Kinetics, Equilibrium, and Thermodynamics for Conjugation of Chitosan with Insulin-Mimetic [meso-Tetrakis(4-sulfonatophenyl)porphyrinato]oxovanadate(IV)(4-) in an Aqueous Solution

This study investigated the conjugation of chitosan with the insulin-mimetic [meso-tetrakis(4-sulfonatophenyl)porphyrinato]oxovanadate(IV)(4-), VO(tpps), in an aqueous medium as a function of conjugation time, VO(tpps) concentrations, and temperatures. To validate the synthesis of chitosan-VO(tpps) conjugate, UV-visible and Fourier transform infrared spectrophotometric techniques were utilized. Conjugate formation is ascribed to the electrostatic interaction between the NH3+ units of chitosan and the SO3- units of VO(tpps). Chitosan enhances the stability of VO(tpps) in an aqueous medium (pH 2.5). VO(tpps) conjugation with chitosan was best explained by pseudo-second-order kinetic and Langmuir isotherm models based on kinetic and isotherm studies. The Langmuir equation determined that the maximal ability of VO(tpps) conjugated with each gram of chitosan was 39.22 μmol at a solution temperature of 45 °C. Activation energy and thermodynamic studies (Ea: 8.78 kJ/mol, ΔG: -24.52 to -27.55 kJ/mol, ΔS: 204.22 J/(mol K), and ΔH: 37.30 kJ/mol) reveal that conjugation is endothermic and physical in nature. The discharge of VO(tpps) from conjugate was analyzed in freshly prepared 0.1 mol/L phosphate buffer (pH 7.4) at 37 °C. The release of VO(tpps) from the conjugate is a two-phase process best explained by the Higuchi model, according to a kinetic analysis of the release data. Taking into consideration all experimental findings, it is proposed that chitosan can be used to formulate both solid and liquid insulin-mimetic chitosan-VO(tpps) conjugates.

Oxovanadium(IV) Complexes with Nitrogen Donors: Synthesis, Characterisation, and Biological Activities

Some oxovanadium(IV) complexes of SA/SSA and 5, 10, 15, and 20-meso-tetraphenylporphyrin (H2tpp) with unidentate and bidentate nitrogen donors have been synthesized and characterized by elemental analysis, conductivity measurements, magnetic susceptibility, UV-Vis, IR, mass spectroscopy, TGA/DTA, and 1H, 13C and 51V NMR studies to investigate the steric and electronic effects of axial ligands on the properties of porphyrins. On the basis of these studies, it has been investigated that the axial ligands bind to the sixth coordination site of the vanadium ion to form a relatively stable six-coordinate-porphyrin complex where as in the case of SA/SSA complexes the nitrogen donors bind to the equatorial position giving square pyramidal geometry. The in vitro cytotoxicity against human cancer cell lines and antimicrobial activities of the synthesized compounds have been done against various fungal and bacterial pathogens. The [VO(SA/SSA)L/L-L] complexes were found to possess higher antibacterial, antifungal activity and in vitro cytotoxicity against human cancer cell lines than VO(tpp)L complexes.

The discovery of vanadyl and zinc complexes for treating diabetes and metabolic syndromes

The incidence of diabetes mellitus has increased over the decades because of lifestyle changes. The number of people with diabetes mellitus worldwide is expected to increase from 150 million to 220 million by 2010 and to 300 million by 2025. There are two main types of diabetes mellitus. Type 1 diabetes mellitus is due to the autoimmune-mediated destruction of pancreatic β cells, resulting in absolute insulin deficiency; the patients require exogenous insulin injections. Type 2 is characterized by insulin resistance and abnormal insulin secretion and the patients require exercise, diet control and/or oral hypoglycemics. However, each treatment has some adverse effects, including physical burden, formation of self-antibodies for insulin injections, the severe side effects of hypoglycemics and the discontinuation of insulin synthesis in the pancreas. To overcome these adverse effects and replace the use of these agents, the author attempted to develop new antidiabetic agents with novel structures and mechanisms. This review focuses on the authors' recent development of vanadium and zinc complexes for antidiabetic and antimetabolic syndromes.

