNF-α, TGF-β, TLR ligands, and type I interferons (IFN-α/β) act as key inducers, often functioning in a synergistic manner to enhance IDO1 expression and thereby strengthen tumor immune evasion [31]. Furthermore, comprehensive transcriptomic profiling indicates that TDO2 mRNA is broadly upregulated in various breast cancer subtypes. Importantly, this upregulation correlates significantly with the expression levels of IDO1 and IDO2, suggesting a shared regulatory axis that contributes to shaping the immunosuppressive tumor landscape [41].

MicroRNAs (miRNAs) and epigenetic mechanisms play critical roles in modulating the expression of tryptophan metabolism enzymes. Specifically, miR-153 binds to the 3’UTR of IDO1, thereby destabilizing its mRNA and reducing protein levels, which subsequently attenuates tryptophan catabolism and exerts anti-tumor effects in tumor cells [31]. Furthermore, the 3’UTR of IDO2 contains conserved binding sites for miR-590-3p, establishing a negative feedback loop that restricts IDO2 mRNA accumulation [42]. In certain tumor contexts, exosomal miR-142-5p can target ARID2 to reduce methylation at the IFN-γ promoter, indirectly enhancing IFN-γ and IDO1 transcription [31].

Regulatory mechanisms at the translation level

IDO1 and IDO2 display distinct regulatory profiles regarding their transcription-translation dependency. While splice variant regulation represents a shared post-transcriptional feature across the IDO/TDO family, their reliance on de novo protein synthesis differs significantly. In human dendritic cells, treatment with the transcription inhibitor actinomycin D or the translation inhibitor cycloheximide completely abrogates IDO1 protein expression, indicating that its maintenance is strictly dependent on sustained transcription and de novo translation [42]. By contrast, IDO2 protein levels remain largely unaffected, suggesting that the protein is highly stable after synthesis and possesses minimal reliance on continuous de novo production.

Extensive transcriptomic analysis in the mouse hippocampus has identified multiple splice variants for all three tryptophan metabolism enzymes. Specifically, IDO1 encodes two transcripts, IDO2 encodes four distinct variants, and TDO2 encodes three transcripts [40]. Notably, all encoded proteins, such as IDO1-FL, IDO2-FL, IDO2-v3, and TDO2-FL, are enzymatically active, although they exhibit distinct catalytic efficiencies [40]. Consequently, the relative abundance and translational status of these splice variants at the post-transcriptional level significantly determine the total enzymatic activity, representing a critical regulatory mechanism for IDO2 and TDO2 [40].

Regulatory mechanisms at the post-translational level

The post-translational regulatory landscape of IDO1, IDO2, and TDO2 exhibits distinct divergence. While IDO1 is governed by a highly defined network, IDO2 displays intrinsic stability, and specific mechanisms for TDO2 remain largely uncharacterized. IDO1 protein contains two canonical immunoreceptor tyrosine-based inhibitory motifs (ITIMs). Phosphorylation of these motifs exerts dual regulatory effects: it recruits suppressor of cytokine signaling 3 (SOCS3) to mediate ubiquitination and proteasomal degradation, thereby rapidly terminating IDO1 expression; simultaneously, it triggers non-enzymatic signaling functions (e.g., phosphatidylinositol 3-kinase [PI3K] activation), induces an immunoregulatory phenotype in specific cell types such as plasmacytoid dendritic cells (pDCs), and facilitates translocation of IDO1 from the cytoplasm to early endosomes. Thus, IDO1 achieves precise dynamic regulation via the “phosphorylation–SOCS3–ubiquitination” axis [31, 42].

The structural divergence between IDO1 and IDO2 dictates their distinct regulatory fates. While IDO1 relies on SOCS3-mediated degradation, IDO2 harbors only a single non-canonical ITIM lacking the full binding site for SOCS3 interaction. Functional assays confirm the absence of SOCS3 binding, and protein degradation studies demonstrate that IDO2 is resistant to SOCS3-dependent proteasomal degradation. As a result, IDO2 exhibits significantly higher intrinsic stability and a slower turnover rate than IDO1, establishing it as a constitutively stable, SOCS3-independent “homeostatic” protein [42].

Turning to TDO2, its expression is largely restricted to hepatocytes and neurons, governed primarily by transcriptional factors such as glucocorticoids and tryptophan availability [31, 40]. Unlike the well-characterized PTM networks of IDO1, specific post-translational modifications of TDO2 remain poorly defined. Current research predominantly focuses on its baseline expression levels and associations with the tumor immune microenvironment and clinical outcomes [31, 41].

Role of IDO in human diseases

The IDO family, comprising IDO1, IDO2, and TDO2, functions as the master regulator of tryptophan metabolism, governing the conversion of tryptophan to kynurenine. This metabolic pathway exerts profound effects on diverse physiological and pathological processes, including immune tolerance, inflammatory responses, and tumor microenvironment modulation. Aberrant activation of these pathways has been implicated in the progression of various diseases, such as cancer, inflammatory disorders, diabetes, and psychiatric conditions, primarily by promoting immunosuppression and pathogenic inflammation.

IDO family and tumor

Members of the IDO family are frequently overexpressed in various malignancies. By depleting tryptophan and accumulating kynurenine within the tumor microenvironment, these enzymes directly inhibit T-cell and NK-cell proliferation and function. Simultaneously, they promote the generation and activation of regulatory T cells (Tregs) and myeloid-derived suppressor cells (MDSCs), thereby facilitating tumor immune evasion. Furthermore, downstream metabolites reinforce this immunosuppressive state by activating the AhR signaling pathway and exerting synergistic inhibitory effects with immune checkpoints such as PD-1/PD-L1. Consequently, the IDO family has emerged as a critical therapeutic target, and combination strategies involving its inhibitors have demonstrated significant clinical potential (Fig. 4).

Fig. 4.

