R176502, a new bafilolide metabolite with potent antiproliferative activity from a novel Micromonospora species.

During the course of a screening program intended to identify new antiproliferative agents, a new bafilolide metabolite was discovered. R176502 (1) was isolated from the liquid fermentation cultures of a novel Micromonospora species found in African river bottom sediment. It was purified from ethyl acetate extracts using a series of countercurrent chromatographic steps. The structure was determined using 1- and 2-D NMR experiments. Three previously described bafilomycins (bafilomycins A1 (2), B1 (3), and B2 (4)) were also isolated (from other microbial strains). R176502 exhibited potency for inhibition of tumor cell proliferation in the nM range of concentrations.