Absorption diffusion cell conditions

Table I. Effect of Diffusion Cell Conditions on the Absorption of Cinnamyl Anthranilate (I) (Cortisone Control) ...

OECD has adopted an in vitro test for skin absorption potential (OECD TG 428, Skin Absorption In Vitro Method). According to this guideline, excised skin from human or animal sources can be used. The skin is positioned in a diffusion cell consisting of a donor chamber and a receptor chamber, between the two chambers. The test substance, which may be radio-labeled, is applied to the surface of the skin sample. The chemical remains on the skin for a specified time under specified conditions, before removal by an appropriate cleansing procedure. The fluid in the receptor chamber is sampled at time points throughout the experiment and analyzed for the test chemical and/or metabolites. 

To estimate percutaneous lethal doses, Dugard and Mawdsley (1978) measured CN transport across human epidermis using a diffusion cell technique, and the data obtained allowed absorption for differing conditions for example, 10% NaCN at pH 11.4 in contact with skin may lead to symptoms within 25 min and death within 1 h. 

The receptor phase of any diffusion cell should provide an accurate simulation of the in vivo permeation conditions. The permeant concentration in the receptor fluid should not exceed 10 percent of saturation solubility (Skelly et al. 1987), as excessive receptor phase concentration can lead to a decrease in absorption rate and result in an underestimate of bioavailability. The most common receptor fluid is pH 7.4 phosphate-buffered saline (PBS), although if a compound has a water solubility below 10 p.g/mL, then a wholly aqueous receptor phase is unsuitable, and addition of solubilizers becomes necessary (Bronaugh 1985). 

The primary approach to assess dermal absorption is the in vitro diffusion cell. In thi,s model, skin sections (full thickness, deimatomed to a specific thickness) are placed in a two-chambered diffusion cell in which receptor fluid is placed in a reservoir (static cells) or perfused through a receiving chamber (flow-through cells) to simulate cutaneous blood flow. Chemical may either be dosed under ambient conditions neat or dissolved in a vehicle (Franz and Bronaugh cells) or in water (sLdc-by-side diffusion... 

If the receptor fluid Is not replaced, the concentration In the receptor phase of the diffusion cell will eventually equilibrate with the donor phase concentration, at which point further net absorption will cease. Thus, this technique Is unsatisfactory for measuring the complete time course of absorption. This limitation may be overcome by Intermittently emptying the receptor fluid and replacing with "fresh" solution In order to maintain "sink" conditions ("dynamic diffusion cell technique). 

Techniques which utilize continuously perfused, "dynamic diffusion cells maintain "sink" conditions, and serial samples may be collected in automated fraction collectors. When these samples are added, the cumulative amount removed ("excreted") from the diffusion cell as a function of time is obtained. This Is not Identical to cumulative absorption, which is the sum of the amount excreted (EXC) plus the residual amount (RA) still remaining In the diffusion cell. Thus, the continuous perfusion technique is limited by the ability of the resulting cumulative excretion curve to approximate the true cumulative percutaneous absorption curve. Two methods of approximation may be employed. Both depend on an understanding of the kinetic parameters which define the diffusion cell system being utilized. 

If a reliable estimate of P Is to be obtained from a cumulative excretion curve generated by a continuously perfused, dynamic" diffusion cell, data points must be selected after the value of t defined by Equation 7 has been surpassed (when the shape of the excretion curve becomes dependent on absorption only), but before 10-15% of the total available dose has been absorbed (l.e., during the "steady state" period of absorption). Inspection of Figure 2 demonstrates that data points selected before or after these boundary conditions may lead to determinations of P which are falsely low. 

Figure 4 compares the cumulative absorption curve for benzoic acid (4.8 pmol deposited in acetone over 3.14 cm of excised human skin) approximated by this technique, compared to the true absorption curve obtained by manually emptying a diffusion cell at hourly Intervals, then refilling with "fresh" solution to maintain sink conditions. The results are essentially Identical. 

