Investigation into the presence of insulin-degrading enzyme in cultured type II alveolar cells and the effects of enzyme inhibitors on pulmonary bioavailability of insulin in rats

Modulation of Insulin Sensitivity by Insulin-Degrading Enzyme

Insulin-degrading enzyme (IDE) is a highly conserved and ubiquitously expressed metalloprotease that degrades insulin and several other intermediate-size peptides. For many decades, IDE had been assumed to be involved primarily in hepatic insulin clearance, a key process that regulates availability of circulating insulin levels for peripheral tissues. Emerging evidence, however, suggests that IDE has several other important physiological functions relevant to glucose and insulin homeostasis, including the regulation of insulin secretion from pancreatic β-cells. Investigation of mice with tissue-specific genetic deletion of Ide in the liver and pancreatic β-cells (L-IDE-KO and B-IDE-KO mice, respectively) has revealed additional roles for IDE in the regulation of hepatic insulin action and sensitivity. In this review, we discuss current knowledge about IDE’s function as a regulator of insulin secretion and hepatic insulin sensitivity, both evaluating the classical view of IDE as an insulin protease and also exploring evidence for several non-proteolytic functions. Insulin proteostasis and insulin sensitivity have both been highlighted as targets controlling blood sugar levels in type 2 diabetes, so a clearer understanding the physiological functions of IDE in pancreas and liver could led to the development of novel therapeutics for the treatment of this disease.

Extreme subcutaneous, intramuscular and inhaled insulin resistance treated by pancreas transplantation alone

Diabetes mellitus with resistance to insulin administered subcutaneously or intramuscularly (DRIASM) is a rare syndrome and is usually treated with continuous intravenous insulin infusion. We present here two cases of DRIASM in 16 and 18 years female patients that were submitted to pancreas transplantation alone (PTA). Both were diagnosed with type 1 diabetes as young children and had labile glycemic control with recurrent episodes of diabetic ketoacidosis. They had prolonged periods of hospitalization and complications related to their central venous access. Exocrine and endocrine drainages were in the bladder and systemic, respectively. Both presented immediate graft function. In patient 1, enteric conversion was necessary due to reflux pancreatitis. Patient 2 developed mild postoperative hyperglycemia in spite of having normal pancreas allograft biopsy and that was attributed to her immunosuppressive regimen. Patient 1 died 9 months after PTA from septic shock related to pneumonia. In 8 months of follow-up, Patient 2 presented optimal glycemic control without the use of antidiabetic agents. In conclusion, PTA may be an alternative treatment for DRIASM patients.

Application of supercritical fluid to preparation of powders of high-molecular weight drugs for inhalation

The application of supercritical carbon dioxide to particle design has recently emerged as a promising way to produce powders of macromolecules such as proteins and genes. Recently, an insulin powder for inhalation was approved by authorities in Europe and the USA. Other macromolecules for inhalation therapy will follow. In the 1990s proteins were precipitated with supercritical CO(2) from solutions in an organic solvent such as dimethylsulfoxide, which caused significant unfolding of protein. Since 2000, aqueous solutions of proteins and genes have generally been used with a cosolvent such as ethanol to precipitate in CO(2). Operating conditions such as temperature, pressure, flow rates, and concentration of ingredients affect the particle size and integrity of proteins or genes. By optimizing these conditions, the precipitation of proteins and genes with supercritical CO(2) is a promising way to produce protein and gene particles for inhalation.

Enhancement of insulin transport across primary rat alveolar epithelial cell monolayers by endogenous cellular factor(s)

PURPOSE

To characterize factor(s) contained in apical medium of primary cultured rat alveolar epithelial type II cell-like monolayers (RAECM-II) that enhance insulin absorption across alveolar epithelial cells.

