Marcy J. Balunas

Natural products as aromatase inhibitors

With the clinical success of several synthetic aromatase inhibitors (AIs) in the treatment of postmenopausal estrogen receptor-positive breast cancer, researchers have also been investigating the potential of natural products as AIs. Natural products from terrestrial and marine organisms provide a chemically diverse array of compounds not always available through current synthetic chemistry techniques. Natural products that have been used traditionally for nutritional or medicinal purposes (e.g., botanical dietary supplements) may also afford AIs with reduced side effects. A thorough review of the literature regarding natural product extracts and secondary metabolites of plant, microbial, and marine origin that have been shown to exhibit aromatase inhibitory activity is presented herein.

Santacruzamate A, a Potent and Selective Histone Deacetylase Inhibitor from the Panamanian Marine Cyanobacterium cf. Symploca sp.

A dark brown tuft-forming cyanobacterium, morphologically resembling the genus Symploca, was collected during an expedition to the Coiba National Park, a UNESCO World Heritage Site on the Pacific coast of Panama. Phylogenetic analysis of its 16S rRNA gene sequence indicated that it is 4.5% divergent from the type strain for Symploca and thus is likely a new genus. Fractionation of the crude extract led to the isolation of a new cytotoxin, designated santacruzamate A (1), which has several structural features in common with suberoylanilide hydroxamic acid [(2), SAHA, trade name Vorinostat], a clinically approved histone deacetylase (HDAC) inhibitor used to treat refractory cutaneous T-cell lymphoma. Recognition of the structural similarly of 1 and SAHA led to the characterization of santacruzamate A as a picomolar level selective inhibitor of HDAC2, a Class I HDAC, with relatively little inhibition of HDAC4 or HDAC6, both Class II HDACs. As a result, chemical syntheses of santacruzamate A as well as a structurally intriguing hybrid molecule, which blends aspects of both agents (1 and 2), were achieved and evaluated for their HDAC activity and specificity.

Natural compounds with aromatase inhibitory activity: an update.

Several synthetic aromatase inhibitors are currently in clinical use for the treatment of postmenopausal women with hormone-receptor positive breast cancer. However, these treatments may lead to untoward side effects and so the search for new aromatase inhibitors continues, especially those for which the activity is promoter-specific, targeting the breast-specific promoters I.3 and II. Recently, numerous natural compounds have been found to inhibit aromatase in noncellular, cellular, and IN VIVO studies. These investigations, covering the last two years, as well as additional studies that have focused on the evaluation of natural compounds as promoter-specific aromatase inhibitors or as aromatase inducers, are described in this review.

Bastimolide A, a Potent Antimalarial Polyhydroxy Macrolide from the Marine Cyanobacterium Okeania hirsuta

Bastimolide A (1), a polyhydroxy macrolide with a 40-membered ring, was isolated from a new genus of the tropical marine cyanobacterium Okeania hirsuta. This novel macrolide was defined by spectroscopy and chemical reactions to possess one 1,3-diol, one 1,3,5-triol, six 1,5-diols, and one tert-butyl group; however, the relationships of these moieties to one another were obscured by a highly degenerate (1)H NMR spectrum. Its complete structure and absolute configuration were therefore unambiguously determined by X-ray diffraction analysis of the nona-p-nitrobenzoate derivative (1d). Pure bastimolide A (1) showed potent antimalarial activity against four resistant strains of Plasmodium falciparum with IC50 values between 80 and 270 nM, although with some toxicity to the control Vero cells (IC50 = 2.1 μM), and thus represents a potentially promising lead for antimalarial drug discovery. Moreover, rigorous establishment of its molecular arrangement gives fresh insight into the structures and biosynthesis of cyanobacterial polyhydroxymacrolides.

Coibacins A and B: Total Synthesis and Stereochemical Revision

The interface between synthetic organic chemistry and natural products was explored in order to unravel the structure of coibacin A, a metabolite isolated from the marine cyanobacterium cf. Oscillatoria sp. that exhibits selective antileishmanial activity and potent anti-inflammatory properties. Our synthetic plan focused on a convergent strategy that allows rapid access to the desired target by coupling of three key fragments involving E-selective Wittig and modified Julia olefinations. CD measurements and comparative HPLC analyses of the natural product and four synthetic stereoisomers led to determination of its absolute configuration, thus correcting the original assignment at C-5 and unambiguously establishing those at C-16 and C-18. Additionally, we synthesized coibacin B on the basis of the assignment of configuration for coibacin A.

Leisingera sp. JC1, a Bacterial Isolate from Hawaiian Bobtail Squid Eggs, Produces Indigoidine and Differentially Inhibits Vibrios.

Female members of many cephalopod species house a bacterial consortium in the accessory nidamental gland (ANG), part of the reproductive system. These bacteria are deposited into eggs that are then laid in the environment where they must develop unprotected from predation, pathogens, and fouling. In this study, we characterized the genome and secondary metabolite production of Leisingera sp. JC1, a member of the roseobacter clade (Rhodobacteraceae) of Alphaproteobacteria isolated from the jelly coat of eggs from the Hawaiian bobtail squid, Euprymna scolopes. Whole genome sequencing and MLSA analysis revealed that Leisingera sp. JC1 falls within a group of roseobacters associated with squid ANGs. Genome and biochemical analyses revealed the potential for and production of a number of secondary metabolites, including siderophores and acyl-homoserine lactones involved with quorum sensing. The complete biosynthetic gene cluster for the pigment indigoidine was detected in the genome and mass spectrometry confirmed the production of this compound. Furthermore, we investigated the production of indigoidine under co-culture conditions with Vibrio fischeri, the light organ symbiont of E. scolopes, and with other vibrios. Finally, both Leisingera sp. JC1 and secondary metabolite extracts of this strain had differential antimicrobial activity against a number of marine vibrios, suggesting that Leisingera sp. JC1 may play a role in host defense against other marine bacteria either in the eggs and/or ANG. These data also suggest that indigoidine may be partially, but not wholly, responsible for the antimicrobial activity of this squid-associated bacterium.

