Effects of enzyme inhibitors and insulin concentration on transepithelial transport of insulin in rats

Targeting Insulin-Degrading Enzyme in Insulin Clearance

Hepatic insulin clearance, a physiological process that in response to nutritional cues clears ~50–80% of circulating insulin, is emerging as an important factor in our understanding of the pathogenesis of type 2 diabetes mellitus (T2DM). Insulin-degrading enzyme (IDE) is a highly conserved Zn²⁺-metalloprotease that degrades insulin and several other intermediate-size peptides. Both, insulin clearance and IDE activity are reduced in diabetic patients, albeit the cause-effect relationship in humans remains unproven. Because historically IDE has been proposed as the main enzyme involved in insulin degradation, efforts in the development of IDE inhibitors as therapeutics in diabetic patients has attracted attention during the last decades. In this review, we retrace the path from Mirsky’s seminal discovery of IDE to the present, highlighting the pros and cons of the development of IDE inhibitors as a pharmacological approach to treating diabetic patients.

Strategies for enhanced peptide and protein delivery

Recent advances in the fields of molecular biology and biotechnology have allowed for the large-scale production and subsequent exploitation of the therapeutic potential of protein- and peptide-based drugs. The facilitation of delivery of this class of drugs must be tailored to meet the requirements and often the limitations dictated by the route of delivery chosen. The aim of this review is to comprehensively discuss several routes of drug delivery, detailing the uses and exploitation of each, from origins to present day approaches. Specific reference is made to the compatibility or incompatibility of each approach in the facilitation of the delivery of drugs of protein origin. Additionally, the physiological nature of the delivery route and the inherent physiological obstacles that must be considered when determining the most suitable approach to drug design and delivery enhancement are also addressed. Examples of novel protein-based drug designs and delivery methodologies that illustrate such enhancement strategies are explored.

Transport of Insulin in Modified Valia-Chien Chambers and Caco-2 Cell Monolayers

The transport characteristics of insulin were investigated using two different absorption models. Using the modified Valia-Chien chambers, permeability coefficients of insulin in the duodenum, jejunum, and ileum were 0.71x10(-7), 7.11x10(-7) and 9.45x10(-7) cm/s, respectively. In the Caco-2 cell monolayers, the bidirectional transepithelial fluxes of insulin across Caco-2 cell line showed symmetry. Confocal laser scanning microscopy visualized that FD-4 and FITC-insulin were mainly located in the paracellular route. It is evident that the lower intestine might be an advantageous region, and absorption enhancer that helps open tight junctions between cells should be used for oral delivery of insulin.

Molecular variants and derivatives of insulin for improved glycemic control in diabetes

Insulin is a historic molecule. It presents many first instances, such as the first protein to be fully sequenced, one of the first proteins to be crystallized in pure form, one among the early proteins whose structure was investigated using X-ray crystallography, the first protein to be chemically synthesized and the first Biotech drug. Therefore, the development of insulin in the early years is intricately intertwined with the progress in molecular and structural biology. In recent years, development of a range of insulin analogs has led to better control of glucose levels, thus preventing secondary complications and improving the quality of life in diabetic patients. Such analogs were obtained by modification of the native insulin sequence. They vary with regard to their pharmacokinetic profile, stability, tissue specificity and mode of administration. In addition, alterations involving incorporation of various chemical moieties in insulin and its co-crystallization with insoluble derivatives are used to modulate the time-action profile of the drug. This article traces the development of molecular variants and derivatives of insulin. It discusses future directions for further improvement in their properties to produce still better insulin therapeutics for tight glycemic control.

Novel Mucosal Insulin Delivery Systems Based on Fusogenic Liposomes

PURPOSE

Fusogenic liposomes (FLs) are unique delivery vehicles capable of introducing their contents directly into the cytoplasm with the aid of envelope glycoproteins of Sendai virus (SeV). The objective of this study was to evaluate the potential of FL to improve the mucosal absorption of insulin from rat intestinal membranes.

METHOD

The FLs containing insulin were prepared by fusing insulin-loaded liposomes with inactivated SeV particles and were administered directly into the ileal, the colonic, and the rectal loops (10 IU/kg).

RESULTS

The FL successfully enhanced the insulin absorption and induced a significant hypoglycemic effect following the colonic and the rectal administration without detectable mucosal damage. This enhancing effect of insulin absorption was further improved by increasing the amount of insulin loaded in the FL and by coencapsulating insulin-degrading enzyme inhibitor. In contrast, the insulin absorption was not increased by the ileal administration of FL because the mucous/glycocalyx layers overlaid on the ileal epithelium impede the fusion of FL to the intestinal mucosa.

CONCLUSION

Our results indicated that FL is a useful carrier for improving the absorption of poorly absorbable drugs, such as insulin, via the intestinal tract.

