Correction For The Dmso Effect By The Ashift Method
6.5.1 DMSO Binding to the Uncharged Form of a Compound It was found that the log S pH curves were altered in the presence of as little as 0.5 v v DMSO, in that the apparent pKa values, pK pp, derived from log S versus pH 481 , were different from the true pKa values by about one log unit. The pKapp values were generally higher than the true pKa values for weak acids positive shift , and lower than those for weak bases negative shift . This has been called the ''A shift'' Avdeef, unpublished ....
Partitioning Into Octanol
In all other sections of this book, we use the term Kp to represent the partition coefficient and Kd, the apparent partition coefficient. These terms were chosen to avoid symbol conflict when discussing permeability and diffusivity. Since this chapter and Chapter 5 are devoted primarily to partition coefficients, we will use the most common terminology P for partition coefficients and D for apparent pH-dependent partition coefficients. Other symbols for these parameters have been used in the...
Charge State
Weak acids and bases ionize in solutions to varying extent, depending on pH. This, in turn, affects the distribution of the chemicals in solution and affects their availability to enter biological reactions. The characteristic thermodynamic parameter relating the pH to the charge state of a molecule is the ionization constant, pKa 34,35 . Knowledge of the pKa of a substance is widely useful. It can predict the absorption, distribution, and excretion of medicinal substances. For example, urine...
Pp
where Pe refers to the measured effective permeability, Pu refers to the total UWL permeability, Pm is the permeability of the membrane which would be measured if the UWL were made vanishingly thin . If it is possible to separate the donor and acceptor contributions to the UWL, then the total Pu can be allocated between its parts according to 1 Pu 1 pUD 1 pU . In Caco-2 literature, equations like Eq. 7.46 often have a fitter, Pf, component, to account for resistance of the water-filled pores of...
R
Note that Eqs. 7.20 and 7.21 are nearly identical to Eqs. 7.12 and 7.13 the differences are the 1 R term to reflect membrane retention and the lag time offset, tLag the time needed to saturate the membrane with solute . These differences warrant the new equations to be denoted with the subscript e. When using the 96-well microtiter plate format, typical metrics are VA 200400 mL, VD 200-400 mL, A 0.3 cm2, VM 4-6 mL, h filter thickness 125 mm, 70 porosity e , t permeation time 3-15 h, tLAG 0-60...
Info Xon
All Pe and SD Pe are in units of 10 6 cm s nd compound not detected in the acceptor compartment. All Pe and SD Pe are in units of 10 6 cm s nd compound not detected in the acceptor compartment. TABLE 7.13 Soy Lecithin in Dodecane PAMPA Models with Sink , pH 7.4a TABLE 7.13 Soy Lecithin in Dodecane PAMPA Models with Sink , pH 7.4a
Other Chromatographic Methods
Capillary electrophoresis CE see Section 3.5 has been used to determine partition coefficients 320-322 . Lipid vesicles or micelles are added to the buffer whose pH is adjusted to different values. Since drug molecules partition to a different extent as a function of pH, the analysis of mobility vs pH data yields log P values. Centrifugal partition chromatography CPC has been used to characterize the partitioning behavior of hydrophilic molecules, where log D values as low as 3 can be obtained...
Bjerrum Plots
The Bjerrum plots are probably the most important graphical tools in the initial stages of titration data analysis. Since one knows how much strong acid and strong base have been added to the solution at any point and also how many dissociable protons the sample substance brings to the solution, one knows the total hydrogen ion concentration in solution, despite what equilibrium reactions are taking place. By measuring the pH and after converting it into pcH log H , one knows the free hydrogen...
O Hc
Figure 4.8 Octanol-water Bjerrum plots for a diprotic a acid, b ampholyte, and c base. The volumes of octanol and water are equal, so that the difference between the apparent pKa and the true pKa is about equal to the partition coefficient. Avdeef, A., Curr. Topics Med. Chem., 1, 277-351 2001 . Reproduced with permission from Bentham Science Publishers, Ltd. method, but all in all, it is well positioned to replace the shake-flask procedure as the primary validation method for ionizable...
The Solubility Problem and Cosolvent Methods
Since many new substances of interest are very poorly soluble in water, the assessment of the pKa in aqueous solution can be difficult and problematic. Potentiometry can be a quick technique for such assessment, provided the solubility of the substance is at least 100 p.M. Solutions as dilute as 10 p.M can still be analyzed, but special attention must be given to electrode calibration, and ambient carbon dioxide must be excluded. If the substance is soluble to only 1-10 p.M and possesses a...
