Improved Bicyclic Pyrrolidine Analogues Inhibit Toxoplasma gondii Growth In Vitro and Cure Infection In Vivo

Inhibition of phenylalanine tRNA synthetase (PheRS) by bicyclic pyrrolidines provides a potent and specific inhibition of parasite growth. Herein, we describe novel bicyclic pyrrolidines designed to explore structure–activity relationships with Toxoplasma gondii vs human PheRS. Modification of the biaryl alkyne extension, which fits into the phenylalanine-binding site, showed a strong preference for ortho hydroxyl addition over meta and para. Further addition of N to both the proximal and distal phenyl rings of the biaryl alkyne and to the methoxyphenyl urea moiety, which fits into a unique auxiliary site present in the parasite enzyme, identified compounds with reduced plasma protein binding and lower hERG activity. Finally, we identified a potent lead with improved pharmacokinetics, extended plasma exposure, central nervous system penetration, and low-dose cure of acute infection in mouse. Collectively, these findings advance new candidates for the treatment of toxoplasmosis based on selective and potent inhibitors of parasite PheRS.