Pharmacokinetics and pharmacodynamics of mibavademab (a leptin receptor agonist): Results from a first-in-human phase I study

Mibavademab (previously known as REGN4461), a fully human monoclonal antibody, is being investigated for the treatment of conditions associated with leptin deficiency. Here, we report pharmacokinetics (PKs), pharmacodynamics, and immunogenicity from a phase I study in healthy participants (NCT03530514). In part A, lean or overweight healthy participants were randomized to single-ascending-dose cohorts of 0.3, 1.0, 3.0, 10, and 30 mg/kg intravenous (i.v.), or 300 and 600 mg subcutaneous doses of mibavademab or placebo. In part B, overweight or obese participants were randomized to receive multiple doses of mibavademab (15 mg/kg i.v. loading dose and 10 mg/kg i.v. at weeks 3, 6, and 9) or placebo, stratified by body mass index and baseline leptin levels: low leptin (<5 ng/mL) or relatively low leptin (5-8 ng/mL in men and 5-24 ng/mL in women). Fifty-six and 55 participants completed the single-ascending-dose and multiple-dose parts, respectively. In the single-ascending-dose cohorts, mibavademab PKs were nonlinear with target-mediated elimination, greater than dose-proportional increases in exposure, and there were no dose-dependent differences in total soluble leptin receptor (sLEPR) levels in serum over time. Following multiple-dose administration of mibavademab in participants with leptin <8 ng/mL, lower mean mibavademab concentrations, higher mean total sLEPR concentrations, and larger mean decreases in body weight than in the relatively low leptin cohorts were observed. Baseline leptin was correlated with mibavademab PKs and pharmacodynamics. No treatment-emergent anti-mibavademab antibodies were observed in any mibavademab-treated participant. Results from this study collectively inform further development of mibavademab to treat conditions associated with leptin deficiency.

Chronic exposure to methylmercury enhances the anorexigenic effects of leptin in C57BL/6J male mice

Several studies have demonstrated that heavy metals disrupt energy homeostasis. Leptin inhibits food intake and decreases body weight through activation of its receptor in the hypothalamus. The impact of heavy metals on leptin signaling in the hypothalamus is unclear. Here, we show that the environmental pollutant, methylmercury (MeHg), favors an anorexigenic profile in wild-type males. C57BL/6J mice were exposed to MeHg via drinking water (5 ppm) up to 30 days. Our data shows that MeHg exposure was associated with changes in leptin induced activation of Janus kinase 2 (JAK2)/signal transducer and activator of transcription 3 (STAT3) signaling pathway in the hypothalamus. In males, the activation of JAK2/STAT3 signaling pathway was sustained by an increase in SOCS3 protein levels. In females, MeHg-activated STAT3 was inhibited by a concomitant increase in PTP1B. Taken together, our data suggest that MeHg enhanced leptin effects in males, favoring an anorexigenic profile in males, which notably, have been shown to be more sensitive to the neurological effects of this organometal than females. A better understanding of MeHg-induced molecular mechanism alterations in the hypothalamus advances the understanding of its neurotoxicity and provides molecular sites for novel therapies.

Role of estrogen receptor signaling in skeletal response to leptin in female ob/ob mice

Leptin, critical in regulation of energy metabolism, is also important for normal bone growth, maturation and turnover. Compared to wild type (WT) mice, bone mass is lower in leptin-deficient ob/ob mice. Osteopenia in growing ob/ob mice is due to decreased bone accrual, and is associated with reduced longitudinal bone growth, impaired cancellous bone maturation and increased marrow adipose tissue (MAT). However, leptin deficiency also results in gonadal dysfunction, disrupting production of gonadal hormones which regulate bone growth and turnover. The present study evaluated the role of increased estrogen in mediating the effects of leptin on bone in ob/ob mice. Three-month-old female ob/ob mice were randomized into one of the 3 groups: (1) ob/ob + vehicle (veh), (2) ob/ob + leptin (leptin) or (3) ob/ob + leptin and the potent estrogen receptor antagonist ICI 182,780 (leptin + ICI). Age-matched WT mice received vehicle. Leptin (40 µg/mouse, daily) and ICI (10 µg/mouse, 2×/week) were administered by subcutaneous injection for 1 month and bone analyzed by X-ray absorptiometry, microcomputed tomography and static and dynamic histomorphometry. Uterine weight did not differ between ob/ob mice and ob/ob mice receiving leptin + ICI, indicating that ICI successfully blocked the uterine response to leptin-induced increases in estrogen levels. Compared to leptin-treated ob/ob mice, ob/ob mice receiving leptin + ICI had lower uterine weight; did not differ in weight loss, MAT or bone formation rate; and had higher longitudinal bone growth rate and cancellous bone volume fraction. We conclude that increased estrogen signaling following leptin treatment is dispensable for the positive actions of leptin on bone and may attenuate leptin-induced bone growth.

Role of autonomic nervous system in chronic CNS-mediated antidiabetic action of leptin

This study tested whether ganglionic blockade or hepatic vagotomy attenuates the chronic central nervous system (CNS)-mediated antidiabetic and cardiovascular effects of leptin. Male Sprague-Dawley rats were instrumented with telemetry probes and arterial and venous catheters for determination of blood pressure (BP), heart rate (HR), blood sampling, and intravenous (iv) infusions. An intracerebroventricular (ICV) cannula was placed into the brain lateral ventricle for infusion of leptin or vehicle. After control measurements, streptozotocin (STZ) was injected iv (50 mg/kg) to induce diabetes, and 5 days later leptin (n = 6) or saline vehicle (n = 5) was infused ICV for 12 days via osmotic pumps. Beginning on day 6 of leptin treatment, the ganglionic blocker hexamethonium (15 mg·kg^-1·day^-1 iv) was infused, while leptin infusion was continued, to assess the role of the autonomic nervous system. Induction of diabetes was associated with increases in blood glucose (98 ± 7 to 350 ± 19 mg/dl), food intake (23 ± 3 to 43 ± 3 g/day), decreases in HR (-70 ± 11 beats/min), polyuria, and increased water consumption, which were all completely normalized by ICV leptin infusion. Although hexamethonium attenuated leptin's effect on HR, it failed to impair leptin's ability to restore euglycemia or to prevent the polyuria or increased water intake in STZ-diabetic rats. We also found that after pretreatment with hexamethonium (n = 8), ICV leptin infusion, during continued ganglionic blockade, completely normalized blood glucose in diabetic rats. In addition, selective hepatic vagotomy did not attenuate leptin's ability to restore euglycemia in diabetic rats. These results suggest that leptin's powerful chronic CNS antidiabetic actions are mediated primarily via nonautonomic mechanisms.

Leptin Matures Aspects of Lung Structure and Function in the Ovine Fetus

In human and ovine fetuses, glucocorticoids stimulate leptin secretion, although the extent to which leptin mediates the maturational effects of glucocorticoids on pulmonary development is unclear. This study investigated the effects of leptin administration on indices of lung structure and function before birth. Chronically catheterized singleton sheep fetuses were infused iv for 5 days with either saline or recombinant ovine leptin (0.5 mg/kg · d leptin (LEP), 0.5 LEP or 1.0 mg/kg · d, 1.0 LEP) from 125 days of gestation (term ∼145 d). Over the infusion, leptin administration increased plasma leptin, but not cortisol, concentrations. On the fifth day of infusion, 0.5 LEP reduced alveolar wall thickness and increased the volume at closing pressure of the pressure-volume deflation curve, interalveolar septal elastin content, secondary septal crest density, and the mRNA abundance of the leptin receptor (Ob-R) and surfactant protein (SP) B. Neither treatment influenced static lung compliance, maximal lung volume at 40 cmH2O, lung compartment volumes, alveolar surface area, pulmonary glycogen, protein content of the long form signaling Ob-Rb or phosphorylated signal transducers and activators of transcription-3, or mRNA levels of SP-A, C, or D, elastin, vascular endothelial growth factor-A, the vascular endothelial growth factor receptor 2, angiotensin-converting enzyme, peroxisome proliferator-activated receptor γ, or parathyroid hormone-related peptide. Leptin administration in the ovine fetus during late gestation promotes aspects of lung maturation, including up-regulation of SP-B.

Effects of cell-type specific leptin receptor mutation on leptin transport across the BBB

The functions of leptin receptors (LRs) are cell-type specific. At the blood-brain barrier, LRs mediate leptin transport that is essential for its CNS actions, and both endothelial and astrocytic LRs may be involved. To test this, we generated endothelia specific LR knockout (ELKO) and astrocyte specific LR knockout (ALKO) mice. ELKO mice were derived from a cross of Tie2-cre recombinase mice with LR-floxed mice, whereas ALKO mice were generated by a cross of GFAP-cre with LR-floxed mice, yielding mutant transmembrane LRs without signaling functions in endothelial cells and astrocytes, respectively. The ELKO mutation did not affect leptin half-life in blood or apparent influx rate to the brain and spinal cord, though there was an increase of brain parenchymal uptake of leptin after in situ brain perfusion. Similarly, the ALKO mutation did not affect blood-brain barrier permeation of leptin or its degradation in blood and brain. The results support our observation from cellular studies that membrane-bound truncated LRs are fully efficient in transporting leptin, and that basal levels of astrocytic LRs do not affect leptin transport across the endothelial monolayer. Nonetheless, the absence of leptin signaling at the BBB appears to enhance the availability of leptin to CNS parenchyma. The ELKO and ALKO mice provide new models to determine the dynamic regulation of leptin transport in metabolic and inflammatory disorders where cellular distribution of LRs is shifted.

