Recent Developments in the Synthesis and Evaluation of Glucose Analog Inhibitors of Glycogen Phosphorylases as Potential Antidiabetic Agents

Diabetes is among the largest contributors to global mortality through its long term complications. The worldwide epidemic of type 2 diabetes has been stimulating the quest for new concepts and targets for the treatment of this incurable disease. A new target is glycogen phosphorylase (GP), the main regulatory enzyme in the liver responsible for the control of blood glucose levels. One of several approaches to influence the action of GP is the use of glucose derivatives as active site inhibitors. This field of research commenced 10-15 years ago and, due to joint efforts in computer aided molecular design, organic synthesis, protein crystallography, and biological assays, resulted in glucopyranosylidenespiro- hydantoin 16 (Ki = 3-4 μM) as the most efficient glucose analog inhibitor of GP of that time. The present paper surveys the recent developments of this field achieved mainly in the last five years: the synthesis and evaluation of glucopyranosylidene-spiro-thiohydantoin 18 (Ki = 5 μM) which has proven equipotent with 16, and is available in gram amounts; furanosylidene- and xylopyranosylidene-spiro-(thio)hydantoins whose ineffectiveness (Ki > 10 μM) confirmed the high specificity of the catalytic site of GP towards the Dglucopyranosyl unit; “open” hydantoins like methyl N-(1-carboxamido-Dglucopyranosyl) carbamate 37 (Ki = 16 μM) and N-acyl-N'-(b-D-glucopyranosyl) ureas among them the to date best glucose analog inhibitor N-(2-naphthoyl)-N'-(b- D-glucopyranosyl)urea (35, Ki = 0.4 μM) which can also bind to the so-called new allosteric site of GP; C-(b-D-glucopyranosyl)heterocycles (tetrazole, 2-methyl- 1,3,4-oxadiazole, benzimidazole (Ki = 11 μM), and benzothiazole). Iminosugars like isofagomine (52, IC50 = 0.7 μM), noeuromycin (60, IC50 = 4 μM), and azafagomine (61, IC50 = 13.5 μM) also bind strongly to the active site of GP, however, substitution on the nitrogens makes the binding weaker. The natural product five-membered iminosugar DAB (63) exhibited IC50 ~ 0.4-0.5 μM. Azoloperhydropyridines which can be regarded iminosugar-annelated heterocycles show moderate inhibition of GP: nojiritetrazole 12 (Ki = 53 μM) is the best inhibitor and fewer nitrogens in the five-membered ring weakens the binding. Physiological investigations have been carried out with N-acetyl-b-Dglucopyranosylamine 6, spiro-thiohydantoin 18, isofagomine 52, and DAB 63 to underline the potential use of these compounds in the treatment of type 2 diabetes. Computational methods suggest to synthesize further anomerically bifunctional glucose derivatives which may be good inhibitors of GP.