IDO1/IDO2/TDO2 overexpression promotes tumor immune escape. Overexpression of IDO1/IDO2/TDO2 increases kynurenine (Kyn), which activates AhR to induce Tregs and MDSCs, while depleting effector T and NK cells, thereby facilitating tumor immune escape

IDO1 and tumors: a key molecule promoting immunosuppression and tumor progression

IDO1 is highly expressed across diverse tumor types and plays a pivotal role in driving tumor immune escape, enhancing invasiveness, and correlating with poor clinical prognosis.

Mechanistically, IDO1 catalyzes the catabolism of tryptophan into kynurenine, thereby inducing tryptophan depletion and kynurenine accumulation within the tumor microenvironment (TME). This metabolic shift impairs the cytotoxic function of effector T cells and natural killer (NK) cells while promoting the expansion of regulatory T cells (Tregs), ultimately establishing an immune-tolerant niche that facilitates tumor immune evasion [31, 43–45]. Beyond its immunomodulatory effects, IDO1 activates downstream signaling cascades, including the kynurenine-aryl hydrocarbon receptor-aquaporin 4 (Kyn-AhR-AQP4) and phosphatidylinositol 3-kinase-protein kinase B (PI3K-Akt) pathways, to promote tumor cell proliferation, migration and resistance to apoptosis [31, 46, 47]. Notably, IDO1 expression is not restricted to tumor cells but is also detected in stromal components of the TME, such as tumor-associated fibroblasts, endothelial cells, and dendritic cells, which collectively synergize to shape a robust immunosuppressive microenvironment [31, 43, 44].

Clinically, elevated IDO1 expression is strongly correlated with aggressive tumor phenotypes, including lymph node metastasis, disease recurrence, poor differentiation, advanced clinical stage, and reduced overall survival, in malignancies such as lung adenocarcinoma, diffuse large B-cell lymphoma, and esophageal cancer [44, 45, 48–50]. However, this association is context-dependent; studies in high-grade serous ovarian cancer have demonstrated that high IDO1 expression correlates with increased tumor-infiltrating lymphocytes and improved prognosis, underscoring the critical role of tumor type and TME heterogeneity in determining its functional outcome [51].

The functional interplay between IDO1 and immune checkpoints, such as PD-1/PD-L1, represents a critical mechanism of tumor immune evasion. Evidence suggests that their co-expression creates a synergistic environment for immune suppression [43, 44, 48, 52]. Consequently, combined inhibition of these pathways elicits superior anti-tumor immune responses compared to monotherapy.While preclinical and clinical studies highlight the potential of IDO1 inhibitors across diverse malignancies [44, 48, 53], the disappointing results observed in single-agent clinical trials [6, 54, 55]underscore the urgent need for combinatorial approaches and more precise patient selection criteria.

As a master regulator of tumor immunosuppression, IDO1 represents a critical therapeutic target whose clinical utility is contingent upon tumor context. To fully harness its potential, future strategies must focus on optimizing combinatorial regimens and implementing precise patient screening protocols.

IDO2 and tumors: immunosuppression mediated by multiple mechanisms and tumor progression

IDO2 functions as a rate-limiting enzyme in the tryptophan metabolism pathway and is frequently overexpressed in diverse malignancies. Its upregulation is closely linked to tumorigenesis, disease progression, and immune microenvironment remodeling, with high expression generally correlating with poor clinical outcomes and immunosuppression. Notably, unlike IDO1, IDO2 exerts oncogenic effects through non-catalytic mechanisms.

As a key driver of tumor immunosuppression, IDO2 orchestrates metabolic reprogramming to suppress T and NK cell function while fostering regulatory T cell (Treg) expansion [56–59]. Critically, its upregulation is linked to increased PD-L1 expression and altered immune infiltration in the tumor microenvironment, thereby playing a pivotal role in immune checkpoint regulation [56, 58, 60, 61].

Additionally, membrane localization and phosphorylation of IDO2 in tumor cells suggest non-enzymatic roles such as signal transduction that influence tumor cell behavior [61]. Clinically, high IDO2 expression correlates with tumor progression, enhanced invasiveness, and poor outcomes in malignancies including non-small cell lung cancer, pancreatic cancer, and medullary thyroid cancer, supporting its utility as a prognostic marker [56–58, 61, 62]. Animal experiments and clinical data further demonstrate that IDO2 inhibition or knockout enhances anti-tumor immunity and suppresses tumor growth, underscoring its potential as a novel immunotherapeutic target [57, 59, 62].

In summary, the dual enzymatic and non-enzymatic roles of IDO2 position it as a key mediator of tumor immunosuppression. Consequently, it represents a valuable target for prognostic assessment and therapeutic intervention, with further research anticipated to significantly advance the field of precision tumor immunotherapy.

TDO2 and tumors: a dual driver connecting neurometabolism and tumor immunosuppression

Positioned as a key nexus between neural regulation and tumor immunity, TDO2 drives tumor progression while maintaining systemic tryptophan balance via tryptophan metabolism. Recent studies highlight its emerging role as a promising therapeutic target, with growing interest in elucidating its molecular mechanisms across various tumor types.

The enzymatic activity of TDO2 drives tumor progression via the tryptophan-kynurenine pathway. Specifically, TDO2 catalyzes tryptophan degradation to produce Kyn, which subsequently binds and activates the AhR, thereby enhancing tumor proliferation and immune evasion [63–66]. This effect is amplified by the co-expression of TDO2 in tumor cells and surrounding stromal elements (e.g., fibroblasts), creating a robust immunosuppressive niche. Clinical data further indicate that high TDO2 expression is a significant predictor of poor outcomes, especially in gliomas [64, 66–68].

