Potent Α-glucosidase Inhibitors Research Articles

Novel cinnamic acid magnolol derivatives as potent α-glucosidase and α-amylase inhibitors: Synthesis, in vitro and in silico studies

In this study, twenty novel cinnamic acid magnolol derivatives were synthesized, and screened for their anti-hyperglycemic potential. All synthesized compounds exhibited good to moderate α-glucosidase and α-amylase inhibitory activities with IC50 values: 5.11 ± 1.46–90.26 ± 1.85 µM and 4.27 ± 1.51–49.28 ± 2.54 µM as compared to the standard acarbose (IC50: 255.44 ± 1.89 μM and 80.33 ± 2.95 μM, respectively). Compound 6j showed the strongest inhibitory activity against α-glucosidase (IC50 = 5.11 ± 1.46 µM) and α-amylase (IC50 = 4.27 ± 1.51 µM). Kinetic study indicated that compound 6j was reversible and a mixed type inhibitor against α-glucosidase and α-amylase. In silico studies revealed the binding interaction between 6j and two enzymes, respectively. Finally, cells cytotoxicity assay revealed that compound 6j showed low toxicity against 3 T3-L1 cells and HepG2 cells.

SMILES-based QSAR and molecular docking study of xanthone derivatives as α-glucosidase inhibitors

Increasing diabetic population is one of the major health concerns all over the world. Inhibition of α-glucosidase is a clinically proved and attractive strategy to manage diabetes. In this study, robust and reliable QSAR models to predict α-glucosidase inhibitory potential of xanthone derivatives are developed by the Monte Carlo technique. The chemical structures are represented by SMILES notation without any 3D-optimization. The significance of the index of ideality correlation (IIC) with applicability domain (AD) is also studied in depth. The models developed using CORAL software by considering IIC criteria are found to be statistically more significant and robust than simple balance of correlation. The QSAR models are validated by both internal and external validation methods. The promoters of increase and decrease of activity are also extracted and interpreted in detail. The interpretation of developed models explains the role of different structural attributes in predicting the pIC50 of xanthone derivatives as α-glucosidase inhibitors. Based on the results of model interpretation, modifications are done on some xanthone derivatives and 15 new molecules are designed. The α-glucosidase inhibitory activity of novel molecules is further supported by docking studies.

Design, synthesis, biological evaluations and in silico studies of sulfonate ester derivatives of 2-(2-benzylidenehydrazono)thiazolidin-4-one as potential α-glucosidase inhibitors

A novel series of hydrazolyl linked sulfonate ester analogues of 4-thiazolidinone nucleus has been rationally designed, synthesized and characterized by various spectroscopic techniques including 1H NMR, 13C NMR and mass spectrometry. All of the synthesized derivatives were tested for in vitro α-glucosidase inhibitory activities and antioxidant potential. The investigated compounds displayed appreciable α-glucosidase inhibition with IC50 values ranging from 42.80 ± 0.48 to 599.04 ± 1.26 μM in comparison to acarbose (478.07 ± 1.53 μM) and structure-activity relationship was established. Further, the safety profile of the most potent derivative (7d) was assessed by MTT assay in normal HEK cells. In vivo disaccharide loading test endorsed the higher efficacy of (7d) over acarbose (at a dose of 20 mg/kg of body weight) for reduction of postprandial hyperglycemia after sucrose administration. In silico procedures i.e. homology modeling, molecular docking, molecular dynamic simulations, binding free energy calculations and ADME predictive studies further justified the results of in vitro and in vivo biological investigations.

Discovery of Amide-Functionalized Benzimidazolium Salts as Potent α-Glucosidase Inhibitors.