MATERIALS AND METHODS

Primary rat alveolar epithelial cell monolayers cultured on Transwells in the presence and absence of 10 ng/ml keratinocyte growth factor for 6 days were dosed from the apical compartment with radiolabeled insulin in: newborn bovine serum-containing medium (SM), conditioned medium from apical compartment of rat alveolar epithelial type I cell-like monolayers (RAECM-I) (CMI), or conditioned medium from apical compartment of RAECM-II (CMII). At the end of 2 h incubation, basolateral medium was collected and amounts of transported radiolabeled insulin were determined using a gamma counter. In order to determine the molecular size range of the enhancing factor(s), CMII was centrifuged in 50 kDa molecular weight cut-off Centricon tubes, and both retentate and filtrate were used as separate dosing solutions. Heat denaturation and ammonium sulphate precipitation were used to determine if the involved factor(s) represent proteins or other smaller soluble factors. Transalveolar transport rates of a paracellular marker, (14)C-mannitol, and fluid-phase marker, horseradish peroxidase, were determined in the presence and absence of the factors. Effects of temperature (4, 16 and 37 degrees C) on radiolabeled insulin fluxes were also measured.

RESULTS

Conditioned medium obtained from the apical compartment of RAECM-II, CMII, increased transport of insulin across the monolayers when compared to SM or CMI. The enhancing effect of CMII was retained in the precipitate following ammonium sulfate treatment and in the retentate after Centricon filtration. The enhancing effect of CMII was significantly decreased when heated at 80 degrees C for 15 min. CMII did not affect the transport of (14)C-mannitol or HRP, while its effect on insulin transport was decreased by 87% when temperature was lowered to 4 degrees C from 37 degrees C.

CONCLUSIONS

Conditioned medium from type II cell-like monolayer cultures appears to contain protein factor(s) which seem to be involved in facilitating active transcellular transport of insulin across primary cultured RAECM-II.

Local and Systemic Delivery of High-Molecular Weight Drugs by Powder Inhalation

The pulmonary route has recently attracted attention as a noninvasive administration route for peptide and protein drugs, and an insulin powder for inhalation was approved by authorities in Europe and the USA. The present study examined usefulness of insulin and gene powders for systemic and local inhalation therapy. We prepared several dry insulin powders by spray drying to examine the effect of additives on insulin absorption. Citric acid appears to be a safe and potent absorption enhancer for insulin in dry powder. However, in the powder with citric acid (MIC0.2 SD) insulin was unstable compared with the other powders examined. To improve insulin stability, a combination of insulin powder and citric acid powder was prepared (MIC Mix). MIC Mix showed hypoglycemic activity comparable to MIC0.2 SD while the insulin stability was much better than that of MIC SD. Next, dry insulin powders with mannitol were prepared with supercritical carbon dioxide (SCF); the powder thus prepared reduced blood glucose level rapidly and was more effective than that prepared by spray drying. Chitosan-pDNA complex powders as a pulmonary gene delivery system were also prepared with SCF and their in vivo activity was evaluated. The addition of chitosan suppressed the degradation of pCMV-Luc during preparation and increased the storage stability. The luciferase activity in mouse lung was evaluated after pulmonary administration of the powders. The chitosan-pDNA powder with an N/P ratio=5 increased the luciferase activity to 27 times that of the pCMV-Luc solution. These results suggest that gene powder with chitosan is a useful pulmonary gene delivery system.

Population Pharmacokinetic–Pharmacodynamic Modeling of Subcutaneous and Pulmonary Insulin in Rats

PURPOSE

To develop a population pharmacokinetic-pharmacodynamic (PKPD) model for insulin in rats.

METHODS

Rats were administered insulin either subcutaneously (s.c) (0.26,1.3,2.6 U/kg) or by pulmonary route (spray-instillation (s.i)) (0.26,1.3,2.6,13,26 U/kg). Insulin (0.26,1.3,2.6 U/kg) combined with different combinations of hydroxy methyl amino propionic acid (HMAP: 5,10,16,25 mg/kg) was also administered by spray-instillation. Plasma insulin and glucose concentrations at pre-determined time points were measured. Population pharmacokinetic-pharmacodynamic modeling was performed using NONMEM.

