Key Takeaways & Executive Findings
- •• • ATZn nanoparticles co-deliver TDO inhibitor and acetaminophen, achieving dual blockade of kynurenine pathway at TDO and HAAO nodes, reducing both Kyn and QA production; statistical significance of immune modulation reached p < 0.01 and p < 0.0001 in flow cytometry analyses. • • QA suppression via HAAO inhibition reduced M2 macrophage polarization (CD45+ F4/80+ CD86− CD206+) and increased M1 macrophages (CD45+ F4/80+ CD86+ CD206−), with differences significant at p < 0.01, indicating a shift toward pro-inflammatory tumor microenvironment. • • Kyn inhibition and Zn2+ supplementation increased CD8+ T cell infiltration (CD45+ CD3+ CD4− CD8+) and reduced Treg cells (CD45+ CD3+ CD4+ CD8− Foxp3+), with statistical significance at p < 0.0001, correlating with enhanced antitumor cytotoxicity. • • The acid-responsive design of ATZn enables targeted release in tumor microenvironment, potentially improving therapeutic index and reducing systemic toxicity compared to free drug combinations, as evidenced by effective immune modulation at tolerated doses.
Abstract
Immunosuppressive metabolites are major drivers of tumor immune suppression. Among these, kynurenine (Kyn) is produced through the catalysis of tryptophan (Trp) 2,3-dioxygenase (TDO) in hepatocellular carcinoma. However, TDO inhibition alone is often insufficient because residual pathway flux sustains the accumulation of the downstream immunosuppressive metabolite quinolinic acid (QA). Here, we propose a strategy to disrupt residual kynurenine pathway activity to enhance metabolism-driven tumor immunotherapy. We develop acid-responsive metal-organic complex nanoparticles (APAP@TDOi-Zn, ATZn) that integrate the TDO inhibitor (TDOi) and Zn2+, while encapsulating acetaminophen (APAP) to inhibit 3-hydroxyanthranilate 3,4-dioxygenase (HAAO), thereby limiting QA production and simultaneously suppressing the residual immunosuppressive metabolite. QA suppression limits M2 macrophage polarization, whereas Kyn inhibition and Zn2+ supplementation promote T cell proliferation and cytotoxicity. Consequently, ATZn rewires Trp-Kyn metabolism and augments antitumor immunotherapy. This work enhances the efficacy of metabolic checkpoint blockade and provides a strategy to overcome metabolism-driven immune resistance.
1. Introduction
Metabolic checkpoints, particularly the kynurenine pathway of tryptophan catabolism, drive profound immunosuppression in hepatocellular carcinoma. TDO converts tryptophan to kynurenine, which directly induces T cell exhaustion and regulatory T cell differentiation. However, clinical TDO inhibitors have failed to meet endpoints, indicating that residual pathway flux through downstream metabolites such as quinolinic acid sustains immunosuppression. Quinolinic acid promotes M2 macrophage polarization and arginine depletion, further suppressing T cell activity. Thus, single-agent TDO blockade is insufficient; concurrent inhibition of downstream enzymes like HAAO is necessary to fully disrupt the pathway.
This study introduces ATZn, acid-responsive metal-organic nanoparticles co-encapsulating a TDO inhibitor and acetaminophen, a clinical HAAO inhibitor. The design simultaneously blocks Kyn production and QA accumulation, addressing the bottleneck of residual pathway activity. By limiting QA, ATZn suppresses M2 polarization; by inhibiting Kyn and providing Zn2+, it enhances CD8+ T cell infiltration and reduces Tregs. This dual-action strategy rewires Trp-Kyn metabolism, offering a more effective approach to metabolic checkpoint blockade and overcoming metabolism-driven immune resistance.
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WU Fan, XIANG Guangming, ZHANG Jing, YING Leilei, RUAN Hang, WU Yiling, WEI Xuehao, WU Yelin, LIU Yanyan, SHI Ruicheng, JIANG Xingwu (2026). Inhibition of Residual Kynurenine Pathway Activity Boosts Antitumor Immune Responses. SCIENCE CHINA Materials. https://doi.org/10.1007/s40843-025-4039-1
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Frequently Asked Questions
What is the mechanistic basis for combining TDO inhibition with HAAO inhibition, and how does this dual blockade overcome resistance to single-agent TDO inhibitors?
Single-agent TDO inhibitors fail because residual flux through the kynurenine pathway continues to produce downstream immunosuppressive metabolites, particularly quinolinic acid (QA). QA promotes M2 macrophage polarization and induces Arg1 expression, depleting arginine and suppressing T cells. By co-inhibiting HAAO with acetaminophen, ATZn blocks QA production, thereby relieving this residual immunosuppression. This dual blockade is essential to fully disrupt the pathway and achieve robust antitumor immunity.
What are the specific immune cell populations modulated by ATZn, and what are the statistical significances of these changes?
Flow cytometric analyses revealed that ATZn significantly reduced M2 macrophages (CD45+ F4/80+ CD86− CD206+) and increased M1 macrophages (CD45+ F4/80+ CD86+ CD206−), with p < 0.01. Additionally, ATZn increased CD8+ T cells (CD45+ CD3+ CD4− CD8+) and decreased Treg cells (CD45+ CD3+ CD4+ CD8− Foxp3+), with p < 0.0001. These changes indicate a shift from an immunosuppressive to an immunostimulatory tumor microenvironment.
How does the acid-responsive release profile of ATZn contribute to its therapeutic efficacy and potential safety profile?
The acid-responsive design ensures that the TDO inhibitor, acetaminophen, and Zn2+ are released preferentially in the acidic tumor microenvironment, minimizing systemic exposure and off-target effects. This targeted release allows for effective local concentrations to modulate immune cells, as evidenced by the significant immune changes observed, while potentially reducing toxicity compared to systemic administration of free drugs.
What are the translational implications of using acetaminophen, a widely used clinical drug, as an HAAO inhibitor in this nanoparticle formulation?
Acetaminophen is already clinically approved and has a well-characterized safety profile, which could accelerate clinical translation. Its incorporation into ATZn nanoparticles allows for repurposing of an existing drug for a new indication—HAAO inhibition in cancer immunotherapy. This strategy leverages known pharmacokinetics and toxicology, potentially reducing development time and cost.
What are the key experimental validations that support the conclusion that ATZn enhances antitumor immune responses?
The conclusions are supported by flow cytometric analyses of tumor tissues, which quantified macrophage polarization (M1/M2), CD8+ T cell infiltration, and Treg populations. Statistical significance (p < 0.01 and p < 0.0001) confirms that ATZn treatment leads to a favorable shift in immune cell composition, correlating with enhanced antitumor immunity. These data provide direct evidence of the immunomodulatory effects of ATZn.
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