Bicyclic azetidines target acute and chronic stages of Toxoplasma gondii by inhibiting parasite phenylalanyl t-RNA synthetase

Abstract

Toxoplasma gondii commonly infects humans and while most infections are controlled by the immune response, currently approved drugs are not capable of clearing chronic infection in humans. Hence, approximately one third of the world’s human population is at risk of reactivation, potentially leading to severe sequelae. To identify new candidates for treating chronic infection, we investigated a series of compounds derived from diversity-oriented synthesis. Bicyclic azetidines are potent low nanomolar inhibitors of phenylalanine tRNA synthetase (PheRS) in T. gondii, with excellent selectivity. Biochemical and genetic studies validate PheRS as the primary target of bicyclic azetidines in T. gondii, providing a structural basis for rational design of improved analogs. Favorable pharmacokinetic properties of a lead compound provide excellent protection from acute infection and partial protection from chronic infection in an immunocompromised mouse model of toxoplasmosis. Collectively, PheRS inhibitors of the bicyclic azetidine series offer promise for treatment of chronic toxoplasmosis.

Bicyclic Pyrrolidine Inhibitors of Toxoplasma gondii Phenylalanine t-RNA Synthetase with Antiparasitic Potency In Vitro and Brain Exposure

“…BRD7929 is a 4,8bicyclic azetidine that was an early lead in T. gondii studies from the HTS at the Broad Institute. 9 The compound BRD7929 has good potency and exposure but toxicityassociated liabilities, including hERG inhibition. These issues are likely exacerbated by high tissue accumulation.…”

“…Hence, the synthesis of the new scaffold provides a more feasible synthesis of an antiparasitic than BRD7929, which requires 16 steps that are less robust and less stereoselective. 9 ■ METHODS Synthesis. General Considerations.…”

“…9,10 Importantly, treatment with BRD7929 was effective at curing chronic infection with T. gondii, albeit in a small percentage of animals (i.e., 15−20%). 9 This broad antiparasitic activity has garnered much attention in the field. 11 Crystallographic structures of bicyclic azetidines in complex with Plasmodium falciparum and Plasmodium vivax cPheRS demonstrated that two molecular appendages interacted with deep pockets in the enzyme target: the biarylacetylene motif was found to occupy the L-Phe site, while the methoxyphenyl urea motif occupied a distinct site of unknown function, the so-called auxiliary site.…”

Exploration of aminoacyl-tRNA synthetases from eukaryotic parasites for drug development

“…BRD7929 was the most potent, possibly due to greater hydrophobicity. Series of bicyclic azetidines inhibit T. gondii growth in vitro and provide protection in a mouse model against acute and chronic toxoplasmosis ( Table 1 ) ( 72 ). They are potent against tachyzoites ( Fig.…”

Bicyclic pyrrolidine inhibitors ofToxoplasma gondiiphenylalanine t-RNA synthetase with antiparasitic potencyin vitroand brain exposure

“…BRD7929 is a 4,8-bicyclic azetidine that was an early lead in T. gondii studies from the HTS at the Broad Institute. 9 This compound has good potency and exposure, but toxicity-associated liabilities. The PK of this and other compounds will be discussed later in this paper.…”