The metabolic products of TDO2, notably Kyn and quinolinic acid, play a pivotal role in both neurotoxicity and immune modulation, significantly impairing tumor-related neuronal integrity and altering immune cell dynamics [65, 66, 68]. Mechanistically, Kyn binds to and activates the AhR, orchestrating the expansion of Tregs and suppressor macrophages while simultaneously inhibiting the effector function of CD8 + T cells, thus establishing a robust immunosuppressive niche [63, 64, 69]. However, the clinical utility of monotherapy targeting IDO1 alone remains unsatisfactory [31, 70, 71]. Consequently, TDO2 and its downstream signaling pathways have emerged as critical therapeutic targets for intervention. Emerging evidence suggests that combined strategie, such as dual inhibition of IDO1/TDO2 or AhR, or direct degradation of Kyn may be more effective in reversing immunosuppression and potentiating immune checkpoint blockade [16, 63, 69]. Currently, several novel agents, including specific TDO2 inhibitors, dual-target inhibitors, and Kyn-degrading enzymes, are actively being investigated in preclinical studies or early-phase clinical trials [16].

TDO2 emerges as a critical nexus linking tumor-associated neurodegeneration and immunosuppression through tryptophan metabolic dysregulation. Although targeting this axis holds significant promise for breaking through immunotherapy bottlenecks, overcoming the hurdles of intricate molecular mechanisms and clinical translation remains imperative. Future directions should focus on multi-target combinatorial inhibition and precise tumor stratification to unlock optimal therapeutic outcomes.

IDO family and autoimmune diseases

The IDO family exerts complex and sometimes opposing roles in autoimmune diseases by fine-tuning immune tolerance and inflammatory responses. Among family members, IDO1 primarily promotes immune tolerance, whereas IDO2 exhibits pro-inflammatory properties, particularly in B cell–mediated autoimmunity. This functional antagonism highlights the critical role of the IDO family in maintaining immune homeostasis and identifies it as a promising therapeutic target for autoimmune diseases (Fig. 5).

Fig. 5.

Roles of IDO family members in autoimmune diseases. This schematic illustrates the distinct functions of IDO family enzymes in immune regulation. IDO1 promotes immune tolerance, and its dysfunction can exacerbate autoimmunity. IDO2 drives proinflammatory, B cell-mediated antibody responses. TDO2, primarily neuroprotective, has limited direct immune regulation

IDO1: a central regulator of immune tolerance

IDO1 plays a protective role in autoimmune diseases by maintaining immune tolerance through tryptophan metabolism. By suppressing effector T cell activity and promoting regulatory T cell differentiation, IDO1 limits excessive immune activation [72, 73]. Consistent evidence from animal models, including experimental autoimmune encephalomyelitis, rheumatoid arthritis, and type 1 diabetes mellitus, has generally shown that IDO1 deficiency or dysfunction aggravates autoimmune pathology [72, 74–77]. In immune cells such as dendritic cells and B cells, IDO1 expression is essential for restraining autoreactive immune responses. Human studies further indicate that genetic polymorphisms or functional impairment of IDO1 are associated with increased susceptibility to autoimmune diseases [78, 79]. Therapeutic strategies aimed at restoring or enhancing IDO1 activity, including pharmacological induction and metabolic pathway modulation, have shown durable immunosuppressive effects in preclinical models, supporting IDO1 as both a biomarker and a therapeutic target [74].

IDO2: a driver of B cell–mediated autoimmunity

Existing research highly consistently indicates that in contrast to IDO1, IDO2 plays a distinct pro-inflammatory role in autoimmune diseases, primarily by promoting B cell activation and autoantibody production [30]. Experimental studies in arthritis models reveal that IDO2 deletion or inhibition markedly reduces inflammation and autoantibody levels without compromising systemic immune competence [29, 30, 80, 81]. Notably, the pathogenic function of IDO2 extends beyond its enzymatic activity and involves non-enzymatic interactions with transcriptional regulators that control B cell responses [82, 83]. The disease specificity of IDO2 is underscored by its prominent role in antibody-driven arthritis, while exerting limited effects in T cell–dominated autoimmune models [29, 30]. Emerging therapeutic approaches targeting IDO2, including selective inhibitors and B cell–directed gene silencing strategies, have demonstrated promising efficacy in preclinical studies [80, 81, 84].

TDO2: a modulator of neuroimmune homeostasis

TDO2 primarily regulates systemic tryptophan homeostasis and contributes indirectly to immune modulation through the generation of neuroactive and immunoregulatory metabolites [85]. Although its direct immunosuppressive effects appear limited compared with IDO1, studies in autoimmune models such as experimental autoimmune encephalomyelitis suggest that TDO2 provides spatially restricted neuroprotection without significantly altering immune cell infiltration or disease severity [72, 86]. These findings indicate that TDO2 may influence autoimmune disease progression by modulating the neuroimmune environment rather than directly controlling immune activation [86]. While its immunological functions remain incompletely defined, TDO2 represents a potential therapeutic target, particularly in autoimmune diseases with prominent neurological involvement [7, 85, 86].

IDO family and infectious diseases

The IDO family plays context-dependent roles in infectious diseases by integrating antimicrobial defense with immune regulation. Through tryptophan degradation, IDO enzymes can directly restrict pathogen growth, while sustained activation of the kynurenine pathway modulates host immunity, shaping infection outcomes. This balance between pathogen control and immune tolerance critically determines whether infections are cleared or persist (Fig. 6).

Fig. 6.

Distinct roles of IDO1, IDO2, and TDO2 in host-pathogen interactions and infection outcomes. IDO1, IDO2, and TDO2 exhibit tissue-specific expression and differential immunomodulatory functions during infections. Tryptophan depletion exerts antimicrobial effects, while kynurenine accumulation can either promote immune clearance via AhR or facilitate pathogen immune evasion, critically determining infection outcomes

IDO1: a multifunctional enzyme with dual immunological roles

IDO1 exerts the most complex and extensively studied functions during infection. By degrading tryptophan, IDO1 limits the growth of diverse pathogens, while kynurenine production promotes regulatory T cell expansion, suppresses excessive inflammation, and maintains immune tolerance [33, 87]. Although these effects protect host tissues, prolonged IDO1 activation in fungal, parasitic, viral, and bacterial infections may also induce immunosuppression, facilitating pathogen persistence [88–90]. Experimental evidence indicates that IDO1 deficiency can trigger compensatory inflammatory pathways that partially control pathogen burden, highlighting its context-dependent role in host defense [87, 91]. Therapeutically, IDO1 inhibition shows potential in tumors and selected infectious settings but may aggravate disease in certain fungal infections, underscoring the need for precision-based targeting strategies [31, 44, 53].

