Development of a Series of Kynurenine 3-Monooxygenase Inhibitors Leading to a Clinical Candidate for the Treatment of Acute Pancreatitis

Recently, we reported a novel role for KMO in the pathogenesis of acute pancreatitis (AP). A number of inhibitors of kynurenine 3-monooxygenase (KMO) have previously been described as potential treatments for neurodegenerative conditions and particularly for Huntington’s disease. However, the inhibitors reported to date have insufficient aqueous solubility relative to their cellular potency to be compatible with the intravenous (iv) dosing route required in AP. We have identified and optimized a novel series of high affinity KMO inhibitors with favorable physicochemical properties. The leading example is exquisitely selective, has low clearance in two species, prevents lung and kidney damage in a rat model of acute pancreatitis, and is progressing into preclinical development

Insights into the mechanism of inhibition of tryptophan 2,3-dioxygenase by isatin derivatives

<p>Tryptophan 2,3-dioxygenase (TDO) is a cytosolic protein with a proven immunomodulatory function that promotes tumoral immune resistance and proliferation. Despite the interest in TDO as a therapeutic target in cancer treatment, the number of biologically useful inhibitors is limited. Herein, we report isatin derivatives as a new class of TDO inhibitors. Through structure–activity relationships and molecular docking studies, we optimized the inhibition potency of isatin derivatives by >130-fold and elucidated the mechanistic details that control their mode of action. Hydrogen bond interactions between the compound and key active site residues of TDO, freedom upon rotation of the C3 chemical moiety and the presence of chlorines in the benzene ring of the compound comprise the properties that an isatin-based inhibitor requires to effectively inhibit the enzymatic activity of TDO.</p

The Mechanism of Substrate Inhibition in Human Indoleamine 2,3-Dioxygenase

Indoleamine 2,3-dioxygenase catalyzes the O₂-dependent oxidation of l-tryptophan (l-Trp) to <i>N</i>-formylkynurenine (NFK) as part of the kynurenine pathway. Inhibition of enzyme activity at high l-Trp concentrations was first noted more than 30 years ago, but the mechanism of inhibition has not been established. Using a combination of kinetic and reduction potential measurements, we present evidence showing that inhibition of enzyme activity in human indoleamine 2,3-dioxygenase (hIDO) and a number of site-directed variants during turnover with l-tryptophan (l-Trp) can be accounted for by the sequential, ordered binding of O₂ and l-Trp. Analysis of the data shows that at low concentrations of l-Trp, O₂ binds first followed by the binding of l-Trp; at higher concentrations of l-Trp, the order of binding is reversed. In addition, we show that the heme reduction potential (<i>E</i>_m⁰) has a regulatory role in controlling the overall rate of catalysis (and hence the extent of inhibition) because there is a quantifiable correlation between <i>E</i>_m⁰ (that increases in the presence of l-Trp) and the rate constant for O₂ binding. This means that the initial formation of ferric superoxide (Fe³⁺–O₂^•–) from Fe²⁺-O₂ becomes thermodynamically less favorable as substrate binds, and we propose that it is the slowing down of this oxidation step at higher concentrations of substrate that is the origin of the inhibition. In contrast, we show that regeneration of the ferrous enzyme (and formation of NFK) in the final step of the mechanism, which formally requires reduction of the heme, is facilitated by the higher reduction potential in the substrate-bound enzyme and the two constants (<i>k</i>_cat and <i>E</i>_m⁰) are shown also to be correlated. Thus, the overall catalytic activity is balanced between the equal and opposite dependencies of the initial and final steps of the mechanism on the heme reduction potential. This tuning of the reduction potential provides a simple mechanism for regulation of the reactivity, which may be used more widely across this family of enzymes

Probing the Ternary Complexes of Indoleamine and Tryptophan 2,3-Dioxygenases by Cryoreduction EPR and ENDOR Spectroscopy

We have applied cryoreduction/EPR/ENDOR techniques to characterize the active-site structure of the ferrous-oxy complexes of human (hIDO) and <i>Shewanella oneidensis</i> (sIDO) indoleamine 2,3-dioxygenases, <i>Xanthomonas campestris</i> (<i>Xc</i>TDO) tryptophan 2,3-dioxygenase, and the H55S variant of <i>Xc</i>TDO in the absence and in the presence of the substrate l-Trp and a substrate analogue, l-Me-Trp. The results reveal the presence of multiple conformations of the binary ferrous-oxy species of the IDOs. In more populated conformers, most likely a water molecule is within hydrogen-bonding distance of the bound ligand, which favors protonation of a cryogenerated ferric peroxy species at 77 K. In contrast to the binary complexes, cryoreduction of all of the studied ternary [enzyme-O₂-Trp] dioxygenase complexes generates a ferric peroxy heme species with very similar EPR and ¹H ENDOR spectra in which protonation of the basic peroxy ligand does not occur at 77 K. Parallel studies with l-Me-Trp, in which the proton of the indole nitrogen is replaced with a methyl group, eliminate the possibility that the indole NH group of the substrate acts as a hydrogen bond donor to the bound O₂, and we suggest instead that the ammonium group of the substrate hydrogen-bonds to the dioxygen ligand. The present data show that substrate binding, primarily through this H-bond, causes the bound dioxygen to adopt a new conformation, which presumably is oriented for insertion of O₂ into the C₂−C₃ double bond of the substrate. This substrate interaction further helps control the reactivity of the heme-bound dioxygen by “shielding” it from water