Studies on biotransformation of elastase. I. Transport of 131 I-labeled elastase across rat intestine in vitro

Intestinal Lymphatic Biology, Drug Delivery, and Therapeutics: Current Status and Future Directions

Historically, the intestinal lymphatics were considered passive conduits for fluids, immune cells, dietary lipids, lipid soluble vitamins, and lipophilic drugs. Studies of intestinal lymphatic drug delivery in the late 20th century focused primarily on the drugs' physicochemical properties, especially high lipophilicity, that resulted in intestinal lymphatic transport. More recent discoveries have changed our traditional view by demonstrating that the lymphatics are active, plastic, and tissue-specific players in a range of biological and pathological processes, including within the intestine. These findings have, in turn, inspired exploration of lymph-specific therapies for a range of diseases, as well as the development of more sophisticated strategies to actively deliver drugs or vaccines to the intestinal lymph, including a range of nanotechnologies, lipid prodrugs, and lipid-conjugated materials that "hitchhike" onto lymphatic transport pathways. With the increasing development of novel therapeutics such as biologics, there has been interest in whether these therapeutics are absorbed and transported through intestinal lymph after oral administration. Here we review the current state of understanding of the anatomy and physiology of the gastrointestinal lymphatic system in health and disease, with a focus on aspects relevant to drug delivery. We summarize the current state-of-the-art approaches to deliver drugs and quantify their uptake into the intestinal lymphatic system. Finally, and excitingly, we discuss recent examples of significant pharmacokinetic and therapeutic benefits achieved via intestinal lymphatic drug delivery. We also propose approaches to advance the development and clinical application of intestinal lymphatic delivery strategies in the future. SIGNIFICANCE STATEMENT: This comprehensive review details the understanding of the anatomy and physiology of the intestinal lymphatic system in health and disease, with a focus on aspects relevant to drug delivery. It highlights current state-of-the-art approaches to deliver drugs to the intestinal lymphatics and the shift toward the use of these strategies to achieve pharmacokinetic and therapeutic benefits for patients.

Elastase in the prevention of arterial aging and the treatment of atherosclerosis

Arterial aging is defined as the age-related structural and functional changes in arteries from the precapillary to the aortic level. These include atheromatous changes. Such changes can be estimated in medium-sized or larger arteries by clinical diagnostic studies, including B-mode echography of the carotid artery (thickening of the intima, plaque formation and increase in luminal diameter) and abdominal aorta (plaque formation and increase in luminal diameter). Doppler echography (pulse wave velocity; PWV) and autopsy studies. These changes include distension of the lumen, increased arterial wall thickness (which may be associated with atherosclerotic plaques) and decreased extensibility of the arterial wall. Since 1981, an anti-atherosclerotic drug containing porcine pancreatic elastase 1 (PPE1) has been used for the prevention of arterial aging and the treatment of atherosclerosis in elderly patients in Japan. So far, the age-related increase in PWV has been found to be lower in those who take PPE1 than in controls. The atherosclerotic index of the carotid artery has also been found to be lower in subjects receiving PPE1 treatment than in control subjects. The pharmacological basis of PPE1 therapy, although paradoxical to the consensus opinion of the pathogenic role of elastase in western countries, is discussed with reference to data gathered in Japan. The pathomechanism of arterial aging and atherosclerosis, with special reference to elastin, is reviewed along with the presentation of some of our data.

Conservation of digestive enzymes

The traditional understanding is that an entirely new complement of digestive enzymes is secreted by the pancreas into the small intestines with each meal. This is thought to be necessary because, like food itself, these enzymes are degraded during digestion. In this review we discuss experiments that bring this point of view into question. They suggest that digestive enzymes can be absorbed into blood, reaccumulated by the pancreas, and reutilized, instead of being reduced to their constituent amino acids in the intestines. This is called an enteropancreatic circulation of digestive enzymes.

Antiatheromatous properties of elastase in cholesterol-fed rabbits and ex vivo suppressive effect of elastase on smooth muscle cell proliferation in the presence of hypercholesterolemia

The effects of a clinical oral dose of elastase on aortic atheroma development were studied in 0.2% cholesterol-fed rabbits, with or without endothelial denudation, using a balloon catheter. Elastase slightly decreased the serum lipids, but there was no significant difference from the serum lipids in the control groups. However, elastase significantly reduced the surface involvement of Sudan IV-positive staining areas of the aortas in rabbits with and without endothelial injury. The effect of elastase on SMC proliferation stimulated by hypercholesterolemia (C-serum or C-plasma) was also investigated in ex vivo experiments. The cholesterol levels in the serum and plasma of the elastase group were not significantly different from those of the 0.5% cholesterol-fed control rabbits. Proliferation of aortic smooth muscle cells was stimulated in the control group, while such stimulation in the elastase group was significantly lower. Therefore, elastase may suppress the stimulation through plasma factors, hence a reduction in atheroma development would ensue.

