Discovery and Optimization of Rationally Designed Bicyclic Inhibitors of Human Arginase to Enhance Cancer Immunotherapy

Abstract

The action of arginase, a metalloenzyme responsible for the hydrolysis of arginine to urea and ornithine, is hypothesized to suppress immune-cell activity within the tumor microenvironment, and thus its inhibition may constitute a means by which to potentiate the efficacy of immunotherapeutics such as anti-PD-1 checkpoint inhibitors. Taking inspiration from reported enzyme–inhibitor cocrystal structures, we designed and synthesized novel inhibitors of human arginase possessing a fused 5,5-bicyclic ring system. The prototypical member of this class, 3, when dosed orally, successfully demonstrated serum arginase inhibition and concomitant arginine elevation in a syngeneic mouse carcinoma model, despite modest oral bioavailability. Structure-based design strategies to improve the bioavailability of this class, including scaffold modification, fluorination, and installation of active-transport recognition motifs were explored.

Arginase as a Potential Biomarker of Disease Progression: A Molecular Imaging Perspective

“…The most recent scaffold of ABH-derived arginase inhibitors was inspired by compound 9, with the cyclopentane moiety being replaced by a rigid bicyclic center ( Figure 5 ) [ 174 ]. The secondary amine from the pyrrolidine group of compound 12 is positioned at an ideal distance from Asp-181 (or potentially Asp-200 from Arg2) to establish an extra electrostatic interaction with the Arg1 binding site.…”

“…The secondary amine from the pyrrolidine group of compound 12 is positioned at an ideal distance from Asp-181 (or potentially Asp-200 from Arg2) to establish an extra electrostatic interaction with the Arg1 binding site. Regardless of the modest oral bioavailability (7%) and reduced membrane permeability, compound 12 was successfully demonstrated to increase serum levels of L-arginine when administered orally in a carcinoma mouse model [ 174 ]. Substitutions in the secondary amine to improve pharmacokinetics proved to be unsuccessful.…”

“…Substitutions in the secondary amine to improve pharmacokinetics proved to be unsuccessful. A reasonable improvement was achieved with a fluorooctahydropentalene bearing a methylamino motif, 13 [ 174 ]. From a molecular imaging point of view, it is interesting to state that, among the few fluorinated arginase inhibitors reported in the literature [ 171 , 173 , 174 ], compound 13 is the most promising.…”

“…A reasonable improvement was achieved with a fluorooctahydropentalene bearing a methylamino motif, 13 [ 174 ]. From a molecular imaging point of view, it is interesting to state that, among the few fluorinated arginase inhibitors reported in the literature [ 171 , 173 , 174 ], compound 13 is the most promising. The synthesis of a radiofluorinated analog of this compound may hold great potential for positron emission tomography (PET) imaging detection of arginase-related processes.…”

“…For instance, the presence of a chlorophenyl group in compound 1 (IC 50 : Arg1 = 330 nM and Arg2 = 1120 nM [ 171 ]) and 7 (IC 50 : Arg1 = 17 nM and Arg2 = 30 nM [ 171 ]) may open perspectives for the development of [ 18 F]fluorophenyl analogs via several late-stage 18 F-fluorination options [ 248 ]. Compound 13 (IC 50 : Arg1 = 6 nM [ 174 ]), which contains a fluorocyclopentane moiety in its structure, may also serve as a lead for the development of a radiolabeled analog.…”