The Marine Sponge Diacarnus bismarckensis as a Source of Peroxiterpene Inhibitors of Trypanosoma brucei, the Causative Agent of Sleeping Sickness

Abstract

Human African trypanosomiasis (HAT), also known as African sleeping sickness, is a neglected tropical disease with inadequate therapeutic options. We have launched a collaborative new lead discovery venture using our repository of extracts and natural product compounds as input into our growth inhibition primary screen against Trypanosoma brucei. Careful evaluation of the spectral data of the natural products and derivatives allowed for the elucidation of the absolute configuration (using the modified Mosher's method) of two new peroxiterpenes: (+)-muqubilone B (1a) and (−)-ent-muqubilone (3a). Five known compounds were also isolated: (+)-sigmosceptrellin A (4a), (+)-sigmosceptrellin A methyl ester (4b), (−)-sigmosceptrellin B (5), (+)-epi-muqubillin A (6) and (−)-epi-nuapapuin B methyl ester (7). The isolated peroxiterpenes demonstrated activities in the range from IC 50 = 0.2 -2 μg/mL.The search for chemotherapeutics to treat human African trypanosomiasis (HAT), also known as African sleeping sickness, has not been a major target of industry drug discovery campaigns, even though the currently available therapeutics are inadequate.1 This is surprising as there are 50,000 annual cases of infection, HAT is the world's third most devastating parasitic disease, and it remains a major threat to more than 60 million people.2 -4 Because it is a serious health problem for resource poor regions of Africa, HAT is designated as a neglected tropical disease (NTD). The causative agent is a protozoan parasite, Trypanosoma brucei, 5 subdivided in two sub-genera, T. brucei gambiense in West Africa and T. brucei rhodesiense in East Africa. 1 Most of the four current chemotherapeutics to combat these parasites at their different development stages are very antiquated. The drugs (by registration date for HAT treatment) consist of suramin4 (1922), pentamidine1 (1941), melarsoprol3 (1949), and eflorithine1 (1990). Another relevant point is that new strains of T. brucei are showing cross-resistance to some of these agents.6 In recent years, only one compound, DB289 7 (a synthetic analog of pentamidine), received serious clinical evaluation against HAT, but further development was discontinued in 2008. 3 * To whom correspondence should be addressed. phil@chemistry.ucsc.edu. Tel: (831)

Relative and Absolute Stereochemistry of Diacarperoxides: Antimalarial Norditerpene Endoperoxides from Marine Sponge Diacarnus megaspinorhabdosa

“…Although no XRD structure of 1 could be obtained, our calculations showed the same conformation (Figure S52 in Supplementary Information) of the endoperoxide core as all of the three closely-related crystal structures [12,13,14]. Back to our calculations, it is the configurationally dependent conformations that thus dictate the chemical shift differences and underlie the empirical rule.…”

“…Compound 6 was likely the acyclic form of nuapapuin A methyl ester. We applied chemical degradation on nuapapuin A methyl ester (which was also obtained by us from the same sponge) under reductive condition (Figure 10), and the product showed identical 1 H NMR spectrum and similar optical rotation with 6 , verifying the structures of both of these two natural products [12,25]. To establish the absolute configuration of 6 , modified Mosher’s method [26,27] was applied, and NMR anisotropic analysis of corresponding product was used to assign the absolute configuration of 6 as 2 R ,3 R ,6 R (Figure 11).…”

“…The signal patterns for a 1,2-dioxane ring and a substituted methyl propionate group were acquired from NMR, establishing a scaffold of the marine-derived endoperoxide class [10,11]. Further analysis of the NMR spectra (Table 1 and Table 2) provided evidence for the existence of monoterpene substitutions different from reported derivatives [12,13]. The stereochemical issues were encountered either because of conformational flexibility or two distal stereogenic centers ( vide infra ).…”

New Metabolites from the South China Sea Sponge <i>Diacarnus megaspinorhabdosa</i>

“…Besides the three new compounds 1-3, the known compounds 4-8 were identified as muquketone (4), 3) aikupikoxide A (5), 4) (−)-ent-muqubilone methyl ester (6), 13) muqubilin (7), 14) ( )-epimuqubilin A methyl ester (8) 3) by the comparison of their spectroscopic data with those in the literatures.…”

“…6) Such ketones are increasingly being identified as minor co-metabolites with norterpene cyclic peroxides. 3,[9][10][11] Recently, attention has been drawn on these metabolites because of their pharmaceutical properties, including antimalarial, 8,12) antimicrobial, 12) Trypanosoma brucei inhibitory, 13) sea urchin egg cell-division inhibitory, 14) cytotoxic, 4,7,15) antiulcer and antihypotensive activities. 16) As our previous chemical research on the sponge D. megaspinorhabdosa, five norterpene cyclic peroxides diacarperoxides H-L have been isolated.…”