Intranasal delivery of mouse [D-Leu-4]-OB3, a synthetic peptide amide with leptin-like activity, improves energy balance, glycaemic control, insulin sensitivity and bone formation in leptin-resistant C57BLK/6-m db/db mice

BACKGROUND

We have recently shown that intranasal administration of mouse [D-Leu-4]-OB3 reconstituted in Intravail(®) to male Swiss Webster mice resulted in significantly higher uptake and bioavailability when compared with commonly used injection methods of delivery.

AIM AND METHODS

In this study, we examined the effects of intranasal delivery of mouse [D-Leu-4]-OB3 in Intravail(®) on energy balance, glucose regulation, insulin secretion and serum levels of osteocalcin, a specific and sensitive marker of bone formation. Genetically obese C57BLK/6-m db/db mice were allowed food and water ad libitum and given either Intravail(®) alone or mouse [D-Leu-4]-OB3 in Intravail(®) for 14 days by intranasal instillation.

RESULTS

Mouse [D-Leu-4]-OB3 reduced body weight gain, daily food intake, daily water intake and serum glucose by 11.5, 2.2, 4.0 and 61.9%, respectively. Serum insulin levels in db/db mice given mouse [D-Leu-4]-OB3 were approximately threefold lower than those in mice receiving Intravail(®) alone. Mouse [D-Leu-4]-OB3 elevated serum osteocalcin in db/db mice by 28.7% over Intravail(®) treated control mice.

CONCLUSIONS

The results of our study indicate that intranasal delivery of biologically active mouse [D-Leu-4]-OB3 in Intravail(®) is feasible and has significant effects on regulating body weight gain, food and water intake, serum glucose, insulin sensitivity and bone formation in leptin-resistant C57BLK/6-m db/db mice.

Enhanced weight loss with pramlintide/metreleptin: an integrated neurohormonal approach to obesity pharmacotherapy

The neurohormonal control of body weight involves a complex interplay between long-term adiposity signals (e.g., leptin), and short-term satiation signals (e.g., amylin). In diet-induced obese (DIO) rodents, amylin/leptin combination treatment led to marked, synergistic, fat-specific weight loss. To evaluate the weight-lowering effect of combined amylin/leptin agonism (with pramlintide/metreleptin) in human obesity, a 24-week, randomized, double-blind, active-drug-controlled, proof-of-concept study was conducted in obese or overweight subjects (N = 177; 63% female; 39 +/- 8 years; BMI 32.0 +/- 2.1 kg/m(2); 93.3 +/- 13.2 kg; mean +/- s.d.). After a 4-week lead-in period with pramlintide (180 microg b.i.d. for 2 weeks, 360 microg b.i.d. thereafter) and diet (40% calorie deficit), subjects achieving 2-8% weight loss were randomized 1:2:2 to 20 weeks of treatment with metreleptin (5 mg b.i.d.), pramlintide (360 microg b.i.d.), or pramlintide/metreleptin (360 microg/5 mg b.i.d.). Combination treatment with pramlintide/metreleptin led to significantly greater weight loss from enrollment to week 20 (-12.7 +/- 0.9%; least squares mean +/- s.e.) than treatment with pramlintide (-8.4 +/- 0.9%; P < 0.001) or metreleptin (-8.2 +/- 1.3%; P < 0.01) alone (evaluable, N = 93). The greater reduction in body weight was significant as early as week 4, and weight loss continued throughout the study, without evidence of a plateau. The most common adverse events with pramlintide/metreleptin were injection site events and nausea, which were mostly mild to moderate and decreased over time. These results support further development of pramlintide/metreleptin as a novel, integrated neurohormonal approach to obesity pharmacotherapy.

Pegylated leptin antagonist is a potent orexigenic agent: preparation and mechanism of activity

Leptin, a pleiotropic adipokine, is a central regulator of appetite and weight and a key immunomodulatory protein. Although inborn leptin deficiency causes weight gain, it is unclear whether induced leptin deficiency in adult wild-type animals would be orexigenic. Previous work with a potent competitive leptin antagonist did not induce a true metabolic state of leptin deficiency in mice because of a short circulating half-life. In this study, we increased the half-life of the leptin antagonist by pegylation, which resulted in significantly increased bioavailability and retaining of antagonistic activity. Mice administered the pegylated antagonist showed a rapid and dramatic increase in food intake with weight gain. Resulting fat was confined to the mesenteric region with no accumulation in the liver. Serum cholesterol, triglyceride, and hepatic aminotransferases remained unaffected. Weight changes were reversible on cessation of leptin antagonist treatment. The mechanism of severe central leptin deficiency was found to be primarily caused by blockade of transport of circulating leptin across the blood-brain barrier with antagonisms at the arcuate nucleus playing a more minor role. Altogether we introduce a novel compound that induces central and peripheral leptin deficiency. This compound should be useful in exploring the involvement of leptin in metabolic and immune processes and could serve as a therapeutic for the treatment of cachexia.

Differential accessibility of circulating leptin to individual hypothalamic sites

Hypothalamic neurons expressing the long form of the leptin receptor (LRb) mediate important leptin actions. Although it has been suggested that leptin crosses the blood-brain barrier (BBB) via a specific transport system, we hypothesized the existence of a population of hypothalamic arcuate nucleus (ARC) neurons that senses leptin independently of this transport system. Indeed, endogenous circulating leptin results in detectable levels of baseline activated signal transducer and activator of transcription 3 (STAT3) phosphorylation in a population of ARC/LRb neurons, consistent with increased sensing of circulating leptin in these neurons compared with other LRb neurons. Furthermore, a population of ARC/LRb neurons that responds more rapidly and sensitively to circulating leptin compared with other hypothalamic LRb neurons detected by leptin activated phosphorylated STAT3. In addition, peripheral application of the BBB-impermeant retrograde tracer fluorogold revealed a population of ARC/LRb neurons that directly contact the circulation (e.g. via neuronal processes reaching outside the BBB). Taken together, these data suggest that a population of ARC/LRb neurons directly contacts the circulation and displays increased sensitivity to circulating leptin compared with neurons residing entirely behind the BBB elsewhere in the hypothalamus.

Effect of leptin on peroxidation and antioxidant defense in ethanol-supplemented Mus musculus heart

The aim of the study was to evaluate the effect of exogenous mouse leptin on ethanol-induced cardiac toxicity in mice. Administering ethanol (6.32 g/kg body weight p.o.) to 4-week-old healthy mice for 45 days resulted in significantly elevated plasma levels of leptin, lactate dehydrogenase (LDH), cardiac lipid hydroperoxides (LOOH) and thiobarbituric acid reactive substances (TBARS) and significantly decreased cardiac superoxide dismutase, catalase, vitamin C, vitamin E, reduced glutathione and glutathione-dependent enzyme levels (glutathione peroxidase and glutathione S-transferase). Subsequent to the experimental induction of toxicity (i.e., after the initial period of 30 days) exogenous leptin was administered (230 microg/kg body weight i.p.) every alternate day for 15 days along with the daily dose of ethanol. Leptin administration to ethanol-treated mice significantly elevated the levels of plasma leptin, LDH and cardiac LOOH, TBARS, whereas the activity of antioxidant enzymes and the concentrations of vitamins C and E were further decreased significantly. These findings were consistent with our histological observations, confirming that leptin enhances cardiac toxicity in ethanol-supplemented mice.

Pharmacokinetics of recombinant methionyl human leptin after subcutaneous administration: variation of concentration-dependent parameters according to assay

CONTEXT

Recombinant human leptin (r-metHuLeptin) has demonstrated efficacy in improving hormonal and metabolic parameters in leptin-deficient states, but pharmacokinetic parameters after sc administration have not yet been published. In addition, the effect of potential variability across different leptin assays on concentration-dependent pharmacokinetic parameters remains unknown.

OBJECTIVE

The objective of the study was to characterize pharmacokinetic parameters after sc r-metHuLeptin administration using three commercially available leptin assays (Linco, Diagnostic Systems Laboratories, and Alpco).

DESIGN, SETTING, PATIENTS, AND INTERVENTION

We analyzed pharmacokinetic profiles in five lean and five obese men after sc administration of physiological (0.01 mg/kg) and pharmacological (0.3 mg/kg) doses of r-metHuLeptin.

MAIN OUTCOME MEASURES

Leptin pharmacokinetic parameters were measured.

