Evaluation of a Cyclic GMP-Dependent Protein Kinase Inhibitor in Treatment of Murine Toxoplasmosis: Gamma Interferon Is Required for Efficacy

Abstract

The trisubstituted pyrrole 4-[2-(4-fluorophenyl)-5-(1-methylpiperidine-4-yl)-1H-pyrrol-3-yl]pyridine (compound 1) is a potent inhibitor of cyclic GMP-dependent protein kinases from Apicomplexan protozoa and displays cytostatic activity againstToxoplasma gondiiin vitro.Compound 1 has now been evaluated againstT. gondiiinfections in the mouse and appeared to protect the animals when given intraperitoneally at 50 mg/kg twice daily for 10 days. However, samples from brain, spleen, and lung taken from infected treated mice revealed the presence of parasites after cessation of administration of compound 1, indicating that a transient asymptomatic parasite recrudescence occurs in all survivors. The ability of mice to controlToxoplasmainfection after compound 1 treatment has been terminated suggested that the mouse immune system plays a synergistic role with chemotherapy in controlling the infection. To explore this possibility, gamma interferon (IFN-γ)-knockout mice were infected with parasites and treated with compound 1, and survival was compared to that of normal mice. IFN-γ-knockout mice were protected againstT. gondiithroughout the treatment phase but died during the posttreatment phase in which peak recrudescence was observed in treated immunocompetent mice. These data suggest that an IFN-γ-dependent immune response was essential for controlling and resolving parasite recrudescence in mice treated with compound 1. In addition, when compound 1-cured immunocompetent mice were rechallenged with a lethal doseof T. gondii,all survived (n= 32). It appears that the cytostatic nature of compound 1 provides an “immunization” phase during chemotherapy which allows the mice to survive the recrudescence and any subsequent challenge with a lethal dose ofT. gondii.