Lead Optimization of TgCDPK1 Inhibitors for the Treatment of Toxoplasmosis

Toxoplasma gondii is an important opportunistic pathogen that infects many individuals and threatens the health of those with compromised immunity. Current therapies are unable to eradicate chronic infections and pose risks of adverse reactions. Using X-ray structure-based drug design, we have developed a new series of biaryl-substituted pyrazolopyrimidine inhibitors of the essential parasite enzyme calcium-dependent protein kinase 1 (TgCDPK1). These inhibitors have excellent potency against the enzyme and in vitro antiparasitic activity. We further optimized the compounds for increased metabolic stability, lowered plasma protein binding, decreased efflux, and improved pharmacokinetics (PK). Several of the inhibitors had desirable PK with high oral bioavailability, low clearance, and extended half-life, leading to excellent compound exposure in the plasma and brain over a 24-h period. Three compounds were tested during acute infection in both immunocompetent and immunocompromised mice. We identified 16c as a promising preclinical candidate to treat toxoplasmosis.