Mini Reviews in Medicinal Chemistry - Volume 10, Issue 12, 2010

Special issue of Mini-Reviews in Medicinal Chemistry dedicated to the memory of Nikos G. Oikonomakos (1945-2008). Glycogenolysis is the phosphorolytic degradation of the storage polysaccharide glycogen to glucose-1-phosphate which, after subsequent transformations to glucose-6-phosphate and glucose, may serve as an energy source for the organism's cells. The rate determining phosphorylation step of the process is catalyzed by glycogen phosphorylases (GP). These enzymes belong to the most studied biocatalysts and archetypical phenomena like phosphorylative and allosteric regulation were described for the first time with GPs. The tissue specific GP isoenzymes are responsible for maintaining blood glucose levels (liver isoform) as well as for direct fuel supply of the given cells/tissues (muscle and brain isoforms). Because of the direct connection between blood sugar levels and elevated hepatic glucose output, and thereby liver GP, in type 2 diabetic patients inhibition of the enzyme was suggested as a possible base for an antidiabetic therapy. Though this is the most intensively investigated therapeutic field, several others e. g. treatment and/or prevention of early cardiac and cardiovascular disease in non-diabetics, stabilizing cardiac arrhythmias, protection against ischemic injury, preventing tumour growth have been suggested to be targeted by inhibitors of GP. Thus, finding good matches between the biological and chemical space represented by the binding sites of GP on one hand and the relevant parts of the small molecule universe on the other has attracted long standing interest in both academia and industry. The roots of these investigations had been in Oxford (United Kingdom) where L. N. Johnson, G. W. J. Fleet, N. G. Oikonomakos and coworkers started to study glucose derived inhibitors of GP by combining methods of organic synthesis, enzyme kinetics, protein crystallography, and computational chemistry. The work was continued and extended by the Oikonomakos group in Athens (Greece) as part of a worldwide collaboration. Although the sudden and untimely passing of Nikos on Aug 31, 2008 brought about a slight break in this activity, the members of the group are carrying on the research. What else than this could demonstrate better how enthusiastic as a scientist, how collaborative as a colleague, and how efficient as an educator Nikos was. The authors and the editor of this special issue pay a tribute to his memory by gathering the latest results of the title field. The most accessed GP isolated from rabbit muscle is presented in the first paper by Evangelia D. Chrysina (Athens, Greece) describing the binding peculiarities of a range of glucose derivatives and analyzing the structure-activity relationships in structural terms supported by crystallographic results. The next survey by Jean-Pierre Praly and Sébastien Vidal (Lyon, France) gives an overview of the glycogen metabolism with its enzymes and regulation in the context of diabetes mellitus followed by a detailed description of syntheses and kinetic studies of glucose based and iminosugar type inhibitors disclosed in the past couple of years. Thanasis Gimisis (Athens, Greece) summarizes an extensive synthetic work supported by enzyme kinetics and crystallography to probe the catalytic site of GP by N-glucopyranosides of oxamic acid derivatives, L-α-amino acids and peptides, purine and pyrimidine nucleobases and related compounds. Wendy A. Loughlin (Brisbane, Australia) focuses on the allosteric inhibition of GP by a large array of compounds of extremely high chemical diversity and also discusses the question of isoform selectivity. The application of computational methods to the design of GP inhibitors is analyzed by Joseph M. Hayes and Demetres D. Leonidas (Athens, Greece) who present several examples on how computations may reduce experimental costs, cost effectiveness of molecular modelling and docking methods themselves, and what are the pros and cons of binding site predictions. Physiological investigations surveyed by Loranne Agius (Newcastle, United Kingdom) have been instrumental in revealing the ways of action of GP inhibitors and validating GP as a therapeutic target against type 2 diabetes. Last but not least, Li Xu and Hongbin Sun (Nanjing, China) raise and explain the possibility of a new non-antidiabetic application of inhibition of glycogenolysis in a therapeutic approach to cerebral ischemia.

The quest for the discovery of new antihyperglycaemic agents has been more intense the last years due to the rapid increase of mortality associated with type 2 diabetes. Glycogen metabolism has been one of the major causes of the elevated blood glucose levels; hence, special attention has been drawn to the control of the enzymes implicated in the relevant pathway. To this end, the allosteric enzyme of glycogen phosphorylase, has been proposed as molecular target for the design of potential new antidiabetic agents by an interdisciplinary approach comprising organic synthesis, kinetic and Xray crystallographic studies and physiological experiments. The results derived from the thorough investigation of the catalytic site of the enzyme with the structure-based inhibitor design approach are summarized with emphasis on the most potent inhibitors identified for different classes of compounds.

