Matthew D. Lebar

Cold-water marine natural products

Marine natural products isolated from organisms collected from cold-water habitats are described. Emphasis is on bioactive compounds from tunicates, sponges, microbes, bryozoans, corals, algae, molluscs and echinoderms. Synthetic studies of several important classes of cold-water compounds are highlighted.

The Diarylheptanoid (+)-aR,11S-Myricanol and Two Flavones from Bayberry (Myrica cerifera) Destabilize the Microtubule Associated Protein Tau

Target-based drug discovery for Alzheimers disease (AD) centered on modulation of the amyloid β peptide has met with limited success. Therefore, recent efforts have focused on targeting the microtubule-associated protein tau. Tau pathologically accumulates in more than 15 neurodegenerative diseases and is most closely linked with postsymptomatic progression in AD. We endeavored to identify compounds that decrease tau stability rather than prevent its aggregation. An extract from Myrica cerifera (bayberry/southern wax myrtle) potently reduced both endogenous and overexpressed tau protein levels in cells and murine brain slices. The bayberry flavonoids myricetin and myricitrin were confirmed to contribute to this potency, but a diarylheptanoid, myricanol, was the most effective anti-tau component in the extract, with potency approaching the best targeted lead therapies. (+)-aR,11S-Myricanol, isolated from M. cerifera and reported here for the first time as the naturally occurring aglycone, was significantly more potent than commercially available (±)-myricanol. Myricanol may represent a novel scaffold for drug development efforts targeting tau turnover in AD.

Screening Mangrove Endophytic Fungi for Antimalarial Natural Products

We conducted a screening campaign to investigate fungi as a source for new antimalarial compounds. A subset of our fungal collection comprising Chinese mangrove endophytes provided over 5000 lipophilic extracts. We developed an accelerated discovery program based on small-scale cultivation for crude extract screening and a high-throughput malaria assay. Criteria for hits were developed and high priority hits were subjected to scale-up cultivation. Extracts from large scale cultivation were fractionated and these fractions subjected to both in vitro malaria and cytotoxicity screening. Criteria for advancing fractions to purification were developed, including the introduction of a selectivity index and by dereplication of known metabolites. From the Chinese mangrove endophytes, four new compounds (14–16, 18) were isolated including a new dimeric tetrahydroxanthone, dicerandrol D (14), which was found to display the most favorable bioactivity profile.

Synthesis and Structure Reassessment of Psammopemmin A

We have isolated meridianins A, B, C, and E from the Antarctic tunicate Synoicum sp. In the process of verifying the structure of these compounds it was noted that the physical data reported for meridianins bore a striking resemblance to that of psammopemmins. The psammopemmins are alkaloids bearing similar structures to the meridianins, but reported from the Antarctic sponge Psammopemma sp. To verify the structure originally proposed for psammopemmin A, the compound was synthesized. By comparing the 1H and 13C NMR data of reported and synthetic psammopemmin A with that of meridianin A, we infer that the correct structure of psammopemmin A isolated from Psammopemma sp. is actually that of meridianin A.

CNS and antimalarial activity of synthetic meridianin and psammopemmin analogs.

The marine invertebrate-derived meridianin A, the originally proposed structure for psammopemmin A, and several related 3-pyrimidylindole analogs were synthesized and subsequently investigated for central nervous system, antimalarial, and cytotoxic activity. A Suzuki coupling of an indoleborate ester to the pyrimidine electrophile was utilized to form the natural product and derivatives thereof. The 3-pyrimidineindoles were found to prevent radioligand binding to several CNS receptors and transporters, most notably, serotonin receptors (<0.2 μM K(i) for 5HT(2B)). Two compounds also inhibited the human malaria parasite Plasmodium falciparum (IC(50) <50 μM). Only the natural product was cytotoxic toward A549 cells (IC(50)=15 μM).

Miniaturized Cultivation of Microbiota for Antimalarial Drug Discovery.

The ongoing search for effective antiplasmodial agents remains essential in the fight against malaria worldwide. Emerging parasitic drug resistance places an urgent need to explore chemotherapies with novel structures and mechanisms of action. Natural products have historically provided effective antimalarial drug scaffolds. In an effort to search natures chemical potential for antiplasmodial agents, unconventionally sourced organisms coupled with innovative cultivation techniques were utilized. Approximately 60,000 niche microbes from various habitats (slow‐growing terrestrial fungi, Antarctic microbes, and mangrove endophytes) were cultivated on a small‐scale, extracted, and used in high‐throughput screening to determine antimalarial activity. About 1% of crude extracts were considered active and 6% partially active (≥67% inhibition at 5 and 50 μg/mL, respectively). Active extracts (685) were cultivated on a large‐scale, fractionated, and screened for both antimalarial activity and cytotoxicity. High interest fractions (397) with an IC50 < 1.11 μg/mL were identified and subjected to chromatographic separation for compound characterization and dereplication. Identifying active compounds with nanomolar antimalarial activity coupled with a selectivity index tenfold higher was accomplished with two of the 52 compounds isolated. This microscale, high‐throughput screening project for antiplasmodial agents is discussed in the context of current natural product drug discovery efforts.