Actinoramide A Identified as a Potent Antimalarial from Titration-Based Screening of Marine Natural Product Extracts

Methods to identify the bioactive
diversity within natural product
extracts (NPEs) continue to evolve. NPEs constitute complex mixtures
of chemical substances varying in structure, composition, and abundance.
NPEs can therefore be challenging to evaluate efficiently with high-throughput
screening approaches designed to test pure substances. Here we facilitate
the rapid identification and prioritization of antimalarial NPEs using
a pharmacologically driven, quantitative high-throughput-screening
(qHTS) paradigm. In qHTS each NPE is tested across a concentration
range from which sigmoidal response, efficacy, and apparent EC50s can be used to rank order NPEs for subsequent organism
reculture, extraction, and fractionation. Using an NPE library derived
from diverse marine microorganisms we observed potent antimalarial
activity from two Streptomyces sp. extracts identified
from thousands tested using qHTS. Seven compounds were isolated from
two phylogenetically related Streptomyces species: Streptomyces ballenaensis collected from Costa Rica and Streptomyces bangulaensis collected from Papua New Guinea.
Among them we identified actinoramides A and B, belonging to the unusually
elaborated nonproteinogenic amino-acid-containing tetrapeptide series
of natural products. In addition, we characterized a series of new
compounds, including an artifact, 25-epi-actinoramide
A, and actinoramides D, E, and F, which are closely related biosynthetic
congeners of the previously reported metabolites.