IDO2: pro-inflammatory and B cell–oriented immune regulation

Unlike IDO1, IDO2 exhibits distinct immunological functions that preferentially modulate inflammatory and B cell–mediated responses [39, 92–94]. IDO2 is mainly expressed in B cells, dendritic cells, and monocytes, where it promotes pro-inflammatory signaling and contributes to immune regulation through both enzymatic and non-enzymatic mechanisms [95]. In infection models, altered IDO2 expression has been associated with dysregulated inflammation, immune tolerance, and tissue damage, particularly in viral and fungal infections [29, 82, 83, 96]. Notably, IDO2 deficiency can exacerbate fungal infections, suggesting a non-redundant role in anti-fungal immunity [29, 30]. Together, these findings indicate that IDO2 complements and counterbalances IDO1 by shaping immune responses through distinct cellular pathways.

TDO2: metabolic homeostasis linking immunity and infection

TDO2 primarily functions in maintaining systemic tryptophan homeostasis but also participates in immune regulation during infection. By depleting tryptophan, TDO2 restricts pathogen growth and suppresses T cell activation and cytokine production, thereby contributing to immune tolerance [7, 13, 97–99]. In viral infection models, TDO2 inhibition modulates antiviral antibody responses and infection-associated autoimmunity without directly affecting viral replication, suggesting a role in regulating immune-mediated pathology rather than pathogen clearance [97, 98]. Although the functional interplay between TDO2 and IDO enzymes remains incompletely defined, accumulating evidence highlights TDO2 as a metabolic regulator with therapeutic potential in infection-related immune dysregulation [13, 99].

IDO family and neurological diseases

The IDO family plays a central role in neurological and psychiatric disorders through dysregulation of the Try–KP pathway. Under inflammatory stress, activation of IDO and TDO2 shifts tryptophan metabolism away from serotonin synthesis toward the production of neuroactive metabolites. Reduced serotonin availability is closely associated with mood disorders, while excessive accumulation of neurotoxic metabolites, such as quinolinic acid, induces excitotoxicity, oxidative stress, and neuroinflammation. Imbalance between neurotoxic and neuroprotective kynurenine metabolites contributes to the pathogenesis of neurodegenerative, demyelinating, epileptic, and psychiatric disorders, positioning the KP pathway as a key neuroimmunomodulatory target (Fig. 7).

Fig. 7.

The role of tryptophan-kynurenine pathway in neuroinflammation and associated neurological disorders. Inflammatory stress and genetic predisposition dysregulate the tryptophan-kynurenine pathway, leading to an imbalance between neurotoxic and neuroprotective metabolites. This imbalance contributes to neuroinflammation, excitotoxicity, and neuronal damage, ultimately driving the pathogenesis of various neurological and psychiatric disorders, highlighting potential therapeutic targets

IDO1: linking neuroinflammation to disease progression

IDO1 primarily influences neurological diseases by modulating neuroinflammation and immune–metabolic interactions. Elevated IDO1 activity has been observed in neurodegenerative and neuroinflammatory conditions, where it promotes the accumulation of neurotoxic metabolites and exacerbates neuronal injury. In contrast, IDO1 deficiency or inhibition can alleviate disease severity in selected models, highlighting its context-dependent role. Overall, IDO1 contributes to neurological disease progression mainly by shaping inflammatory responses and the local metabolic microenvironment, making it a potential target for immune metabolic intervention (Table 2).

Table 2.

Roles and therapeutic prospects of IDO1 in a variety of neurological diseases

Disease/Model	The function/mechanism of IDO1	Therapeutic prospects/target suggestions	References
AD	Elevated IDO1 activity promotes neuroinflammation and metabolic disorders, Inhibition can improve cognition and metabolism.	Elevated IDO1 activity promotes neuroinflammation and metabolic disorders, Inhibition can improve cognition and metabolism.	[100–102]
PD	IDO1 + dendritic cells/monocytes regulate immunity and influence the progression of inflammation.	Immunomodulation and IDO1-targeted therapy are worth exploring.	[103, 104]
MS	IDO1 has an immune protective effect, and its deficiency aggravates the condition.	IDO1 positive regulators or PAM have potential.	[75, 76, 105]
Epilepsy	IDO1 promotes the production of neurotoxic metabolites, and its deficiency can alleviate seizures and injuries.	The combination of IDO1 inhibitors and immunotherapy has attracted attention.	[106]
Glioma/brain tumor	IDO1 promotes the production of neurotoxic metabolites, and its deficiency can alleviate seizures and injuries.	The combination of IDO1 inhibitors and immunotherapy has attracted attention.	[46, 107]
Neuropathic pain	IDO1 inhibitors can relieve pain and related emotional disorders.	IDO1 inhibitors can relieve pain and related emotional disorders.	[108, 109]

IDO1 plays a key role in the pathogenesis and progression of various neurological diseases by regulating tryptophan metabolism and the immune microenvironment. The drug development and mechanism research of IDO1 provide an important theoretical and practical basis for the innovative treatment of neurodegenerative diseases, autoimmune diseases and brain tumors, etc.

IDO2: context-dependent neuroimmune modulation

Studies have shown that compared with IDO1, IDO2 plays a more context-dependent role in nervous system diseases, which can protect or promote nervous system diseases. The literature suggests that IDO2 is more likely to regulate disease susceptibility and inflammatory thresholds than to drive the course of disease. In demyelinating and inflammatory models [110], IDO2 has been reported to exert protective functions by restraining excessive inflammation, whereas in other settings its deficiency shows limited impact, underscoring strong context dependence [30, 75, 111]. Altered IDO2 expression and epigenetic regulation have also been observed in neurodegenerative and neurodevelopmental disorders, suggesting its potential involvement as a biomarker and modulator of disease susceptibility rather than a universal driver of pathology [5, 33, 112].

