Improvement of intestinal absorption of peptides: adsorption of B1-Phe monoglucosylated insulin to rat intestinal brush-border membrane vesicles

Impact of physiochemical properties on pharmacokinetics of protein therapeutics

Physicochemical properties, such as molecular weight, size, partition coefficient, acid dissociation constant and solubility have a great impact on pharmacokinetics of traditional small molecule drugs and substantially used in development of small drugs. However, predicting pharmacokinetic fate (absorption, distribution, metabolism and elimination) of protein therapeutics from their physicochemical parameters is extremely difficult due to the macromolecular nature of therapeutic proteins and peptides. Their structural complexity and immunogenicity are other contributing factors that determine their biological fate. Therefore, to develop generalized strategies concerning development of therapeutic proteins and peptides are highly challenging. However, reviewing the literature, authors found that physiochemical properties, such as molecular weight, charge and structural modification are having great impact on pharmacokinetics of protein therapeutics and an attempt is made to provide the major findings in this manuscript. This manuscript will serve to provide some bases for developing protein therapeutics with desired pharmacokinetic profile.

Liposomes containing glycocholate as potential oral insulin delivery systems: preparation, in vitro characterization, and improved protection against enzymatic degradation

Background:

Oral delivery of insulin is challenging and must overcome the barriers of gastric and enzymatic degradation as well as low permeation across the intestinal epithelium. The present study aimed to develop a liposomal delivery system containing glycocholate as an enzyme inhibitor and permeation enhancer for oral insulin delivery.

Methods:

Liposomes containing sodium glycocholate were prepared by a reversed-phase evaporation method followed by homogenization. The particle size and entrapment efficiency of recombinant human insulin (rhINS)-loaded sodium glycocholate liposomes can be easily adjusted by tuning the homogenization parameters, phospholipid:sodium glycocholate ratio, insulin:phospholipid ratio, water:ether volume ratio, interior water phase pH, and the hydration buffer pH.

Results:

The optimal formulation showed an insulin entrapment efficiency of 30% ± 2% and a particle size of 154 ± 18 nm. A conformational study by circular dichroism spectroscopy and a bioactivity study confirmed the preserved integrity of rhINS against preparative stress. Transmission electron micrographs revealed a nearly spherical and deformed structure with discernable lamella for sodium glycocholate liposomes. Sodium glycocholate liposomes showed better protection of insulin against enzymatic degradation by pepsin, trypsin, and α-chymotrypsin than liposomes containing the bile salt counterparts of sodium taurocholate and sodium deoxycholate.

Conclusion:

Sodium glycocholate liposomes showed promising in vitro characteristics and have the potential to be able to deliver insulin orally.

Improved Intestinal Membrane Permeability of Hexose-Quinoline Derivatives via the Hexose Transporter, SGLT1

Intestinal membrane permeability is an important factor affecting the bioavailability of drugs. As a strategy to improve membrane permeability, membrane transporters are useful targets since essential nutrients are absorbed efficiently via specific transporters. For example, there are reports that intestinal hexose transporters could be used as a tool to improve permeability; however, there has been no direct evidence that the transporter protein, sodium/glucose cotransporter 1 (SGLT1), is involved in the transport of hexose analogs. Accordingly, we examined directly whether the intestinal membrane permeability of hexose analogs can be improved by utilizing SGLT1. Three hexose-quinoline derivatives were synthesized and their interactions with SGLT1 were evaluated. Among the three derivatives, the glucose-quinoline molecule exhibited an inhibitory effect on D-glucose uptake by both rat intestinal brush-border membrane vesicles (BBMVs) and Xenopus oocytes expressing SGLT1. In addition, significant uptake of the glucose-quinoline derivative by Xenopus oocytes expressing SGLT1 was observed by both an electrophysiological assay and direct measurement of the uptake of the compound, while the galactose-quinoline derivative did not show significant uptake via SGLT1. Thus, it was directly demonstrated that SGLT1 could be used as a tool for the improvement of intestinal membrane permeability of drugs by modification to the glucose analogs.

In vitro and in situ evidence for the contribution of Labrasol and Gelucire 44/14 on transport of cephalexin and cefoperazone by rat intestine

In vitro and in situ intestinal transport of beta-lactam antibiotics in the presence of two novel pharmaceutical excipients, caprylocaproyl and lauroyl macrogolglycerides (Labrasol and Gelucire 44/14), is described. The objective was to compare the effects of both macrogolglycerides on the intestinal transport of cephalexin, a substrate of oligopeptide transporters, and cefoperazone, a non-substrate of them. The in vitro transport studies were performed using a sheet of rat jejunum mounted in Ussing-type diffusion chambers. The in situ studies used an isolated internal loop model in the rat. Labrasol and Gelucire 44/14 were used as the excipients at low concentrations (0.01-0.5%, w/v). The membrane permeability of both drugs was compared by apparent permeability coefficients (P(app)) determined from changes in the amount of permeation vs. time in in vitro studies and by apparent absorptive clearance (CL(app)) determined from changes in the steady state drug concentration of perfusate in in situ studies. The P(app) value of cephalexin increased with an increase in the concentration of Labrasol (0.05-0.5%) compared to the value without Labrasol. The enhancing effect of Labrasol on cephalexin transport was similarly observed in in situ studies, and when 0.5% Labrasol was used in the presence of glycyl-L-leucine or L-alanyl-L-alanine, 60 or 46% enhancement of the active transport of cephalexin by Labrasol was obtained. On the other hand, Gelucire 44/14 did not affect the P(app) and CL(app) of either drug. The different effects of the excipients on cephalexin transport were thought to be due to the influences of size parameters such as a polydispersity index and particle size, and the change in the short-circuit current of jejunum by the addition of the excipient.

Nutrient absorption

Some interesting advances in mechanisms and regulation of nutrient absorption were reported last year. Further evidence was obtained that the rate-limiting step in triacylglycerol absorption, especially with large doses of lipid, is transport of prechylomicrons from the endoplasmic reticulum to the Golgi apparatus. Targeted disruption of the adenosine triphosphate-binding cassette transporter in mice produced changes similar to human Tangier disease and suggested that this mouse may be a model for studying intestinal high-density lipoprotein assembly and secretion. A new mechanism for carbohydrate malabsorption was discovered: in sucrase-isomaltase deficiency, the enzyme fails to anchor in the brush border membrane and so is secreted into the lumen, where it is ineffective. Glycosylating insulin at B1 phenylalanine permitted it to bind to the brush border membrane and greatly enhanced its hypoglycemic activity when given orally. CaCo-2 cells and normal human enterocytes were shown to have two variants of the human sodium-dependent vitamin C transporter, hSVCT1; one is active and the other is an inactive splice variant. In rats, the divalent metal ion transporter, DMT1, appeared to be important for regulation of both absorption of iron and its movement into the liver.