Analogs of Marinopyrrole A Show Enhancement to Observed In Vitro Potency against Acute Toxoplasma gondii Infection

The apicomplexan parasite Toxoplasma gondii is the causative agent of toxoplasmosis, a globally distributed infection with severe clinical consequences for immunocompromised individuals and developing fetuses. There are few available treatments, and these are associated with potentially severe adverse effects. Marinopyrrole A, a compound discovered in a marine Streptomyces species, has previously been found to exhibit potent antimicrobial activity, prompting our interest in exploring efficacy against Toxoplasma gondii. We found that marinopyrrole A was a highly potent anti-Toxoplasma molecule, with an in vitro 50% maximal inhibitory concentration (IC50) of 0.31 mM, corresponding to a higher potency than that of the current standard of care (pyrimethamine); however, addition of 20% serum led to abrogation of potency, and toxicity to human cell lines was observed. Yet, application of marinopyrrole A to an in vivo lethal acute infection model facilitated significantly enhanced survival at doses of 5, 10, and 20 mg/kg. We then tested a series of marinopyrrole A analogs (RL002, RL003, and RL125) and demonstrated significantly increased potency in vitro, with IC50 values ranging from 0.09 to 0.17 mM (3.6- to 6.8-fold increase relative to pyrimethamine). No detectable cytotoxicity was observed up to 50 mM in human foreskin fibroblasts, with cytotoxicity in HepG2 cells ranging from ;28 to 50 mM, corresponding to .200-fold selectivity for parasites over host cells. All analogs additionally showed reduced sensitivity to serum. Further, RL003 potently inhibited in vitro-generated bradyzoites at 0.245 mM. Taken together, these data support further development of marinopyrrole A analogs as promising anti- Toxoplasma molecules to further combat this prevalent infection.