TDO2: a key regulator of neurotoxic–neuroprotective balance

TDO2 critically regulates the balance between neurotoxic and neuroprotective kynurenine metabolites [113–115]. Increased TDO2 activity favors the accumulation of neurotoxic compounds, contributing to neuronal damage and degeneration, whereas genetic or pharmacological inhibition of TDO2 shifts metabolism toward neuroprotective pathways [114, 116, 117]. Consistent evidence from animal models of Alzheimer’s disease, Parkinson’s disease, Huntington’s disease, and multiple sclerosis supports the neuroprotective potential of TDO2 inhibition, including improvements in motor, cognitive, and neuroinflammatory outcomes [113, 116, 118, 119]. These findings highlight TDO2 as an emerging therapeutic target in neurodegenerative diseases, although the long-term safety and clinical feasibility of sustained TDO2 inhibition remain to be established [7, 120].

IDO and other diseases

IDO family members, especially IDO1 and IDO2, have been widely studied in tumor, autoimmune, infectious and neurological diseases, and also play an important role in a variety of other diseases, including inflammation, cardiovascular, liver, transplantation and metabolic diseases (Table 3).

Table 3.

The association effect of IDO/TDO2 with different diseases

Types of disease	Core content explanation
Cardiovascular and metabolic diseases	Elevated IDO1 activity confers atheroprotection by maintaining vascular immune homeostasis and regulates diabetic inflammatory progression [45, 97, 121, 122].
Liver disease	IDO and TDO2 have dual effects on liver diseases [97, 99], supporting immunosuppression-dependent anti-infection/transplant tolerance while potentially promoting hepatic inflammation and tumorigenesis via aberrant activity.
Allergic and transplantation-related diseases	Highly expressed in allergic diseases and transplant rejection, IDO1 preserves graft survival via tryptophan depletion, kynurenine metabolite production, T cell suppression, and immune tolerance induction [45, 97].
Musculoskeletal disorders	IDO1 mitigates progression of musculoskeletal diseases via regulating inflammation and Treg/Th17 subsets, with its metabolite ratios acting as inflammatory biomarkers [5, 123].
Pregnancy and reproductive immunity	IDO1, initially discovered in pregnancy immune tolerance, maintains gestational immune homeostasis via dendritic cell tolerance and regulatory T-cell induction, preventing maternal T-cell fetal targeting [33, 97].

Research progress of the IDO family as therapeutic targets

The IDO family (IDO1, IDO2, TDO2), as the key regulators of tryptophan metabolism, has shown important therapeutic potential in tumors, autoimmune and nervous system diseases. In the field of cancer, IDO inhibitors can reverse the immunosuppressive microenvironment and significantly enhance anti-tumor immune responses when combined with PD-1/PD-L1 inhibitors. In autoimmune diseases, IDO1 agonists promote immune tolerance, whereas IDO2 inhibitors inhibit abnormal B cell activation. In nervous system diseases, TDO2 inhibitors can regulate the balance of neurotoxic/protective metabolites and improve the pathological process of diseases such as Alzheimer’s disease and depression. Current research priorities include the development of dual inhibitors to overcome the compensatory mechanism, the use of nanotechnology to improve targeting, and the establishment of biomarks-guided precision treatment strategies. Despite the challenges of clinical translation, the IDO family is still regarded as an important bridge connecting immune metabolism and disease treatment, and its regulatory strategy provides a new treatment paradigm for a variety of refractory diseases.

Main targeting strategies and drug types

Small-molecule inhibitor

IDO1 small-molecule inhibitors have become a drug development hotspot due to their potential in tumor immunotherapy, yet no candidates have been approved for marketing, with disappointing outcomes in several key clinical trials. Developed inhibitors span structural classes including hydroxamidines (e.g., epacadostat), benzimidazoles, imidazopyridines, isoxazopyrimidinones, and triazoles. The most of act by binding heme iron at IDO1’s active site, while novel molecules selectively target apo-IDO1 (heme-free form) for enhanced selectivity and persistence [124–131]. Representative agents like epacadostat, indoximod, and navoximod have entered clinical trials, but epacadostat’s phase III ECHO-301 trial (in combination with pembrolizumab) lacked clinical benefit, halting multiple related projects [71, 124, 132–137]. To overcome single-target resistance and compensation, next-generation inhibitors (PROTACs, apo-IDO1 inhibitors, IDO1/TDO2 dual/multi-target agents) are under development [124, 125, 127, 128, 133] Future efforts will focus on structural innovation (novel scaffolds/binding modes) [125, 126, 129, 130, 138, 139], target combinations (e.g., IDO2, TDO2, AhR) or dual-acting molecules [71, 124, 127, 128, 133], and optimizing clinical trial design, patient stratification, and biomarker screening [71, 124, 133–135].The comparison of the structural types and clinical progress of IDO1 inhibitors is as follows (Table 4).

Table 4.

