Spongian Diterpenoids Derived from the Antarctic Sponge Dendrilla antarctica Are Potent Inhibitors of the Leishmania Parasite

From the CH2Cl2 extract of the Antarctic sponge Dendrilla antarctica we found spongian diterpenes, including previously reported aplysulphurin (1), tetrahydroaplysulphurin-1 (2), membranolide (3), and darwinolide (4), utilizing a CH2Cl2/MeOH extraction scheme. However, the extracts also yielded diterpenes bearing one or more methyl acetal functionalities (5–9), two of which are previously unreported, while others are revised here. Further investigation of diterpene reactivity led to additional new metabolites (10–12), which identified them as well as the methyl acetals as artifacts from methanolysis of aplysulphurin. The bioactivity of the methanolysis products, membranoids A–H (5–12), as well as natural products 1–4, were assessed for activity against Leishmania donovani-infected J774A.1 macrophages, revealing insights into their structure/activity relationships. Four diterpenes, tetrahydroaplysulphurin-1 (2) as well as membranoids B (6), D (8), and G (11), displayed low micromolar activity against L. donovani with no discernible cytotoxicity against uninfected J774A.1 cells. Leishmaniasis is a neglected tropical disease that affects one million people every year and can be fatal if left untreated.