Synthesis of bilocularin A carbamate derivatives and their evaluation as leucine transport inhibitors in prostate cancer cells

Large-scale extraction of the leaves of the Australian rainforest tree Maytenus bilocularis followed by extensive purification studies afforded the targeted and abundant dihydro-β-agarofuran, bilocularin A, in sufficient quantities (>500 mg) for detailed semi-synthetic chemistry. Eight bilocularin A carbamate analogues were synthesised using a series of commercially available isocyanate reagents in high purity (>95%) and variable yields (9-91%). All previously undescribed analogues were spectroscopically characterised using NMR, UV, IR and MS data. One compound afforded crystalline material and subsequent single crystal X-ray analysis (Cu-Kα) confirmed the chemical structure along with the absolute configuration. All compounds were evaluated for anti-proliferative activity against the human prostate cancer cell line LNCaP; none of the compounds showed significant (>50%) growth inhibition at 20 μM. Compounds were also tested for their ability to inhibit leucine transport in LNCaP cells, and two analogues showed moderate activity with IC₅₀ values of 8.9 and 8.5 μM. This is the first reported synthesis of dihydro-β-agarofuran carbamate derivatives.

Using UHPLC-MS Profiling for the Discovery of New Dihydro-β-Agarofurans from Australian Celastraceae Plant Extracts

An analytical method using UHPLC-MS was developed and applied to 16 crude CH₂Cl₂ extracts from Australian Celastraceae plants; the endemic plant materials were accessed from Griffith University’s NatureBank resource and included bark, fruit, leaf, root, twig and mixed samples, all of which were collected from Queensland, Australia. The generated UHPLC-MS data were analysed and dereplicated using the scientific databases Dictionary of Natural Products and SciFinder Scholar in order to potentially identify new dihydro-β-agarofurans from local Celastraceae plants. These investigations led to the large-scale extraction and isolation work on a prioritised fruit sample that belonged to the rainforest plant Denhamia celastroides. Chemical investigations resulted in the purification of four new natural products, denhaminols O–R (1–4), along with the related and known compound, denhaminol G (5). The structures of all the new compounds were determined via detailed analysis of NMR and MS data.

Dihydro-β-agarofurans from the Australian rainforest plant Denhamia celastroides that inhibit leucine transport in prostate cancer cells

Four new dihydro-β-agarofurans, denhaminols K-N (4-7), along with three known secondary metabolites, denhaminols A-C (1-3) were obtained from the large-scale isolation studies of the leaves of the Australian endemic rainforest plant, Denhamia celastroides. The structures of the previously undescribed compounds were determined by detailed 1D and 2D nuclear magnetic resonance spectroscopy, mass spectrometry, ultraviolet, and infrared data analysis. All compounds were found to inhibit the activity of leucine transport in a human prostate cancer cell line with IC50 values ranging from 5.1-74.9 μM. Dihydro-β-agarofurans 1-7 showed better potency than the L-type amino acid transporter family inhibitor, 2-aminobicyclo[2.2.1]-heptane-2-carboxylic acid (BCH).

Dihydro-β-agarofurans from the roots of the Australian endemic rainforest tree Maytenus bilocularis act as leucine transport inhibitors

Phytochemical studies of the roots of the Australian plant, Maytenus bilocularis, resulted in the identification of six previously undescribed dihydro-β-agarofuran sesquiterpenoids, bilocularins D-I, along with three known natural products, namely 1α,2α,6β,15-tetraacetoxy-9β-benzoyloxydihydro-β-agarofuran, pristimerin, and celastrol. The structures of all compounds were characterized via analysis of 1D/2D NMR and MS data. The absolute configuration of bilocularin D was defined by X-ray crystallography analysis. Bilocularins D and G, 1α,2α,6β,15-tetraacetoxy-9β-benzoyloxydihydro-β-agarofuran, and celastrol inhibited leucine transport in the human prostate cancer cell line LNCaP with IC₅₀ values ranging from 2.5-27.9 μM, which were more potent than the L-type amino acid transporter (LAT) family inhibitor 2-aminobicyclo[2,2,1]-heptane-2-carboxylic acid (BCH). Bilocularins D-F are the first examples of dihydro-β-agarofurans bearing a hydroxyacetate group.

Celastrofurans A-G: Dihydro-β-agarofurans from the Australian Rainforest Vine Celastrus subspicata and Their Inhibitory Effect on Leucine Transport in Prostate Cancer Cells

Seven new dihydro-β-agarofurans, celastrofurans A-G (1-7), along with two known secondary metabolites, 9β-benzoyloxy-1α-furoyloxydihydro-β-agarofuran (8) and (1R,2R,4R,5S,7R,9S,10R)-2-acetoxy-9-benzoyloxy-1-furoyloxydihydro-β-agarofuran (9), were obtained from the leaves of the Australian rainforest vine, Celastrus subspicata. The structures of the new compounds were determined by detailed spectroscopic (1D/2D NMR) and MS data analysis. The absolute configurations of compounds 1-4 were defined by ECD and single-crystal X-ray diffraction studies. All compounds were found to exhibit inhibitory activity on leucine transport in the human prostate cancer cell line LNCaP with IC₅₀ values ranging from 7.0 to 98.9 μM. Dihydro-β-agarofurans 1-9 showed better potency than the L-type amino acid transporter (LAT) family inhibitor, 2-aminobicyclo[2.2.1]-heptane-2-carboxylic acid (BCH).

Natural Products as Alternative Choices for P-Glycoprotein (P-gp) Inhibition

Multidrug resistance (MDR) is regarded as one of the bottlenecks of successful clinical treatment for numerous chemotherapeutic agents. Multiple key regulators are alleged to be responsible for MDR and making the treatment regimens ineffective. In this review, we discuss MDR in relation to P-glycoprotein (P-gp) and its down-regulation by natural bioactive molecules. P-gp, a unique ATP-dependent membrane transport protein, is one of those key regulators which are present in the lining of the colon, endothelial cells of the blood brain barrier (BBB), bile duct, adrenal gland, kidney tubules, small intestine, pancreatic ducts and in many other tissues like heart, lungs, spleen, skeletal muscles, etc. Due to its diverse tissue distribution, P-gp is a novel protective barrier to stop the intake of xenobiotics into the human body. Over-expression of P-gp leads to decreased intracellular accretion of many chemotherapeutic agents thus assisting in the development of MDR. Eventually, the effectiveness of these drugs is decreased. P-gp inhibitors act by altering intracellular ATP levels which are the source of energy and/or by affecting membrane contours to increase permeability. However, the use of synthetic inhibitors is known to cause serious toxicities. For this reason, the search for more potent and less toxic P-gp inhibitors of natural origin is underway. The present review aims to recapitulate the research findings on bioactive constituents of natural origin with P-gp inhibition characteristics. Natural bioactive constituents with P-gp modulating effects offer great potential for semi-synthetic modification to produce new scaffolds which could serve as valuable investigative tools to recognize the function of complex ABC transporters apart from evading the systemic toxicities shown by synthetic counterparts. Despite the many published scientific findings encompassing P-gp inhibitors, however, this article stand alones because it provides a vivid picture to the readers pertaining to Pgp inhibitors obtained from natural sources coupled with their mode of action and structures. It provides first-hand information to the scientists working in the field of drug discovery to further synthesise and discover new P-gp inhibitors with less toxicity and more efficacies.