OBJECTIVE: Fibroids are characterized by marked overexpression of tryptophan 2,3 dioxygenase (TDO2). The objective of this study was to determine the effectiveness of in vivo administration of an inhibitor of TDO2 (680C91) on fibroid size and gene expression.
METHODS: Animal and ex vivo human study.
METHODS: Academic Research Institution.
METHODS: Severe combined immunodeficiency mice bearing human fibroid xenografts treated with vehicle and TDO2 inhibitor.
METHODS: Daily intraperitoneal administration of 680C91 or vehicle for 2 months and in vitro studies with fibroid explants.
METHODS: Tumor weight and gene expression profile of xenografts and in vitro mechanistic experiments using fibroid explants.
RESULTS: Compound 680C91 was well-tolerated with no effects on blood chemistry and body weight. Treatment of mice with 680C91 resulted in 30% reduction in the weight of fibroid xenografts after 2 months of treatment and as expected lower levels of kynurenine, the byproduct of tryptophan degradation and an endogenous ligand of aryl hydrocarbon receptor (AhR) in the xenografts. The expression of cytochrome P450 family 1 subfamily B member 1 (CYP1B1), transforming growth factor β3 (TGF-β3), fibronectin (FN1), cyclin-dependent kinase 2 (CDK2), E2F transcription factor 1 (E2F1), interleukin 8 (IL-8) and secreted protein acidic and cysteine rich (SPARC) mRNA were lower in the xenografts of mice treated with 680C91 compared with vehicle controls. Similarly, the protein abundance of collagen, FN1, CYP1B1, and SPARC were lower in the xenografts of 680C9- treated mice compared with vehicle controls. Immunohistochemical analysis of xenografts indicated decreased expression of collagen, Ki67 and E2F1 but no significant changes in cleaved caspase 3 expression in mice treated with 680C91. The levels of kynurenine in the xenografts showed a direct correlation with the tumor weight and FN1 levels. In vitro studies with fibroid explants showed a significant induction of CYP1B1, TGF-β3, FN1, CDK2, E2F1, IL8, and SPARC mRNA by tryptophan, which could be blocked by cotreatment with 680C91 and the AhR antagonist CH-223191.
CONCLUSIONS: The results indicate that correction of aberrant tryptophan catabolism in fibroids could be an effective treatment through its effect to reduce cell proliferation and extracellular matrix accumulation.

With immunotherapy enjoying a rapid resurgence based on the achievement of durable remissions in some patients with agents that derepress immune function, commonly referred to as \"checkpoint inhibitors,\" enormous attention developed around the IDO1 enzyme as a metabolic mediator of immune escape in cancer. In particular, outcomes of multiple phase 1/2 trials encouraged the idea that small molecule inhibitors of IDO1 may improve patient responses to anti-PD1 immune checkpoint therapy. However, recent results from ECHO-301, the first large phase 3 trial to evaluate an IDO1-selective enzyme inhibitor (epacadostat) in combination with an anti-PD1 antibody (pembrolizumab) in advanced melanoma, showed no indication that epacadostat provided an increased benefit. Here we discuss several caveats associated with this failed trial. First is the uncertainty as to whether the target was adequately inhibited. In particular, there remains a lack of direct evidence regarding the degree of IDO1 inhibition within the tumor, and previous trial data suggest that sufficient drug exposure may not have been achieved at the dose tested in ECHO-301. Second, while there is a mechanistic rationale for the combination tested, the preclinical data were not particularly compelling. More efficacious combinations have been demonstrated with DNA damaging modalities which may therefore be a more attractive alternative. Third, as a highly selective IDO1 inhibitor, epacadostat was advanced aggressively despite preclinical genetic evidence of tumors bypassing IDO1 blockade. Indeed, a well-grounded literature starting in 2011 points to targeting strategies that account for both IDO and tryptophan 2,3-dioxygenase as more appealing directions to pursue, including dual inhibitors and inhibitors of nodal downstream effector pathways such as aryl hydrocarbon receptor blockade. Overall, the clinical readout from a single trial with significant limitations is by no means a definitive test for the field. While biomarker information yet to be gleaned from ECHO-301 may yet reveal useful information regarding IDO1 pathway drugs, better rationalized compounds and better rationalized trial designs will be important in the future to accurately gauge medical impact.