Permeability permeation experiment

As described previously in this chapter, efforts have been made to develop methods for quantification of skin permeability, validation of diffusion cell setups, and correlation of in vitro data with the in vivo situation. However, the average drug permeation experiment does not provide insight into the temporal and local disposition within the tissue, that is, the skin penetration. The following discussion will give an overview of methods tackling this kind of problem. 

Recently it has been claimed that the tissue can be considered viable if the drug permeability does not change over the course of the experiment, and thus the actual permeability experiments themselves may provide insight into the viability of the tissue [109, 157], This method was employed in permeation experiments using porcine buccal mucosa, where the permeability of compounds was assessed in two consecutive permeability experiments to ensure the nature of the barrier was not compromised [111, 112]. While this demonstrates that the barrier nature of the tissue was unaltered between the permeation experiments, the tissue may have already undergone tissue death in the time between the excision and the commencement of the initial permeation experiment, and thus the permeability rate obtained in vitro may not be representative of the in vivo situation. Therefore, more studies assessing the dependence of the barrier nature of the buccal mucosa on tissue viability are... 

The successful application of in vitro models of intestinal drug absorption depends on the ability of the in vitro model to mimic the relevant characteristics of the in vivo biological barrier. Most compounds are absorbed by passive transcellular diffusion. To undergo tran-scellular transport a molecule must cross the lipid bilayer of the apical and basolateral cell membranes. In recent years, there has been a widespread acceptance of a technique, artificial membrane permeation assay (PAMPA), to estimate intestinal permeability.117118 The principle of the PAMPA is that, diffusion across a lipid layer, mimics transepithelial permeation. Experiments are conducted by applying a drug solution on top of a lipid layer covering a filter that separates top (donor) and bottom (receiver) chambers. The rate of drug appearance in the bottom wells should reflect the diffusion across the lipid layer, and by extrapolation, across the epithelial cell layer. 

Permeability coefficients were calculated from the data obtained in the permeation experiments using the following equation ... 

The integral permeability coefficient P may be determined directly from permeation steady-state flux measurements or indirectly from sorption kinetic measurements 27 521 activity is usually replaced by gas concentration or pressure (unless the gas deviates substantially from ideal behaviour and it is desired to allow for this) and a<>, ax (p0, Pi) are the boundary high and low activities (pressures) respectively in a permeation experiment, or the final (initial) and initial (final) activities (pressures) respectively in an absorption (desorption) experiment. 

Solution of Fick s first law under the boundary conditions of a steady state permeation experiment yields an expression for the permeability constant, P (27),... 

The presence of the term y) makes the permeability coefficient a function of the solvent used as the liquid phase. Some experimental data illustrating this effect are shown in Figure 2.7 [11], which is a plot of the product of the progesterone flux and the membrane thickness, 7, against the concentration difference across the membrane, (cio — cif ). From Equation (2.28), the slope of this line is the permeability, P]. Three sets of dialysis permeation experiments are reported, in which the solvent used to dissolve the progesterone is water, silicone oil and poly(ethylene glycol) MW 600 (PEG 600), respectively. The permeability calculated from these plots varies from 9.5 x 10 7 cm2/s for water to 6.5 x 10 10 cm2/s for PEG 600. This difference reflects the activity term yj/ in Equation (2.28). However, when the driving force across the membrane is... 

Figure 2.40 Blocking of hydrogen in hydrogen/sulfur dioxide gas mixture permeation experiments with finely microporous membranes [63] as a function of the amount of sulfur dioxide adsorbed by the membrane. As sulfur dioxide sorption increases the hydrogen permeability is reduced until at about 140 cm3 (SO2) (STP) /g, the membrane is completely blocked and only sulfur dioxide permeates. Data obtained at several temperatures fall on the same master curve ( , 0°C A. —10 °C , — 20.7 °C A, —33.6°C). Reprinted from R. Ash, R.M. Barrer and C.G. Pope, Flow of Adsorbable Gases and Vapours in Microporous Medium, Proc. R. Soc. London, Ser. A, 271, 19 (1963) with permission from The Royal Society...

Also the fact that only transcellular permeation is regarded in PAMPA experiments is valuable as it allows comparing PAMPA results to cellular permeation experiments that feature all possible permeation mechanism including paracellular or active transport and active efflux. Kerns (2004) recommended this comparison to get an insight into the permeation mechanism applied by a compound under investigation. The possibility to obtain permeability pH profiles is also really helpful to identify the relevant permeability value of a compound and cannot be determined by cellular assays due to the limited pH range usable with living cells. 

A major breakthrough in the study of gas and v or transport in polymer membranes was achieved by Daynes in 1920 He pointed out that steady-state permeability measurements could only lead to the determination of the product EMcd and not their separate values. He showed that, under boundary conditions which were easy to achieve experimentally, D is related to the time retired to achieve steady state permeation throu an initially degassed membrane. The so-called diffusion time lag , 6, is obtained by back-extrapolation to the time axis of the pseudo-steady-state portion of the pressure buildup in a low pressure downstream receiving vdume for a transient permeation experiment. As shown in Eq. (6), the time lag is quantitatively related to the diffusion coefficient and the membrane thickness, , for the simple case where both ko and D are constants. 

Some methods to determine the permeability of a drug from the GI tract include 1) in vivo intestinal perfusion studies in humans 2) in vivo or in situ intestinal perfusion studies in animals 3) in vitro permeation experiments using excised human or animal intestinal tissues and 4) in vitro permeation experiments across a monolayer of cultured human intestinal cells. 