RESULTS

Measurement of leptin produced typical pharmacokinetic profiles in all assays with time to maximal concentration and half-life of approximately 3 h. Diagnostic Systems Laboratories consistently measured leptin higher than Linco, with Alpco measuring intermediate between or similar to Linco. There was high correlation among assays (R(2) ranging from 0.89 to 0.98, all P < 0.01). Concentration-dependent parameters such as maximal concentration, area under the curve, and clearance were significantly different among assays, whereas concentration-independent parameters such as time to maximal concentration and half-life were generally not different.

CONCLUSIONS

We report novel data on leptin pharmacokinetic parameters after sc administration, which will be relevant for the future therapeutic use of r-metHuLeptin. Although commercially available assays demonstrated high correlation, they can provide substantially different measures of leptin levels. This demonstrates the importance of standardizing leptin assays for diagnosing patients with relative leptin deficiency, determining appropriate doses of r-metHuLeptin for administration, and monitoring response to therapy.

Altered hypothalamic leptin, insulin, and melanocortin binding associated with moderate-fat diet and predisposition to obesity

Rats with a genetic predisposition to develop diet-induced obesity (DIO) have a preexisting reduction in central leptin and insulin sensitivity. High-fat diets also reduce sensitivity to leptin, insulin, and melanocortin agonists. We postulated that such reduced sensitivities would be associated with decreased binding to the hypothalamic leptin, insulin, and melanocortin receptors in selectively bred DIO rats and in rats fed a high-energy (HE; 31% fat) diet for 7 wk. On HE diet, DIO rats gained 15% more weight and had 121% heavier fat pads and 70% higher leptin levels than low fat chow-fed DIO rats. Diet-resistant (DR) rats gained no more weight on HE diet but had 48% heavier fat pads and 70% higher leptin levels than chow-fed DR rats. Compared with DR rats, DIO (125)I-leptin binding was 41, 36, and 40% lower in the hypothalamic dorsomedial, arcuate, and dorsomedial portion of the ventromedial nuclei, respectively, and arcuate (125)I-insulin binding was 31% lower independent of diet. In contrast, hypothalamic melanocortin binding did not differ between DIO and DR rats. However, HE diet intake lowered lateral hypothalamic melanocortin-3 and melanocortin-4 receptor and hippocampal insulin binding of both DIO and DR rats and hypothalamic paraventricular nucleus melanocortin-4 receptor binding only in DR rats. Neither genotype nor diet affected substantia nigra or ventral tegmental area binding. These results corroborate our previous findings demonstrating a preexisting decrease in DIO hypothalamic leptin and insulin signaling and demonstrate that HE diet intake reduces hypothalamic melanocortin and hippocampal insulin binding.

Brain uptake of intranasally applied radioiodinated leptin in Wistar rats

Leptin is mainly synthesized and secreted by fat cells in proportion to adipose tissue mass. Under physiological conditions, this hormone reduces food intake and increases thermogenesis through interactions with neurons in the central nervous system. However, transport of leptin into the central nervous system via the blood-brain barrier is saturable, and in obesity the feedback signal to the brain is markedly insufficient. In recent experiments we have shown, that intranasal (i.n.) delivery of leptin reduces food intake in rats. The aim of the present study was to explore the distribution of i.n. delivered leptin within brain, blood, and peripheral tissues. Application of [(125)I]leptin (0.03, 0.1, and 0.2 mg/kg) into male Wistar rats' nares (n = 8 per group) leads to supraphysiological brain leptin concentrations 30 min after application, with contents in the hypothalamus (7.3 +/- 2.6, 5.9 +/- 1.6, and 13.8 +/- 5.7 ng/g; P = 0.023; F = 6.157) being significantly higher than the brain average (1.2 +/- 0.2, 3.9 +/- 1.0, and 6.0 +/- 1.9 ng/g). In contrast, contents in the occipital/entorhinal cortex were lower than the brain average, indicating a minor participation of the transport via cerebrospinal fluid, which would have favored cerebrospinal fluid-exposed surfaces. In experiments employing the application of unlabeled leptin administered iv, we were able to show that excess blood leptin does not diminish brain uptake of i.n. leptin (as indicated by [(125)I]leptin), supporting a direct transport from nose to brain by circumvention of the blood-brain barrier. This study thus clearly demonstrates a rapid and highly effective transport of leptin from nose to brain.

Engineering a pharmacologically superior form of leptin for the treatment of obesity

Leptin plays a central role in the homeostasis of body weight through its regulatory effects on appetite and energy expenditure, yet in trials as a therapeutic agent for the treatment of obesity in humans it has been disappointing. The poor clinical efficacy of leptin results from its short circulating half-life, low potency and poor solubility, necessitating large and frequent doses to obtain even modest clinical benefit. Engineered Fc-leptin immunofusins, consisting of the Fc fragment of an immunoglobulin gamma chain followed by leptin, exhibit improved pharmacological properties with very consistent and potent biological activities. Furthermore, in extending the circulating half-life of the protein in vivo from a few minutes for leptin to many hours for Fc-leptin, these proteins have the potential to reduce drastically the dosage and frequency of administration required to obtain clinical benefit. The results of this study show that the engineered leptin immunofusins described here have significantly enhanced pharmacological properties in comparison with the recombinant leptin that was used in clinical trials. As such, they could represent an important step towards a therapeutically superior form of leptin if the disappointing performance of leptin in early clinical trials was due to its poor pharmacological properties rather than any conceptual weakness in the strategy of using leptin for the treatment of obesity and its related disorders.

The neurobiology of human obesity

Earlier ideas that sympathetic nervous system activity is low in human obesity, contributing to weight gain through absence of sympathetically mediated thermogenesis, can now be discounted. The application of sympathetic nerve recording techniques and isotope dilution methodology quantifying neurotransmitter release from sympathetic nerves has established that the sympathetic outflows to the kidneys and skeletal muscle vasculature are activated in obese humans. The cause remains unclear. The adipocyte hormone, leptin, stimulates the sympathetic nervous system in rodents, but whether this applies in humans is uncertain. Cross-sectional studies suggest a quantitative link exists between regional sympathetic nervous tone (most notably in the kidneys) and rates of leptin release, but definitive studies documenting that leptin administration activates the human sympathetic nervous system have not been done. What might be the clinical implications of these new findings? The demonstration that the suppressed sympathetic tone characterizing many experimental models of obesity does not exist in human obesity weakens the case for the use of beta3-adrenergic agonists as thermogenic agents to facilitate weight loss. Although the neurogenic character of obesity-related hypertension is now established, whether antiadrenergic antihypertensive drugs are the preferred agents for blood pressure reduction has not been adequately tested. Multiple site central venous sampling, disclosing release of leptin into the internal jugular veins, led to the demonstration that the leptin gene is also expressed in the brain, in addition to adipocytes. Brain resistance to leptin has been inferred in human obesity, given that overweight is accompanied by high plasma leptin levels. The fact that the genes for leptin and its receptors are normally expressed in the brain in human obesity, and that release of leptin from the brain is actually increased, argues against this. Brain leptin release has the potential to override the peripheral, adipocyte leptin system.

Premature emission

An actor who misses his cue can spoil a scene. Timing is equally important to keep metabolism running smoothly, according to a new study. The work suggests that a premature jump in the hormone leptin soon after birth boosts susceptibility to obesity later in life. The results might help explain why babies whose mothers are undernourished often grow into hefty adults.

Leptin orally supplied to neonate rats is directly uptaken by the immature stomach and may regulate short-term feeding

Although leptin is a hormone mainly produced by the adipose tissue, it is also produced by the gastric mucosa and the mammary epithelium and is present in maternal milk. The effects of milk leptin on the neonate are not known. The purpose of the investigation was to evaluate the short-term effects of the administration of a single oral dose of leptin on 4-d-old rats as well as the effects of chronic supplementation during the lactation period with a daily oral dose of leptin (equivalent to 5 times the amount of leptin ingested normally from maternal milk during the suckling period) on body weight, the gastric leptin system, gastric food content, and thermogenic capacity. Our results show that the administration of a single oral dose of 4 ng of leptin to 4-d-old rats produces a short-term increase in leptin levels in the stomach and serum and a decrease in the weight of the gastric contents. Pups treated with a daily oral dose of leptin during the whole lactation period showed, at the end of the suckling period, compared with controls, lower gastric contents, lower leptin production by the stomach and the sc adipose tissue, and lower thermogenic capacity in brown adipose tissue. We conclude that oral leptin is absorbed by the immature gastric epithelium of the neonate, and this leptin exerts clear biological effects, down-regulating endogenous leptin production and playing a potential role in the short-term control on food intake during the lactation period.

Correlation of tetradecylmaltoside induced increases in nasal peptide drug delivery with morphological changes in nasal epithelial cells

The effect of tetradecylmaltoside (TDM) on nasal peptide drug absorption was assessed with four peptides of distinct molecular size: insulin (5.7 kDa), leptin (16 kDa), somatropin (22.1 kDa), and epoetin alfa (30.4 kDa). The nasal uptake of the smallest peptides, insulin and leptin, was significantly increased at a TDM concentration of only 0.06%. The uptake of somatropin was significantly increased when concentrations of 0.125% or more were used. The uptake of the largest peptide, epoetin alfa, was not significantly increased, in the presence of 0.125-0.5% TDM. Light microscopy revealed that formulations containing 0.125% TDM caused moderate alterations in nasal epithelial cell morphology, while higher concentrations of TDM (0.5%), caused more extensive morphological changes. Following treatment with 0.125% TDM, the distribution of cilia was altered and the number of pinocytotic vesicles was increased, at a time that correlated with increased nasal absorption of insulin. Consistent with these findings, FITC-insulin applied nasally in the absence of TDM did not enter nasal epithelial cells, whereas FITC-insulin co-administered with 0.125% TDM was internalized into the cells, with a uniform distribution, consistent with transcellular movement of the peptide through the cells.