Mucoadhesive polymers with immobilized proteinase inhibitors for oral administration of protein drugs

A new approach to overcome the degradation of protein drugs by proteolytic enzymes and their targeting to the blood through the digestive apparatus was developed. The approach is based on the immobilization of drugs into the polymeric hydrogel containing glycoprotein--ovomucoid from duck egg whites. This glycoprotein inhibits the activity of proteolytic enzymes and acts as a biospecific ligand to lectins on the walls of the gastrointestinal tract.

Enhancement of absorption of insulin-loaded polyisobutylcyanoacrylate nanospheres by sodium cholate after oral and subcutaneous administration in diabetic rats

Polyisobutylcyanoacrylate (PIBCA) nanospheres were employed as biodegradable polymeric carriers for oral (p.o.) and subcutaneous (s.c.) delivery of insulin. The polymerization technique used was able to hold 65%-95% of insulin added 30 min after initiation of polymerization. The percentage drug loading was monomer concentration dependent. Insulin adsorption to the nanospheres was measured by radioimmunoassay. Although Pluronic F68 (0.5%) did not significantly alter the in vitro insulin degradation half-life T50%, sodium cholate (0.5%) increased the degradation T50% of insulin by 56% (from 13.6 +/- 1.6 to 22.1 +/- 2 min). This study also investigated the in vivo performance of insulin-loaded PIBCA in aqueous suspension with or without sodium cholate (0.5%) and Pluronic F68 (0.5%) surfactants after oral and subcutaneous administration to alloxan-induced diabetic rats. Insulin absorption was evaluated by its hypoglycemic effect. Insulin associated with PIBCA nanospheres retains its biological activity up to 15 h and 24 h after oral and subcutaneous administrations, respectively. Administered orally insulin-loaded (75 U/kg) nanospheres, in the presence of surfactants, significantly reduced the mean blood glucose level from 392 +/- 32 to 80 +/- 13 mg/dl within 2 h and maintained it at 100 mg/dl or less for more than 8 h. On the other hand, the subcutaneous administration of insulin-loaded (25 U/kg) nanospheres significantly decreased the blood glucose level from 406 +/- 33 to 88.5 +/- 12.8 mg/dl within 1 h, and the lowered glucose level was maintained at 100 mg/dl or less for more than 12 h; it returned to its initial value 24 h after administration. Insulin-loaded nanospheres with surfactants showed significant (P < .05) pharmacological availability (PA%) of 37.6% +/- 3.7% and 65.2% +/- 2.7% after oral and subcutaneous dosages, respectively. The existence of surfactants with PIBCA nanospheres improved the oral PA% by 49.2%. These findings suggest that the developed PIBCA, in the presence of surfactants, would be useful not only in improving insulin gastrointestinal absorption, but also in sustaining its systemic action by lowering the blood glucose to an acceptable level.

Transport studies of insulin across rat jejunum in the presence of chicken and duck ovomucoids

Our aim was to evaluate the transport of insulin across rat jejunum in the presence of ovomucoids and to assess the effect of ovomucoids on intestinal tissue by studying the permeation of a lipophilic and a hydrophilic marker. Rat jejunal segments were mounted in a side-by-side diffusion chamber filled with Krebs bicarbonate buffer, bubbled with 95% O2/5% CO2 at a fixed flow rate and maintained at 37 degrees C. The permeation of insulin, a lipophilic marker ([7- 3H] testosterone) and a hydrophilic marker (D-[1- 14C] mannitol) was evaluated in the presence of 0.5-1.5 microM duck ovomucoid (DkOVM) or chicken ovomucoid (CkOVM). For stability and permeation of insulin in the presence of alpha-chymotrypsin, an enzyme-to-inhibitor ratio of 1:1 and 1:2 was used. In the absence of alpha-chymotrypsin, the permeability coefficient (Papp) of insulin at pH 7.4 was 0.922+/- 0.168 x 10(-7) cm s(-1), which decreased with increasing concentrations of DkOVM or CkOVM. Conversely, the permeation of the hydrophilic and lipophilic marker increased with increasing concentrations of CkOVM and DkOVM. In stability studies, the percentage of drug remaining was found to be 2-fold higher at the 1:2 ratio than with the 1:1 ratio of enzyme to inhibitor. This was in agreement with the 2-fold increase in flux values of insulin in the presence of alpha-chymotrypsin and DkOVM at the 1:2 ratio of enzyme to inhibitor. The decrease in permeation of insulin in ovomucoids was unexpected. Marker transport studies indicated that ovomucoids have the potential to modulate transcellular and paracellular permeability. The flux enhancement of insulin in the presence of alpha-chymotrypsin and DkOVM is encouraging. The use of ovomucoids offers potential to enhance oral delivery of insulin and warrants further investigation.

