Isolation, Derivative Synthesis, and Structure-Activity Relationships of Antiparasitic Bromopyrrole Alkaloids from the Marine Sponge Tedania brasiliensis

Abstract

The isolation and identification of a series of new pseudoceratidine (1) derivatives from the sponge Tedania brasiliensis enabled the evaluation of their antiparasitic activity against Plasmodium falciparum, Leishmania (Leishmania) amazonensis, Leishmania (Leishmania) infantum, and Trypanosoma cruzi, the causative agents of malaria, cutaneous leishmaniasis, visceral leishmaniasis, and Chagas disease, respectively. The new 3-debromopseudoceratidine (4), 20-debromopseudoceratidine (5), 4-bromopseudoceratidine (6), 19-bromopseudoceratidine (7), and 4,19-dibromopseudoceratidine (8) are reported. New tedamides A-D (9-12), with an unprecedented 4-bromo-4-methoxy-5-oxo-4,5-dihydro-1H-pyrrole-2-carboxamide moiety, are also described. Compounds 4 and 5, 6 and 7, 9 and 10, and 11 and 12 have been isolated as pairs of inseparable structural isomers differing in their sites of bromination or oxidation. Tedamides 9+10 and 11+12 were obtained as optically active pairs, indicating an enzymatic formation rather than an artifactual origin. N¹²-Acetylpseudoceratidine (2) and N¹²-formylpseudoceratidine (3) were obtained by derivatization of pseudoceratidine (1). The antiparasitic activity of pseudoceratidine (1) led us to synthesize 23 derivatives (16, 17, 20, 21, 23, 25, 27-29, 31, 33, 35, 38, 39, 42, 43, 46, 47, 50, and 51) with variations in the polyamine chain and aromatic moiety in sufficient amounts for biological evaluation in antiparasitic assays. The measured antimalarial activity of pseudoceratidine (1) and derivatives 4, 5, 16, 23, 25, 31, and 50 provided an initial SAR evaluation of these compounds as potential leads for antiparasitics against Leishmania amastigotes and against P. falciparum. The results obtained indicate that pseudoceratidine represents a promising scaffold for the development of new antimalarial drugs.

Figure 1

NOE observed in the 1D NOESY spectrum of tedamides A (9) and B (10) that supports the presence of fragment A instead of fragment B in the structures of 9 and 10.

Figure 2

Structures and anti-plasmodial activities of pseudoceratidine derivatives against Plasmodium falciparum.

Figure 3

Structures and anti-leishmanial activities of pseudoceratidine derivatives against Leishmania (L.) infantum.

Figure 4

In vitro activity of 23, 42 and 50 against intracellular L. (L.) amazonensis amastigotes. Macrophages derived from BALB/c mice bone marrow were infected with L. (L.) amazonensis stationary promastigotes for 1 h (MOI = 10). After 24 h, infected cells were incubated with 6.125, 25, 50 or 100 μM of each compound for 24 h. MeOH-fixed cells were stained and infection was determined by counting 300 cells/coverslip. Experiments were performed in triplicate. The results shown are representative of two independent experiments. A. Bars indicate the number of intracellular amastigotes. Numbers above each bar indicate the percentage of reduction over control untreated infected macrophages. (B) Photomicrograph examples showing untreated infected macrophages (a) and infected macrophages incubated with compound 42 at 100 μM (b). Arrows point to intracellular amastigotes. Bar = 10 μm.

Figure 5

HepG2 cells morphology before (left) and after (right) treatment with pseudoceratidine (1) at 10 μM.

Figure 6

Microscopy of synchronized parasites continuously treated with pseudoceratidine (1) at concentration 10-fold the IC₅₀ value (top line) and DMSO (control, bottom line). Images are representative of three independent experiments.

Figure 7

Isobologram plot for drug interaction analysis of pseudoceratidine (1) and sodium artesunate.

Scheme 1

Syntheses of pseudoceratidine derivatives with variations in the tether.

Scheme 2

Syntheses of pseudoceratidine derivatives with variations in the pyrrole (aromatic) moiety.