Comparison of IDO1 inhibitor structure types and clinical progress

Molecule/Type	Structural features/mechanisms	Clinical progress/challenges	References
Epacadostat	Hydroxamidines, heme binding	Phase III failure, drug resistance issue	[71, 124, 132, 135, 136]
Linrodostat	Heme translocation type	Phase III failure, drug resistance issue	[128, 134]
PROTACs/apo-IDO1	Heme translocation type	Phase III failure, drug resistance issue	[124, 127, 128, 133]
Novel triazole/benzimidazole	Diverse new skeletons	Outstanding in vitro/animal experimental activity	[125, 126, 129, 130, 138, 139]

As an IDO1 homologue, IDO2 has emerged as a novel drug target for tumor immune escape and autoimmune diseases, with its small-molecule inhibitor development in the early stage but achieving breakthroughs. Compound 22 (reported in 2022), the first selective IDO2 inhibitor (in vitro IC₅₀ = 112 nM), reduces disease severity and inflammatory factors in RA animal models [81]. Combined IDO1/IDO2 inhibition exerts synergistic anti-tumor effects. Compound 4t (IDO1 IC50 = 28 nM, IDO2 IC50 = 144 nM) was superior to epacadostat in a mouse tumor model [140]. Additionally, low-molecular-weight IDO2 inhibitors based on the 1,2,3-triazole scaffold have been developed via structural simulation and molecular docking, showing partial selectivity but requiring improved activity [141, 142]. Challenges include IDO2-IDO1 structural homology (hindering selective inhibitor development) and unclear physiological/pathological roles of IDO2 (which exhibits protective effects in some diseases). To date, IDO2 inhibitors remain limited to in vitro or animal studies, with no clinical reports. Representative IDO2 inhibitors and their characteristics are as follows (Table 5).

Table 5.

Representative molecules of IDO2 small molecule inhibitors and their characteristic

Molecule/Type	Selectivity/Target	In vitro activity (IC50)	In vivo action/model	Main application prospects
Compound 22	Highly selective IDO2	112 nM	Animal model of rheumatoid arthritis	Autoimmune and mechanism research
Compound 4t	IDO1/IDO2 double inhibition	28/144 nM	Tumor mouse model	tumor immunotherapy
1,2, 3-triazole derivatives	IDO2 predominated	51 µM	cell experiment	Molecular probes, structural optimization

TDO2 small-molecule inhibitors are pivotal for tumor immunotherapy and neurological disease intervention, with recent focus on developing highly selective, orally available, blood-brain barrier-penetrant novel inhibitors and TDO2/IDO1 dual-target agents. Virtual screening and molecular docking have identified potent, selective TDO2 inhibitors (e.g., substituted naphthotriazolone, IC₅₀ = 30 nM, 3-phenylindoles), validated in animal models [143, 144], with efficient inhibition rooted in hydrogen/coordination bonds with active-site residues (His55, Thr254, Fe atom) [145–147]. TDO2/IDO1 dual inhibitors (e.g., PVZB3001, RG70099, NPACT00380) reduce kynurenine levels, restore NK cell function, and enhance antitumor immunity in vitro/in vivo [148–151], with optimization via machine learning and AI-driven molecular generation [151, 152]. Notably, TDO2 inhibitors improve motor/cognitive/intestinal functions in Parkinson’s disease mouse models (neuroprotective/anti-inflammatory effects) [144] and enhance immune checkpoint inhibitor efficacy in tumors, with some candidates entering preclinical/early clinical trials [153]. Rapidly advancing research covers structural optimization, dual-target design, and preclinical validation. Future priorities include boosting selectivity, pharmacokinetics, and clinical translational potential for precision tumor and neurological disease treatment. The types and representative molecules of TDO2 inhibitors are summarized as follows (Table 6).

Table 6.

Comparison of TDO2 inhibitor types, representative molecules and their main properties

Type	Representing molecules/structures	Main features and progress
Selective TDO2 inhibitors	3-phenylindole, LM10, etc.	High selectivity, brain penetration, and effective in animal models
Dual-target TDO2/IDO1 inhibitor	PVZB3001, RG70099, etc.	Dual enzyme inhibition in vivo and in vitro, and immune enhancement
Derivatives of natural products	PVZB3001, RG70099, etc.	It has diverse structures and some activities are superior to those of known inhibitors

Dual and multiple inhibitors targeting the IDO family: a new direction in tumor immunotherapy

IDO1-driven tryptophan metabolism contributes to immunosuppression in a wide range of tumors and has long been considered an attractive target for cancer immunotherapy. However, the limited clinical efficacy of single-agent IDO1 inhibitors has highlighted the presence of compensatory mechanisms involving related enzymes such as TDO2 and IDO2, prompting the development of dual and multiple inhibitors to achieve more effective pathway blockade [154].

Dual inhibition of IDO1 and TDO2 has emerged as a particularly promising strategy. Preclinical studies consistently demonstrate that selective IDO1 inhibition can induce compensatory upregulation of TDO2, whereas simultaneous targeting of both enzymes more effectively suppresses kynurenine production and enhances antitumor immune responses [155, 156]. Several IDO1/TDO2 dual inhibitors have shown superior antitumor efficacy in resistant tumor models, supporting the rationale for combined enzymatic blockade.

Beyond dual IDO1/TDO2 targeting, multiple inhibitors simultaneously engaging IDO1 and additional pathways involved in tumor immune evasion have been developed. These include IDO1/HDAC dual inhibitors integrating immunometabolic and epigenetic regulation, IDO1/TrxR inhibitors modulating redox homeostasis and immune activation, and IDO1/NAMPT inhibitors disrupting NAD⁺ biosynthesis [55, 157]. Such multi-target strategies aim to amplify antitumor immunity by converging metabolic, transcriptional, and survival pathways.

Given that many tumors co-express IDO1, IDO2, and TDO2, inhibitors targeting multiple IDO family members provide a more comprehensive approach to suppressing tryptophan–kynurenine metabolism. Experimental compounds capable of simultaneously inhibiting IDO1, IDO2, and TDO2 demonstrate enhanced antitumor activity compared with single-target agents, underscoring the therapeutic potential of broader pathway inhibition [158].

Collectively, dual and multiple IDO family inhibitors represent an emerging frontier in tumor immunotherapy. While early studies indicate acceptable safety profiles and encouraging preclinical efficacy, further optimization of selectivity, pharmacokinetics, patient stratification, and combination strategies will be essential to translate these agents into meaningful clinical benefit. Representative inhibitors and their activities are as follows (Table 7).

Table 7.