Plasma clearance and tissue distribution of radiolabeled leptin in the chicken

Leptin is an adipose and liver tissue-derived secreted protein in chickens that has been implicated in the regulation of food intake and whole-body energy balance. In this study, the metabolic clearance and tissue uptake of leptin were examined in the chicken (Gallus gallus). Four-week-old broiler males were infused with (125)I-labeled mouse leptin. Chromatography of radiolabeled leptin in plasma produced two peaks, one at 16 kDa (free leptin) and a free iodine peak. No leptin binding protein in blood was detected. Leptin was cleared with a half-life estimate of 23 min. In order to investigate the tissue distribution and uptake of radiolabeled leptin, multiple tissues were removed from infused birds at 15 and 240 min post-infusion, and trichloroacetic acid (TCA)-precipitable radioactivity was determined. The amounts of radioactivity at 15 min post-infusion in the tissues in rank order were: kidney, testis, lung, spleen, heart, liver, small and large intestine, gizzard, pancreas, bursa, leg and breast muscle, adrenals, and brain. A slightly different pattern of distribution was observed at 240 min post-infusion. We conclude from these studies that unlike mammals, no circulating leptin binding protein is present in chickens. Leptin is metabolized and cleared very rapidly from blood by the kidney.

Leptin and post-prandial satiety: acute central leptin more potently reduces meal frequency than meal size in the rat

RATIONALE

Many attempts to understand ingestion have sought to clarify the control of meals. Little is known about the effects of the anorexogenic hormone leptin on meal patterning.

OBJECTIVE

The present study sought to perform a dose-response analysis of the effects of acute central leptin administration on meal patterning using a validated, objective meal definition and to compare these results to those obtained with a previously used, subjective meal definition.

METHODS

To validate the objective meal definition pharmacologically, the microstructural effects of the well-studied compound fenfluramine (SC 0, 1, 2, 4 mg/kg) on spontaneous nocturnal intake were determined in mature, non-deprived male Wistar rats (n=8) using a full Latin square design. The effects of intracerebroventricular leptin administration (0, 0.3, 1, 3, 6.25 microg; n=10) were also examined, and perceived meal patterns obtained from the objective and subjective definitions were compared.

RESULTS

Fenfluramine reduced meal size and eating rate at doses that did not reduce meal frequency or duration. In contrast, comparably anorectic doses of leptin had potent post-meal satiety-like effects, reducing meal frequency and prolonging the intermeal interval without reducing average meal size, a finding opposite to that suggested by the previously used subjective meal definition. Unlike comparably and more anorectic doses of fenfluramine, leptin non-specifically reduced both prandial and non-prandial drinking.

CONCLUSIONS

Acute increases in central leptin levels may potently augment post-prandial satiety and influence body-fluid homeostasis. The results reveal unappreciated central modes of action for the ob protein which qualitatively differ from the intra-meal satiating-like effects of fenfluramine.

In vivo and in vitro evidence for a hepatic modulation of the leptin signal in rats

BACKGROUND

Leptin is primarily secreted by the adipose tissue. It binds not only to hypothalamic structures involved in energy regulation but also to many peripheral tissues including the liver. Leptin circulates in free and receptor-bound forms. Both components are differentially regulated under various pathophysiological conditions and serve different physiological functions. They are released from adipose tissue but previous data suggest an additional formation outside the fat compartment. Here we tested the contribution of the liver in binding and modulating leptin in the circulation.

MATERIALS AND METHODS

In vivo experiments were performed with radioactive labelled leptin with and without pretreatment with unlabelled leptin in freely moving, chronic intravenously cannulated male rats. Livers were investigated by immunohistochemistry and in situ hybridization and immunoblotting was performed, followed by ex vivo liver perfusion studies with human recombinant leptin.

RESULTS

In in vivo experiments radioactively labelled leptin (at low concentrations) is avidly bound to rat liver (greater than 80% of basal serum values 90 min following i.v. infusion). Pre-treatment with excess of unlabelled leptin in vivo revealed a rapid hepatic down-regulation of leptin receptor isoforms when tested by in situ hybridization, immunoblotting or immunohistochemistry. Ex vivo perfusion of rat liver with human recombinant leptin induced a dose- and time-dependent formation of receptor-bound leptin in the perfusate.

CONCLUSIONS

The present data support an active role of the liver in the modulation of the leptin signal through different regulation of the soluble leptin receptor, the bound and free forms of the hormone, which may have important implications for leptin's central efficacy and the development of 'leptin resistance'.

Leptin hormonal kinetics in the fed state: effects of adiposity, age, and gender on endogenous leptin production and clearance rates

Leptin is an adipocyte-secreted hormone that regulates energy homeostasis and neuroendocrine function. Replacement therapy with recombinant methionyl human leptin (r-metHuLeptin) improves obesity, insulin resistance, hyperlipidemia, and neuroendocrine dysfunction associated with low-leptin states. We administered three doses of r-metHuLeptin (0.1, 0.3, and 1.0 mg/kg) to healthy subjects to determine r-metHuLeptin pharmacokinetics in the fed state, to determine endogenous leptin production and clearance rates, and to study the effects of age, body mass index, gender, and race on r-metHuLeptin pharmacokinetics. We detected no dose-dependent effects on elimination half-life (t(1/2)), dose-normalized area under the curve (nAUC(0- infinity)), total body clearance (CL), or volume of distribution at steady state. The mean t(1/2), CL, and volume of distribution at steady state of r-metHuLeptin are 3.4 +/- 1.5 h, 79 +/- 16 ml/kg.h, and 150 +/- 39 ml/kg, respectively. Older subjects have a higher nAUC(0- infinity) (P = 0.003) and tend to have a decreased leptin production rate (Rsyn) and CL (P = 0.01). Increased body mass index is associated with higher baseline endogenous leptin levels (P < 0.0001), higher Rsyn (P < 0.0001), and longer t(1/2) (P = 0.008). Females have significantly greater baseline endogenous leptin levels and Rsyn than males (P < 0.0001). In summary, the leptin production rate is increased in females and with increasing adiposity, whereas leptin clearance is decreased with increasing adiposity, and nAUC(0- infinity) is increased with age. Elucidation of leptin pharmacokinetic parameters allows the accurate calculation of exogenous leptin replacement doses for humans in the fed state.

Role of brain-gut axis in healing of gastric ulcers

The previous studies demonstrated the pivotal role of capsaicin-sensitive peptidergic sensory neurons and vagal nerves in the maintenance of gastric mucosal integrity. The aim of the present study was: 1). to examine the effect of the functional ablation of sensory neurons with neurotoxic dose of capsaicin and surgical vagotomy on the course of healing of gastric ulcer in rat, and 2). to compare the ulcer healing action of leptin in rats with or without capsaicin-induced inactivation of sensory neurons. Three series of experiments (A, B and C) were performed in Wistar rats with gastric ulcers induced by acetic acid method. In series A, the course of ulcer healing was compared in rats with intact and capsaicin-inactivated sensory neurons. In the series B, the effect of vagotomy on the ulcer healing and accompanying changes in GBF were determined at day 8 and 16 after ulcer induction. The rats of series C, consisting of animals with intact nerves or those with capsaicin-denervation, received the 7-day treatment with exogenous leptin (10 microg/kg i.p. twice daily) to check whether blockade of sensory nerves could influence the acceleration of ulcer healing by this peptide. Capsaicin-induced ablation of sensory neurons significantly delayed ulcer healing and this was accompanied by the significant fall in the GBF and the significant rise in the gastric mucosal gene expression of IL-1beta and TNF-alpha. Vagotomy significantly delayed ulcer healing and led to decrease in GBF at ulcer margin. Treatment with exogenous leptin significantly accelerated ulcer healing, increased the GBF at ulcer margin and upregulated mRNA for iNOS and these effects were attenuated in rats with capsaicin-deactivation of sensory neurons. We conclude that: 1). vagal and sensory neurons contribute to the gastric ulcer healing process possibly due to the increase of GBF, the limitation of inflammatory response, and overexpression of TGFalpha and iNOS resulting in NO release, and 2). the acceleration of ulcer healing by leptin was attenuated in animals with capsaicin-denervation suggesting an involvement of neuropeptides released from sensory afferent nerves in the ulcer healing effect of this hormone.

