Spatial and temporal studies of metabolic activity: contrasting biochemical kinetics in tissues and pathways during fasted and fed states.

The regulation of nutrient homeostasis, i.e., the ability to transition between fasted and fed states, is fundamental in maintaining health. Since food is typically consumed over limited (anabolic) periods, dietary components must be processed and stored to counterbalance the catabolic stress that occurs between meals. Herein, we contrast tissue- and pathway-specific metabolic activity in fasted and fed states. We demonstrate that knowledge of biochemical kinetics that is obtained from opposite ends of the energetic spectrum can allow mechanism-based differentiation of healthy and disease phenotypes. Rat models of type 1 and type 2 diabetes serve as case studies for probing spatial and temporal patterns of metabolic activity via [2H]water labeling. Experimental designs that capture integrative whole body metabolism, including meal-induced substrate partitioning, can support an array of research surrounding metabolic disease; the relative simplicity of the approach that is discussed here should enable routine applications in preclinical models.

Functionally selective signaling and broad metabolic benefits by novel insulin receptor partial agonists.

Insulin analogs have been developed to treat diabetes with focus primarily on improving the time action profile without affecting ligand-receptor interaction or functional selectivity. As a result, inherent liabilities (e.g. hypoglycemia) of injectable insulin continue to limit the true therapeutic potential of related agents. Insulin dimers were synthesized to investigate whether partial agonism of the insulin receptor (IR) tyrosine kinase is achievable, and to explore the potential for tissue-selective systemic insulin pharmacology. The insulin dimers induced distinct IR conformational changes compared to native monomeric insulin and substrate phosphorylation assays demonstrated partial agonism. Structurally distinct dimers with differences in conjugation sites and linkers were prepared to deliver desirable IR partial agonist (IRPA). Systemic infusions of a B29-B29 dimer in vivo revealed sharp differences compared to native insulin. Suppression of hepatic glucose production and lipolysis were like that attained with regular insulin, albeit with a distinctly shallower dose-response. In contrast, there was highly attenuated stimulation of glucose uptake into muscle. Mechanistic studies indicated that IRPAs exploit tissue differences in receptor density and have additional distinctions pertaining to drug clearance and distribution. The hepato-adipose selective action of IRPAs is a potentially safer approach for treatment of diabetes.

Discovery of Insulin Receptor Partial Agonists MK-5160 and MK-1092 as Novel Basal Insulins with Potential to Improve Therapeutic Index.

We have identified a series of novel insulin receptor partial agonists (IRPAs) with a potential to mitigate the risk of hypoglycemia associated with the use of insulin as an antidiabetic treatment. These molecules were designed as dimers of native insulin connected via chemical linkers of variable lengths with optional capping groups at the N-terminals of insulin chains. Depending on the structure, the maximal activation level (%Max) varied in the range of ∼20-70% of native insulin, and EC50 values remained in sub-nM range. Studies in minipig and dog demonstrated that IRPAs had sufficient efficacy to normalize plasma glucose levels in diabetes, while providing reduction of hypoglycemia risk. IRPAs had a prolonged duration of action, potentially making them suitable for once-daily dosing. Two lead compounds with %Max values of 30 and 40% relative to native insulin were selected for follow up studies in the clinic.

Superior Glycemic Control With a Glucose-Responsive Insulin Analog: Hepatic and Nonhepatic Impacts.

We evaluated the hepatic and nonhepatic responses to glucose-responsive insulin (GRI). Eight dogs received GRI or regular human insulin (HI) in random order. A primed, continuous intravenous infusion of [3-3H]glucose began at -120 min. Basal sampling (-30 to 0 min) was followed by two study periods (150 min each), clamp period 1 (P1) and clamp period 2 (P2). At 0 min, somatostatin and GRI (36 ± 3 pmol/kg/min) or HI (1.8 pmol/kg/min) were infused intravenously; basal glucagon was replaced intraportally. Glucose was infused intravenously to clamp plasma glucose at 80 mg/dL (P1) and 240 mg/dL (P2). Whole-body insulin clearance and insulin concentrations were not different in P1 versus P2 with HI, but whole-body insulin clearance was 23% higher and arterial insulin 16% lower in P1 versus P2 with GRI. Net hepatic glucose output was similar between treatments in P1. In P2, both treatments induced net hepatic glucose uptake (HGU) (HI mean ± SEM 2.1 ± 0.5 vs. 3.3 ± 0.4 GRI mg/kg/min). Nonhepatic glucose uptake in P1 and P2, respectively, differed between treatments (2.6 ± 0.3 and 7.4 ± 0.6 mg/kg/min with HI vs. 2.0 ± 0.2 and 8.1 ± 0.8 mg/kg/min with GRI). Thus, glycemia affected GRI but not HI clearance, with resultant differential effects on HGU and nonHGU. GRI holds promise for decreasing hypoglycemia risk while enhancing glucose uptake under hyperglycemic conditions.

Engineering Glucose Responsiveness Into Insulin.

Insulin has a narrow therapeutic index, reflected in a small margin between a dose that achieves good glycemic control and one that causes hypoglycemia. Once injected, the clearance of exogenous insulin is invariant regardless of blood glucose, aggravating the potential to cause hypoglycemia. We sought to create a "smart" insulin, one that can alter insulin clearance and hence insulin action in response to blood glucose, mitigating risk for hypoglycemia. The approach added saccharide units to insulin to create insulin analogs with affinity for both the insulin receptor (IR) and mannose receptor C-type 1 (MR), which functions to clear endogenous mannosylated proteins, a principle used to endow insulin analogs with glucose responsivity. Iteration of these efforts culminated in the discovery of MK-2640, and its in vitro and in vivo preclinical properties are detailed in this report. In glucose clamp experiments conducted in healthy dogs, as plasma glucose was lowered stepwise from 280 mg/dL to 80 mg/dL, progressively more MK-2640 was cleared via MR, reducing by ∼30% its availability for binding to the IR. In dose escalations studies in diabetic minipigs, a higher therapeutic index for MK-2640 (threefold) was observed versus regular insulin (1.3-fold).