RESULTS

Insulin exhibited dose-disproportional PK across formulations and routes of administration. The kinetic model suggested monoexponential disposition with simultaneous first order (64%-dimeric form of insulin) - zero order (36%-hexameric form of insulin) absorption. Maximum relative bioavailability (relative to s.c - 0.26 U/kg) of spray-instilled insulin was 46%. Addition of HMAP increased the relative bioavailability of insulin administered via spray-instilled route by 40%. The insulin-glucose relationship was characterized using an indirect response model, wherein, insulin stimulation of glucose uptake into muscle cells was assumed. The basal zero order production rate of glucose (k(G, prod)) was estimated as 0.98 mg/dl/min. The SC50 was fixed at 80 mu U/ml based on literature reports and the S(max) was estimated to be 6.

CONCLUSIONS

The proposed PKPD model satisfactorily describes insulin disposition and glucose concentrations across range of doses with and without HMAP.

The pharmacokinetics of pulmonary insulin in the in vitro isolated perfused rat lung: Implications of metabolism and regional deposition

The pharmacokinetics of several lung disposition pathways for pulmonary insulin were studied and modeled in the isolated perfused rat lung (IPRL). Insulin solution was administered by forced instillation into the airways of the IPRL as 0.1 or 0.02 ml doses of coarse spray, with or without bacitracin (BAC), N-ethylmaleimide (NEM) and atrial natriuretic peptide (ANP). Each insulin absorption profile was fitted to a kinetic model that incorporated the distribution fraction of the dose reaching the lobar region (DF) and the rate constants for absorption into perfusate (k(a)) and non-absorptive loss (k(nal)); k(nal) was shown to be due to the sum of mucociliary clearance and metabolism. Insulin absorption occurred largely by passive diffusion with values for k(a) = 0.39-0.50 h(-1). With DF = 0.91 following 0.1 ml doses, 11.9 +/- 3.4% of bioavailabilities were observed in 1h. In contrast, derived values for k(nal) = 2.34-3.45 h(-1) were significantly larger than the rate constant for mucociliary clearance determined previously in this IPRL (0.96-1.74 h(-1)) due to lung metabolism. Indeed, BAC, but neither NEM nor ANP, was found to decrease the value of k(nal), which suggested that BAC-inhibitable lung ectopeptidases, and not insulin degrading enzyme (IDE), were responsible for this pulmonary metabolism. Shallower lung distribution with DF = 0.73 following 0.02 ml doses resulted in reduced values for k(a) = 0.27 h(-1) and k(nal) = 2.79 h(-1), indicating that these kinetic processes may be lung-region dependent, even within this model and emphasizing the likely importance of reliable lung deposition in vivo.

Insulin disposition in the lung following oral inhalation in humans : a meta-analysis of its pharmacokinetics

BACKGROUND

Oral inhalation of insulin potentially offers non-invasive treatment and better glycaemic control in diabetes by virtue of its apparently faster absorption into the systemic circulation compared with subcutaneous injection. Nevertheless, the lung kinetics of inhaled insulin in humans have yet to be fully clarified because of the complexity of insulin-glucose (patho)physiology and the difficulty in approximating the inhaled dose. As a result, there remains considerable debate on the mechanisms of absorption and metabolism of insulin in the lung.

OBJECTIVES

To develop and apply a physiologically realistic insulin-glucose kinetic model to a meta-analysis of insulin-glucose profiles from well-controlled clinical studies of inhaled insulin published in the literature, and thereby, to derive the kinetic descriptors of insulin in the lung following inhalation through curve fitting.

MODEL DEVELOPMENT

The model assumed first-order absorption (k(a,L)) and parallel non-absorptive loss (k(mm,L)), the latter primarily occurring via metabolism and mucociliary clearance in the lung, alongside two systemic compartments. Where necessary, glucose-dependent endogenous pancreatic insulin secretion was also taken into account by using blood glucose data as the second independent variable.

