Investigation of the Anti-Toxoplasma gondii Potential of Loratadine

Toxoplasmosis remains a major global health concern, particularly in immunocompromised individuals and pregnant women, while currently available therapies are associated with significant toxicity and limited efficacy against chronic infection. Drug repurposing represents an attractive strategy for the identification of safer and more effective anti-Toxoplasma gondii agents. This study investigated the antiparasitic potential of loratadine, a second-generation antihistamine, using an integrated in silico, in vitro, and in vivo approach. The anti-T. gondii activity of loratadine was evaluated through β-galactosidase-based proliferation assays, reversibility assays, and experiments using pretreated tachyzoites. In vivo efficacy was assessed in murine models of acute and chronic toxoplasmosis. Disease progression, survival, parasite burden, and cerebral cyst formation were analyzed following treatment. Loratadine inhibited the intracellular proliferation of T. gondii tachyzoites in a concentration-dependent manner and induced partially irreversible effects on parasite viability after drug withdrawal. Pretreatment of extracellular tachyzoites produced limited effects, suggesting that loratadine predominantly acts during the intracellular stage of infection. In the acute toxoplasmosis model, loratadine treatment delayed disease progression, prolonged animal survival, and significantly reduced the number of tachyzoites recovered from the peritoneal cavity. In the chronic infection model, treatment significantly decreased both the number and size of cerebral cysts, although these findings do not demonstrate cyst eradication or direct bradyzoite killing. Despite its measurable biological activity, loratadine exhibited a relatively low in vitro selectivity index, highlighting the need for further optimization. These exploratory findings identify loratadine as a promising lead compound (hit) for further optimization rather than an immediately translatable therapeutic candidate. Additional medicinal chemistry, pharmacokinetic, mechanistic, and confirmatory preclinical studies will be required to determine its potential for anti-T. gondii drug development.