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in Fig 22 (ie, intrinsic permeabilities P0 of drugs) Also, a new model for gastrointestinal (oral) absorption based on permeability measurements using arti cial membranes will be presented Approximately 1400 measurements of permeability are presented in tables and gures in this chapter Most of the data are original, not published previously Unless otherwise noted, the permeability and membrane retention data are from pION s laboratories, based on the permeation cell design developed at pION Cells of different designs, employing different lter and phospholipid membrane materials, produce different permeability values for reasons discussed below Although the analysis of the measurements is the basis of the presentation in this chapter, much of the data can be further mined for useful quantitative structure property information, and the reader is encouraged to do so First-person references in this chapter, such as our laboratory, refer speci cally to pION s laboratory, and our results are those of several colleagues who have contributed to the effort, covering a period of >4 years, as cited in the acknowledgment section Where possible, comparisons to published permeability results from other laboratories will be made The survey of over 50 arti cial lipid membrane models (pION) in this chapter reveals a new and very promising in vitro GIT model, based on the use of levels of lecithin membrane components higher than those previously reported, the use of negatively charged phospholipid membrane components, pH gradients, and arti cial sink conditions Also, a novel direction is suggested in the search for an ideal in vitro BBB model, based on the salient differences between the properties of the GIT and the BBB We return to using the Kp and Kd symbols to represent the partition coef cient and the apparent partition (distribution) coef cient, respectively The effective, apparent, membrane, and intrinsic permeability coef cients are denoted Pe, Pa, Pm, and P0, respectively, and D refers to the diffusivity of molecules The coverage of permeability in this book is more comprehensive than that of solubility, lipophilicity, and ionization This decision was made because permeability is not as thoroughly treated in the pharmaceutical literature as the other topics, and also because much emphasis is placed on the PAMPA in this book, which is indeed a very new technique [547] in need of elaboration
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72 HISTORICAL DEVELOPMENTS IN ARTIFICIAL-MEMBRANE PERMEABILITY MEASUREMENT 721 Lipid Bilayer Concept
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The history of the development of the bilayer membrane model is fascinating, and spans at least 300 years, beginning with studies of soap bubbles and oil layers on water [517 519] In 1672 Robert Hooke observed under a microscope the growth of black spots on soap bubbles [520] Three years later Isaac Newton [521], studying the images
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DEVELOPMENTS IN ARTIFICIAL-MEMBRANE PERMEABILITY MEASUREMENT
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of the Sun very faintly re ected [off the black patched on the surface of soap bubbles], calculated the thickness of the black patches to be equivalent to 95 A ngstrom, father of spectroscopy, who taught at the University of (Anders Jonas A Uppsala, after whom the A unit is named, did not appear until about 150 years later) Ben Franklin, a self-trained scientist of eclectic interests, but better known for his role in American political history, was visiting England in the early 1770s He published in the Philosophical Transactions of the Royal Society in 1774 [552]:
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At length being at Clapham where there is, on the common, a large pond, which I observed to be one day very rough with the wind, I fetched out a cruet of oil, and dropt a little of it on the water and there the oil, though not more than a tea spoonful, spread amazingly, and extended itself gradually till it reached the lee side, making all that quarter of the pond, perhaps half an acre, as smooth as a looking glass so thin as to produce prismatic colors and beyond them so much thinner as to be invisible
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Franklin mentioned Pliny s account of sherman pouring oil on troubled waters in ancient times, a practice that survives to the present (Franklin s experiment was reenacted by the author at the pond on Clapham Common with a teaspoon of olive oil The spreading oil covered a surface not larger than that of a beach towel it appears that technique and/or choice of oil is important The olive oil quickly spread out in circular patterns of brilliant prismatic colors, but then dissolved from sight Indeed, the pond itself has shrunken considerably over the intervening 230 years) More than 100 years later, in 1890, Lord Rayleigh, a professor of natural philosophy at the Royal Institution of London, was conducting a series of quantitative experiments with water and oil, where he carefully measured the area to which a volume of oil would expand This led him to calculate the thickness of the oil lm [517,518] A year after publishing his work, he was contacted by a German woman named Agnes Pockels, who had done extensive experiments in oil lms in her kitchen sink She developed a device for carefully measuring the exact area of an oil lm Lord Rayleigh helped Agnes Pockels in publishing her results in scienti c journals (1891 1894) [517,518] Franklin s teaspoon of oil (assuming a density 09 g/mL and average fatty-acid molecular weight 280 g/mol) would contain 10 22 fatty-acid tails The half-acre pond surface covered by the oil, $2000 m2, is about 2 10 23 A2 So, each tail 2, assuming that a single monolayer would be expected to occupy about 20 A (25 A calculated thickness) of oil formed on the surface of the pond Pfeffer in 1877 [523] subjected plant cell suspensions to different amounts of salt and observed the cells to shrink under hypertonic conditions and swell in hypotonic conditions He concluded there was a semipermeable membrane separating the cell interior from the external solution, an invisible (under light microscope) plasma membrane Overton in the 1890s at the University of Zurich carried out some 10,000 experiments with more than 500 different chemical compounds [518,524] He measured
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