Novel pyxinol amide derivatives bearing an aliphatic heterocycle as P-glycoprotein modulators for overcoming multidrug resistance.

P-glycoprotein (Pgp) modulators are promising agents for overcoming multidrug resistance (MDR) in cancer chemotherapy. In this study, via structural optimization of our lead compound S54 (nonsubstrate allosteric inhibitor of Pgp), 29 novel pyxinol amide derivatives bearing an aliphatic heterocycle were designed, synthesized, and screened for MDR reversal activity in KBV cells. Unlike S54, these active derivatives were shown to transport substrates of Pgp. The most potent derivative 4c exhibited promising MDR reversal activity (IC50 of paclitaxel = 8.80 ± 0.56 nM, reversal fold = 211.8), which was slightly better than that of third-generation Pgp modulator tariquidar (IC50 of paclitaxel = 9.02 ± 0.35 nM, reversal fold = 206.6). Moreover, the cytotoxicity of this derivative was 8-fold lower than that of tariquidar in human normal HK-2 cells. Furthermore, 4c blocked the efflux function of Pgp and displayed high selectivity for Pgp but had no effect on its expression and distribution. Molecular docking revealed that 4c bound preferentially to the drug-binding domain of Pgp. Overall, 4c is a promising lead compound for developing Pgp modulators.

A Combination of Biocatalysis and Fenton-Like Reaction Induced OH• Burst for Cascade Amplification of Cancer Chemodynamic Therapy.

Chemodynamic therapy (CDT) is a novel antitumor strategy that employs Fenton or Fenton-like reactions to generate highly toxic hydroxyl radical (OH•) from hydrogen peroxide (H2O2) for inducing tumor cell death. However, the antitumor efficacy of the CDT strategy is harshly limited by the redox homeostasis of tumor cells; especially the OH • is easily scavenged by glutathione (GSH) and the intracellular H2O2 level is insufficient in the tumor cells. Herein, we propose the Mn2+-menadione (also known as vitamin K3, MK3) cascade biocatalysis strategy to disrupt the redox homeostasis of tumor cells and induce a OH• storm, resulting in enhanced CDT effect. A nanoliposome encapsulating Mn-MK3 (Mn-MK3@LP) was prepared for the treatment of hepatic tumors in this study. After Mn-MK3@LPs were taken up by tumor cells, menadione could facilitate the production of intracellular H2O2 via redox cycling, and further the cytotoxic OH • burst was induced by Mn2+-mediated Fenton-like reaction. Moreover, high-valent manganese ions were reduced by GSH and the depletion of GSH further disrupted the redox homeostasis of tumor cells, thus achieving synergistically enhanced CDT. Overall, both cellular and animal experiments confirmed that the Mn-MK3@LP cascade biocatalysis nanoliposome exhibited excellent biosafety and tumor suppression efficacy. This study may provide deep insights for developing novel CDT-based strategies for tumor therapy.

Supramolecular Hydrogel Dexamethasone-Diclofenac for the Treatment of Rheumatoid Arthritis.

Rheumatoid arthritis (RA) severely affects patients' quality of life and is commonly treated with glucocorticosteroids injections, like dexamethasone, which may have side effects. This study aimed to create a novel low dose of twin-drug hydrogel containing dexamethasone and diclofenac and explore its potential as a drug delivery system for an enhanced anti-inflammatory effect. Its characterization involved rheology, transmission electron microscope (TEM), Fourier-transform infrared spectroscopy (FTIR), and X-ray diffraction (XRD). Furthermore, the hydrogel demonstrated thixotropic properties. The hydrogel exhibited no cytotoxicity against RAW 264.7 macrophages. Furthermore, the hydrogel demonstrated a significant anti-inflammatory efficacy by effectively downregulating the levels of NO, TNF-α, and IL-6 in lipopolysaccharide (LPS)-activated RAW 264.7 macrophages. The co-delivery approach, when intra-articularly injected in adjuvant-induced arthritis (AIA) rats, significantly alleviated chronic inflammation leading to reduced synovitis, delayed bone erosion onset, and the downregulation of inflammatory cytokines. The biocompatibility and adverse effect evaluation indicated good biological safety. Furthermore, the hydrogel demonstrated efficacy in reducing NF-κB nuclear translocation in LPS-induced RAW 264.7 macrophages and inhibited p-NF-kB, COX-2, and iNOS expression both in RAW 264.7 macrophages and the joints of AIA rats. In conclusion, the findings indicate that the hydrogel possesses potent anti-inflammatory activity, which effectively addresses the limitations associated with free forms. It presents a promising therapeutic strategy for the management of RA.

