An inhibitor of human liver glycogen phosphorylase a (HLGPa) has been identified and characterized in vitro and in vivo. This substance, [R-(R*,S*)]-5-chloro-N-[3-(dimethylamino)-2-hydroxy-3-oxo-1-(phenylmethyl)propyl]-1H-indole-2-carboxamide (CP-91149), inhibited HLGPa with an IC 50 of 0.13 M in the presence of 7.5 mM glucose. CP-91149 resembles caffeine, a known allosteric phosphorylase inhibitor, in that it is 5-to 10-fold less potent in the absence of glucose. Further analysis, however, suggests that CP-91149 and caffeine are kinetically distinct. Functionally, CP-91149 inhibited glucagon-stimulated glycogenolysis in isolated rat hepatocytes (P < 0.05 at 10-100 M) and in primary human hepatocytes (2.1 M IC 50 ). In vivo, oral administration of CP-91149 to diabetic ob͞ob mice at 25-50 mg͞kg resulted in rapid (3 h) glucose lowering by 100-120 mg͞dl (P < 0.001) without producing hypoglycemia. Further, CP-91149 treatment did not lower glucose levels in normoglycemic, nondiabetic mice. In ob͞ob mice pretreated with 14 C-glucose to label liver glycogen, CP-91149 administration reduced 14 C-glycogen breakdown, confirming that glucose lowering resulted from inhibition of glycogenolysis in vivo. These findings support the use of CP-91149 in investigating glycogenolytic versus gluconeogenic f lux in hepatic glucose production, and they demonstrate that glycogenolysis inhibitors may be useful in the treatment of type 2 diabetes.

Type 2 diabetes mellitus is a severe disease with large economic consequences, which is significantly under-diagnosed and incompletely treated in the general population. Control of blood glucose levels is a key objective in treating diabetic patients, who are most often prescribed one or more oral hypoglycaemic agents in addition to diet and exercise modification as well as insulin. In spite of the availability of different classes of hypoglycaemic drugs, treatment regimens are often unable to achieve an intensive degree of glucose control known to most effectively reduce the incidence and severity of diabetic complications. Hepatic glucose output is elevated in type 2 diabetic patients and current evidence indicates that glycogenolysis (release of monomeric glucose from the glycogen polymer storage form) is an important contributor to the abnormally high production of glucose by the liver. Glycogen phosphorylase is the enzyme that catalyses this release and recent advances in new inhibitors of this structurally and kinetically well studied enzyme have enabled work which further delineate the pharmacological and physiological consequences of inhibiting glucose production by this pathway. Most notably, these agents lower glucose in diabetic animal models, both acutely and chronically, appear to affect both gluconeogenic and glycogenolytic pathways and demonstrate potential for a beneficial effect on cardiovascular risk factors. Cumulatively, this information has bolstered interest and promise in glycogen phosphorylase inhibitors (GPIs) as potential new hypoglycaemic agents for treatment of type 2 diabetes mellitus.

Glucokinase is a key regulator of glucose homeostasis, and small molecule allosteric activators of this enzyme represent a promising opportunity for the treatment of type 2 diabetes. Systemically acting glucokinase activators (liver and pancreas) have been reported to be efficacious but in many cases present hypoglycaemia risk due to activation of the enzyme at low glucose levels in the pancreas, leading to inappropriately excessive insulin secretion. It was therefore postulated that a liver selective activator may offer effective glycemic control with reduced hypoglycemia risk. Herein, we report structure-activity studies on a carboxylic acid containing series of glucokinase activators with preferential activity in hepatocytes versus pancreatic β-cells. These activators were designed to have low passive permeability thereby minimizing distribution into extrahepatic tissues; concurrently, they were also optimized as substrates for active liver uptake via members of the organic anion transporting polypeptide (OATP) family. These studies lead to the identification of 19 as a potent glucokinase activator with a greater than 50-fold liver-to-pancreas ratio of tissue distribution in rodent and non-rodent species. In preclinical diabetic animals, 19 was found to robustly lower fasting and postprandial glucose with no hypoglycemia, leading to its selection as a clinical development candidate for treating type 2 diabetes.

Glycogen in the brain is localized almost exclusively to astrocytes. The physiological function of this energy store has been difficult to establish because of the difficulty in manipulating brain glycogen concentrations in vivo. Here, we used a novel glycogen phosphorylase inhibitor, CP-316,819 ([R-R*,S*]-5-chloro-N-[2-hydroxy-3-(methoxymethylamino)-3-oxo-1-(phenylmethyl)propyl]-1H-indole-2-carboxamide), that causes glycogen accumulation under normoglycemic conditions but permits glycogen utilization when glucose concentrations are low. Rats treated with CP-316,819 had an 88 Ϯ 3% increase in brain glycogen content. When subjected to hypoglycemia, these rats maintained brain electrical activity 91 Ϯ 14 min longer than rats with normal brain glycogen levels and showed markedly reduced neuronal death. These studies establish a novel approach for manipulating brain glycogen concentration in normal, awake animals and provide in vivo confirmation that astrocyte glycogen supports neuronal function and survival during glucose deprivation. These findings also suggest an approach for forestalling hypoglycemic coma and brain injury in diabetic patients.

The ability to non-invasively measure endogenous pancreatic β-cell mass (BCM) would accelerate research on the pathophysiology of diabetes and revolutionize the preclinical development of new treatments, the clinical assessment of therapeutic efficacy, and the early diagnosis and subsequent monitoring of disease progression. The vesicular monoamine transporter type 2 (VMAT2) is co-expressed with insulin in β-cells and represents a promising target for BCM imaging. Methods We evaluated the VMAT2 radiotracer 18F-fluoropropyl-dihydrotetrabenazine ([18F]FP-(+)-DTBZ, also known as [18F]AV-133) for quantitative positron emission tomography (PET) imaging of BCM in healthy control subjects and patients with type 1 diabetes mellitus (T1DM). Standardized uptake value (SUV) was calculated as the net tracer uptake in pancreas normalized by injected dose and body weight. Total volume of distribution (VT), the equilibrium ratio of tracer concentration in tissue relative to plasma, was estimated by kinetic modeling with arterial input functions. Binding potential (BPND), the steady-state ratio of specific binding to non-displaceable uptake, was calculated using the renal cortex as a reference tissue devoid of specific VMAT2 binding. Results Mean pancreatic SUV, VT, and BPND were reduced by 38%, 20% and 40%, respectively, in T1DM. The radiotracer binding parameters correlated with insulin secretion capacity as determined by arginine-stimulus tests. Group differences and correlations with β-cell function were enhanced for total pancreas binding parameters that accounted for tracer binding density as well as organ volume. Conclusion These findings demonstrate that quantitative evaluation of islet β-cell density and aggregate BCM can be performed clinically with [18F]FP-(+)-DTBZ PET.