Improvement of hyperglycaemia and metabolic syndromes in type 2 diabetic KKAy mice by oral treatment with [meso-tetrakis(4-sulfonatophenyl) porphyrinato]oxovanadium(IV)(4-) complex

Recently, we reported that [meso-tetrakis(4-sulfonatophenyl)porphyrinato]oxovanadium(IV)(4-), VO(tpps), shows in-vitro insulin-mimetic and in-vivo anti-diabetic activity in streptozotocin (STZ)-induced type 1 diabetic mice. This result prompted us to examine its ability in type 2 diabetic model KKAy mice with insulin resistance. We studied the in-vivo anti-diabetic activity of VO(tpps), compared with that of vanadium(IV) oxide sulfate, VS, as control. Both compounds were orally administered at doses of 5–10 mg (0.1-0.2 mmol) V/kg body weight to the KKAy mice for 28 days. VO(tpps) normalized the hyperglycaemia within 15 days, while VS lowered the blood glucose concentration only by a small degree. In addition, metabolic syndromes characterized by insulin and leptin resistance were significantly improved in VO(tpps)-treated KKAy mice compared with those treated with VS. The improvement in diabetes was validated by oral glucose tolerance test and decrease in HbA1c concentration. Based on these observations, VO(tpps) is proposed to be an orally active oxovanadium(IV)-porphyrin complex for treating not only type 2 diabetes but also metabolic syndromes in animals.

Evaluation of insulin-mimetic activities of vanadyl and zinc(II) complexes from the viewpoint of heterocyclic bidentate ligands

Vanadyl sulfate (VOSO(4)) has been clinically tested in diabetic patients since 1995. Oral administrations of VOSO(4) improved the type 2 diabetic state with respect to plasma glucose, HbA(1c), and fructosamine levels. The development of toxicity by increasing the administration of VOSO(4) should be avoided. One method was the utilization of vanadyl complexes with coordination compounds that are low-toxic and low-molecular-weight ligands to enhance the permeation of the metal ion to lipid bilayer membrane. Over a decade we have focused on a variety of heterocyclic compounds as bidentate ligands for metal ions. Vanadyl and zinc(II) complexes of 1-substituted 3-hydroxy-2-methyl-4(1H)-pyridinethiones, 4,5,6-substituted 1-hydroxy-2(1H)-pyrimidinones, 4-(p-substituted)phenyl-3-hydroxythiazole-2(3H)-thiones, 3-hydroxypyrone, 1-alkyl- or 1-phenylalkyl-3-hydroxy-2(1H)-pyridinethiones, optically active 1-substituted 3-hydroxy-4(1H)-pyridinethiones, and 5-dialkylsulfonamido- or 5,7-bis(dialkylsulfonamido)-8-hydroxyquinolines were prepared, and their insulin-mimetic activities were evaluated in terms of IC(50) values which stand for a 50% inhibitory concentration of the free fatty acid release from isolated rat adipocytes. In this article, the relationship between the insulin-mimetic activity and the partition coefficient, the chirality, the substituent effect, molecular weight, the pK(a) value, and the coordination mode was discussed. In vivo blood glucose-lowering effects of the vanadyl complex with 1-hydroxy-4,6-dimethyl-2(1H)-pyrimidinone in streptozotocin (STZ)-induced diabetic rats and the zinc(II) complexes with 4-(p-chlorophenyl)thiazole- and 4-methylthiazole-2(3H)-thione in KK-A(y) mice were also discussed.

A [meso-tetrakis(4-sulfonatophenyl)porphyrinato]zinc(ii) complex as an oral therapeutic for the treatment of type 2 diabetic KKA(y) mice

