Development of Drug Candidates for Diabetic Complications

Aldose reductase (AR) is an enzyme that catalyzes reduction of glucose into sorbitol in the polyol pathway in many tissues, and believed to be strongly concerned with diabetic complications. Aldose reductase inhibitors (ARIs), therefore, have attracted much attention of medicinal chemists as therapeutics for the diabetic complications.

Recently, we have synthesized botryllazine B (1, isolated from the red ascidian Botryllus leachi) and its analogues of diverse substitution patterns have been prepared, and their in vitro inhibitory activities against recombinant human aldose reductase (h-ALR2) evaluated.  Among tested compounds (15 analogues), the amino analogue (2) proved to be the most potent inhibitor, with IC50 = 0.91 uM.  This efficacy is higher than that of sorbinil, known as a highly potent ARI (IC50 = 0.6–9.1 uM against various ALR2).  Kinetic analyses of 2 and botryllazine B revealed that these inhibitors exhibit an unprecedented mixed-type inhibition on h-ALR2 with respect to the substrate D,L-glyceraldehyde, in the presence of NADPH at inhibitor concentrations near the IC50 values.

We have also developed pterin-based aldose reductase inhibitors that were readily prepared by condensation of methyl pterin-7-carboxylate and various free amino acids. ,Their in vitro inhibitory activity tests against h-ALR2 revealed that the derivative conjugated with glycine was found to be a good ARI (IC50 = 1.97 uM).

More recently, we have synthesized a number of (Z)-4-arylmethylidene-1H-imidazol-5(4H)-ones, which are related to the fluorescent chromophore of the Aequorea green fluorescent protein (GFP), have been synthesized and evaluated their in vitro inhibitory activity against h-ALR2 for the first time. Among them, the derivative having 2-naphtylmethylidene exhibited the best inhibitory effect with an IC50 value of 0.10 uM. Structure-activity relationship studies combined with docking simulations revealed the interaction mode of the newly synthesized inhibitors toward the target protein as well as the structural features required to gain a high inhibitory activity. Thus, the GFP chromophore model compounds synthesized in this study have proved to be potential drugs for diabetic complications.

Design and Synthesis of Biologically Active Hydroxyazine-type Heterocyclic Compounds

Zinc(II) ion has attracted increasing attention of medicinal chemists because of its insulin-mimetic activity.  Recently, many organozinc complexes with heterocyclic compounds as ligands have been synthesized to exhibit higher insulin-mimetic activities than the inorganic ZnSO4. The role of the heterocyclic ligands is enhancement of the gastrointestinal absorptivity of zinc(II) ion, and, therefore, the ligands become unnecessary after the absorption. 

1-(Arylmethyl)-2,5-dihydro-4-hydroxy-5-oxo-1H-pyrrol-3-carboxylates (5) are known as potential drugs for diabetic complications. Although they have metal-chelatable oxygen atoms, no attempt to prepare zinc(II) complexes with 5 (6) have been performed. Thus, we have synthesized the zinc(II) complexes (6) and evaluated their insulin-mimetic activity in order to develop new chemotherapeutics having dual effectives against diabetes mellitus and diabetic complications. It was found that the zinc(II) complexes (6) showed higher insulin-mimetic activities (IC50 = 0.26-0.82 mM) than ZnSO4 (IC50 = 0.7-1.0 mM).

Design and Synthesis of Intensely Luminescent Compounds

Fluorescent organic molecules continue to arouse strong interest because of their fascinating applications such as electroluminescent materials, molecular probes for clinical uses, and molecular sensors for chemical and biochemical analytes. Although there is already a wide choice of fluorescent molecules for particular applications, development of organic molecules exhibiting brilliant fluorescence have still be sought because new types of fluorophores have still been desired in the field of bioimaging, organic electroluminescence, and so on. Our research group has been interested in the chemistry and fluorescent properties of nitrogen-containing heterocyclic molecules, especially pyrazines.

We have synthesized 2,5-bis(benzimidazol-2-yl)pyrazine (BBIP), which exhibited extremely intensive blue-fluorescence with a fluorescence quantum yield of 0.90 in dimethyl sulfoxide (DMSO).  In the presence of sodium methoxide as a base, the fluorescence maximum of BBIP showed a bathochromic shift to 477 nm with a fluorescence quantum yield of almost unity.

Our current interest focuses on utilizing BBIP for microenvironment probes, ion sensors, organic luminescent devices and so on.

Design and Synthesis of Chemiluminescent Compounds for Detecting Reactive Oxygen Species

Imidazo[1,2-a]pyrazin-3(7H)-ones are widely distributed in luminescent substrates of marine luminous organisms.  They can specifically react with superoxide anion immediately after its production to emit light (465 nm) without any light sources for excitation. Therefore, the imidazopyrazinones have been utilized as good chemosensors for the real-time detection of superoxide anion. 

Most of the imidazopyrazinones developed thus far emit blue light, which sometimes is absorbed by some biomaterials. To prevent this problem, development of imidazopyrazinones that exhibit red-shifted emission has been desired.  Besides, if the imidazopyrazinones are used to detect superoxide anion in cells, they should be lipophilic to permeate cell membranes. Current work focuses on the development of imidazopyrazinone-based lipophilic chemiluminescent probes with emission at a longer wavelength than 500 nm for the successful detection of superoxide anion inside cells.