RESULTS

Despite the model's simplicity and the use of mean data, 16 insulin-glucose profiles from ten clinical studies were successfully fitted to the model, yielding values for the rate constants k(a,L) and k(mm,L). Whole serum insulin profiles were rate-determined by k(a,L) and k(mm,L) combined, representing 'flip-flop' pharmacokinetics. The best estimate for k(a,L) was found to be 0.020-0.032 h(-1), effectively unchanged across doses (0.3-1.8 IU/kg), formulations (powder vs liquid) and subjects (healthy vs diabetic), suggesting passive diffusive absorption of insulin from the lung. In contrast, the values for k(mm,L) were much larger (0.5-1.6 h(-1)) and decreased with increasing inhaled dose. Therefore, it is likely that dose-dependent saturable lung metabolism controls the value of k(mm,L), alongside mucociliary clearance. As a result, the absolute bioavailability ranged from 1.5% to 4.8%. The modelling analysis also enabled derivation of increased values for both k(a,L) and k(mm,L) as a possible cause of faster absorption for deep inspiratory manoeuvres and increased absorption in smokers, and faster and increased absorption for insulin lispro.

CONCLUSIONS

Although some of these results need to be substantiated experimentally, it appears that this modelling analysis has enabled unification of the literature information associated with the kinetics and mechanisms of insulin disposition in the lung following inhalation in humans.

Mechanisms for absorption enhancement of inhaled insulin by sodium taurocholate

The effects of sodium taurocholate (NaTC) on the absorption of inhaled insulin was investigated using both in vivo and in vitro experiments. The absolute bioavailability of insulin when given as a nebulized solution (0.6 mM) to anesthetized intubated beagle dogs was low (2.6+/-0.3%). However, when NaTC at different concentrations (2-32 mM) were included in the formulations the bioavailability increased and at 32 mM it was about nine times higher (23.2+/-4.4%) than for pure insulin. In a similar concentration interval (20-25 mM) NaTC decreased the transepithelial electrical resistance (TEER) across Caco-2 cell monolayers leading to an increased permeability of insulin. At higher concentrations (above 30 mM) the viability of the Caco-2 cells decreased and the insulin permeability increased dramatically. Furthermore, we show that NaTC in the concentration range 2-15 mM gradually decreases the aggregation state of insulin, i.e., produces mono- or dimeric insulin from hexameric insulin. In conclusion, NaTC increases the bioavailability of nebulized insulin, increases the permeability of insulin across Caco-2 cell monolayers, and decreases the aggregation state of insulin at similar concentrations. We suggest that the main mechanisms behind the absorption enhancement of inhaled insulin by NaTC are the production of insulin monomers and an opening of tight junctions between adjacent airway epithelial cells.

Proteolytic enzymes as a limitation for pulmonary absorption of insulin: in vitro and in vivo investigations

In vitro biodegradation of insulin in lung cytosol and subcellular pellets of normal and diabetic rats was investigated. Rat lung was homogenized and subcellular fractions were isolated by ultracentrifugation. Degradations of [125I]-insulin after incubation with lung cytosol or subcellular pellets was determined using the trichloroacetic acid method. The results show that insulin is highly degraded in cytosol and subcellular pellets. Cytosolic insulin degradation was strongly inhibited by bacitracin or sodium cholate. The degradation of insulin in the lung cytosol from diabetic rats was significantly less than from normal rat. The lung protease activity reached a maximum at pH 7.4. Enzyme inhibitors like bacitracin and sodium cholate noticeably enhanced the relative pharmacological bioavailability of insulin when given intratracheally with insulin to normal rats. Acidic insulin solutions (pH 3.0) had more pronounced hypoglycaemic effects than neutrol solution (pH 7.0). These in vitro and in vivo results suggest that the proteolytic enzymes in the lung limit pulmonary delivery of insulin. The coadministration of protease inhibitors would be a useful approach for improving the pulmonary absorption of insulin.