Antiplasmodial Compounds from Deep-Water Marine Invertebrates

Abstract

Novel drug leads for malaria therapy are urgently needed because of the widespread emergence of resistance to all available drugs. Screening of the Harbor Branch enriched fraction library against the Plasmodium falciparum chloroquine-resistant strain (Dd2) followed by bioassay-guided fractionation led to the identification of two potent antiplasmodials; a novel diterpene designated as bebrycin A (1) and the known C21 degraded terpene nitenin (2). A SYBR Green I assay was used to establish a Dd2 EC₅₀ of 1.08 ± 0.21 and 0.29 ± 0.02 µM for bebrycin A and nitenin, respectively. Further analysis was then performed to assess the stage specificity of the inhibitors antiplasmodial effects on the Dd2 intraerythrocytic life cycle. Exposure to bebrycin A was found to block parasite maturation at the schizont stage if added any time prior to late schizogony at 42 hours post invasion, (HPI). In contrast, early life cycle exposure to nitenin (prior to 18 HPI) was identified as crucial to parasite inhibition, suggesting nitenin may target the maturation of the parasite during the transition from ring to early trophozoite (6-18 HPI), a novel property among known antimalarials.

Keywords: Bebryce grandis; P. falciparum; Spongia lamella; antiplasmodial; bebrycin A; malaria; marine natural product; nitenin.

Figure 1

Structures of bebrycin A (1) and nitenin (2).

Figure 2

Key NMR data for 1. (a) Bolded bonds indicate spin systems defined for 1 from interpretation of the 2D-DQF COSY spectra. Black arrows indicate key ¹H-¹³C long-range couplings observed in the 2D-gHMBC spectrum that tie the spin systems together. (b) Red arrows indicate key nuclear Overhauser enhancements (nOe) observed in the 2D-NOESY and 1D-dpfgse-noe spectra.

Figure 3

Nitenin and bebrycin A exhibit distinct profiles of inhibition during P. falciparum intraerythrocytic maturation. Synchronized Dd2 culture was exposed to the test compounds at 5 × EC₅₀ starting at 6, 18, and 30 HPI and monitored into the next life cycle stage up to 54 HPI. Untreated wells (containing DMSO vehicle) or dihydroartemisinin (DHA) were include as controls. Giemsa smears (inset) and flow cytometry with nucleic acid staining fluorophore YOYO-1 were collected every 12 h following compound addition at 6 (a), 18 (b), and (c) 30 HPI. Results represent the combination of three independent replicates.