Die formale kinetik der resorption unter berücksichtigung der darmdurchblutung

Gastroretentive fibrous dosage forms for prolonged delivery of sparingly-soluble tyrosine kinase inhibitors. Part 3: Theoretical models of drug concentration in blood

In this part, drug concentration in blood after ingesting slow-release gastroretentive fibrous dosage forms and immediate-release particulate forms is modeled. The tyrosine kinase inhibitor nilotinib, which is slightly soluble in low-pH gastric fluid but practically insoluble in pH-neutral intestinal fluid is used as drug. The models suggest that upon ingestion, the fibrous dosage form expands, is retained in the stomach for prolonged time, and releases drug into the gastric fluid at a constant rate. The released drug molecules flow into the duodenum with the gastric fluid, and are absorbed by the blood. The drug is eliminated from the blood by the liver at a rate proportional to its concentration. Eventually, the elimination and absorption rates will be equal, and the drug concentration in blood plateaus out. After the gastric residence time drug absorption stops, and the drug concentration in blood drops to zero. By contrast, after administering an immediate-release particulate dosage form the drug particles are swept out of the stomach rapidly, and drug absorption stops much earlier. The drug concentration in blood rises and falls without attaining steady state. The gastroretentive fibrous dosage forms enable a constant drug concentration in blood for drugs that are insoluble in intestinal fluids.

Theoretical model of gastrointestinal absorption of lead

A theoretical model is presented which is used to explain gastrointestinal absorption of ingested lead from use of traditional medicines. Dissolution processes are discussed and the relative contributions of pore and passive diffusion through the intestinal epithelium for the resulting species examined. The determination of the rate of absorption is described in terms of a mathematical three-compartment system and effects of particle size are considered by reference to the Noyes-Whitney dissolution rate law.

The longitudinal intraluminal concentration gradient in the perfused rat jejunum and the appropriate mean concentration for calculation of the absorption rate

In anaesthetized rats, jejunal segments 30 cm in length were perfused (rates:0.1, 0.2,0.5 ml/min) with solutions containing antipyrine, salicylic acid, urea, L-lysine, L- and D-phenylalanine. The intraluminal concentration was determined approx. 10, 20, and 30 cm from the inflow cannula. The intraluminal concentration gradients were almost exponential and corresponded to the gradient predicted for a laminar flow through a solute-permeable circular tube. This gradient has a slightly greater curvature than the exponential one. Theoretical considerations showed that the logarithmic mean (Co-Cz)/ln(Co/Cz) of the in- and outflow concentration is the best average value for application in non-integrated absorption equations. This is also valid for the temporal concentration gradient after single administration of a substance into an intestinal segment.

Mechanistic and quantitative evaluation of precorneal pilocarpine disposition in albino rabbits

The low ocular bioavailability of topically applied pilocarpine is attributed to extensive precorneal drug loss in conjection with the resistance to corneal penetration. Several elements of precorneal loss were reported earlier, but a complete mechanistic understanding has not been available. The present study was designed to gain a better understanding of the mechanisms governing pilocarpine disposition in the precorneal areas as well as the relative influence of these parameters on ocular drug bioavailability. Radioactive pilocarpine and glycerin solutions were instilled into the precorneal area of the albino rabbit eye under various experimental conditions, and the drug concentration in the lacrimal lake was monitored as a function of time. The results demonstrated that nonconjunctival loss of pilocarpine, vasodilation due to the drug, and lacrimation due to vehicle formulation are additional aspects of precorneal drug disposition. The individual influence of all precorneal loss parameters on drug bioavailability was then assessed using a mathematical model formulated from experimental findings on both precorneal and intraocular drug disposition. Drainage and vasodilation, as well as nonconjunctival pilocarpine loss, exerted major influences on drug loss at the absorption site.

The permeability coefficient of the wall of a villous membrane

The equations hitherto used to correct the permeability coefficient for the unstirred layer influence are valid only for flat membranes. Therefore, appropriate equations for membranes with a villous surface (e.g., small intestine) have been derived. They take into account the non-linear concentration gradient in the intervillous part of the unstirred layer. Quantitative information about the geometry of the villous surface and the unstirred layer thickness are needed to calculate the permeability coefficient of the membrane wall (e.g., intestinal epithelium). The concentration of highly permeable substances drops sharply already in the upper part of intervillous space, so that the tips of the villi function as effective absorbing area. The intervillous concentration gradient of a substance with a low permeability coefficient is so small, that such a substance is absorbed by the total surface area of the villous membrane. The effective absorbing area of substances with intermediate permeability coefficient lies between the described limits.

[Insorption of tritiated water in hepatic cirrhosis (author's transl)]

In 10 patients with hepatic cirrhosis of various etiology both the insorption and the volume of distribution of tritiated water are determined. Indicator for the velocity of insorption is the time interval between the oral application of 200 mu-Ci THO distributed in 100 ml tap water and the appearance of a peak concentration of the isotope in the serum. In comparison with normal controls the insorption of THO is protracted in patients with hepatic cirrhosis from 37.0 plus or minus 4.4 min (S.E.) to a mean of 69.0 plus or minus 29.6 min. Individually the "insorption time" can be quite normal. The prolongation of the process of insorption is independent of the apparent volume of distribution of THO or of the etiology of hepatic cirrhosis. In the early phases of the insorption which are characterized by the main increase of the radioactivity in serum there is no difference between patients with hepatic cirrhosis and normal controls. Although an alteration of body fluid metabolism can not be excluded as a source of the prolonged equilibration, it is suggested that changes of the intestinal blood flow, of the tissue pressure and of the colloidosmotic pressure are due to a disturbance of the insorption of tritiated water in patients with hepatic cirrhosis.