Receptor fluid

Water-miscible semisolids, such as some gels, in direct contact with an aqueous donor or receptor fluid represent a free boundary system, with transport occurring across a liquid/gel interface. 

A device was designed for use in studying the release of corticosteroids suspended in an oleaginous ointment base [20], The apparatus consists of a Teflon dish which floats on the surface of the stirred receptor fluid. After the system has been brought to thermal equilibrium, a known amount of ointment is evenly spread on the bottom surface of the Teflon dish. At particular time intervals,... 

A device designed to determine the transport of an epidermal growth factor from a Pluronic gel, a Carbopol gel, and a vanishing cream base was shown capable of demonstrating formulation-dependent release kinetics [22], This device consists of a release cell which has a membrane to minimize release due to mechanical breakdown and is placed in a stirred and temperature-controlled receptor fluid. 

In a similar diffusional device, the ointment containing the drug sample was placed on a membrane over a stirred and thermostated receptor fluid [23], Potential advantages of this device are that the unit construction fixes the distance between the stirrer and the membrane and the stirrer symmetry relative to the membrane is less susceptible to variation. This device was used to evaluate the... 

The permeability of human skin to n-hexane has been determined in vitro in flow-through diffusion cells (Loden 1986). Pieces of full-thickness human skin were exposed to 3H -hcxane in human serum, and the appearance of label in the trans compartment measured for 0.5 or 12 hours. The skin was then sectioned with a microtome into 0.25 mm slices and the quantity of label in the skin measured. The rate of resorption (uptake of substance by the receptor fluid beneath the skin [i.e., the amount that passes through the skin]) was calculated. The rate of resorption for n-hexane through human skin was calculated to be 0.83 ( g cm2/hr). The permeability of n-hexane through human skin was much lower (approximately 100-fold) than for other chemicals tested in this study. For example, rates of resorption (in g cm2/hr) were 99 for benzene and 118 for ethylene glycol. 

Given the overwhelming influence of the physical properties of skin in determining bioavailabilities via the dermal route, assessment of dermal penetration is one area in metabolism and toxicology where in vitro methods can be effectively used to predict in vivo results and to screen chemicals. Apparatus and equipment exist that one can use to maintain sections of skin (obtained from euthanized animals or from human cadavers or surgical discard) for such experiments (Holland et al., 1984). These apparatus are set up to maintain the metabolic integrity of the skin sample between two reservoirs the one on the stratum comeum side, called the application reservoir and the one on the subcutaneous side, called the receptor reservoir. One simply places radiolabeled test material in the application reservoir and collects samples from the receptor fluid at various time points. 

Polybrominated Diphenyl Ethers. No information was located regarding dermal absorption of PBDEs in humans. The only information regarding dermal absorption in animals is that from a study of absorption in an in vitro preparation (Hughes et al. 2001). In that study, " C-dccaBDE dissolved in tetrahydrofuran was applied to dorsal skin (three dose levels) excised from adult hairless female mice and fractions of receptor fluid were collected over a 24-hour period. Transfer of radioactivity to the receptor fluid was minimal, only 0.07 to 0.34% of the applied radioactivity. Two to 20% of the radioactivity was found in the skin, and the lowest dose applied had the highest percentage of the dose in the skin. Washing the skin with solvent 24 hours after application removed 77-92% of the applied dose. In this study, decaDBE did not easily penetrate the skin, but inferences to dermal absorption in humans based on these limited results may not be appropriate. 

Transdermal transport of PG was followed vitro. Fullthickness skin, excised from hairless mice (StCH HR-1, Skin Cancer Hospital, Philadelphia, PA) and used immediately, was interposed between delivery system and receptor chamber. Serial samples of receptor fluid were collected and analyzed as before. Experiments were performed in quadruplicate. 

The choice of receptor fluid can influence the outcome of the study considerably (Ramsey et al., 1994 Bronaugh, 1995). In order to avoid underestimation of skin absorption, the test compound should be soluble in the receptor fluid. On the other hand, the receptor fluid should not damage the barrier properties of the skin membrane. Various receptor fluids have been used, including saline (for hydrophilic test substances) and water/ethanol mixtures, or saline supplemented with bovine serum albumin or poly(ethylene glycol) 20 oleyl ether (for testing of lipophilic compounds). When performing studies with metabolicaUy active skin preparations, the receptor fluid should support the viability of the skin. In these cases, a tissue culture medium is normally used. 

To illnstrate the differences between skin membrane types, Fignre 9.1 shows a series of independent experiments with the lipophilic reference compound testosterone performed in onr laboratory. The data demonstrate that epidermal membranes were approximately 3.5 times more permeable than fnll-thickness skin, based on the percentage of the applied compound reaching the receptor fluid during 24 h. Each experiment was performed with skin from a different donor, and the variation between the in vitro experiments was within the same range as reported for hnman volunteers in vivo (Schaefer and Redelmeier, 1996). 

Receptor fluid Solubility, maintenance of skin viability/ barrier integrity... 

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). 

Receptor fluids described in the literature range from water alone to isotonic phosphate buffers containing albumin, which increases solubility (Dick et al. 1996). Microbial growth can produce problems due to partitioning of the permeant into, or metabolism of the... 

Figure 14.10 Release rates of betamethasone dipropionate from ointments into various receptor fluids. 0), 5% hexane in acetonitrile (2), octanol (3), acetonitrile (4), 60°/o acetonitrile in water (5), 95% ethanol Cdata from Zatz et al. 1996).

Dick, I. P., P. G. Blain, and F. M. Williams. 1996. Improved in vitro skin absorption for lipophilic compounds following the addition of albumin to the receptor fluid in flow-through cells. In Prediction of percutaneous penetration, vol. 4b. edited by K. R. Brain, V. J. James, and K. A. Walters. Cardiff, UK STS Publishing, pp. 267-270. 

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... 

Most studies today are conducted in one-chamber diffusion cells that hold receptor fluid beneath the skin. The top surface of the skin is exposed to the environment and is surroimded by a short wall. A tube extends upward from the receptor fluid for manual sample removal. The Franz cell is the most widely known cell of this type (Franz, 1975). A flow-through diffusion cell (Figure 2.1) is a modification of this design that should have a much smaller receptor fluid chamber to permit easy removal of contents with a moderate flow (1 to 2 ml/h) of receptor fluid (Bronaugh and Stewart, 1985). The continual replacement of the receptor fluid pomits maintenance of skin viability when a physiological buffer is used (Collier etal., 1989). This diffusion cell also has the advantage of automatic samphng with the use of a fraction collector. 

In vitro skin absorption smdies often differ in the receptor fluid used. A buffered saline solution may simply be used in a study with nonviable skin however, a more physiological solution such as HEPES-buffered Hanks balanced salt solution is required to maintain the viability of skin in the diffusion cells (Collier et al., 1989). The viabihty of skin can be maintained for at least 24 h based on glucose utilization of skin, histological evaluations, and the maintenance of estadiol and testosterone metabolism (Collier et al., 1989). 

Modifications of the receptor fluid are sometimes made to facilitate the partitioning of hpophihc chemicals from skin into the receptor fluid to simulate the... 

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