Info Neo
log PmemN - lo9 P TN 204 - 059 lo9 PoctN log PmemN - lo9 P TN 204 - 059 lo9 PoctN
Permeability In The Gastrointestinal Tract And At The Bloodbrain Barrier
Measured permeability especially when combined with solubility and charge state can be viewed as a surrogate property for predicting oral gastrointestinal absorption of preclinical drug candidate molecules. This chapter considers the transport of molecules by passive diffusion through phospholipid bilayers. The emphasis is on 1 the current state-of-the-art measurement of permeabilities by the so-called PAMPA method and 2 the theoretical physicochemical models that attempt to rationalize the...
b
Figure 4.4 a Lipophilicity profile of an ampholyte at two values of background salt and b log-log speciation plot at 0.15 M KCl. Avdeef, A., Curr. Topics Med. Chem., 1, 277-351 2001 . Reproduced with permission from Bentham Science Publishers, Ltd. Figures 4.2b, 4.3b, and 4.4b are log-log speciation plots, indicating the concentrations of species in units of the total aqueous sample concentration. Similar plots were described by Scherrer 280 . The uppermost curve in Fig. 4.2b shows the...
Acknowledgments
Professor Norman Ho University of Utah was very kind to critically read the Chapter 7 and comment on the various derivations and concepts of permeability. His unique expertise on the topic spans many decades. His thoughts and advice 30 pages of handwritten notes inspired me to rewrite some of the sections in that chapter. I am very grateful to him. Special thanks go to Per Nielsen and Cynthia Berger of pION for critically reading and commenting on the manuscript. I am grateful to other...
Pc
lipid components of biological membranes . For simplicity, all acyl charns axe shown as oleyl rescue, in vitro. It is considered the seminal work on the self-assembly of planar lipid bilayers 516,518,519,531,532 . Their research led them to the conclusion that a soap film in its final stages of thinning has a structure of a single bilayer, with the oily tails of detergent molecules pointing to the side of air, and the polar heads sandwiching a layer of water. Their experimental model drew on...
e
Figure 7.67 Human intestinal absorption compared to a pION's double-sink sum-Pe PAMPA GIT model and b human jejunal permeabilities 56 . about two and a half orders for the HJP data. Such a spread in the PAMPA data could facilitate the selection of well-absorbed molecules from those poorly absorbed. In conclusion, the double-sink sum-Pe PAMPA in vitro GIT assay seems to predict human absorption as well as in vivo human permeability measurements see Figs. 7.66a,b and in vitro Caco-2 permeability...
Spectrophotometric Measurements
The most effective spectrophotometric procedures for pKa determination are based on the processing of whole absorption curves over a broad range of wavelengths, with data collected over a suitable range of pH. Most of the approaches are based on mass balance equations incorporating absorbance data of solutions adjusted to various pH values as dependent variables and equilibrium constants as parameters, refined by nonlinear least-squares refinement, using Gauss-Newton, Marquardt, or Simplex...
Info Mcx
0.000 0.005 0.010 0.015 0.020 Titrant Volume mL 0.5M HCl Figure 6.7 Dissolution template titration DTT curve of propoxyphene 0.51 mg of the hydrochloride salt was dissolved in 5.1 mL of 0.15 M KCl solution, with 0.0084 mL of 0.5 M KOH used to raise the pH to 10.5. compound is anticipated to be based on the template the longer the assay time. An entire solubility-pH profile is deduced from the assay. A graphical analysis follows, based on Bjerrum plots see Sections 3.3.1 and 4.14 . The Bjerrum...
Info Elb
Figure 7.44 Collander relationship between intrinsic permeabilities of 20 soy lecithin versus 2 DOPC PAMPA models. octanol-water system might suggest that, given a background solution of a lipophilic counterion, ion pair transmembrane transport takes place e.g., Section 4.5 . Such hypotheses can be tested in a direct way in the PAMPA assay. If the charged species, especially quaternary ammonium drugs, appear in the acceptor compartment, the case for charged species transport could be further...
Partitioning Into Octanol42
4.5 Partitioning of Quaternary Ammonium Drugs 50 4.6 log D of Multiprotic Drugs and the Common-Ion Effect 50 4.7 Summary of Charged-Species Partitioning in Octanol-Water 53 4.8 Ion Pair Absorption of Ionized Drugs Fact or Fiction 53 4.12 Liposome Chromatography 55 4.13 Other Chromatographic Methods 55 4.14 pH-Metric log P Method 55 4.15 High-Throughput log P Methods 59 4.16 Octanol-Water log PN, log P', and log D7 4 ''Gold Standard'' for Drug Molecules 59