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 glucopyranosylidene-spiro-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 D-glucopyranosyl unit; “open” hydantoins like methyl N-(1-carboxamido-D-glucopyranosyl)carbamate 37 (Ki = 16 > M) and N-acyl-N-(β-Dglucopyranosyl) ureas among them the to date best glucose analog inhibitor N-(2-naphthoyl)-N-(β-D-glucopyranosyl)urea (35, Ki = 0.4 μM) which can also bind to the so-called new allosteric site of GP; C-(β-D-glucopyranosyl)heterocycles (tetrazole, 1,3,4-oxadiazoles, benzimidazole (Ki = 11 μM), and benzothiazole). Iminosugars like isofagomine (45, IC50 = 0.7 μM), noeuromycin (53, IC50 = 4 μM), and azafagomine (54, 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 (56) 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-β-Dglucopyranosylamine 6, spiro-thiohydantoin 18, isofagomine 45, and DAB 56 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.

The protein glycogen phosphorylase has been linked to type 2 diabetes, indicating the importance of this target to human health. Hence, the search for potent and selective inhibitors of this enzyme, which may lead to antihyperglycaemic drugs, has received particular attention. Glycogen phosphorylase is a typical allosteric protein with five different ligand binding sites, thus offering multiple opportunities for modulation of enzyme activity. The present survey is focused on recent new molecules, potential inhibitors of the enzyme. The biological activity can be modified by these molecules through direct binding, allosteric effects or other structural changes. Progress in our understanding of the mechanism of action of these inhibitors has been made by the determination of high-resolution enzyme inhibitor structures (both muscle and liver). The knowledge of the three-dimensional structures of protein-ligand complexes allows analysis of how the ligands interact with the target and has the potential to facilitate structure-based drug design. In this review, the synthesis, structure determination and computational studies of the most recent inhibitors of glycogen phosphorylase at the different binding sites are presented and analyzed.