Synthesis, Anti-Inflammatory Activities, and Molecular Docking Study of Novel Pyxinol Derivatives as Inhibitors of NF-κB Activation.

Pyxinol, an active metabolite of ginsenosides in human hepatocytes, exhibits various pharmacological activities. Here, a series of C-3 modified pyxinol derivatives was designed and virtually screened by molecular docking with the key inflammation-related proteins of the nuclear factor kappa B (NF-κB) pathway. Some of the novel derivatives were synthesized to assess their effects in inhibiting the production of nitric oxide (NO) and mitochondrial reactive oxygen species (MtROS) in lipopolysaccharide-triggered RAW264.7 cells. Derivative 2c exhibited the highest NO and MtROS inhibitory activities with low cytotoxicity. Furthermore, 2c decreased the protein levels of interleukin 1ß, tumor necrosis factor α, inducible nitric oxide synthase, and cyclooxygenase 2 and suppressed the activation of NF-κB signaling. Cellular thermal shift assays indicated that 2c could directly bind with p65 and p50 in situ. Molecular docking revealed that 2c's binding to the p65-p50 heterodimer and p50 homodimer was close to their DNA binding sites. In summary, pyxinol derivatives possess potential for development as NF-κB inhibitors.

Discovery of Novel Antitumor Small-Molecule Agent with Dual Action of CDK2/p-RB and MDM2/p53.

Cell cycle-dependent kinase 2 (CDK2) is located downstream of CDK4/6 in the cell cycle and regulates cell entry into S-phase by binding to Cyclin E and hyper-phosphorylating Rb. Proto-oncogene murine double minute 2 (MDM2) is a key negative regulator of p53, which is highly expressed in tumors and plays an important role in tumorigenesis and progression. In this study, we identified a dual inhibitor of CDK2 and MDM2, III-13, which had good selectivity for inhibiting CDK2 activity and significantly reduced MDM2 expression. In vitro results showed that III-13 inhibited proliferation of a wide range of tumor cells, regardless of whether Cyclin E1 (CCNE1) was overexpressed or not. The results of in vivo experiments showed that III-13 significantly inhibited proliferation of tumor cells and did not affect body weight of mice. The results of the druggability evaluation showed that III-13 was characterized by low bioavailability and poor membrane permeability when orally administered, suggesting the necessity of further structural modifications. Therefore, this study provided a lead compound for antitumor drugs, especially those against CCNE1-amplified tumor proliferation.

Facile fluorescence detection of malachite green in fish using molecularly imprinted polymers doped CdTe quantum dots based system.

Malachite green (MG) possesses high toxicity, therefore, the detection of MG in fish tissues is of vital importance. A novel core-shell MIPs doped CdTe quantum dots coated silica nanoparticles (CdTe-MIP/SiO2 NPs) were synthesized via a simple one-pot strategy. The materials were characterized carefully. The resulting CdTe-MIP/SiO2 NPs were coated on the thin layer chromatography plate, and coupled with miniaturized fluorimeter for fluorescence detection of MG in fish samples. The resulting CdTe-MIP/SiO2 NPs based system possessed good linearity (0.01 âˆ¼ 20 µmol/L), high recoveries (98.36 %∼101.45 %) and low detection limit (3.7 nmol/L) for MG. Furthermore, CdTe-MIP/SiO2 NPs based system were employed to measure fish samples spiked with MG, meanwhile, HPLC was utilized to evaluate the accuracy and reliability. And the paired t-test was conducted to evaluate differences between fluorescence method and HPLC, P > 0.05 means no significant difference was observed, the results demonstrated that both fluorescence method and HPLC are suitable for MG analysis.

An outward-rectifying plant K+ channel SPORK2 exhibits temperature-sensitive ion-transport activity.