The effect of absorption enhancers on the initial degradation kinetics of insulin by alpha-chymotrypsin

The goal of this investigation was to establish a fast method to screen various insulin absorption enhancers by following their effect on the initial kinetics of insulin incubated with alpha-chymotrypsin at 37 degrees C. A simple, sensitive and reproducible reversed phase high performance liquid chromatography (HPLC) method has been developed to carry out this goal. Linear responses (r > 0.999) were observed over the range of 0.4-4 U/ml for insulin. There was no significant difference (P < 0.05) between inter- and intra-day studies for insulin. The mean relative standard deviations (RSD%) of the results of within-day precision and accuracy of insulin were 12%. The assay was sensitive to detect the existence of any metabolite due to the addition of any absorption enhancers, even if it was not seen with insulin alone. Three metabolites (A-C) were detected only when insulin was incubated with alpha-chymotrypsin at 37 degrees C. Metabolite D was observed when either glycocholic acid (0.5, 1%) or taurochenodeoxycholate (0.5, 1%) was incubated with insulin in the absence of alpha-chymotrypsin at 37 degrees C. The compounds that significantly increased insulin T50% were glycyrrhizic acid (0.5%) > deoxycholic acid (1%) > deoxycholic acid (0.5%) > glycyrrhizic acid (1%) > cholic acid (0.5, 1%). Capric acid (0.5%), hydroxypropyl-alpha-cyclodextrin (0.5, 1%) and dimethyl-alpha-cyclodextrin (0.5, 1, 5%) did not significantly affect insulin T50%. The bile salts increased insulin T50% in this order: deoxycholate > cholate > glycocholate > taurocholate > taurodeoxycholate > taurochenodeoxycholate > glycodeoxycholate. The results obtained would support the feasibility of utilizing such method for screening any compound incorporated in insulin formulation. These compounds should be used in the minimum possible concentration to avoid or minimize insulin degradation.

Controlled release opportunities for oral peptide delivery in aquaculture

Over the last decade, fish supplies for human consumption have reached over 100 million tons. Due to overfishing, future increases in demand can only be met from the aquaculture industry. This will require increased research in areas such as the control and manipulation of fish reproduction. There is increasing interest in the oral delivery of peptides that control gamete reproduction. However, compared to mammalian species, little is known about the barriers to peptide delivery and methods to improve such delivery. The three major barriers to peptide delivery are the enzymatic barriers sourced from the host luminal and membrane bound peptidases, the immunological cells present within both the enterocytes and underlying connective tissue and the physical barrier of the epithelial cells. Furthermore, the anatomy and physiology of the gastrointestinal tract of these species are markedly different when compared to higher vertebrates and therefore must be considered when designing appropriate delivery systems. Research to date has focused on the oral delivery and subsequent pharmacodynamic responses to the peptides associated with growth and reproduction. However, minimal work has been undertaken to overcome the identified barriers and therefore any future investigations need to attend to these obstacles before the oral delivery of bioactive peptides can become a commercial reality.

Innovative treatment programs against cancer: II. Nuclear factor-kappaB (NF-kappaB) as a molecular target

Nuclear factor-kappaB (NF-kappaB) activity affects cell survival and determines the sensitivity of cancer cells to cytotoxic agents as well as to ionizing radiation. Preventing the protective function of NF-kappaB may result in chemo- and radio-sensitization of cancer cells. Therefore, NF-kappaB has emerged as one of the most promising molecular targets in rational drug design efforts of translational cancer research programs.

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

The purpose of this study was to investigate the role of insulin-degrading enzyme (IDE, EC 3.4.22.11) in insulin degradation in alveolar epithelium. The primary culture of isolated rat type-II pneumocytes was used for the in-vitro characterization of IDE. Insulin was then administered intratracheally with various inhibitors to assess the improvement in its pulmonary bioavailability. In cultured type-II pneumocytes, the cytosolic insulin-degrading activity contributed 81% of total insulin degradation, reached a maximum at pH 7.5 and had an apparent Michaelis-Menten constant (Km) of 135 nM. N-Ethylmaleimide, p-chloromercuribenzoic acid and 1,10-phenanthroline inhibited insulin-degrading activity almost completely in both crude homogenate and cytosol. An immunoprecipitation study showed that IDE contributed 74% of cytosolic insulin-degrading activity. Western blot analysis showing a single band of 110 kDa on reduced SDS (sodium dodecylsulphate) gels confirmed the presence of IDE in cultured type-II cells. When given intratracheally with insulin, inhibitors including N-ethylmaleimide, p-chloromercuribenzoic acid, and 1,10-phenanthroline significantly enhanced the absolute bioavailability of insulin and the compound's hypoglycaemic effects. These results suggest that IDE is present in alveolar epithelium and might be involved in limiting insulin absorption in the lung.