Current Therapeutic Role and Medicinal Potential of Scutellaria barbata in Traditional Chinese Medicine and Western Research

ETHNOPHARMACOLOGICAL RELEVANCE Scutellaria barbata is a common herb in Traditional Chinese Medicine (TCM) most often used to treat cancer. S. barbata has been found to exhibit efficacy both in vitro and in vivo on a variety of cancer types. Similarly encouraging results have been shown in patients with metastatic breast cancer from Phase Ia and Ib clinical trials. This study aims to elucidate the current use of S. barbata by TCM practitioners and in current Western research. MATERIALS AND METHODS Semi-structured interviews were conducted with fifteen TCM practitioners in Beijing and Nanjing, China to understand their clinical use of S. barbata. Practitioners were also asked to comment on the future development of TCM using Western research methods and the potential for integration of the two types of medicine in clinical therapy. Statistical analyses were conducted to compare use of S. barbata by disease and in conjunction with other herbs. RESULTS Current Western research related to S. barbata is focused on cancer treatment, which corresponds to the most common use of S. barbata by TCM practitioners. Other common uses that practitioners reported included infection and inflammation, for which Beijing practitioners reported use of S. barbata more often than Nanjing practitioners (p<0.05). Hedyotis diffusa was found to be the most commonly cited herb to pair with S. barbata for cancer treatment (p<0.05). When compared to Western clinical trials of BZL101, an S. barbata extract, TCM practitioners reported using smaller doses of S. barbata in shorter durations, in combination with numerous other herbs with the goal to potentiate therapeutic efficacy and mitigate side effects. In addition, TCM practitioners repeatedly emphasized symptom differentiating as the key to achieving maximum therapeutic potential of S. barbata, a factor typically overlooked in Western research. CONCLUSION Similarities and differences in diagnosis and treatment regimens between TCM practitioners and Western research have the potential to shed light on possible new avenues of research. Additional research may prove beneficial on possible synergistic effects of herbs commonly used with S. barbata, such as Hedyotis diffusa and Lobelia chinensis.

Natural products as exquisitely potent cytotoxic payloads for antibody- drug conjugates.

Antibody-drug conjugates (ADCs) are an emerging area of study within medicinal chemistry and are thought of as sophisticated drug delivery systems due to their specificity to a disease-targeted antigen. ADCs have been actively utilized as therapeutics for hematological and solid tumor cancers due to their capability to deliver a cytotoxic compound to a specific cancer cell without affecting normal cells. An antibody drug conjugate has three major constituents: a monoclonal antibody (mAb), a chemical linker, and a potent cytotoxic payload. There has been a continuing effort to optimize antibody-drug conjugates, with the primary focus of design and development directed at either the mAb or the chemical linker, with little effort devoted to the optimization of payload compounds. In fact, among the 114 ongoing or recently completed clinical trials, there is generally a lack of diversity in the cytotoxic payloads that are utilized, with only seven payload compounds reported (four additional trials are ongoing with structures that have not been reported). Six of these seven payload compounds are derived from natural product sources, highlighting the importance of natural products as cytotoxic payloads for ADC.

Flow Cytometry Enables Multiplexed Measurements of Genetically Encoded Intramolecular FRET Sensors Suitable for Screening.

Genetically encoded sensors based on intramolecular FRET between CFP and YFP are used extensively in cell biology research. Flow cytometry has been shown to offer a means to measure CFP-YFP FRET; we suspected it would provide a unique way to conduct multiplexed measurements from cells expressing different FRET sensors, which is difficult to do with microscopy, and that this could be used for screening. We confirmed that flow cytometry accurately measures FRET signals using cells transiently transfected with an ERK activity reporter, comparing responses measured with imaging and cytometry. We created polyclonal long-term transfectant lines, each expressing a different intramolecular FRET sensor, and devised a way to bar-code four distinct populations of cells. We demonstrated the feasibility of multiplexed measurements and determined that robust multiplexed measurements can be conducted in plate format. To validate the suitability of the method for screening, we measured responses from a plate of bacterial extracts that in unrelated experiments we had determined contained the protein kinase C (PKC)–activating compound teleocidin A-1. The multiplexed assay correctly identifying the teleocidin A-1-containing well. We propose that multiplexed cytometric FRET measurements will be useful for analyzing cellular function and for screening compound collections.

Synthesis and biological evaluation of santacruzamate A analogues for anti-proliferative and immunomodulatory activity

Santacruzamate A (SCA) is a natural product isolated from a Panamanian marine cyanobacterium, previously reported to have potent and selective histone deacetylase (HDAC) activity. To optimize the enzymatic and cellular activity, 40 SCA analogues were synthesized in a systematic exploration of the zinc-binding group (ZBG), cap terminus, and linker region. Two cap group analogues inhibited proliferation of MCF-7 breast cancer cells, with analogous increased degranulation of cytotoxic T cells (CTLs), while one cap group analogue reduced CTL degranulation, indicative of suppression of the immune response. Additional testing of these analogues resulted in reevaluation of the previously reported SCA mechanism of action. These analogues and the resulting structure-activity relationships will be of interest for future studies on cell proliferation and immune modulation.