Temperature sensing is critical for the survival of living organisms.1,2 Thermosensitive transient receptor-potential (TRP) cation channels function as thermosensors in mammals.2,3,4,5,6 In contrast to animals, land plants lack TRP genes.7,8,9 Previous patch-clamp studies in plant cells suggested the presence of ion channels whose activities are related to temperature, implying the presence of TRP-like channels.10,11,12,13,14 However, the molecular entities of such temperature-sensitive ion channels were still unknown in land plants. In this study, we observed that the unique rainfall-induced leaf-folding movement of the legume tree Samanea saman15 was temperature-sensitive by using a rainfall-mimicking assay. Chilling-induced leaf folding in S. saman was shown to be related to the swelling of the motor cells16,17 at the base of the leaflet. This swelling suggested involvement of temperature-sensitive inactivation of K+ currents, independent of fluctuations in ion channel gene expression in motor cells. These findings led us to examine the temperature sensitivity of an outward-rectifying K+ channel, SPORK2, which was reported as an ion channel responsible for the nyctinastic (circadian-rhythmic) leaf movement of S. saman.18 We also discovered that SPORK2 exhibits temperature-sensitive K+ transport activity in the Xenopus oocyte expression system. Using chimeric channels, we showed that two domains of SPORK2 regulated the temperature sensitivity. Furthermore, heterologously expressed SPORK2 in Arabidopsis guard cells induced temperature-dependent stomatal closure. Therefore, SPORK2 is an ion channel in land plants with temperature-sensitive ion-transport activity that functions similarly to mammalian TRP channels. Our current findings advance the molecular understanding of temperature-sensing mechanisms in plants.

A Multi-Path MAC Scheduling Scheme for Multi-Channel Wireless Sensor Networks.

Designing reasonable MAC scheduling strategies is an important means to ensure transmission quality in wireless sensor networks (WSNs). When there exist multiple available routes from the source to the destination, it is necessary to combine a data traffic allocation mechanism and design a multi-path MAC scheduling scheme in order to ensure QoS. This paper develops a multi-path resource allocation method for multi-channel wireless sensor networks, which uses random-access technology to complete MAC scheduling and selects the transmission path for each packet according to the probability. Through theoretical analysis and simulation experiments, it can be found that the proposed strategy can provide a reliable throughput capacity region. Meanwhile, due to the use of random-access technology, the computational complexity of the proposed algorithm can be independent of the number of links and channels.

Discovery of Pyxinol Amide Derivatives Bearing Amino Acid Residues as Nonsubstrate Allosteric Inhibitors of P-Glycoprotein-Mediated Multidrug Resistance.

Nonsubstrate allosteric inhibitors of P-glycoprotein (Pgp), which are considered promising modulators for overcoming multidrug resistance (MDR), are relatively unknown. Herein, we designed and synthesized amino acids bearing amide derivatives of pyxinol, the main ginsenoside metabolite produced by the human liver, and examined their MDR reversal abilities. A potential nonsubstrate inhibitor (7a) was identified to undergo high-affinity binding to the putative allosteric site of Pgp at the nucleotide-binding domains. Subsequent assays confirmed that 7a (25 µM) was able to suppress both basal and verapamil-stimulated Pgp-ATPase activities (inhibition rates of 87 and 60%, respectively) and could not be pumped out by Pgp, indicating that it was a rare nonsubstrate allosteric inhibitor. Moreover, 7a interfered with Pgp-mediated Rhodamine123 efflux while exhibiting high selectivity for Pgp. Notably, 7a also markedly enhanced the therapeutic efficacy of paclitaxel, with a tumor inhibition ratio of 58.1%, when used to treat nude mice bearing KBV xenograft tumors.

Fusion of Michael-acceptors enhances the anti-inflammatory activity of ginsenosides as potential modulators of the NLRP3 signaling pathway.

Ginsenosides are a promising group of secondary metabolites for developing anti-inflammatory agents. In this study, Michael acceptor was fused into the aglycone A-ring of protopanoxadiol (PPD)-type ginsenosides (MAAG), the main pharmacophore of ginseng, and its liver metabolites to produce novel derivatives and assess their anti-inflammatory activity in vitro. The structure-activity relationship of MAAG derivatives was assessed based on their NO-inhibition activities. Of these, a 4-nitrobenzylidene derivative of PPD (2a) was the most effective and dose-dependently inhibited the release of proinflammatory cytokines. Further studies indicated that 2a-induced downregulation on lipopolysaccharide (LPS)-induced iNOS protein expression and cytokine release may be related to its inhibitory effect on MAPK and NF-κB signaling pathways. Importantly, 2a almost completely inhibited LPS-induced production of mitochondrial reactive oxygen species (mtROS) and LPS-induced NLRP3 upregulation. This inhibition was higher than that by hydrocortisone sodium succinate, a glucocorticoid drug. Overall, the fusion of Michael acceptors into the aglycone of ginsenosides greatly enhanced the anti-inflammatory activities of the derivatives, and 2a alleviated inflammation considerably. These findings could be attributed to the inhibition of LPS-induced mtROS to block abnormal activation of the NLRP3 pathway.