α-Glucosidase inhibitors (AGIs) are used as medicines for the treatment of diabetes mellitus. The α-Glucosidase enzyme is present in the small intestine and is responsible for the breakdown of carbohydrates into sugars. The process results in an increase in blood sugar levels. AGIs slow down the digestion of carbohydrates that is helpful in controlling the sugar levels in the blood after meals. Among heterocyclic compounds, benzimidazole moiety is recognized as a potent bioactive scaffold for its wide range of biologically active derivatives. The aim of this study is to explore the α-glucosidase inhibition ability of benzimidazolium salts. In this study, two novel series of benzimidazolium salts, i.e., 1-benzyl-3-{2-(substituted) amino-2-oxoethyl}-1H-benzo[d]imidazol-3-ium bromide 9a–m and 1-benzyl-3-{2-substituted) amino-2-oxoethyl}-2-methyl-1H-benzo[d] imidazol-3-ium bromide 10a–m were screened for their in vitro α-glucosidase inhibitory potential. These compounds were synthesized through a multistep procedure and were characterized by 1H-NMR, 13C-NMR, and EI-MS techniques. Compound 10d was identified as the potent α-glucosidase inhibitor among the series with an IC50 value of 14 ± 0.013 μM, which is 4-fold higher than the standard drug, acarbose. In addition, compounds 10a, 10e, 10h, 10g, 10k, 10l, and 10m also exhibited pronounced potential for α-glucosidase inhibition with IC50 value ranging from 15 ± 0.037 to 32.27 ± 0.050 µM when compared with the reference drug acarbose (IC50 = 58.8 ± 0.12 μM). A molecular docking study was performed to rationalize the binding interactions of potent inhibitors with the active site of the α-glucosidase enzyme.

Design, Synthesis and Bioactivity Investigation of Novel 2,3-Diarylthiazolidine-4-Ones as Potent α-Glucosidase Inhibitors

Herein, we have designed and synthesized sixteen novel 2,3-diarylthiazolidin-4-one derivatives 6a-p and tested their activity as α-Glucosidase inhibitors. Target compounds 6a-p were characterized using spectroscopic methods (1H-NMR, 13C-NMR, MS, IR), elemental analysis, and melting point. α-Glucosidase inhibition activity was evaluated using the α-Glucosidase enzyme inhibition kit. All 6a-p showed higher α-Glucosidase inhibition activity (90 to 704 µM) in comparison to acarbose as a standard (IC50: 750 µM). 6p, 6m, and 6f exerted the best activity with the IC50 value of 90, 100, and 149 µM respectively. Enzyme kinetic studies showed a competitive mode of inhibition for the most active compound, 6p; molecular docking study revealed the mode of interactions between the most active compounds and enzyme active site. To evaluate the cytotoxicity profile of the synthesized compounds, an MTT assay was done on three different cell lines which showed all 6a-p are safe and nontoxic with IC50 values higher than 750 µM.

Inhibitory Activity and Mechanism Investigation of Hypericin as a Novel α-Glucosidase Inhibitor.

α-glucosidase is a major enzyme that is involved in starch digestion and type 2 diabetes mellitus. In this study, the inhibition of hypericin by α-glucosidase and its mechanism were firstly investigated using enzyme kinetics analysis, real-time interaction analysis between hypericin and α-glucosidase by surface plasmon resonance (SPR), and molecular docking simulation. The results showed that hypericin was a high potential reversible and competitive α-glucosidase inhibitor, with a maximum half inhibitory concentration (IC50) of 4.66 ± 0.27 mg/L. The binding affinities of hypericin with α-glucosidase were assessed using an SPR detection system, which indicated that these were strong and fast, with balances dissociation constant (KD) values of 6.56 × 10−5 M and exhibited a slow dissociation reaction. Analysis by molecular docking further revealed that hydrophobic forces are generated by interactions between hypericin and amino acid residues Arg-315 and Tyr-316. In addition, hydrogen bonding occurred between hypericin and α-glucosidase amino acid residues Lys-156, Ser-157, Gly-160, Ser-240, His-280, Asp-242, and Asp-307. The structure and micro-environment of α-glucosidase enzymes were altered, which led to a decrease in α-glucosidase activity. This research identified that hypericin, an anthracene ketone compound, could be a novel α-glucosidase inhibitor and further applied to the development of potential anti-diabetic drugs.