Obesity-prone rats have normal blood-brain barrier transport but defective central leptin signaling before obesity onset

Rats selectively bred to develop diet-induced obesity (DIO) were compared with those bred to be diet resistant (DR) on a 31% fat high-energy diet with regard to their central leptin signaling and blood-brain barrier (BBB) transport. Peripheral leptin injection (15 mg/kg ip) into lean 4- to 5-wk-old rats produced 54% less anorexia in DIO than DR rats. DIO rats also had 21, 63, and 64% less leptin-induced immunoreactive phosphorylated signal transducer and activator of transcription 3 (pSTAT3) expression in the hypothalamic arcuate, ventromedial, and dorsomedial nuclei, respectively. However, hindbrain leptin-induced nucleus tractus solitarius pSTAT3 and generalized sympathetic (24-h urine norepinephrine) activation were comparable. Reduced central leptin signaling was not due to defective BBB transport since transport did not differ between lean 4- to 5-wk-old DIO and DR rats. Conversely, DIO leptin BBB transport was reduced when they became obese at 23 wk of age on low-fat chow or after 6 wk on high-energy diet. In addition, leptin receptor mRNA expression was 23% lower in the arcuate nuclei of 4- to 5-wk-old DIO compared with DR rats. Thus a preexisting reduction in hypothalamic but not brain stem leptin signaling might contribute to the development of DIO when dietary fat and caloric density are increased. Defects in leptin transport appear to be an acquired defect associated with the development of obesity and possibly age.

Modulatory role of testosterone in plasma leptin turnover in rats

We explored whether testosterone influences circulating leptin turnover in rats. Sham-operated male and female rats and 21-d gonadectomized rats treated or not treated with testosterone propionate were used. Anesthetized rats were implanted with an iv catheter, and then blood samples were drawn before and throughout a 60-min period following systemic leptin administration. Plasma testosterone, estradiol, and leptin concentrations were monitored. The results indicated that while gonadectomy blunted circulating concentrations of the homologous sex steroid, testosterone therapy, in gonadectomized rats, restored plasma testosterone concentrations to values found in normal male rats. Pharmacokinetic parameters evaluated during the test indicated the following: First, in the overall pharmacokinetic analyses, testosterone therapy in gonadectomized rats induced a more rapid disappearance of leptin from the circulation. Second, orchidectomy significantly enhanced the area under the curve (AUC) of circulating leptin, an effect fully reversed by testosterone treatment. Third, testosterone treatment in ovariectomized rats significantly decreased the AUC of leptin concentrations. Fourth, while gonadectomy alone did not modify circulating leptin half-life, conversely, testosterone therapy in gonadectomized rats decreased leptin halflife in the circulation. Finally, while orchidectomy reduced leptin body clearance, this parameter was increased by androgen therapy in gonadectomized rats. Our results strongly support that testosterone could play a main role in plasma leptin turnover by increasing leptin clearance rate and shortening plasma leptin half-life.

Sugar coating extends half-lives and improves effectiveness of cytokine hormones

Recombinant human erythropoietin (rhEPO) is an effective and widely used therapeutic agent that is produced by bioengineering. Modification of the rhEPO protein by glycoengineering increased its already abundant N-glycosylation, which enhances its erythropoietic activity in vivo by decreasing its metabolic clearance. Elliott et al. recently reported increased in vivo activities of thrombopoietin (Mpl ligand) and leptin following carbohydrate addition to both, which suggests that such glycoengineering could be applied to a variety of hormones, cytokines and growth factors.

Lipid profile in mice fed a high-fat diet after exogenous leptin administration

Previous studies suggest a possible link between leptin and decreased lipid levels, however, the role of leptin in high-fat diet-induced hyperlipidemia remains unclear. The aim of our study was to evaluate the effect of administering leptin on plasma and tissue lipids in mice fed a high-fat diet. Feeding a high-fat diet (2% cholesterol, 0.125% bile salts, 5% peanut oil) to four-week-old healthy mice for a period of 45 days, resulted in significantly elevated levels of plasma and tissue total cholesterol, phospholipids, free fatty acids and triglycerides as compared with those of the control mice. Subsequently after thirty days, exogenous leptin (230 microg/kg i.p.) was administered simultaneously with the daily dose of high-fat diet every alternate day for fifteen days. Leptin administration significantly reduced the levels of total cholesterol, phospholipids, free fatty acids and triglycerides in the plasma, liver, heart and kidney of both the control and high-fat diet fed mice. Moreover, leptin administration markedly reduced the levels of plasma LDL, VLDL and elevated plasma HDL and the activity of lipoprotein lipase as compared with the untreated control and high-fat diet fed mice. Thus, leptin administration was found to have a marked protective effect against hyperlipidemia and thus obesity, by virtue of its lipid lowering effects.

Pharmacokinetic model to describe the lymphatic absorption of r-metHu-leptin after subcutaneous injection to sheep

PURPOSE

The purpose of this work was to develop a pharmacokinetic model to describe the contribution of the lymphatics to the absorption and bioavailability of r-metHu-Leptin administered by subcutaneous (SC) injection to sheep.

METHODS

r-metHu-Leptin was administered either by bolus intravenous injection (0.1 mg/kg) into the jugular vein or by SC injection (0.15 mg/kg) into the interdigital space of the hind leg. The SC groups included a non-cannulated control group and a lymph-cannulated group, in which peripheral lymph was continuously collected from a cannula in the efferent popliteal lymph duct. Serum and lymph concentrations were determined by enzyme-linked immunosorbent assay and profiles were modeled using compartmental pharmacokinetic methods. The fraction of the dose reaching the systemic circulation (Fsys) and the proportions of the absorbed dose taken up via the blood (Fblood) and lymph (Flymph) were determined.

RESULTS

Serum and lymph concentration vs. time profiles were well described by a two compartment model with parallel first order absorption into blood and lymph. Fsys for the SC control group was 60.4 +/- 8.4%. In the lymph-cannulated group, 21.7 +/- 6.4% of the dose was recovered in serum and 34.4 +/- 9.7% was recovered in peripheral lymph giving a total fraction absorbed (Fabs) of 56.0 +/- 10.3%. Fsys for the SC control group was not significantly different to Fabs in the lymph-cannulated group.

CONCLUSION

This study has shown that the lymph represents the predominant pathway for absorption of r-metHu-Leptin after SC administration.

Leptin distribution and metabolism in the pregnant rat: transplacental leptin passage increases in late gestation but is reduced by excess glucocorticoids

Leptin is essential for the establishment of pregnancy and appears to promote fetal growth, but the mechanisms regulating fetal leptin exposure remain unclear. In rodents, indirect evidence suggests that fetal leptin is partly derived from the maternal circulation via transplacental passage. Indeed, the placenta expresses mRNA for Ob-Ra, one of the short forms of the leptin receptor (Ob-R(S)) important in leptin transport, and this expression increases markedly in late pregnancy. Therefore, we determined the transplacental passage of maternal leptin to the fetus in the rat and whether this transport increases near term in association with a rise in placental expression of Ob-R(S) protein. Because of the proposed role of leptin in promoting fetal growth, we also assessed the effect of glucocorticoid-induced fetal growth retardation on placental leptin transport. Anesthetized rats received a constant infusion of (125)I-leptin via a jugular cannula before and at d 16 and 22 of pregnancy (term = d 23); plasma samples were obtained at 10, 20, 40, 60, 80, and 100 min, and fetuses and placentas were collected at the time of the final sample. The metabolic clearance rate of leptin fell (P < 0.01) from 3.08 +/- 0.23 ml/min per kg in nonpregnant rats to 2.36 +/- 0.13 ml/min per kg by d 22. Transplacental passage of (125)I-leptin, estimated from its concentration in the whole fetus relative to maternal plasma, increased 10-fold (P < 0.005) between d 16 and d 22 of pregnancy. Over this same period, Ob-R(S) protein expression in the placental labyrinth zone increased by almost 2-fold. Transplacental leptin passage was reduced (P < 0.05) by 77% after maternal dexamethasone treatment, whereas suppression of endogenous glucocorticoid synthesis (by metyrapone) increased (P < 0.05) the transfer of maternal leptin to the fetus by 55%. These data show that transplacental passage of maternal leptin is a significant source of fetal leptin and increases markedly during late pregnancy. Consistent with the proposed role of leptin as a fetal growth factor, transplacental leptin passage is reduced in association with glucocorticoid-induced fetal growth retardation.

Impaired transport of leptin across the blood-brain barrier in obesity is acquired and reversible

Leptin resistance is a major cause of obesity in humans. A major component of this resistance is likely an impaired transport of leptin across the blood-brain barrier (BBB). The fattest subgroup of otherwise normal 12-mo-old CD-1 mice have severely impaired transport of leptin across the BBB. However, it is unknown whether these mice are born with a BBB impairment or acquire it with aging and obesity. Here, we found within an otherwise normal population of CD-1 mice that the 10% fattest mice gained weight throughout a 12-mo-life span, whereas the 10% thinnest mice gained little weight after 3 mo of age. The fattest mice acquired a progressive impairment in their ability to transport leptin across the BBB, whereas the thinnest mice had a rate of transport that did not change with age. Fasting fat mice for 24 h or treating them with leptin resulted in modest weight reduction and development of transport rates for leptin across the BBB similar to those of thin mice. These results show that, in obese CD-1 mice, the impaired transport of leptin across the BBB develops in tandem with obesity and is reversible with even modest weight reduction.