Synthesis and evaluation of ginsenosides imprinted polymer-based chromatographic stationary phase.

The molecular imprinting technique has aroused great interest in preparing novel stationary phases, and the resulting materials named molecularly imprinted polymers coated silica packing materials exhibit good performance in separating diverse analytes based on their good characteristics (including high selectivity, simple synthesis, and good chemical stability). To date, mono-template is commonly used in synthesizing molecularly imprinted polymers-based stationary phases. The resulting materials always own the disadvantages of low column efficiency and restricted analytes, and the price of ginsenosides with high purity was very high. In this study, to overcome the weaknesses of molecularly imprinted polymers-based stationary phases mentioned above, the multi-templates (total saponins of folium ginseng) strategy was used to prepare ginsenosides imprinted polymer-based stationary phase. The resulting ginsenosides imprinted polymer-coated silica stationary phase has a good spherical shape and suitable pore structures. Additionally, the total saponins of folium ginseng were cheaper than other kinds of ginsenosides. Moreover, the ginsenosides imprinted polymer-coated silica stationary phase-packed column performed well in the separation of ginsenosides, nucleosides, and sulfonamides. The ginsenosides imprinted polymer-coated silica stationary phase possesses good reproducibility, repeatability, and stability for seven days. Therefore, a multi-templates strategy for synthesizing the ginsenosides imprinted polymer-coated silica stationary phase is considered in the future.

Screening for α-glucosidase inhibitors from Selaginella uncinata based on the ligand fishing combined with ultra-high-performance liquid chromatography-quadrupole time-of-flight-tandem mass spectrometry.

Biflavonoids are naturally occurring compounds consisting of two flavonoid moieties that have received substantial attention from researchers. Although many kinds of biflavonoids are typically distributed in Selaginella uncinata with hypoglycemic effect, their anti-α-glucosidase activities are not yet clear. In this study, a ligand fishing strategy for fast screening of α-glucosidase inhibitors from S. uncinata was proposed. α-Glucosidase was first immobilized on Fe3 O4 magnetic nanoparticles (MNPs) and then the α-glucosidase-functionalized MNPs were incubated with crude extracts of S. uncinata to fish out the ligands. Furthermore, considering the similarity and easy confusion of the structures of biflavonoids, the fragmentation patterns of different types of biflavonoids were studied. Based on this, 11 biflavonoids ligands with α-glucosidase inhibitory activities were accurately and quickly identified from S. uncinata with ultra-high-performance liquid chromatography-quadrupole time-of-flight-tandem mass spectrometry. Furthermore, these ligands were confirmed to be potential inhibitors through the in vitro inhibitory assay and molecular docking.

Design, Synthesis, and Anti-Inflammatory Activities of 12-Dehydropyxinol Derivatives.

Pyxinol skeleton is a promising framework of anti-inflammatory agents formed in the human liver from 20S-protopanaxadiol, the main active aglycone of ginsenosides. In the present study, a new series of amino acid-containing derivatives were produced from 12-dehydropyxinol, a pyxinol oxidation metabolite, and its anti-inflammatory activity was assessed using an NO inhibition assay. Interestingly, the dehydrogenation at C-12 of pyxinol derivatives improved their potency greatly. Furthermore, half of the derivatives exhibited better NO inhibitory activity than hydrocortisone sodium succinate, a glucocorticoid drug. The structure-activity relationship analysis indicated that the kinds of amino acid residues and their hydrophilicity influenced the activity to a great extent, as did R/S stereochemistry at C-24. Of the various derivatives, 5c with an N-Boc-protected phenylalanine residue showed the highest NO inhibitory activity and relatively low cytotoxicity. Moreover, derivative 5c could dose-dependently suppress iNOS, IL-1ß, and TNF-α via the MAPK and NF-κB pathways, but not the GR pathway. Overall, pyxinol derivatives hold potential for application as anti-inflammatory agents.

