Taken together the results reveal a complex feedback and fee

Taken together, the results reveal a complex feedback and feedforward signaling network between the tryptophan metabolic enzymes IDO1/TDO2, the KP metabolite KYN, and the ligand-activated transcription factor AhR (Figure 2). Cancer N6-Methyl-dATP highjack this signaling circuitry to deregulate immunological responses and to attain resistance to anticancer therapies including immune checkpoint inhibitors. Furthermore, cancer cells may collude with the host microbiota to produce AhR agonists (e.g., indole-3-carbinol) from dietary sources. In the following sections we outline strategies in the development of IO therapeutic interventions targeting the IDO1/TDO2–KYN–AhR signaling circuitry. We will discuss three different approaches that intercept unique nodes of the enzymatic/signaling pathway: (i) inhibition of IDO1/TDO2 to prevent the formation of the endogenous AhR ligand KYN, (ii) systemic depletion of KYN to prevent its engagement with AhR, and (iii) inhibition of AhR activation by synthetic AhR modulators (Figure 4).
Clinical Development of IDO1 Inhibitors Since the first description of indoximod (D-1MT, 1-methyl-D-tryptophan) as an IDO1 pathway inhibitor, several selective IDO1 inhibitors have been reported. There is a large body of preclinical and clinical data for indoximod, which is not included in this review because the mode of action of indoximod remains debatable. We suggest articles 50, 51, 52, 53 to our readers who are interested in the topics. A large number of selective IDO1 or TDO2 inhibitors and dual IDO1/TDO2 inhibitors are still in preclinical evaluation, and these are the subject of recent review articles 54, 55, 56 and will not be discussed in detail. We summarize here the pharmacology of the clinical development candidates – epacadostat (originator: Incyte), navoximod (originator: NewLink), BMS-986205 (originator: Flexus), and PF-06840003 (originator: iTeos) (Figure 5). It is interesting to note that all four clinical development candidates were originally discovered by a biotech company, and then codeveloped with or licensed to a pharmaceutical company.
Depletion of KYN by Engineered Kynureninase (KYNase) Once produced from tryptophan via IDO1/TDO2 catalyzed oxidation, KYN is a substrate of multiple downstream intracellular reactions along the KYN metabolic pathway (Figure 1). KYNase is an enzyme that catalyzes the synthesis of anthranilic acid from KYN, and of 3-hydroxyanthranilic acid (3-HAA) from 3-hydroxykynurenine. KYN is readily exported and taken up by cells via the large neutral amino acid transporter system that also regulates the cellular uptakes of tryptophan. One emerging approach to inhibition of KYN-mediated tumor-promoting immunity is to enzymatically deplete the extracellular pool of KYN via engineered KYNase 92, 93, 94. KYNase engineered from the bacterial kynureninase enzyme more effectively converts KYN (>1000 higher Kcat/Km) into immunologically benign anthranilic acid than does the endogenous KYNase. In mice bearing the syngeneic melanoma B16F10 tumor, depletion of KYN in both plasma and tumor tissue was achieved by a single subcutaneous (SC) dose of KYNase, and this was accompanied by increased effector T cells in the tumor. Peritumoral injection of polyethylene glycol-conjugated (PEGylated) KYNase resulted in complete depletion of KYN in serum as well as in tumor tissue. Administration of KYNase did not alter tryptophan concentrations in either compartment. In mice bearing CT26 colon carcinoma, treatment with KYNase as a monotherapy led to durable responses and resistance to tumor rechallenge. KYNase treatment resulted in increased accumulation of proliferative CD8+ cells deeply inside the tumor interior, as well as increased concentration of IFN-γ in the TME. The antitumor effect by KYNase was shown to be dependent on IDO1 because it did not demonstrate an inhibitory effect on tumor growth in IDO1 knockout (KO) mice. The antitumor activity of KYNase was abolished in Rag−/− mice and in mice depleted for CD8+ T cells, indicating its dependency on functional immunity. KYNase demonstrated significant tumor growth inhibition and survival benefit when combined with immune checkpoint inhibitors in B16F10, CT26, and breast cancer 4T1 models, and the KYNase+anti-PD-1 combination was more efficacious than the epacadostat+anti-PD-1 combination in CT26 tumor-bearing mice. An optimized human KYNase is expected to progress toward clinical development for the treatment of advanced cancers. Because KYNase can deplete KYN generated by IDO1- and/or TDO2-positive cancer cells, there is a unique opportunity to exploit KYNase in the treatment of patients whose cancers overexpress both IDO1 and TDO2.