Optimizing Pyrazolopyrimidine Inhibitors of Calcium Dependent Protein Kinase 1 for Treatment of Acute and Chronic Toxoplasmosis

Calcium
dependent protein kinase 1 (CDPK1) is an essential Ser/Thr
kinase that controls invasion and egress by the protozoan parasite Toxoplasma gondii. The Gly gatekeeper of CDPK1 makes it
exquisitely sensitive to inhibition by small molecule 1H-pyrazolo­[3,4-d]­pyrimidine-4-amine (PP) compounds
that are bulky ATP mimetics. Here we rationally designed, synthesized,
and tested a series of novel PP analogs that were evaluated for inhibition
of CDPK1 enzyme activity in vitro and parasite growth
in cell culture. Optimal substitution on the PP scaffold included
2-pyridyl ethers directed into the hydrophobic pocket and small carbocyclic
rings accessing the ribose-binding pocket. Further optimization of
the series led to identification of the lead compound 3a that displayed excellent potency, selectivity, safety profile, and
efficacy in vivo. The results of these studies provide
a foundation for further work to optimize CDPK1 inhibitors for the
treatment of acute and chronic toxoplasmosis.