Changes in cerebral endothelial barrier antigen, without alteration of permeability for intravenously injected leptin in diet-induced obesity in rats

Leptin, a potent anorectic, 16-kDa, adipose tissue-derived protein, predominantly acts in hypothalamic nuclei, signaling obesity and modulating ingestive behavior. To reach this brain area, leptin, probably has to cross the blood-brain barrier (BBB). In some cases of obesity, enhanced leptin levels in the blood do not result in anorectic effects, probably due to an altered leptin transport across the BBB. Therefore, we investigated the BBB in lean and diet-induced obese Lewis rats. To obtain information about the presence of microvessels with barrier dysfunction we examined three brain areas (hypothalamus, cortex, hippocampus) using a monoclonal antibody which detects intact microvessels of the BBB (anti-endothelial barrier antigen, anti-EBA). The results showed a significantly reduced EBA staining in the brain sections of the obese animals, except the hippocampus, compared to the control group. In a second step we injected I125-labeled leptin intravenously (i.v.) in permanent i.v.-cannulated, unrestrained Lewis rats (lean and obese). We measured the radioactivity in the cerebrospinal fluid after puncture of the cisterna magna, in the blood and brain tissue 90 min after injection. The leptin content in the cerebrospinal fluid and brain was not reduced in obese compared to lean rats, thus showing a similar transport capacity of the BBB in both experimental groups. Therefore, the results of the in vivo investigations do not indicate an impairment of the BBB in diet-induced obesity, despite the immunohistological findings. Further functional and morphological studies are necessary to evaluate the specific role of other organs and distinct forms of leptin (free and protein-bound) in the pathogenesis of diet-induced obesity.

Pegylated human recombinant leptin (PEG-OB) causes additional weight loss in severely energy-restricted, overweight men

BACKGROUND

Increasing evidence suggests that falling leptin concentrations observed during fasting act as a peripheral signal of starvation, which serves to conserve energy in the face of limited reserves. An extension of this hypothesis is that exogenous leptin should affect energy regulation during severe energy restriction.

OBJECTIVE

To explore this hypothesis, we assessed whether elevated leptin concentrations achieved with the use of long-acting pegylated human recombinant leptin [polyethylene glycol-OB protein (PEG-OB)] affected weight loss and changes in body composition, energy expenditure, appetite, and metabolic variables during semistarvation in healthy overweight men.

DESIGN

A randomized, double-blind, placebo-controlled study was executed in overweight men with a mean (+/- SEM) age of 34.8 +/- 1.3 y and body mass index (in kg/m2) of 28.8 +/- 0.5. All subjects received weekly treatment with 80 mg PEG-OB (n = 12) or matching placebo (n = 10) for 46 d while their energy intake was reduced to 2.1 MJ/d by means of a very-low-energy diet. Body composition (hydrodensitometry and deuterium dilution), energy expenditure (ventilated hood), and appetite (visual analogue scales) were evaluated at the start and the end of the study. Metabolic variables were measured throughout the study period.

RESULTS

Compared with placebo treatment, treatment with PEG-OB led to significant (P < 0.03) additional weight loss (14.6 +/- 0.8 compared with 11.8 +/- 0.9 kg) and a reduction in appetite (P < 0.05) after 46 d, but the 2 treatment groups did not differ significantly in changes in body composition, energy expenditure, and metabolic variables.

CONCLUSION

Our observations support the hypothesis that the decrease in leptin concentrations during starvation increases appetite in humans.

Leptin transport across the blood-brain barrier of the Koletsky rat is not mediated by a product of the leptin receptor gene

Obesity in humans is thought to be caused by a resistance to leptin. Currently, the evidence suggests that this resistance is caused by an impaired transport of leptin across the blood-brain barrier (BBB). It has been assumed that the short form of the leptin receptor, which is a splice variant of the gene which produces all known leptin receptors, is the leptin transporter, but evidence for this is mixed. The Koletsky rat model should provide a clear answer as to whether transport is dependent on leptin receptors as it does not express any functional receptors. The transport of intravenous leptin across the BBB of the Koletsky rat has been found to be greatly reduced, but evidence for a residual of transport makes it unclear whether the transporter is essentially absent or simply saturated by the high levels of leptin in the serum. Here we used the brain perfusion method to negate the influence of serum levels. We found that, whereas no transport of intravenous leptin occurred in the obese Koletsky, the rate of transport was no different from controls when brain perfusion was used. Leptin was transported completely across the BBB, was saturable, and had the same distribution among brain regions as previously found in normal weight mice (highest transport into the hippocampus and hypothalamus, lowest in the frontal cortex). We conclude that a leptin transporter and possibly its gene have yet to be identified and that the short form likely plays a role in the modulation of transport activity.

Leptin uptake by serotonergic neurones of the dorsal raphe

The effects of leptin on food intake, metabolism, sleep patterns and reproduction may be mediated, in part, by the midbrain serotonergic systems. Here, we report on the distribution of neurones that accumulate leptin in the raphe nuclei of male and female rats after intracerebroventricular administration of mouse recombinant leptin labelled with digoxigenin. Direct leptin-targeted cells were present in the periventricular grey, pontine and raphe nuclei. Confocal microscopy revealed that raphe neurones which accumulated leptin were predominantly serotonergic. The temporal pattern of leptin accumulation by raphe neurones showed a marked gender difference: 6 h after leptin administration, all male and female rats showed massive leptin binding in the dorsal raphe, while 30 min after leptin treatment, only 10% of male rats exhibited leptin-labelled cells in contrast to 50% of females. The present observations reveal that leptin can be selectively accumulated by serotonergic neurones in the raphe nuclei and that this mechanism is gender specific. These findings support the idea that the midbrain serotonergic system is an important mediator of the effects of leptin on brain function and may provide an explanation for gender differences in metabolism regulation and its coordination with higher functions of the brain.

Strategies for the delivery of leptin to the CNS

Leptin is the major regulator of body fat. It is a 16 kD protein released by fat cells into the blood and crosses the blood-brain barrier (BBB) to interact with its receptors at the arcuate nucleus to affect feeding, thermogenesis, and other functions. Within normal and obese body weight ranges, serum and cerebrospinal fluid (CSF) levels of leptin directly correlate with body mass index and adiposity. In animals, leptin at high levels exerts effects on appetite and at low levels informs the brain when fat reserves are adequate to switch behavioral, endocrine, and immune functions from starvation mode. Leptin offers a unique therapeutic opportunity for conditions related to body weight control, such as reversal of obesity and anorexia, and as an indirect treatment for diseases related to being over- or under-weight, such as insulin resistant diabetes and the endocrine changes accompanying starvation. In humans and in many rodent models, obesity may be a consequence of leptin resistance. More specifically, resistance likely results from an impaired transport of leptin across the BBB. Peripheral administration of native leptin results in weight reduction in moderately obese individuals and weight loss and reversal of insulin resistance and dyslipidemia in individuals with low leptin levels. The peripheral pharmacokinetic and BBB transport characteristics of native leptin suggests strategies for improving the therapeutic profile of leptin. These strategies include the development of longer lasting and more permeable analogs, development of antagonists, enhancing the activity of the leptin transporter, and delivering leptin by intrathecal administration.

Positron emission tomography shows that intrathecal leptin reaches the hypothalamus in baboons

Human obesity may be caused by a resistance to circulating leptin. Evidence from rodents and humans suggests that a major component of this resistance is an impairment in the ability of the blood-brain barrier (BBB) to transport leptin from the blood to the brain. One potential way to bypass the BBB is by administering leptin into the intrathecal (i.t.) space. To be effective, i.t. leptin would have to move caudally from the site of injection, enter the cranium, and reach the hypothalamic arcuate nucleus at the base of the pituitary fossa. However, many substances, especially small, lipid-soluble molecules, do not diffuse far from the site of i.t. injection but are resorbed back into blood. To determine whether i.t. leptin can move caudally, we injected leptin conjugated to diethylenetriaminepentaacetic acid (DTPA) and labeled with (68)Ga (G-Ob) into the lumbar space of three baboons. We also studied unconjugated DTPA labeled with (68)Ga, which did not move up the spinal cord but rapidly appeared in blood after i.t. injection. In contrast, G-Ob steadily moved toward the cranium and had reached the hypothalamus 91 and 139 min after i.t. injection in two baboons. We estimated the concentration of leptin in the hypothalamic region to be at least 8 ng/ml, which is about 40 times higher than cerebrospinal fluid levels in normal weight humans and about 4 times higher than the highest level ever recorded after the peripheral administration of leptin. In a third baboon, the leptin neither moved caudally nor appeared in the blood. We conclude that leptin administered i.t. can reach the hypothalamus in therapeutic concentrations, although there is considerable individual variation.

Leptin: a new growth factor for the small intestine

PURPOSE

This study was designed to evaluate the potential growth factor effects of systemic administration of leptin on mucosal mass and absorptive function in normal rat intestine.