Synthesis, crystal structure, DFT, α-glucosidase and α-amylase inhibition and molecular docking studies of (E)-N'-(4-chlorobenzylidene)-5-phenyl-1H-pyrazole-3-carbohydrazide

In this work, a novel crystal i.e. (E)-N'-(4-chlorobenzylidene)-5-phenyl-1H-pyrazole-3-carbohydrazide has been synthesized and characterized using various spectroscopic techniques. The (E)-configuration of the azomethine (NCH) was confirmed by single crystal X-ray analysis. The molecule crystallizes in the monoclinic space group, P21/c, a = 15.629(9) Å, b = 7.152(4) Å, c = 14.707(9) Å, β = 111.061(15)°, V = 1534.1(6) Å3 and Z = 4. In addition, the elucidated molecular structure was confirmed by comparing the predicted Z-matrix geometries and spectroscopic data with the experimental ones. DFT calculations have been carried out in gas and IEFPCM solvent at the B3LYP/6–31+G(d,p). The in vitro anti-diabetic potential of the title compound was evaluated against α-glucosidase and α-amylase enzymes. Molecular docking studies showed that the various interactions tightly anchored the title compound to the active site, which makes it a more potent α-glucosidase inhibitor compared to well-known Acarbose.

Pressurized Hot Liquid Extraction with 15% v/v Glycerol-Water as An Effective Environment-Friendly Process to Obtain Durvillaea incurvata and Lessonia spicata Phlorotannin Extracts with Antioxidant and Antihyperglycemic Potential.

Brown seaweed phlorotannins have shown the potential to promote several health benefits. Durvillaea incurvata and Lessonia spicata—species that are widely distributed in central and southern Chile—were investigated to obtain phlorotannin extracts with antioxidant and antihyperglycemic potential. The use of an environmentally friendly and food-grade glycerol-based pressurized hot liquid extraction (PHLE) process (15% v/v glycerol water) was assessed for the first time to obtain phlorotannins. Multiple effects were analyzed, including the effect of the species, harvesting area (Las Cruces and Niebla), and anatomical part (holdfast, stipe, and frond) on the extracts’ polyphenol content (TPC), antioxidant capacity (AC), and carbohydrate-hydrolyzing enzyme—α-glucosidase and α-amylase—inhibitory activity. Contaminants, such as mannitol, heavy metals (As, Cd, Pb, Hg, and Sn), and 5-hydroxymethylfurfural (HMF), were also determined. The anatomical part used demonstrated a significant impact on the extracts’ TPC and AC, with holdfasts showing the highest values (TPC: 95 ± 24 mg phloroglucinol equivalents/g dry extract; DPPH: 400 ± 140 μmol Trolox equivalents/g dry extract; ORAC: 560 ± 130 μmol TE/g dry extract). Accordingly, holdfast extracts presented the most potent α-glucosidase inhibition, with D. incurvata from Niebla showing an activity equivalent to fifteen times that of acarbose. Only one frond and stipe extract showed significant α-glucosidase inhibitory capacity. No α-amylase inhibition was found in any extract. Although no HMF was detected, potentially hazardous cadmium levels (over the French limit) and substantial mannitol concentrations—reaching up to 50% of the extract dry weight—were found in most seaweed samples and extracts. Therefore, further purification steps are suggested if food or pharmaceutical applications are intended for the seaweed PHLE extracts obtained in this study.

Synthesis, Biological Evaluation of ortho-Carboxamidostilbenes as Potential Inhibitors of Hyperglycemic Enzymes, and Molecular Docking Study

A new series of ortho-carboxamidostilbenes derivatives were synthesized via Heck Coupling and screened for their α-amylase and α-glucosidase inhibitory potential. The results indicated that all the synthesized compounds showed a substantial α-glucosidase inhibitory potential (IC50 between 3-78 µg/mL) compared to acarbose (IC50 = 16.14 ± 1.05 µg/mL) as the standard. In addition, the IC50 values against α-amylase varying from 3-300 µg/mL when compared to a standard drug acarbose (IC50 = 21.12 ± 0.29 µg/mL). In silico studies were carried out to understand the binding interaction between active compounds and enzymes. The results indicated that the binding energy displayed by compound 5a-5e ranging from -7.9 to -9.0 and -7.2 to -8.5 kcal/mol for the C-terminal subunit of human maltase glucoamylase, ctMGAM (α-glucosidase) and α-amylase, respectively. The interaction modes of 5d (IC50 = 2.90 ± 0.58 µg/mL) in ctMGAM, showed that alkyl and methoxy group interacted with TRP1369 via hydrogen bond and hydrophobic interaction, respectively. Meanwhile, the interaction modes of 5e with (IC50 = 2.94 ± 0.69 µg/mL) in α-amylase showed that the methoxy group interacted with TYR151 via conventional hydrogen bond. These compounds may be considered promising candidates for the development of new anti-diabetic agents.