Among the variety of approaches for pharmacological intervention in T2DM, the inhibition of GP with the aim of reducing hepatic glucose output is a validated and thoroughly investigated strategy. Both the academia and health companies participate in the search of potent inhibitors, that might be suitable for long-term treatment. As several inhibitory sites have been identified for GP, interest focuses mainly on structures that can bind at either the catalytic, the allosteric, or the new allosteric sites. Glucose-based motifs and azasugars that bind at the active site constitute the most populated class of GPis. During the last two years, significant progresses have been made, since newly proposed motifs have Ki values in the low micromolar and even sub- micromolar range. Without ignoring previously reported structures, new series based on β-D-glucopyranosyl-pyrimidine, D-glucopyranosylidene-spiro-isoxazoline and D-glucopyranosylidene-spirooxathiazole motifs appear promising. A representative from this last series, with a 2-naphthyl residue was identified as the most potent GPi to date (Ki = 0.16 μM). While no inhibition was found for sulfonium analogs, D-DAB remains the best inhibitor among five and six-membered iminosugars that showed inhibitory properties toward GP. A study of glucagoninduced glucose production in primary rat hepatocytes has suggested that amylo-1,6-glucosidase inhibitors in combination with GPis may lower glucose level in T2DM. Considering the limitations found for other potent GPis binding at other sites and the complexity of pharmacological development, the potential of glucose-based GPis is still not established firmly and more tests with cells, tissues, animals are required to better establish the risks and merits of these structures, as antidiabetic drugs. Further studies might also confirm other directions where glucose-based GPis could be useful.

Glycogen phosphorylase (GP) is a promising molecular target for the treatment of Type 2 diabetes. The design of potential inhibitors for the catalytic site of the enzyme is based on the high affinity for β-D-glucopyranose and the presence of a β-cavity that extends from the sugar anomeric position forming a 15 x 7.5 x 10 A available space. This review is focused on our efforts towards the design and synthesis of various families of potential inhibitors, including N-β-Dglucopyranosyl oxamic acid esters and oxamides, N-β-D-glucopyranosylaminocarbonyl L-aminoacids and peptides, as well as glucose-derived purine and pyrimidine nucleosides, spiro- and other bicyclic derivatives. Kinetic and crystallographic study of the interactions of these inhibitors with GP has increased our understanding of the importance of the various functional groups within the catalytic site and has pointed the way towards the in silico prediction and design of potent inhibitors, which are both synthetically viable and pharmacologically relevant.

Glycogen Phosphorylase (GP) is an important target for the development of anti-hyperglycaemic drugs. GP is an enzyme which is moderated allosterically with multiple ligand binding sites where inhibitors can potentially modulate enzyme activity. The search for potent and isoform selective inhibitors of GP is ongoing with an increasing focus on allosteric inhibition. In this review, the structural diversity, and enzyme interactions of the most recent inhibitors, and in particular allosteric inhibitors, of GP at the different key binding sites are explored. A range of inhibitors of GP, with known as well as unknown binding site or mechanism is presented.

Glycogen phosphorylase is an important therapeutic target for the potential treatment of type 2 diabetes. The importance of computation in the search for potent, selective and drug-like glycogen phosphorylase inhibitors which may eventually lead to antihyperglycemic drugs is now firmly established. Acting solo or more effectively in combination with experiment in a multidisciplinary approach to structure based drug design, current day modeling methods are an effective means of reducing the time and money spent on costly experimental procedures. Glycogen phosphorylase is an allosteric protein with five different ligand binding sites, hence offering multiple opportunities for modulation of enzyme activity. However, the binding sites have their own individual characteristics, so that different modeling approaches may be more effective for each. This review is focused on advances in the modeling and design of new inhibitors of the enzyme aimed towards providing the reader with some useful hints towards more successful computer-aided inhibitor (drug) design targeting glycogen phosphorylase.

Liver glycogen is synthesized in the postprandial state in response to elevated concentrations of glucose and insulin or by activation of neuroendocrine signals and it is degraded in the postabsorptive state in response to changes in the concentrations of insulin and counter-regulatory hormones. Dysregulation of either glycogen degradation or synthesis through changes in allosteric control or covalent modification of glycogen phosphorylase and glycogen synthase leads to disturbance of blood glucose homeostasis. Liver glycogen phosphorylase has a dual role in the control of glycogen metabolism by regulation of both glycogen degradation and synthesis. The phosphorylated form (GPa) is the active form and determines the rate of degradation of glycogen and it is also a potent allosteric inhibitor of the protein complex, involving the glycogen targeting protein GL and protein phosphatase-1, which catalyses dephosphorylation (activation) of glycogen synthase. Drug discovery programmes exploring the validity of glycogen phosphorylase as a therapeutic target for type 2 diabetes have generated a wide array of selective phosphorylase ligands that modulate the catalytic activity and / or the phosphorylation state (interconversion of GPa and GPb) as well as the binding of GPa to the allosteric site of GL. Glycogen phosphorylase inhibitors that act in hepatocytes either exclusively by dephosphorylating GPa (e.g. indole carboxamides) or by allosteric inhibition of GPa (1,4-dideoxy-1,4-D-arabinitol) are very powerful experimental tools to determine the relative roles of covalent modification of glycogen phosphorylase and / or cycling between glycogen synthesis and degradation in the mechanism(s) by which insulin and neurotransmitters regulate hepatic glycogen metabolism.

Brain ischemia resulting from multiple disease states including cardiac arrest, stroke and traumatic brain injury, is a leading cause of death and disability. Despite significant resources dedicated to developing pharmacological interventions, few effective therapeutic options are currently available. The basic consequence of cerebral ischemia, characterized by energy failure and subsequent brain metabolic abnormalities, enables the protective effects by pharmacological manipulation of brain metabolism. We present here the important roles of brain glycogen metabolism and propose inhibition of glycogenolysis as a therapeutic approach to cerebral ischemia.