Toxoplasma gondii is sensitive to bulky pyrazolo [3,4-d] pyrimidine (PP) inhibitors due to the presence of a Gly gatekeeper in the essential calcium dependent protein kinase 1 (CDPK1). Here we synthesized a number of new derivatives of 3-methyl-benzyl-PP (3-MB-PP, or 1). The potency of PP analogues in inhibiting CDPK1 enzyme activity in vitro (low nM IC(50) values) and blocking parasite growth in host cell monolayers in vivo (low μM EC(50) values) were highly correlated and occurred in a CDPK1-specific manner. Chemical modification of the PP scaffold to increase half-life in the presence of microsomes in vitro led to identification of compounds with enhanced stability while retaining activity. Several of these more potent compounds were able to prevent lethal infection with T. gondii in the mouse model. Collectively, the strategies outlined here provide a route for development of more effective compounds for treatment of toxoplasmosis and perhaps related parasitic diseases.