Bioassay-guided isolation and characterization of potential α-glucosidase inhibitors from the seeds of Quassia indica Gaetrn.

Bioassay-guided isolation and characterization of potential α-glucosidase inhibitors from the seeds of Quassia indica Gaetrn.

Design and synthesis of novel quinazolinone-pyrazole derivatives as potential α-glucosidase inhibitors: Structure-activity relationship, molecular modeling and kinetic study

In this study, a new series of quinazolinone-pyrazole hybrids were designed, synthesized and screened for their α-glucosidase inhibitory activity. The results of the in vitro screening indicated that all the molecular hybrids exhibited more inhibitory activity (IC50 values ranging from 60.5 ± 0.3 µM-186.6 ± 20 μM) in comparison to standard acarbose (IC50 = 750.0 ± 10.0 µM). Limited structure–activity relationship suggested that the variation in the inhibitory activities of the compounds affected by different substitutions on phenyl rings of diphenyl pyrazole moiety. The enzyme kinetic studies of the most potent compound 9i revealed that it inhibited α-glucosidase in a competitive mode with a Ki of 56 μM. Molecular docking study was performed to predict the putative binding interaction. As expected, all pharmacophoric moieties used in the initial structure design playing a pivotal role in the interaction with the binding site of the enzyme. In addition, by performing molecular dynamic investigation and MM-GBSA calculation, we investigated the difference in structural perturbation and dynamic behavior that is observed over α-glycosidase in complex with the most active compound and acarbose relative to unbound α-glycosidase enzyme.

Angiotensin I-converting enzyme, dipeptidyl peptidase-IV, and α-glucosidase inhibitory potential of hazelnut meal protein hydrolysates

Angiotensin I-converting enzyme, dipeptidyl peptidase-IV, and α-glucosidase inhibitory potential of hazelnut meal protein hydrolysates

Evaluation of chemical constituents and biological properties of two endemic Verbascum species

Members from the Verbascum genus are well appraised for their traditional uses against a variety of ailments including anti-inflammatory related disorders. This study thus attempts at validating the biological properties of two underexplored Verbascum members namely Verbascum euphraticum Benth. (V. euphraticum) and Verbascum oocarpum Murb. (V. oocarpum) in terms of their antioxidant and enzymatic inhibitory activities. Concentration of specific bioactive compounds was also established and molecular dynamic analysis were conducted on amassed data. Results showed that total phenolic and total flavonoid contents were higher for the methanolic extract of V. oocarpum compared to the methanolic extract of V. euphraticum. Phytochemical quantification revealed that the extraction solvents influenced the phytochemical composition of the extracts. Derivatives of phenolic acids such as 3-caffeoylquinic acid and 3,5-dicaffeoylquinic acid together with some phenolic acids i.e., gallic, vanillic, caffeic, syringic, p-coumaric and ferulic acid, have been detected in the extracts of both Verbascum species. Methanolic extracts of V. euphraticum and V. oocarpum yielded the highest total content of monitored compounds resulting in 162.758 and 3716.861 μg/g of dried weight extract, respectively. The methanolic extracts of both plants showed highest antioxidant potential while the ethyl acetate and hexane extracts were potent inhibitors of cholinesterase, α-amylase and α-glucosidase. Molecular dynamic results indicated that vanillic acid is able to make strong interactions with copper ions of tyrosinase, resulting into an effective anti-tyrosinase activity. In light of the above, both Verbascum species could be considered for future phytopharmaceutical development.

A facile approach synthesis of benzoylaryl benzimidazole as potential α-amylase and α-glucosidase inhibitor with antioxidant activity

Synthetic routes to a series of benzoylarylbenzimidazol 3a-h have been derived from 3,4-diaminobenzophenone and an appropriate arylaldehyde in the presence of ammonium chloride or a mixture of ammonium chloride and sodium metabisulfite as catalyst. The antioxidant activity of targeted compounds 3a-h has been measured by four different methods and the overall antioxidant evaluation of the compounds indicated the significant MCA, FRAP, and (DPPH-SA) of the compounds except for the compound 3h. In vitro antidiabetic assay of α-amylase and α-glucosidase suggest a good to excellent activity for most tested compounds. The target benzimidazole 3f containing hydroxyl motif at para-position of phenyl revealed an important activity inhibitor against α- amylase (IC50 = 12.09 ± 0.38 µM) and α-glucosidase (IC50 = 11.02 ± 0.04 µM) comparable to the reference drug acarbose. The results of the anti hyperglycemic activity were supported by means of in silico molecular docking calculations showing strong binding affinity of compounds 3a-h with human pancreatic α-amylase (HPA) and human lysosomal acid-α-glucosidase (HLAG) active sites that confirm a good to excellent activity for most of tested compounds.