The effects of feed pressure on C02 flux and permeability, H2 flux, and C02/H2 selectivity were investigated using a membrane with a thickness of -60pm on the BHA microporous Teflon support. The feed pressures ranged from 1.5 to 2.8atm. Temperature was maintained at 110°C, and water rates were kept at 0.03cc/min for both the feed side and sweep side. The feed gas consisted of 20% C02,40% H2, and 40% N2 (on dry basis) with a feed gas rate of about 60cc/min in the gas permeation experiments. 

Permeation-skin-gas chromatography (GC)/MS A silastic membrane was coated onto a fiber to be used as a permeation membrane. The MCF was immersed in the donor phase to partition the compounds into the membrane. At a given partition time, the MCF was transferred into a GC injector to evaporate the partitioned compounds for quantitative and qualitative analyses. This technique was developed and demonstrated to study the percutaneous permeation of a complex mixture consisting of 30 compounds. Each compound permeated into the membrane was identified and quantified with GC/MS. The standard deviation was less than 10% in 12 repeated permeation experiments. The partition coefficients and permeation rates in static and stirred donor solutions were obtained for each compound. The partition coefficients measured by this technique were well correlated (Pf — 0.93) with the reported octanol/water partition coefficients. This technique can be used to study the percutaneous permeation of chemical mixtures. No expensive radiolabeled chemicals were required. Each compound permeated into the membrane can be identified and quantified. The initial permeation rate and equilibrium time can be obtained for each compound, which could serve as characteristic parameters regarding the skin permeability of the compound. 

In an experiment with the EVOH film and ethylvalerate at 110 C, 0.25 milliliters of water was injected into the three liter flask after the permeation experiment had reached steady state. Within seconds, the permeation rate increased to above the detection limit of the mass spectrometer. This was consistent with prior experience wherein the permeability and the diffusivity in an EVOH film increase by a factor of about 1000 in the presence of moisture (6). No more experiments were done with humidity for this study. 

The permeability coefBcient, P, is calculated from the slope of the permeation experiment at steady state by considering the area of the membrane. The diffusion coefficiait, D, is reported to be correlated to the time-lag, 0, according to the following equation ). 

Gas permeation experiment A constant volume technique was used to measure the gas permeability (12). After mounting a membrane in a permeation cell, the cell system was evacuated and its leak rate was checked, which was typically 45 mTorr/hr. The true pressure increase data were obtained by substracting the leak from the measured pressure increase. The permeability P was calculated from the slope of a plot of pressure vs time t at steady state. 

Gas permeation experiments were carried out for carbon dioxide, oxygen and nitrogen. The method used to measure the gas permeability was the constant pressure or variable volume method. For the measurements a home-made system was used [32]. It comprised a two-compartment flat sheet permeability cell. The feed stream circulates in the bottom compartment tangentially to the composite films and permeates through them to be collected in the top compartment where a flowmeter is connected. Due to the asymmetry of the composites the permeation experiments were carried out both for the top and bottom surfaces of the composite films facing the feed stream. 

A permeation experiment is relatively easy to implement as it requires only a pressure gradient Ap across the sample and a device to determine the fluid flow rate J (Figure 21.35). The permeability coefficient kp is then given by... 

The extraordinarily low permeability can be explained by the fact that polyethylene as a non-polar medium can only be very weakly polarized and diffusion cannot lead to a separation of charge carrier. The ions are surrounded in the aqueous solution by a cloud of water molecules shielding the ion s charge. Cations and anions would therefore have to recombine from this hydrate shell to the molecule and become dissolved in the polyethylene or both become dissolved with their hydrate shell and diffuse. Such processes are thermodynamically rather unfavourable. The importance of dissociation of inorganic molecules for the migration becomes clear by permeation tests performed with concentrated hydrochloric acid. Undissociated HCl molecules are found to some extent in concentrated hydrochloric acid while the molecules are fully dissociated in aqueous NaCl or metallic salt solution. The available undissociated HCl molecules can become dissolved in the polyethylene and only then diffuse similarly to water molecules or undissociated acetic acid molecules. While no permeation of chlorine can be observed in permeation experiments with metal salts, diffused chlorine can be proven when using concentrated hydrochloric acid. 

The time-lag method is very suitable for studying ideal systems with a constant diffusion coefficient. The permeability coefficient P can be obtained from the steady-state part of this permeation experiment (eq, V - 106), which means that both the diffusion coefficient and the permeability coefficient can be determined fiom one experiment More... 

The permeability coefficient P is a very characteristic parameter which is often described as a constant intrinsic parameter easily available from simple permeation experiments with membranes of known thickness (using eq. VI - 46). The permeabiliiy coefficient is often given in Barrer units. (lBarrer = lO" cm3(STP).cm.cm. s .ciiikg = 0.76 10 m3(STP).m.m-2.s- .Pa->). 

The hydraulic or water permeability coefficient (L ) can be determined from a simple permeation experiment Assume for a given membrane a Lp value of 5 lO m/hr. bar. The membrane has a rejection coefficient of 95% for NaCl and of 99.8% for NajSO< at 40 bar and 10000 ppm salt Calculate the solute permeability coefficient for both salts. 

A homogeneous cellulosic ester membrane with a thickness of 20 im is placed in a pervaporation cell with a diameter of 10 cm. The permeate side is kept at a vacuum of I mbar.. In a steady state permeation experiment at 20°C 12.0 g of water is collected in 1 hours. Calculate the water permeability coefficient in moljn/m. s.Pa en in cm (STP).cm/cm7.s.cmHg. 

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