Norditerpene Natural Products from Subterranean Fungi with Anti-Parasitic Activity

Cryptosporidiosis, caused by the parasite Cryptosporidium parvum, remains a global health threat without effective, widely applicable therapeutics, particularly for immunocompromised populations. This study sought to identify novel anti-parasitic compounds by screening a library of natural products derived from subterranean fungi and bacteria collected from the Soudan Iron Mine in Minnesota.

The researchers screened 192 crude extracts and pure compounds using an in vitro growth inhibition assay against C. parvum-infected HCT-8 cells. The most promising candidates—a series of norditerpene lactones called oidiolactones—were evaluated for potency, selectivity against related Apicomplexan parasites (Toxoplasma gondii and Plasmodium falciparum), and cytotoxicity. The lead compound, oidiolactone A, was characterized through mechanism-of-action assays, in vitro pharmacokinetic testing, and an in vivo acute infection study in IFN-gamma knockout mice.

Seven structurally related oidiolactones displayed significant anti-parasitic activity. Oidiolactones A and B emerged as the most potent, with EC50 values in the submicromolar range against both C. parvum and T. gondii. Notably, oidiolactone A achieved a 70% reduction in oocyst shedding in an immunocompromised mouse model of C. parvum infection, showing no overt toxicity. The compound acts rapidly, killing parasites within 8–12 hours, and specifically targets the asexual and transition-to-sexual stages of the C. parvum life cycle. Structure-activity relationship (SAR) analysis indicated that specific chemical features, such as the C-13 O-methyl ether and the integrity of the lactone rings, are critical for maintaining this potent activity.

Currently, few therapeutic options exist for cryptosporidiosis, and existing drugs like nitazoxanide often fail in vulnerable populations. The discovery of oidiolactones provides a new, synthetically accessible chemical scaffold for drug development. Given their high potency, favorable safety profile, and in vivo activity, these compounds are strong candidates for further optimization as anti-parasitic therapeutics.