Biological evaluation the 2-aryl-2,3-dihydrobenzodiazaborinin-4(1H)-ones as potential dual α-glucosidase and α-amylase inhibitors with antioxidant properties.

The 2-aryl-2,3-dihydrobenzodiazaborinin-4(1H)-ones (azaborininone) were synthesized as analogues of the 2-arylquinazoline-4-ones and screened through enzymatic assay in vitro for inhibitory effect against α-glucosidase and α-amylase activities. These azaborininones exhibited moderate to good inhibitory effect against these enzymes compared to acarbose used as a reference standard. The results are supported by the enzyme-ligand interactions through kinetics (in vitro) and molecular docking (in silico) studies. The test compounds also exhibited significant antioxidant activity through the 2,2-diphenyl-1-picrylhydrazyl (DPPH) and nitric oxide (NO) free radical scavenging assays. These azaborininone derivatives exhibited no effect on the viability of the human lung cancer (A549) cell line after 24hr and were also not toxic towards the Vero cells.

Synthesis, in vitro, and in silico evaluation of Indazole Schiff bases as potential α-glucosidase inhibitors

Indazole Schiff bases were synthesized 1–24 and structurally characterized by different spectroscopic techniques such as EI-MS, HREI-MS, 1H- and 13C-NMR. Stereochemistry of azomethine moiety in synthesized compounds was confirmed by 2D-NOESY. Among the twenty-four compounds, fourteen compounds 1–5, 7, 9–14, 17, and 20 are structurally new. Compounds 1–24 were screened for in vitro α-glucosidase enzyme inhibitory activity. All compounds were In vitro α-glucosidase inhibitory assay results identified a number of molecules including 1, 2, 4, 7, 9, 10, 12, 13, 18, 19, 21, and 23 as potent α-glucosidase inhibitors with IC50 values 9.4 ± 0.1 to 303.7 ± 0.1 μM as compared to the standard acarbose (IC50 = 750 ± 10 µM). Compound 1 (IC50 = 9.43 ± 0.1 µM) was found to be the most potent molecule of this library. Kinetic studies on most active compound 1 suggested the competitive inhibition mechanism. In silico studies indicated the interaction details between analogs (ligands) and active site of α-glucosidase enzyme.

The In Vitro α-Glucosidase Inhibition Activity of Various Solvent Fractions of Tamarix dioica and 1H-NMR Based Metabolite Identification and Molecular Docking Analysis.

The Tamarix dioica (T. dioica) is widely used medicinal plant to cure many chronic ailments. T. dioica is being used to manage diabetes mellitus in traditional medicinal system; however, very little scientific evidence is available on this plant in this context. The current study involves the fractionation of crude methanolic extract of T. dioica using n-hexane, ethyl acetate, chloroform, and n-butanol. The screening for antioxidant activity using 2,2-diphenyl-1-picrylhydrazyl (DPPH) assay was carried out. The in vitro antidiabetic potential was assessed by measuring α-glucosidase inhibition. Total phenolic and flavonoid contents were also determined for each fraction. The metabolites were identified using highly sensitive and emerging 1H-NMR technique. The results revealed the ethyl acetate fraction as the most potent with DPPH scavenging activity of 84.44 ± 0.21% and α-glucosidase inhibition with IC50 value of 122.81 ± 2.05 µg/mL. The total phenolic and flavonoid content values of 205.45 ± 1.36 mg gallic acid equivalent per gram dried extract and 156.85 ± 1.33 mg quercetin equivalent per gram dried extract were obtained for ethyl acetate fraction. The bucketing of 1H-NMR spectra identified 22 metabolites including some pharmacologically important like tamarixetin, tamaridone, quercetin, rutin, apigenin, catechin, kaempferol, myricetin and isorhamnetin. Leucine, lysine, glutamic acid, aspartic acid, serine, and tyrosine were the major amino acids identified in ethyl acetate fraction. The molecular docking analysis provided significant information on the binding affinity among secondary metabolites and α-glucosidase. These metabolites were most probably responsible for the antioxidant activity and α-glucosidase inhibitory potential of ethyl acetate fraction. The study ascertained the ethnomedicinal use of T. dioica to manage diabetes mellitus and may be a helpful lead towards naturopathic mode for anti-hyperglycemia.