METHODS

Twenty male Sprague-Dawley rats underwent placement of a jugular venous catheter connected to a subcutaneous osmotic pump designed to deliver its contents at a constant rate. The rats were divided into 4 groups (n = 5 per group) based on the contents of the osmotic pump: group 1, 0.1% bovine serum albumin; group 2, leptin, 6.25 microgram/kg/d; Group 3, leptin, 18.75 microgram/kg/d; Group 4, leptin, 43.75 microgram/kg/d. After a 14-day infusion, [(14)C] galactose and [(14)C] glycine absorption were determined using a closed, recirculation technique. DNA content was determined from mucosal biopsies. Total RNA was extracted from mucosal samples, reverse transcribed, and amplified via polymerase chain reaction for the following primer pairs: sodium/glucose cotransporter (SGLT-1), fructose transporter (GLUT-5), and glyceraldehyde-3-phosphate dehydrogenase (GAPDH, internal standard). Statistical analysis was performed by analysis of variance and expressed as mean plus minus SEM.

RESULTS

Systemic administration of increasing doses of leptin enhanced DNA content when compared with the appropriate control (group 2, 1.06 plus minus 0.04 [P <.05]; group 3, 1.1 plus minus 0.05 [P <.01]; and group 4, 1.07 plus minus 0.06 [P <.05]. Leptin enhanced mucosal absorptive function (galactose: group 2, 2.31 plus minus 0.15 [P <.01]; group 3, 2.71 plus minus 0.06 [P <.01]; group 4, 2.19 plus minus 0.28 [P <.05]; glycine: group 2, 2.34 plus minus 0.31 [P <.05]; group 3, 3.32 plus minus 0.14 [P <.01]; group 4, 3.1 plus minus 0.27 [P <.01]) in the normal intestine when compared with the appropriate control animals. Also, leptin enhanced the gene expression of the carbohydrate transporters when compared with the appropriate control rats.

CONCLUSIONS

These data show that systemic leptin administration enhances mucosal mass and absorptive function in normal rat intestine. Thus, leptin appears to be a growth factor for normal small intestine and may play a role in patients who acquire intestinal dysfunction.

Validity of multiple-time regression analysis in measurement of tritiated and iodinated leptin crossing the blood-brain barrier: meaningful controls

Multiple-time regression analysis has been used to study the influx of radiolabeled peptides and polypeptides across the blood-brain barrier (BBB). This study used both tritiated and iodinated leptin to clarify several issues associated with these measurements. Recombinant murine leptin was radiolabeled with 3H by derivatization or with 125I by the iodobead method and each studied separately in mice. Intact 3H-leptin had a higher apparent influx rate from blood to brain than did intact 125I-leptin, correlating with its higher proportion of reversible association with the capillary lumen that would misleadingly appear to reflect entry. Yet the majority of 3H-leptin and 125I-leptin reached brain parenchyma. There was no significant difference in the influx rate between cerebral cortex and the subcortical regions, thus ruling out a predominant contribution of simple diffusion through the circumventricular organs or choroid plexuses outside the BBB. The influx of radiolabeled leptin, especially 125I-leptin, was decreased by excess unlabeled leptin, supporting the presence of a saturable transport system for leptin at the BBB. To identify the specificity of the transport system and determine whether it is shared by 3H-leptin and 125I-leptin, these radioactively labeled leptins were heat-denatured. Denaturation had no effect on the fast influx of 3H-leptin, but abolished the entry of 125I-leptin into brain; excess denatured leptin failed to inhibit the influx of either 3H-leptin or 125I-leptin. This indicates that the conformation of 125I-leptin is similar to that of native unlabeled leptin, so that iodination would be the better choice for investigating the interaction of leptin with the BBB. However, 3H-leptin can use the same transport system, as shown by inhibition of its influx by unlabeled leptin, whereas the derivatization procedure altered its biophysical properties such that its non-saturated influx was greatly enhanced. Finally, the rapid influx of radioactively labeled leptin contrasted greatly with that of the reference compounds 99mTc-albumin and 3H-inulin which had no significant penetration of the BBB. Thus, with additional considerations such as stability and interactions with the vasculature, multiple-time regression analysis is sensitive and selective for study of the penetration of peptides across the BBB.

Decreased leptin uptake in hypothalamic nuclei with ageing in Wistar rats

Leptin interacts with specific receptors in hypothalamic nuclei and modulates energy balance. Growing evidence has shown the association of obesity and hyperleptinaemia with non-insulin-dependent diabetes mellitus and insulin resistance. The aged Wistar rat shows peripheral insulin resistance in the absence of obesity and alterations of glucose homeostasis. However, it is not known whether, in these animals, the leptin action is altered. Here we studied the effect of ageing on plasma leptin concentration and the ability of hypothalamic nuclei to capture i.c.v.-injected digoxigenin-labelled leptin. Our data indicate that 24-month-old animals are hyperleptinaemic. However, daily food intake was greater in old animals, suggesting that they are leptin resistant. Leptin uptake in the hypothalamus was reduced in old rats. This uptake was a receptor-mediated process as demonstrated by displacement. Leptin accumulation in hypothalamic nuclei was partially colocalized with neuropeptide Y fibres. Immunohistochemical and western blot analyses showed a lower amount of the long form of leptin receptors in the hypothalamus of aged rats. Analysis by RT-PCR also demonstrated a decreased expression of leptin receptor mRNA in old animals. We conclude that the lower leptin uptake may be explained, at least in part, by a decreased amount of receptors in hypothalamic neurones of the aged rats.

Glucose and insulin increase the transport of leptin through the blood-brain barrier in normal mice but not in streptozotocin-diabetic mice

Since fasting is one of the few factors found to change the rate of entry of leptin into brain, we used multiple-time regression analysis to study the effects of pretreatment with glucose or insulin on leptin transport across the blood-brain barrier (BBB). Two hours after intraperitoneal injection of glucose (3 g/kg), there was a statistically significant increase in the entry rate (K(i)) of leptin in fasted (from 4.91 +/- 0.70 x 10(-4) ml/g x min to 9.03 +/- 1.00 x 10(-4) ml/g x min) but not (p = 0.15) in nonfasted normal (from 4.90 +/- 1.21 x 10(-4) ml/g x min to 6.42 +/- 1.79 x 10(-4) ml/g x min) or fasted streptozotocin (STZ)-treated diabetic mice (from 4.043 +/- 0.959 x 10(-4) ml/g min to 5.395 +/- 1.355 x 10(-4) ml/g min). Insulin (10 U/kg) increased leptin influx in fasted (from 4.77 +/- 0.26 x 10(-4) ml/g x min to 10.6 +/- 0.15 x 10(-4) ml/g x min at 0.5 h) and nonfasted (from 4.64 +/- 0.75 x 10(-4) ml/g x min to 7.46 +/- 1.48 x 10(-4) ml/g x min at 0.5 h) normal mice, but not in STZ-diabetic mice deficient in insulin (and leptin), even though basal concentrations of glucose were similarly increased in the nonfasted normal and STZ-treated mice. Moreover, the basal rate of leptin influx was the same in overnight fasted normal mice, nonfasted normal mice and STZ-diabetic mice. The results indicate that glucose and insulin can increase leptin transport, but they probably are not the principal factors responsible for the regulatory effect of the BBB on leptin entry into the brain.

Leptin transport at the blood--cerebrospinal fluid barrier using the perfused sheep choroid plexus model

Leptin is secreted by adipose tissue and thought to regulate appetite at the central level. Several studies have explored the central nervous system (CNS) entry of this peptide across the blood-brain and blood-cerebrospinal fluid (CSF) barriers in parallel, but this is the first to explore the transport kinetics of leptin across the choroid plexus (blood-CSF barrier) in isolation from the blood-brain barrier (BBB). This is important as the presence of both barriers can lead to ambiguous results from transport studies. The model used was the isolated Ringer perfused sheep choroid plexus. The steady-state extraction of [(125)I]leptin (7.5 pmol l(-1)) at the blood face of the choroid plexus was 21.1+/-5.7%, which was greater than extraction of the extracellular marker, giving a net cellular uptake for [(125)I]leptin (14.0+/-3.7%). In addition, trichloroacetic acid precipitable [(125)I] was detected in newly formed CSF, indicating intact protein transfer across the blood-CSF barrier. Human plasma concentrations of leptin are reported to be 0.5 nM. Experiments using 0.5 nM leptin in the Ringer produced a concentration of leptin in the CSF of 12 pM (similar to that measured in humans). [(125)I]Leptin uptake at the blood-plexus interface using the single-circulation paired tracer dilution technique (uptake in <60 s) indicated the presence of a saturable transport system, which followed Michaelis-Menten-type kinetics (K(m)=16.3+/-1.8 nM, V(max)=41.2+/-1.4 pmol min(-1) g(-1)), and a non-saturable component (K(d)=0.065+/-0.002 ml min(-1) g(-1)). In addition, secretion of new CSF by the choroid plexuses was significantly decreased with leptin present. This study indicates that leptin transport at the blood-CSF barrier is via saturable and non-saturable mechanisms and that the choroid plexus is involved in the regulation of leptin availability to the brain.