Major TDO2/IDO1 dual inhibitors and comparison of their propertie

Inhibitor/Compound	Main target	In vivo/in vitro activity (IC50)	Indications/models	Key findings/advantages	Refrence
1-(Hetero)aryl-β-carboline derivative	IDO1/TDO2	3.53/1.15 µM	Mouse model of depression	Antidepressant and anti-inflammatory response	[159]
4, 6-substituted − 1 H-indazole derivatives	IDO1/TDO2	0.74/2.93 µM	CT26 tumor model	Significant anti-tumor activity	[160]
N-benzyl/aryl indirubin derivatives	IDO1/TDO2/IDO2	circuit inhibit efficient	LLC/H22 tumor model	Significant anti-tumor activity	[161]
RY103	IDO1/TDO2	unpublished	Mouse model of pancreatic cancer	Significant anti-tumor activity	[162]
PVZB3001	IDO1/TDO2	Both cells/in vitro were effective	Tumor models	Restore the function of NK cells and inhibit tumor growth	[148]
TD34 et al.	IDO1/TDO2	3.42 µM (BT549 cells)	Breast cancer cell line	Restore the function of NK cells and inhibit tumor growth	[163]

New mechanism

In recent years, researchers have developed a variety of novel by molecular mechanisms are activated to restrain activity of IDO family, break through the limitations of the traditional inhibitors and show a more selective and clinical potential.

Traditional IDO1 inhibitors mostly bind to the heme iron of the enzyme, while new inhibitors, such as B37 and some pyrrolic acid derivatives, competitively bind to apo-IDO1 to prevent heme from entering the active site of the enzyme, thereby achieve efficient inhibition. This mechanism is independent of the enzyme activity state and the inhibitory effect is more durable [164, 165]. By linking IDO1 inhibitors with E3 ligase ligands, PROTACs can achieve selective degradation of IDO1 protein, significantly reduce the level of IDO1, and block the tumor immunosuppressive microenvironment [124, 166, 167]. Preclinical studies have shown remarkable results, particularly in combination with PD-1, and represent the next generation of technology.

Immunomodulatory strategies

In recent years, peptide vaccines against IDO1 have attracted wide attention as a new tumor immunotherapy strategy, especially showing synergistic anti-tumor potential in combination with immune checkpoint inhibitors (such as PD-1 antibody).

IDO1 peptide vaccine can induce specific CD8 + and CD4 + T cell responses, target IDO1-expressing immune cells, reverse the immunosuppressive microenvironment, and enhance antitumor immunity [44, 168]. Animal experiments and early clinical studies have shown that IDO1 peptide vaccine alone or in combination with anti-PD-1 antibody can significantly inhibit tumor growth, and the combination of the two can produce a synergistic effect [44, 168, 169]. Various strategies, such as combining IDO1 peptide vaccine with PD-1/PD-L1 inhibitors, IDO1 small molecule inhibitors or mRNA vaccines, have shown the potential to enhance anti-tumor immunity and improve patient prognosis in preclinical and clinical studies [44, 169]. In the future, optimizing the combination regimen and in-depth mechanism research will promote the clinical application of IDO1 peptide vaccine in tumor immunotherapy.

Nano delivery system

IDO catalyzes tryptophan metabolism, promotes the activation of immunosuppressive cells (such as Treg and MDSC), inhibits the function of effector T cells, and leads to tumor immune escape. Nano-delivery systems (such as liposomes, polymer nanoparticles, metal organic frameworks, etc.) can achieve tumor-targeted delivery and controlled release of IDO inhibitors, enhance drug accumulation at the tumor site, reduce systemic toxicity, and cooperate with photothermal/photodynamic therapy, chemotherapy, etc., to induce immunogenic cell death (ICD) and further activate anti-tumor immunity [170–174].

Most of the nano platforms realize the synergistic delivery of IDO inhibitors with chemotherapy drugs, photosensitizers, NO donors, etc., which significantly enhance the tumor inhibition and immune activation effects [175–177]. Combination of immune checkpoint inhibitors (such as PD-1/PD-L1 antibodies) further improved the efficacy, and some nanosystems achieved complete tumor regression and distal suppression in animal models [174, 176, 178–180]. Intelligent responsive nanocatellors (triggered by pH, GSH, red oxygen, etc.) can accurately control drug release and improve therapeutic specificity [172, 177, 181].

However, some IDO inhibitors have limited efficacy as monotherapy or combined with immune checkpoint inhibitors, suggesting the need to optimize delivery systems and patient selection [6, 171, 176]. Moreover, the biological safety, large-scale production and pharmacokinetics of nanomedicine still need to be further studied [171, 176, 180]. There is no doubt that multi-target/multimodal collaborative delivery, individualized treatment and the development of novel nanomaterials are the priorities for the future [171, 176, 182].

Clinical challenges and key lessons

In the field of immuno-oncology, epacadostat was once regarded as a highly promising dark horse. As the first IDO1 inhibitor to enter phase III clinical trials, it was combined with pembrolizumab (Keytruda), the blockbuster drug developed by Merck & Co., Ltd., in an attempt to combat metastatic melanoma. However, the results of the ECHO-301/KEYNOTE-252 trial, published in April 2018, marked a pivotal setback. Despite the encouraging findings from phase I/II trials, large-scale validation in phase III failed to achieve the expected endpoints, which dealt a heavy blow to the development of this agent and highlighted the formidable challenges confronting the development and clinical application of IDO family inhibitors.

Fundamentally, these setbacks primarily stem from the oversimplified interpretation of target biology, insufficient evidence regarding dose-pharmacodynamic relationships, the absence of precise patient stratification, and the failure to control pathway compensation. To address these challenges and advance the development of IDO-targeted therapies, future priorities should focus on three core strategies: pathway-level and multi-target regimens, biomarker-guided precision trial design, and mechanism-based combination therapies (Table 8).

Table 8.