New 4,5-diphenylimidazole-acetamide-1,2,3-triazole hybrids as potent α-glucosidase inhibitors: synthesis, in vitro and in silico enzymatic and toxicity evaluations

New 4,5-diphenylimidazole-acetamide-1,2,3-triazole hybrids as potent α-glucosidase inhibitors: synthesis, in vitro and in silico enzymatic and toxicity evaluations

Trans-Tiliroside: A potent α-glucosidase inhibitor from the leaves of Elaeagnus angustifolia L

Elaeagnus angustifolia L. (Elaeagnaceae) is an important medicinal plant associated with numerous pharmacological activities. Its leaves are used as a therapeutic agent in traditional medicinal systems to treat diabetes. However, the active compounds responsible for the beneficial effects of E. angustifolia remain unclear. In this study, we determined the bioactive profile of E. angustifolia leaves using open column chromatography and semi-preparative HPLC. Further, we sought to determine its α-glucosidase and α-amylase inhibitory activities, and its DPPH and ABTS radical-scavenging activities. Four undescribed flavonol glycosides, igdoside A–D, and four known glucosides were isolated from the ethyl acetate and n-butanol extracts of E. angustifolia leaves. Thereafter, the compound structures were identified using spectroscopic methods, including NMR and mass spectrometry. Of the compounds extracted, kaempferol-3-O-(6″-trans-p-coumaroyl)-β-D-glucopyranoside (trans-tiliroside), exhibited the highest α-glucosidase inhibitory activity with an IC50 value of 2128 ± 63 μM compared to the positive control, acarbose (IC50 = 6561 ± 207 μM). trans-Tiliroside was also found to exhibit potent scavenging activity against the ABTS radical, with an IC50 value of 5 ± 0 μM, compared to the positive controls, trolox (31 ± 1 μM) and α-tocopherol (50 ± 1 μM). In addition, isorhamnetin-3-O-β-D-galactopyranoside (IC50 = 6 ± 0 μM) and astragalin (IC50 = 6 ± 0 μM) showed similar ABTS radical-scavenging activity as trans-tiliroside. Based on HPLC, the content of trans-tiliroside was 9.69% in the ethyl acetate extract, 1.04% in decoction, 0.34% in 70% methanol extract, and 0.23% in infusion. None of the extracts and compounds showed α-amylase inhibition or DPPH-scavenging activity.

Rational Design of Novel Inhibitors of α-Glucosidase: An Application of Quantitative Structure Activity Relationship and Structure-Based Virtual Screening.

α-Glucosidase is considered a prime drug target for Diabetes Mellitus and its inhibitors are used to delay carbohydrate digestion for the treatment of diabetes mellitus. With the aim to design α-glucosidase inhibitors with novel chemical scaffolds, three folds ligand and structure based virtual screening was applied. Initially linear quantitative structure activity relationship (QSAR) model was developed by a molecular operating environment (MOE) using a training set of thirty-two known inhibitors, which showed good correlation coefficient (r2 = 0.88), low root mean square error (RMSE = 0.23), and cross-validated correlation coefficient r2 (q2 = 0.71 and RMSE = 0.31). The model was validated by predicting the biological activities of the test set which depicted r2 value of 0.82, indicating the robustness of the model. For virtual screening, compounds were retrieved from zinc is not commercial (ZINC) database and screened by molecular docking. The best docked compounds were chosen to assess their pharmacokinetic behavior. Later, the α-glucosidase inhibitory potential of the selected compounds was predicted by their mode of binding interactions. The predicted pharmacokinetic profile, docking scores and protein-ligand interactions revealed that eight compounds preferentially target the catalytic site of α-glucosidase thus exhibit potential α-glucosidase inhibition in silico. The α-glucosidase inhibitory activities of those Hits were predicted by QSAR model, which reflect good inhibitory activities of these compounds. These results serve as a guidelines for the rational drug design and development of potential novel anti-diabetic agents.