Design, synthesis, and discovery of Eudistomin Y derivatives as lysosome-targeted antiproliferation agents.

Eudistomin Y is a novel class of ß-carbolines of marine origin with potential antiproliferation activity against MDA-MB-231 cells (triple-negative breast carcinoma). However, the subcellular target or the detailed mechanism against cancer cell proliferation has not yet been identified. In this study, based on its special structure, a novel series of Eudistomin Y fluorescent derivatives were designed and synthesized by enhancing the electron-donor effect of N-9 to endow it with fluorescent properties through N-alkylation. The structure-activity relationships against the proliferation of cancer cells were also analyzed. A quarter of Eudistomin Y derivatives showed much higher potency against cancer cell proliferation than the original Eudistomin Y1. Fluorescent derivative H1k with robust antiproliferative activity could arrest MDA-MB-231 cells in the G2-M phase. The subcellular localization studies of the probes, including H1k, and Eudistomin Y1 were performed in MDA-MB-231 cells, and the co-localization and competitive inhibition assays revealed their lysosome-specific localization. Moreover, H1k could dose-dependently increase the autophagy signal and downregulate the expression of cyclin-dependent kinase (CDK1) and cyclin B1 which principally regulated the G2-M transition. Furthermore, the specific autophagy inhibitor 3-methyladenine significantly inhibited the H1k-triggered antiproliferation of cancer cells and the downregulation of CDK1 and cyclin B1. Overall, the lysosome is identified as the subcellular target of Eudistomin Y for the first time, and derivative H1k showed robust antiproliferative activity against MDA-MB-231 cells by decreasing Cyclin B1-CDK1 complex via a lysosome-dependent pathway.

Identification of yeast α-glucosidase inhibitors from Pueraria lobata by ligand fishing based on magnetic mesoporous silicon combined with knock-out/knock-in technology.

In this study, a ligand fishing technique based on magnetic mesoporous silicon was established and used to screen α-glucosidase inhibitors from Pueraria lobata. To clarify quantity-activity relationships in a holistic view, the knock-out/knock-in technology was used to analyse the interactions of several active constituents in P. lobata. Magnetic mesoporous silicon with a large specific surface area and better biocompatibility was synthesised. Subsequently, α-glucosidase was immobilised on -NH2-modified magnetic mesoporous silicon, and the compounds in the crude extract of P. lobata were screened across enzyme binding. The structures of the ligands were elucidated using UPLC-Q-TOF-MS/MS, and their activities were verified by knock-out/knock-in experiments and molecular docking. Daidzein and puerarin showed α-glucosidase inhibitory activities with an IC50 of 0.088 ± 0.003 mg mL-1 and 0.414 ± 0.005 mg mL-1, respectively. Among them, puerarin, which accounted for more than 40% of the total content, showed synergistic effects with other components and was the main contributor to the α-glucosidase inhibitory activity of P. lobata.

Host-Selective Phytotoxins Incorporating the Epoxy-Triene-Decacarboxylate Moiety Function through the Hijacking of the Plant-Microbe Interaction System.

Host selective toxins (HSTs) are small molecule phytotoxins that control the pathogenicity of microbes in the host plant, but the mechanistic basis for their selectivity is unknown. We developed AcIle-EDA (Aclle-(+)-9,10-epoxy-8-hydroxy-9-methyldeca-trienoic acid) as a molecular probe of an HST, examined its mode of action in genetically modified Oryza sativa, and found it to trigger ROS production through NADPH-oxidase OsRBOHB, causing the emergence of pathogenic traits. This result strongly suggests that AcIle-EDA functions through the hijacking of the plant-microbe interaction system.

Can digital finance reduce carbon emission intensity? A perspective based on factor allocation distortions: evidence from Chinese cities.

