Drug development for congenital toxoplasmosis is challenging since first line antifolate therapy has a high rate of adverse effects. Besides, several studies in humans highlight the importance of drug application early after seroconversion to reduce fetal damage. Calcium-dependent protein kinase 1 (CDPK1) of apicomplexan parasites represents a promising drug target and bumped kinase inhibitors (BKIs) inhibit the activity of CDPK1 and perhaps other targets in Toxoplasma gondii. BKI-1748 displayed an excellent safety profile at the therapeutic level and efficacy features in vitro (parasitostatic effect) and in T. gondii infected mice. For ovine toxoplasmosis, full protection against abortion and congenital infection has been demonstrated with BKI-1748 treatment beginning already at 2 days post-infection (pi). However, the efficacy of the compound applied later after infection (when signs of infection are recognized) has not been investigated so far and it may better reflect real-world use of a therapeutic. Nineteen pregnant sheep were distributed in three experimental groups. Group 1 (G1, n=8) and group 2 (G2, n=8) were dosed orally with 10 TgShSp1 sporulated oocysts at 90 days of gestation (dg). Animals from group 3 (G3, n=3) were mock dosed with PBS at 90 dg. In G1, ten doses at 15 mg/kg of BKI-1748 was administered every other day from day 7 pi (fever peak) onwards. Treatment reduced parasite proliferation in G1 compared to G2 since treated animals exhibited significantly lower rectal temperatures (on days 8 and 9 pi, p<0.0001), serum IFN-gamma levels (on day 10 pi, p<0.05) and IgG levels (none of the treated animals seroconverted). Regarding survival of the lambs, significantly higher percentages of healthy lambs were found in G1 (73.3%, 11/15) and G3 (80%, 4/5) compared to G2 (31.3%, 5/16, p<0.05). Similarly, lambs from G1 (4.2 ± 0.8 kg) and G3 (4.5 ± 0.7 kg, p<0.01) were significantly heavier than those in G2 (3.2 ± 0.7 kg, p<0.01). Looking at the congenital infection, parasite DNA was not detected in cotyledons nor target tissues from the lambs in G1 and G3. By contrast, parasite DNA was detected in all cotyledons and lambs from G2, except those from one sheep that aborted on day 13 pi. Therefore, results obtained here demonstrate that BKI-1748 applied one week after infection, when infection is systemic and fever is maximal, confers protection against congenital toxoplasmosis in sheep.

Funded by United States Department of Agriculture (USDA) (grant number 2020–67015–30881), US National Institutes of Health (NIH) (grant number R01 HD102487) and the Swiss National Science Foundation (grant numbers 310030_214897).