The intestinal absorption of dietary cholesterol esters occurs only after hydrolysis by sterol esterase steryl-ester acylhydrolase (cholesterol esterase, EC 3.1.1.13) in the presence of taurocholate [113][114], This enzyme is synthesized and secreted by the pancreas. The free cholesterol so produced then diffuses through the lumen to the plasma membrane of the intestinal epithelial cells, where it is re-esterified. The resulting cholesterol esters are then transported into the intestinal lymph [115]. The mechanism of cholesterol reesterification remained unclear until it was shown that cholesterol esterase EC 3.1.1.13 has both bile-salt-independent and bile-salt-dependent cholesterol ester synthetic activities, and that it may catalyze the net synthesis of cholesterol esters under physiological conditions [116-118], It seems that cholesterol esterase can switch between hydrolytic and synthetic activities, controlled by the bile salt and/or proton concentration in the enzyme s microenvironment. Cholesterol esterase is also found in other tissues, e.g., in the liver and testis [119][120], The enzyme is able to catalyze the hydrolysis of acylglycerols and phospholipids at the micellar interface, but also to act as a cholesterol transfer protein in phospholipid vesicles independently of esterase activity [121],... 

UV-VIS-NIR diffuse reflectance (DR) spectra were measured using a Perkin-Elmer UV-VIS-NIR spectrometer Lambda 19 equipped with a diffuse reflectance attachment with an integrating sphere coated by BaS04. Spectra of sample in 5 mm thick silica cell were recorded in a differential mode with the parent zeolite treated at the same conditions as a reference. For details see Ref. [5], The absorption intensity was calculated from the Schuster-Kubelka-Munk equation F(R ,) = (l-R< )2/2Roo, where R is the diffuse reflectance from a semi-infinite layer and F(R00) is proportional to the absorption coefficient. 

Simultaneous heat and mass transfer plays an important role in various physical, chemical, and biological processes hence, a vast amount of published research is available in the literature. Heat and mass transfer occurs in absorption, distillation extraction, drying, melting and crystallization, evaporation, and condensation. Mass flow due to the temperature gradient is known as the thermal diffusion or Soret effect. Heat flow due to the isothermal chemical potential gradient is known as the diffusion thermoeffect or the Dufour effect. The Dufour effect is characterized by the heat of transport, which represents the heat flow due to the diffusion of component / under isothermal conditions. Soret effect and Dufour effect represent the coupled phenomena between the vectorial flows of heat and mass. Since many chemical reactions within a biological cell produce or consume heat, local temperature gradients may contribute in the transport of materials across biomembranes. 

All exposure pathways can ultimately result in the absorption of soluble substances across the body s membranes (skin, eyes, respiratory, or digestive tracts), by passive or active diffusion, active transport, or cellular pinocytosis/phagocytosis (the engulfment of foreign particles by cells). The proportion of a substance in contact with a membrane that is absorbed is a complex function of many factors, including the concentration and chemical form of the substance, the relative chemical conditions ambient on either side of the membrane, and the surface area of the membrane with which the substance is in contact. 

Variations in ki and with the respective phase stirring rates are determined first. Determination of as a function of liquid-side stirrer speed is carried out by physical absorption, absorption with slow chemical reaction, or absorption with an instantaneous chemical reaction of a pure gas for a well-known geometric interfacial area (generally the cross-sectional area of the stirred cell minus that of the blade-type stirrer). To determine ka as a function of gas-side stirrer speed Nq, absorption with instantaneous chemical reaction of a dilute solute is carried out. These values of L and k are specific to the geometry of the laboratory apparatus and to the solute gas. For other gases, k under the same conditions can be estimated by assuming it to vary as the square root of the diffusivity Dq of the solute gas. In the same way, for other liquids under the same conditions may be assumed to vary as the square root of the diffusivity of the dissolved gas. 

Absorption from the intestines back into the blood occurs by three mechanisms active transport, diffusion, and solvent drag. Active transport and diffusion are the mechanisms of sodium transport. Because of the high luminal sodium concentration (142 mEq/L), sodium diffuses from the sodium-rich gut into epithelial cells, where it is actively pumped into the blood and exchanged with chloride to maintain an isoelectric condition across the epithelial membrane. 

---