The world is facing the challenges of climate change and energy structure adjustments. The role of digital finance, a new branch of business that combines digital technology and traditional financial products, in reducing global carbon emissions needs to be studied. This paper uses panel data on 280 cities in China from 2011 to 2019 to empirically examine the efficacy of digital finance for governing carbon emission reductions and the mechanism by which it does so. The results show that (1) digital finance can facilitate carbon emission reductions and help reduce carbon emission intensity within regions; (2) digital finance helps promote the rational allocation of resources and alleviates factor distortions by encouraging firms to rationally use their own factor endowments so as to reduce carbon emission intensity, which holds robustly after considering the endogenous issues such as possibly omitting variables and collinearity; and (3) differences in geographical location, the vitality of regional innovation and entrepreneurship, regional willingness to protect the environment, and environmental protection levels lead to heterogeneity in the effect of digital finance on carbon emission intensity. Therefore, it is necessary to vigorously develop digital finance as a long-term tool for carbon governance.

In Silico and In Vivo Pharmacokinetic Evaluation of 84-B10, a Novel Drug Candidate against Acute Kidney Injury and Chronic Kidney Disease.

Acute kidney injury (AKI) and chronic kidney disease (CKD) have become public health problems due to high morbidity and mortality. Currently, drugs recommended for patients with AKI or CKD are extremely limited, and candidates based on a new mechanism need to be explored. 84-B10 is a novel 3-phenylglutaric acid derivative that can activate the mitochondrial protease, Lon protease 1 (LONP1), and may protect against cisplatin-induced AKI and unilateral ureteral obstruction- or 5/6 nephrectomy [5/6Nx]-induced CKD model. Preclinical studies have shown that 84-B10 has a good therapeutic effect, low toxicity, and is a good prospect for further development. In the present study, the UHPLC-MS/MS method was first validated then applied to the pharmacokinetic study and tissue distribution of 84-B10 in rats. Physicochemical properties of 84-B10 were then acquired in silico. Based on these physicochemical and integral physiological parameters, a physiological based pharmacokinetic (PBPK) model was developed using the PK-Sim platform. The fitting accuracy was estimated with the obtained experimental data. Subsequently, the validated model was employed to predict the pharmacokinetic profiles in healthy and chronic kidney injury patients to evaluate potential clinical outcomes. Cmax in CKD patients was about 3250 ng/mL after a single dose of 84-B10 (0.41 mg/kg), and Cmax,ss was 1360 ng/mL after multiple doses. This study may serve in clinical dosage setting in the future.

Research on Throughput-Guaranteed MAC Scheduling Policies in Wireless Networks.

In wireless networks, MAC scheduling methods can be divided into two types according to the implementation model: centralized and distributed scheduling. By reasonably designing MAC scheduling policies, both centralized and distributed schedulers can ensure a reliable throughput capacity region, i.e., realizing throughput-guaranteed. However, it can be found that some existing throughput-guaranteed scheduling schemes cannot further ensure bounded end-to-end average delay, and the reason for this phenomenon has not been deeply analyzed. In practical communication networks, throughput and delay are equally important. Based on this idea, the existing MAC scheduling strategies are investigated systematically in this paper from two aspects of throughput and delay, and their performances are evaluated and compared through both theoretical analysis and simulation experiments. The work of this paper provides a theoretical basis for the improvement of MAC scheduling technology in the next-generation wireless networks.

Rapid Screening of Lipase Inhibitors in Scutellaria baicalensis by Using Porcine Pancreatic Lipase Immobilized on Magnetic Core-Shell Metal-Organic Frameworks.

As for ligand fishing, the current immobilization approaches have some potential drawbacks such as the small protein loading capacity and difficult recycle process. The core-shell metal-organic frameworks composite (Fe3O4-COOH@UiO-66-NH2), which exhibited both magnetic characteristics and large specific surface area, was herein fabricated and used as magnetic support for the covalent immobilization of porcine pancreatic lipase (PPL). The resultant composite Fe3O4-COOH@UiO-66-NH2@PPL manifested a high loading capacity (247.8 mg/g) and relative activity recovery (101.5%). In addition, PPL exhibited enhanced tolerance to temperature and pH after immobilization. Then, the composite Fe3O4-COOH@UiO-66-NH2@PPL was incubated with the extract of Scutellaria baicalensis to fish out the ligands. Eight lipase inhibitors were obtained and identified by UPLC-Q-TOF-MS/MS. The feasibility of the method was further confirmed through an in vitro inhibitory assay and molecular docking. The proposed ligand fishing technique based on Fe3O4-COOH@UiO-66-NH2@PPL provided a feasible, selective, and effective platform for discovering enzyme inhibitors from natural products.