Pegylated Fgf21 rapidly normalizes insulin-stimulated glucose utilization in diet-induced insulin resistant mice.

Fibroblast growth factor 21 (FGF21) is a novel hormone-like polypeptide that when administered exogenously, has been shown to have beneficial effects on food intake, body weight, and metabolism. The in vivo mechanisms of action for its positive metabolic effects remain to be fully elucidated. It has been shown that PEGylation of human FGF21 at specific and preferred sites confer superior metabolic pharmacology. We therefore hypothesized that low doses of PEGylated (30K PEG on position Q108) FGF21 (PEG30-Q108) would improve insulin action, independent of any effect on food intake or body weight. We identified a dose (0.25mg/kg) that had no effect on food intake or body weight, yet did show beneficial metabolic effects. Four groups of 12 weeks, high-fat fed, insulin resistant mice were studied: mice dosed subcutaneously once with vehicle or 0.25mg/kg of PEG30-Q108 24h before the experiment, or mice dosed 4 times over 2 weeks with vehicle or PEG30-Q108. Conscious, unrestrained mice were fasted for 5h and underwent a hyperinsulinemic-euglycemic clamp. Both PEG30-Q108 treatments significantly lowered fasting insulin compared to vehicle, with no difference in food intake or body weight. Insulin-stimulated whole body glucose utilization was normalized to that of lean mice with both PEG30-Q108 treatments compared to vehicle. This accounted for all of the enhanced insulin action, as there was no improvement in insulin's ability to suppress endogenous glucose production. In line with these findings, neither PEG30-Q108 treatment lowered hepatic triglycerides. These results demonstrate the profound ability of PEG30-Q108 to increase whole body insulin sensitivity.

Relationship between glucose volume of distribution and the extracellular space: a multiple tracer study.

Although the use of radioisotopes in the investigation of glucose metabolism dates back more than 50 years, several relevant quantitative aspects have not been definitively determined. These include the volume of distribution (V(d)) of glucose and recycling of glucose radioisotopes from liver glycogen. These problems are further complicated by methodological issues such as the following: (1) glucose tracers have different metabolic fates that may influence volume estimates, and (2) the calculation method needs to be based on physical principles to avoid some limitations of compartmental models. To address these issues, we administered boluses of an extracellular marker ([1-(14)C]-l-glucose, 30 µCi) and 2 glucose tracers ([2-(3)H]-d-glucose and [3-(3)H]-d-glucose, 120 µCi of each), followed by a 1-mg glucagon bolus (in the presence of somatostatin) 245 minutes later, in conscious beagles to account for potential problems in recycling of the label through glycogen. We used modeling methods based on physical principles (circulatory model), which yield volume estimates with a clear physiological interpretation. Glucose V(d) (mL/kg) were 204 ([1-(14)C]-l-glucose), 191 ([2-(3)H]-d-glucose), and 206 ([3-(3)H]-d-glucose). These values were not different and correlated. The amount of recycled [3-(3)H]-d-glucose in response to glucagon was small (∼1.7% of the injected tracer dose). An additional result of this analysis is the determination of the parameters of the circulatory model in beagles for the standard [3-(3)H]-d-glucose tracer. Using multiple tracers in beagles and calculation methods based on physical principles, we have provided direct proof that the glucose V(d) equals the extracellular space in beagles under basal conditions.

Estrogen receptor (ER) subtype agonists alter monoamine levels in the female rat brain.

We assessed the effects of subtype-selective ER agonists on monoamine levels in discrete regions of the female rat brain. Ovariectomized (ovx) rats were treated for 4 days with vehicle, 17ß-estradiol (E; 0.05mg/kg), an ERß agonist (C19; 3mg/kg) or an ERα agonist (PPT; 3mg/kg) and samples from brain regions were assessed for monoamines and metabolites. We also assessed effects of ERß modulation on baseline and fenfluramine-induced release of monoamines in hippocampus using microdialysis. In the first study, E and the ERα agonist increased norepinephrine in cortex and all three ER ligands increased it in the ventral hippocampus. Changes in levels of the noradrenergic metabolite, MHPG and the dopaminergic metabolite, DOPAC were noted in brain areas of ER ligand-treated animals. E also increased levels of 5HIAA in three brain areas. In the microdialysis study, there were no differences among groups in baseline levels of monoamines. However, E and the ERß agonist increased levels of the dopaminergic metabolite, HVA following fenfluramine. In summary, activation of the two nuclear ERs with selective agonists affects monoamine and metabolite levels in discrete brain areas, a number of which are known to play key roles in cognitive and affective function.

Discovery and investigation of a novel class of thiophene-derived antagonists of the human glucagon receptor.

A novel class of antagonists of the human glucagon receptor (hGCGR) has been discovered. Systematic modification of the lead compound identified substituents that were essential for activity and those that were amenable to further optimization. This SAR exploration resulted in the synthesis of 13, which exhibited good potency as an hGCGR functional antagonist (IC50 = 34 nM) and moderate bioavailability (36% in mice).