Main causes of clinical failure of IDO inhibitors and ideas for improvement

Challenges/Lessons	Key point	Corresponding improvement directions	Reference
ECHO‑301 failed	The compensation and non-enzymatic functions of TDO2/IDO2 were not blocked	Strengthen the pharmacodynamics of tissues and gradually advance to Phase III	[135, 158, 183]
Limitations of single-target IDO1	The compensation and non-enzymatic functions of TDO2/IDO2 were not blocked	Develop dual-target or pan-TRP pathway inhibitors and PROTAC degraders	[6, 124, 132, 184, 185]
The patients were not stratified	The expression of IDO1 varies greatly across cancer types and patients	Develop dual-target or pan-TRP pathway inhibitors and PROTAC degraders	[6, 158, 186, 187]
Off-target and safety	The multi-target effects of TRP analogues are difficult to resolve	Develop dual-target or pan-TRP pathway inhibitors and PROTAC degraders	[188, 189]

Future outlook and new directions

The future of IDO family inhibitors will focus on combination therapy, multi-target inhibition, delivery system innovation, precision medicine and downstream pathway target development. Continuous optimization of molecular structure, rational design of clinical trials and accurate patient screening will be the key to promote its clinical transformation and improve its efficacy.

Continued optimization of combination therapy strategies

The efficacy of single-agent IDO1 inhibitors in clinical trials is limited, especially after the failure of the phase III trial of epacadostat combined with PD-1 inhibitors, the research focus has turned to the combination of multiple immunotherapy, chemotherapy, radiotherapy and other multimodal treatment in order to overcome drug resistance and improve efficacy. For example, IDO inhibitors have shown stronger synergistic anti-tumor effects in combination with immune checkpoint inhibitors, chemotherapy, radiotherapy, and photodynamic/photothermal therapy.

Diversification of targets and design of novel molecules

Future development of IDO family inhibitors will focus more on multi-target inhibitors (such as simultaneous inhibition of IDO1, IDO2, and TDO2) and new molecular mechanisms. For example, new strategies such as dual-target or all-target inhibitors, PROTACs (protein degradation-targeted chimeras), apo-IDO1 inhibitors are emerging to overcome the limitations of single-target inhibitors and tumor escape.

Innovation in nanomedicine and delivery systems

Nanotechnology provides a new direction for the targeted delivery and improvement of the efficacy of IDO inhibitors. Nanocapsules can achieve efficient delivery to the tumor site, reduce systemic toxicity, and synergistically with other therapeutic approaches to enhance anti-tumor immune responses.

Biomarkers and precision medicine

The expression level of IDO1 is significantly different among different tumor types and patients. Future clinical trials and applications need to strengthen the screening of biomarkers and patient stratification to improve efficacy prediction and individualized treatment.

Exploration of downstream pathways and new targets

In addition to directly inhibiting IDO1/2/TDO2, downstream targets of the Trp-Kyn-AhR pathway (such as AhR antagonists, Kyn degrading enzymes, and tryptophan analogues) have also become emerging research directions, which are likely to expand the means of immune regulation.

Summarize and prospect

As the key rate-limiting enzymes in the tryptophan-kynurenine metabolic pathway, the IDO family (including IDO1, IDO2, and TDO2) functions as a central hub bridging systemic metabolism and immune regulation. Their multifaceted roles in the pathogenesis and progression of diverse diseases, including tumors, autoimmune disorders, infectious diseases, and neurological conditions, have been extensively characterized. Specifically, IDO1 predominantly mediates local immunosuppression and tumor immune escape; IDO2 drives B cell-related inflammation and autoimmune responses; while TDO2 maintains systemic tryptophan homeostasis and links neurometabolism to immunity. These distinct yet interconnected functions provide a robust theoretical foundation for the development of targeted therapies.

Therapeutic strategies targeting the IDO family have evolved from initial single-target exploration to multi-dimensional innovation. Current advancements include the development of IDO1/TDO2 dual-target and pan-family multi-target inhibitors, the exploration of novel mechanism-based modulators (e.g., PROTAC degraders and apo-IDO1 binders), the application of immunomodulatory regimens such as IDO1 peptide vaccines, and the construction of nano-delivery systems for precise drug targeting. These approaches effectively address the limitations of single-target inhibition, overcome compensatory mechanisms, and enhance therapeutic specificity, highlighting the IDO family’s significant potential as diagnostic biomarkers and core therapeutic targets across multiple disease fields.

Although significant progress has been made, the translation of IDO-targeted therapies into clinical success remains impeded by critical barriers, primarily stemming from incomplete pathway blockade, inadequate patient stratification, and off-target effects. To unlock its full therapeutic potential, future research must pivot toward three strategic pillars: first, the development of high-selectivity modulators with optimized pharmacokinetic profiles, integrated with nano-delivery systems to achieve site-specific release and maximize efficacy while mitigating systemic toxicity; second, the refinement of combinatorial strategies by dissecting synergistic mechanisms with immune checkpoint inhibitors, chemotherapy, and radiotherapy, alongside the creation of robust biomarker panels for precise patient stratification; third, the investigation of non-enzymatic functions of the IDO family, their downstream signaling networks, and dynamic regulatory crosstalk within distinct disease microenvironments to identify novel therapeutic targets.

The convergence of immunology, molecular biology, and nanotechnology heralds a new era for IDO family-targeted therapy, characterized by multi-targeted strategies, diverse modalities, and precision medicine. This transformative approach promises to overcome existing limitations in treating refractory diseases while offering renewed optimism for enhancing therapeutic outcomes across a wide range of pathological conditions.

Author contributions

Xijie Wang: Writing – original draft. Zhe Chen: Writing – review & editing. Linxi Chen: Writing – review & editing, Funding acquisition. Chengfeng Qiu: Writing – review & editing, Funding acquisition.

Funding

This work was supported by the National Natural Science Foundation of China [Grant Numbers 82160075 and 82173813] and the Chenzhou National Sustainable Development Agenda Innovation Demonstration Areas Construction Provincial Special Funding [Grant Number 2023sfq05].

Data availability

Not applicable.

Declarations

Ethics approval and consent to participate

Not applicable.

Consent for publication

Not applicable.

Competing interests

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.