We prepared and characterized [meso-tetrakis(4-sulfonatophenyl)porphyrinato]zinc(II) ([Zn(tpps)]), and investigated its in vitro insulin-mimetic activity and in vivo hypoglycemic effect in type 2 diabetic KKA(y) mice. The results were compared with those of previously proposed insulin-mimetic zinc(II) complexes and zinc sulfate (ZnSO(4)). The in vitro insulin-mimetic activity of [Zn(tpps)] was considerably better than that of bis(allixinato)zinc(II) ([Zn(alx)(2)]), bis(maltolato)zinc(II) ([Zn(mal)(2)]), bis(2-aminomethylpyridinato)zinc(II) ([Zn(2-ampy)(2)](2+)), and ZnSO(4). In particular, the order of in vitro insulin-mimetic activity of the complexes was determined to be: [Zn(tpps)]>[Zn(alx)(2)]>[Zn(mal)(2)]>[Zn(2-ampy)](2+)>ZnSO(4). [Zn(tpps)] normalized the hyperglycemia of KKA(y) mice within 21 days when administered orally at doses of 10-20 mg (0.15-0.31 mmol) Zn per kg body mass for 28 days. In addition, metabolic syndromes such as insulin resistance, the degree of renal disturbance, and the degree of liver disturbance were significantly improved in [Zn(tpps)]-treated KKA(y) mice relative to those administered with saline and ZnSO(4). The improvement in diabetes was validated by the results of oral glucose-tolerance tests and the decrease in the HbA(1c) level observed. In contrast, ZnSO(4) and the ligand H(2)tpps did not lower the elevated blood glucose level under the same experimental conditions. Based on these observations, [Zn(tpps)] is proposed to be the first orally active zinc(II)-porphyrin complex for the efficacious treatment of not only type 2 diabetes but also metabolic syndromes in animals.

Metallokinetic characteristics of antidiabetic bis(allixinato)oxovanadium(IV)-related complexes in the blood of rat

The antidiabetic effect of vanadium is a widely accepted phenomenon; some oxovanadium(IV) complexes have been found to normalize high blood glucose levels in both type 1 and type 2 diabetic animals. In light of the future clinical use of these complexes, the relationship among their chemical structures, physicochemical properties, metallokinetics, and antidiabetic activities must be closely investigated. Recently, we found that among bis(3-hydroxypyronato)oxovanadium(IV) [VO(3hp)(2)] related complexes, bis(allixinato)oxovanadium(IV) [VO(alx)(2)] exhibits a relatively strong hypoglycemic effect in diabetic animals. Next, we examined its metallokinetics in the blood of rats that received five VO(3hp)(2)-related complexes by the blood circulation monitoring-electron paramagnetic resonance method. The metallokinetic parameters were obtained from the blood clearance curves based on a two-compartment model; most parameters, such as area under the concentration curve and mean residence time, correlated significantly with the in vitro insulinomimetic activity in terms of 1/IC(50) (IC(50) is the 50% inhibitory concentration of the complex required for the release of free fatty acids in adipocytes) and the lipophilicity of the complex (log P (com)). The oxovanadium(IV) concentration was significantly higher and the species resided longer in the blood of rats that received VO(alx)(2) than in the blood of rats that received VO(3hp)(2) or bis(kojato)oxovanadium(IV); VO(alx)(2) also exhibited higher log P (com) and 1/IC(50) values. On the basis of these results, we propose that the introduction of lipophilic groups at the C2 and C6 positions of the 3hp ligand is an effective method to enhance the hypoglycemic effect of the complexes, as supported by the observed in vivo exposure and residence in the blood.

Antidiabetic Activity of the Orally Effective Vanadyl-Poly (γ-Glutamic Acid) Complex in Streptozotocin(STZ)-induced Type 1 Diabetic Mice

Newly synthesized vanadyl-poly(γ-glutamic acid) complex (VO-γ-PGA) with a VO(O4) coordination mode was found to have potent antidiabetic activity in streptozotocin (STZ)-induced type 1 diabetic mice (STZ-mice), compared with that of a solution containing only vanadyl sulfate, VOSO 4. This was the first example of orally active vanadyl complex of γ-PGA for treating STZ-mice. To better define its efficacy, we examined here the effects of VO-γ-PGA treatment in STZ-mice by oral administration at the dose of 10 mg V/kg body mass for a longer period time than our previous study. The improvement in diabetic states in STZ-mice compared with saline-treated nondiabetic normal Std ddY mice. It was found that the elevated blood glucose levels in STZ-mice significantly decreased after 3 days and sustained the normalized blood glucose level around 180—200 mg/dL (10—11.1 mM) for the last 14 days, which is close to the blood glucose levels 100—200 mg/dL (5.6—11.1 mM) in nondiabetic normal Std ddY mice. The improvement in diabetes was strongly corelated by the improvement in oral glucose tolerance ability, glycosylated hemoglobin (HbA1c) levels and blood pressure, and serum parameters. The present results confirmed that VO-γ-PGA complex is a promising, orally active insulin-mimetic agent to treat type 1 diabetic mice.