Differentiation of endopeptidases and aminopeptidases by high-performance liquid chromatography of reaction products from chromogenic peptide p-nitroanilines as substrates

A method for differentiating endopeptidases and aminopeptidases on the basis of substrate specificity is presented. Various synthetic chromogenic substrates, succinyl-(Ala)3-p-nitroaniline, succinyl-(Ala)2-p-nitroaniline, (Ala)3-p-nitroaniline, and (Ala)2-p-nitroaniline, were incubated with various peptidases and the incubation mixtures were directly analyzed by high-performance liquid chromatography to determine the splitting patterns of these substrates by the enzymes. The substrates and hydrolyzed products containing the chromophore were separated on a reverse-phase column under isocratic conditions, and the chromophore was specifically detected in the effluent fractions by absorbance measurement at 314 nm. Endopeptidases, leucine aminopeptidase, and dipeptidyl aminopeptidase showed different patterns of cleavage of the substrates. This simple and rapid high-performance liquid chromatographic procedure is suitable for identifying the above activities in different fractions obtained during separation and purification studies. The same approach was applied to the simultaneous determination of three types of endopeptidase activities in rat tissues based on the ability of the enzymes to hydrolyze different sites in succinyl-(Ala)3-p-nitroaniline.

Absorption of elastase through the jejunal mucosa of the rat. An immunocytochemical study

In order to reveal the absorption process of elastase from the intestine, hog pancreatic elastase was injected into the ligated jejunum lumen of the rat, and the tissues were cytochemically observed at various times after injection. The peroxidase anti-peroxidase (PAP) method using anti-hog-elastase rabbit antibody was used for light microscopy, and the anti-elastase Fab'-peroxidase conjugate was used for electron microscopy. The tissues stained by the PAP method exhibited a dense deposition of reaction products on the luminal surface of epithelial cells and a moderate deposition in the blood and lymph capillaries of the intestinal villi. Immunoelectron microscopy revealed that the reaction product was deposited on the surface of the microvilli and in their pocketing; some was found in the pinocytotic vesicles in the terminal-web area and on the inner surface of the enlarged smooth endoplasmic reticulum. Round droplets which gave a positive reaction were found in the widened intercellular cleft and the thick basement membrane lining the blood capillaries and lymphatics. The jejunum retained its normal ultrastructure. The results indicate that the elastase molecules, which were introduced into the rat jejunum lumen, were absorbed without being decomposed through healthy intestinal epithelial cells by pinocytosis and translocated into blood and lymph capillaries.

Origin of circulating serum immunoreactive trypsin in man

Serum trypsin concentrations within the portal venous system have been measured in man during transhepatic portal venography in an attempt to determine its source. In eight experiments, mean serum trypsin concentration at the splenic hilum was 180 +/- 25 ng/ml (mean +/- SEM). Trypsin concentration in the rest of the splenic vein was not significantly different. The mean concentrations in the portal vein (210 +/- 32 ng/ml) and within the superior mesenteric vein (233 +/-- 29 ng/ml) were, however, significantly higher than at the hilum (P less than 0.05). Following cholecystokinin-pancreozymin (CCK-PZ) and secretin stimulation, marked increases in serum trypsin concentration were seen within the portal vein (two patients) and deep within the superior mesenteric (two out of three patients). We conclude that circulating serum trypsin is derived, at least in part, from intestinal reabsorption.

Enteropancreatic circulation of trypsin in man

Following the intraduodenal installation of purified 125I-labeled human trypsin up to about 4--6% of the label was measured after 15--30 min in blood plasma and found to separate in a dextran-gel filtration system similar to purified human trypsin (-125I) after incubation with human serum. About 1% of the installed trypsin-(-125I)-dose was found already after 20 min in 100 ml of aspirated pancreatic secretion and later on also in the duodenal content. The results support the concept of the existence of an enteropancreatic circulation of trypsin also in man and explain in part the low to non-detectable levels of immunoreactive serum trypsin observed in patients with exocrine pancreatic insufficiency.

Amylase transport across ileal epithelium in vitro

Amylase transport was measured across the rabbit ileum in vitro employing a modified Ussing chamber. Amylase was moved preferentially in the mucosal to serosal direction. Its rate of transfer was 2--3 orders of magnitude greater than that for inulin. Mucosal to serosal transport of exogenous amylase was completely inhibited in the absence of oxygen. There was also a constant release of endogenous amylase from intestinal tissue into both mucosal and serosal compartments in the absence of an exogenous source. An estimate of the rate of amylase absorption indicates that it may be of sufficient magnitude to account for the enteropancreatic circulation of amylase secreted by the pancreas during augmented secretion.

Absorption of pancreatic lipase from the duodenum into lymphatics

A significantly higher lipase activity was measured in the duodenal lymph samples of 15 dogs than in each of corresponding arterial blood plasma samples collected prior to, during and after maximal hormonal stimulation of pancreatic secretion. The result may be evaluated as a sign of pancreatic lipase absorption by the duodenum into lymphatics.

The fate of 131 I-labelled human pancreatic elastase after intraduodenal administration

Human pancreatic elastase labelled with 131 I was administered into the duodenum through the fiberoptic gastro-duodenoscope in eight persons. Radioactivity was measured in their plasma, urine and feces during 72 h. 131 I was present in the plasma in a dialyzable form 15 min after administration in all but one person. In one case labelled elastase bound by alpha1-antitrypsin and alpha2-macroglobulin was recovered from plasma indicating absorption of elastase from the intestine. In all individuals most of the radioactivity (28 - 73%) was recovered in the urine. Their feces contained low amounts of labelled substance (0.9 - 7.0%), except in two individuals where large amounts of radioactivity were found in the feces of one person with diarrhoea (43%) and of another, who received oral treatment with antibiotics two months previously (22%). In the latter case endogenous elastase was also present in feces in considerable amounts.