In vitro screening of the open source Pathogen Box identifies novel compounds with profound activities against Neospora caninum.

Neospora caninum is a major cause of abortion in cattle and represents an important veterinary health problem of great economic significance. The Medicines for Malaria Venture (MMV) Pathogen Box, an open-source collection of 400 compounds with proven anti-infective properties against a wide range of pathogens, was screened against a N. caninum beta-galactosidase reporter strain grown in human foreskin fibroblasts. A primary screening carried out at 1µM yielded 40 compounds that were effective against N. caninum tachyzoites. However, 30 of these compounds also affected the viability of the host cells. The 10 remaining compounds exhibited IC50 values between 4 and 43nM. Three compounds with IC50 values below 10nM, namely MMV676602, MMV688762 and MMV671636, were further characterized in vitro in more detail with respect to inhibition of invasion versus intracellular proliferation, and only MMV671636 had an impact on intracellular proliferation of tachyzoites. This was confirmed by transmission electron microscopy, showing that the primary target of MMV671636 was the mitochondrion. MMV671636 treatment of experimentally infected mice significantly reduced the number of animals with lung and brain infection, and these mice also exhibited a significantly reduced titer of antibodies directed against N. caninum antigens. Thus, MMV671636 is a promising starting point for the development of a future neosporosis therapy.