Increased tryptophan (Trp) catabolism in the tumor microenvironment (TME) can mediate immune suppression by upregulation of interferon (IFN)-γ-inducible indoleamine 2,3-dioxygenase (IDO1) and/or ectopic expression of the predominantly liver-restricted enzyme tryptophan 2,3-dioxygenase (TDO)1,2,3,4,5. Whether these effects are due to Trp depletion in the TME or mediated by the accumulation of the IDO1 and/or TDO (hereafter referred to as IDO1/TDO) product kynurenine (Kyn) remains controversial5,6,7,8,9,10,11,12,13. Here we show that administration of a pharmacologically optimized enzyme (PEGylated kynureninase; hereafter referred to as PEG-KYNase) that degrades Kyn into immunologically inert, nontoxic and readily cleared metabolites inhibits tumor growth. Enzyme treatment was associated with a marked increase in the tumor infiltration and proliferation of polyfunctional CD8+ lymphocytes. We show that PEG-KYNase administration had substantial therapeutic effects when combined with approved checkpoint inhibitors or with a cancer vaccine for the treatment of large B16-F10 melanoma, 4T1 breast carcinoma or CT26 colon carcinoma tumors. PEG-KYNase mediated prolonged depletion of Kyn in the TME and reversed the modulatory effects of IDO1/TDO upregulation in the TME.

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We are grateful to N. Ashoura for assistance with various aspects of this work, data interpretation and comments on the manuscript. This work was supported by funding from NIH 1 RO1 CA189623 (E. Stone and G.G.), the Cancer Prevention and Research Institute of Texas grant DP150061 (E. Stone, L.I.R.E. and G.G.) and Kyn Therapeutics (G.G. and E. Stone). Support from the American Cancer Society was provided to T.A.T. (Postdoctoral Fellowship 126584-PF-14-216-01-TBF), N.M. (Postdoctoral Fellowship 123506-PF-13-354-01-CDD) and J.B. (Postdoctoral Fellowship 128252-PF-15-143-01-CDD). M.D. acknowledges support by a postdoctoral fellowship from the Cancer Prevention and Research Institute of Texas (RP140108).

Author information

Author notes

    • Todd A Triplett
    • , Kendra C Garrison
    • , Nicholas Marshall
    •  & Moses Donkor

    These authors contributed equally to this work.


  1. Department of Molecular Biosciences, University of Texas at Austin (UT Austin), Austin, Texas, USA.

    • Todd A Triplett
    • , Lauren I R Ehrlich
    • , Everett Stone
    •  & George Georgiou
  2. Department of Oncology, University of Texas Dell Medical School, LiveSTRONG Cancer Institutes, Austin, Texas, USA.

    • Todd A Triplett
    •  & George Georgiou
  3. Department of Chemical Engineering, University of Texas at Austin, Austin, Texas, USA.

    • Kendra C Garrison
    • , Nicholas Marshall
    • , Moses Donkor
    • , John Blazeck
    • , Candice Lamb
    • , Ahlam Qerqez
    • , Joseph D Dekker
    • , Yuri Tanno
    • , Wei-Cheng Lu
    • , Christos S Karamitros
    • , Kyle Ford
    • , Bing Tan
    • , Mena S Yamany
    •  & George Georgiou
  4. Merck Research Laboratories, Rahway, New Jersey, USA.

    • Nicholas Marshall
  5. MedImmune LLC, Gaithersburg, Maryland, USA.

    • Moses Donkor
  6. Kyn Therapeutics, Cambridge, Massachusetts, USA.

    • Xiaoyan M Zhang
    • , Karen McGovern
    • , Silvia Coma
    •  & Mark Manfredi
  7. Department of Molecular Chemistry and Engineering, Kyoto Institute of Technology, Kyoto, Japan.

    • Yoichi Kumada
  8. Department of Nutritional Sciences, University of Texas at Austin, Austin, Texas, USA.

    • Enrique Sentandreu
    •  & Stefano Tiziani
  9. Heat Biologics Inc., Durham, North Carolina, USA.

    • George Fromm
    •  & Taylor H Schreiber


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T.A.T., K.C.G., N.M. and M.D. designed and performed key experiments; J.B., C.L., A.Q., B.T., Y.T., W.-C.L., C.S.K., K.F. and M.S.Y. expressed, cloned, characterized and prepared enzymes for in vitro and in vivo studies; B.T. and Y.K. developed and performed pharmacokinetic assays; J.D.D., M.M., X.M.Z., G.F., K.M., S.C. and T.H.S. designed and performed certain in vivo and in vitro experiments; E. Sentandreu and S.T. performed metabolomics analyses; T.A.T., K.C.G., M.D., J.D.D., L.I.R.E., G.G. and E. Stone interpreted the data; and G.G., T.A.T., K.C.G. and E. Stone wrote the manuscript.

Competing interests

G.G. and E. Stone are inventors on intellectual property related to this work, and G.G., E. Stone, X.M.Z., K.M., S.C. and M.M. have equity interest in Kyn Therapeutics, a company pursuing the commercial development of this technology.

Corresponding authors

Correspondence to Everett Stone or George Georgiou.

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