Pharmacokinetics of human leptin in mice and rhesus monkeys

OBJECTIVE

The pharmacokinetic characteristics of human leptin were examined in rhesus monkeys and in C57BL/6J mice fed a normal chow or a high-fat diet.

DESIGN

For the monkey study, in nine rhesus monkeys (body weight 12.4 +/- 2.4 kg; mean +/- s.d.), recombinant met-human leptin was injected intravenously or subcutaneously (1 mg/kg). For the mouse study, after 6 months of feeding C57BL/6J mice a high-fat diet (body weight 32.9 +/- 3.6 g; n = 8) or a control diet (24.5 +/- 1.2 g; n = 6), recombinant met-human leptin was administered intraperitoneally (10 microg/g). Blood samples were collected for leptin measurement at specific time points after leptin administration.

MEASUREMENTS

Plasma leptin concentrations were determined by radioimmunoassay and pharmacokinetic analysis was performed.

RESULTS

Disposition of human leptin in rhesus monkeys was biphasic following intravenous administration, with a terminal phase half-life of 96.4 +/- 16.5 min and clearance of 1.8 +/- 0.2 ml/min/kg. Subcutaneously administered leptin was absorbed slowly, perhaps by a zero-order process as leptin levels appeared to plateau and remained elevated throughout the 8 h sampling period. In C57BL/6J mice, the absorption and elimination of human leptin were both first-order following intraperitoneal administration. Pharmacokinetic parameters did not differ between normal-weight mice fed a chow diet and obese mice fed a high-fat diet. The elimination half-life was 47.0 +/- 26.4 min in mice fed a high-fat diet and 49.5 +/- 12.0 min in mice fed a control diet.

CONCLUSION

The kinetics of leptin in rhesus monkeys were biphasic and clearance was similar to values previously reported in humans. The estimated half-life was 96.4 min in rhesus monkeys and 49.5 min in normal weight mice. The was no difference in leptin kinetics between high-fat fed and control mice, suggesting that the increased baseline leptin levels in the obese mice are due to increased leptin production and secretion.

An immunoadsorption strategy to produce specific antisera against analogs of human proteins: development of sensitive and specific radioimmunoassays for two analogs of human leptin

Immunoassay technology is routinely used to measure concentrations of proteins and polypeptides in biological matrices. Increasingly, research efforts have sought to create analogs of human proteins with the aim of improving efficacy or pharmaceutical properties relative to the native protein. Pharmacokinetic assessment of these polypeptide analogs, however, can be greatly confounded by the presence of endogenous native protein. This report describes an immunization and immunoabsorption strategy that was used to create monospecific polyclonal antibodies against analogs of human leptin (LY355101 and LY396623, one and two amino acid changes relative to native human leptin, respectively). Rabbits were immunized with either LY355101 or LY396623. Antisera were screened to determine if any showed increased specificity for the analog relative to native human leptin. Antisera showing increased specificity for the leptin analog were then treated by immunoabsorption against native human leptin, thus depleting human leptin cross-reactivity. The antibodies developed in this process were used in radioimmunoassays. which were validated for use in clinical studies. Both assays proved to be highly specific for LY355101 or LY396623 in the presence of native human leptin. Use of this procedure permitted the measurement of LY355101 and LY396623 pharmacokinetics that were not confounded by the high levels of endogenous human leptin found in obese subjects. This technique has the potential for broad application in the development of assays capable of specifically measuring protein analogs without cross-reactivity to an endogenous substance.

Persistence of blood-to-brain transport of leptin in obese leptin-deficient and leptin receptor-deficient mice

In lean CD-1 mice, leptin is delivered into the brain by a saturable transport mechanism. Previous work has shown that obesity is associated with decreased leptin transport. Here, we investigated the transport of leptin across the blood-brain barrier (BBB) in two murine models of obesity. Radioiodinated leptin was intravenously injected into ob/ob (no leptin production) and db/db (high leptin levels, but no long-form leptin receptor) mutant mice and their lean controls. In all groups, the labeled polypeptide was transported across the BBB by a saturable mechanism. The rates of transport were not significantly different between the mutant strains and their lean controls. The results demonstrate that leptin transport persists in the absence of production of the endogenous polypeptide or its signal-transducing receptor and suggest that the impaired transport previously seen is not directly explained by only obesity or alterations in serum plasma levels.

Biologic response to peripheral and central administration of recombinant human leptin in dogs

OBJECTIVE

Because leptin is believed to act within the central nervous system, the objective of this study was to test that presumption by comparing the biologic responses to recombinant human leptin (rHuLeptin) when delivered either subcutaneously or intrathecally in a large animal species, the beagle dog.

METHODS AND PROCEDURES

Adult beagle dogs were used for all studies (n=3 to 14). Treatment with rHuLeptin was either as daily subcutaneous or intermittent intrathecal injections.

RESULTS

Subcutaneously administered rHuLeptin was absorbed with peak concentrations appearing at 2 to 4 hours. After intrathecal administration, cerebral spinal fluid concentrations declined in a bi-phasic manner with a terminal half-life of -6 to 8 hours. When lean beagles were given leptin subcutaneously, at 0.05 to 5 g/kg/day for up to 6 months, reductions in body weight (up to 30%) and food intake (up to 75%) were observed. Body fat loss was observed in both lean and obese dogs, and confirmed by dual energy X-ray absorptiometry and histology of adipose tissue. When rHuleptin was delivered intrathecally at 4 to 1000 microg/dose for up to 3 months, the primary effects observed were reductions in body weight and food intake. In general all findings reported in the intrathecal studies were consistent with those noted in the subcutaneous studies; however, the required intrathecal dose was substantially lower than that for subcutaneous delivery.

DISCUSSION

These studies demonstrate that both subcutaneous and intrathecal treatment of rHuLeptin was associated with effects on body weight, food intake, and body fat in dogs. These results support the concept that the central nervous system is the probable primary site of action for leptin and suggest that rHuLeptin has similar physiologic activities that influence body weight, body fat, and metabolism in large animals to those reported previously in rodents.

Recombinant leptin for weight loss in obese and lean adults: a randomized, controlled, dose-escalation trial

CONTEXT

The protein hormone leptin is important to the homeostatic regulation of body weight. Treatment with exogenous leptin may affect weight loss.

OBJECTIVE

To determine the relationship between increasing doses of exogenous leptin administration and weight loss in both lean and obese adults.

DESIGN

A randomized, double-blind, placebo-controlled, multicenter, escalating dose cohort trial conducted from April 1997 to October 1998.

SETTING

Four university nutrition and obesity clinics and 2 contract clinical research clinics.

PARTICIPANTS

Fifty-four lean (body mass index, 20.0-27.5 kg/m2; mean [SD] body weight, 72.0 [9.7] kg) and 73 obese (body mass index, 27.6-36.0 kg/m2; mean [SD] body weight, 89.8 [11.4] kg) predominantly white (80%) men (n = 67) and women (n = 60) with mean (SD) age of 39 (10.3) years.

INTERVENTIONS

Recombinant methionyl human leptin self-administered by daily morning subcutaneous injection (0 [placebo], 0.01, 0.03, 0.10, or 0.30 mg/kg). In part A, lean and obese subjects were treated for 4 weeks; in part B, obese subjects were treated for an additional 20 weeks. Lean subjects consumed a eucaloric diet to maintain body weight at the current value, and obese subjects were prescribed a diet that reduced their daily energy intake by 2100 kJ/d (500-kcal/d) from the amount needed to maintain a stable weight.

MAIN OUTCOME MEASURES

Body weight, body fat, and incidence of adverse events.

RESULTS

Weight loss from baseline increased with increasing dose of leptin among all subjects at 4 weeks (P = .02) and among obese subjects at 24 weeks (P = .01) of treatment. Mean (SD) weight changes at 4 weeks ranged from -0.4 (2.0) kg for placebo (n = 36) to -1.9 kg (1.6) kg for the 0.1 mg/kg dose (n = 29). Mean (SD) weight changes at 24 weeks ranged from -0.7 (5.4) kg for the 0.01 mg/kg dose (n = 6) to -7.1 (8.5) kg for the 0.30 mg/kg dose (n = 8). Fat mass declined from baseline as dose increased among all subjects at 4 weeks (P = .002) and among obese subjects at 24 weeks of treatment (P = .004); more than 95% of weight loss was fat loss in the 2 highest dose cohorts at 24 weeks. Baseline serum leptin concentrations were not related to weight loss at week 4 (P = .88) or at week 24 (P = .76). No clinically significant adverse effects were observed on major organ systems. Mild-to-moderate reactions at the injection site were the most commonly reported adverse effects.

CONCLUSIONS

A dose-response relationship with weight and fat loss was observed with subcutaneous recombinant leptin injections in both lean and obese subjects. Based on this study, administration of exogenous leptin appears to induce weight loss in some obese subjects with elevated endogenous serum leptin concentrations. Additional research into the potential role for leptin and related hormones in the treatment of human obesity is warranted.