Pentacyclic Ingamine Alkaloids, a New Antiplasmodial Pharmacophore from the Marine Sponge Petrosid Ng5 Sp5

Two new pentacyclic ingamine alkaloids, namely 22(S)-hydroxyingamine A (2) and dihydroingenamine D (3), together with the known ingamine A (1), have been isolated from marine sponge Petrosid Ng5 Sp5 (family Petrosiidae) obtained from the open repository of the National Cancer Institute, USA. The structures of compounds 1-3 were determined using 1D and 2D NMR, and HRESIMS techniques. The absolute configuration of both the C9 and C22 of 2 was determined as (S) using a modified Mosher esterification method. Compounds 1 and 3 showed strong antiplasmodial activity against chloroquine-sensitive (D6) and resistant (W2) strains of Plasmodium falciparum with IC50 values of 90 and 78 ng/mL and 72 and 57 ng/mL, respectively, while 2 was found to be less active (IC50 values of 200 and 140 ng/mL, respectively). Compounds 1-3 were found to be de-void of in vitro cytotoxicity against human solid tumor cells of breast (BT-549), ovary (SK-OV-3), and epidermoid (KB) carcinomas and skin melanoma (SK-MEL), as well as against noncancerous monkey kidney fibroblasts (VERO) and pig kidney epithelial (LLC-PK11) cells, up to a maximum concentration of 10 mu g/mL. Compounds 1-3 also displayed weak antimicrobial and moderate antileishmanial activities against Leishmania donovani promastigotes. These polycyclic ingamine alkaloids represent the first example of antiplasmodial leads without a beta-carboline ring, which is known to be responsible for the cytotoxicity of the well-known manzamine class of marine alkaloids related to 1-3.