Antimalarial spirooxindole alkaloids with a rare 6/5/5/6/6 polycyclic skeleton from the fungus Penicillium citrinum YSC-1 isolated from a medicinal plant.

OSMAC (one strain many compounds) provides a convenient strategy to produce chemically diverse and novel natural products. In the study, the application of an OSMAC approach on a fungus Penicillium citrinum YSC-1 isolated from a medicinal plant Chloranthus japonicus, using different culture media, led to the isolation and identification of seven new spirooxindole alkaloids penicitrimicins A-G (1-7) with a rare 6/5/5/6/6 polycyclic skeleton, along with two known compounds (8-9). The new structures were characterized based on the comprehensive spectroscopic analyses, including 1D, 2D NMR and HRESIMS data. The absolute configurations of compounds 1-7 were determined by modified Mosher ester methodology, PGME derivatization, X-ray crystallographic analysis, and quantum chemical calculations. Biological evaluation revealed that these spirooxindole alkaloids exhibited good biocompatibility (<5 % hemolysis and > 80 % cell viability) while displaying obvious antimalarial activity against Plasmodium falciparum Dd2 strain, with EC50 values spanning 0.9-2.4 μM. Furthermore, stage-specific assays revealed that compound 5 displayed significant inhibitory effects on the developmental transition of asexual blood-stage parasites, effectively blocking their progression to subsequent lifecycle stages.