Apparent partition coefficient, log

In the traditional shake-flask method, the apparent partition coefficient, log D, is measured, usually at pH 7.4 (sometimes at pH 6.5). Different buffers are used to control each pH used in the determinations [70]. Usually, in a comprehensive study, several pH measurements are made, and values of log are plotted against the pH. This plot is often called the lipophilicity profile . One can determine the true partition coefficients (log P ) and the ionization constants from the features in such a curve. 

Figure 7.17 shows the asymmetry ratios of a series of compounds (acids, bases, and neutrals) determined at iso-pH 7.4, under the influence of sink conditions created not by pH, but by anionic surfactant added to the acceptor wells (discuss later in the chapter). The membrane barrier was constructed from 20% soy lecithin in dodecane. All molecules show an upward dependence on lipophilicity, as estimated by octanol-water apparent partition coefficients, log KdaA). The bases are extensively cationic at pH 7.4, as well as being lipophilic, and so display the highest responses to the sink condition. They are driven to interact with the surfactant by both hydrophobic and electrostatic forces. The anionic acids are largely indifferent... 

The membrane permeabilities Pm may be converted to intrinsic permeabilities P(h when the pKa is taken into consideration. An ionizable molecule exhibits its intrinsic permeability when it is in its uncharged form and there is no water layer resistance. The relationship between Pm and P0 is like that between the pH-dependent apparent partition coefficient (log Kd) and the true partition coefficient (log Kp), respectively. This relationship can be rationalized by the mass balance. Take, for example, the case of a monoprotic acid, HA. The total substance concentration is... 

The apparent partition coefficient (log P) and the calculated molar re-fractivity (CMR) are physicochemical parameters that correlate with the ability of a compound to be involved in MDR, and nonobvious structural homologies between compounds as dissimilar as verapamil (148) and vinblastine (1) exist as revealed by computer modeling studies (115). A far... 

The apparent partition coefficients (log D) of CQ and FQ were measured at vacuolar (5.2) and cytosolic (7.4) pHs (Table 1, [128]). At cytosolic pH, FQ was over 100-fold more lipophilic than CQ, whereas the difference in lipophilicity is only slight at acidic DV pH (about threefold). 

The dissociation equilibrium depends on the pH value of the aqueous phase and on the pA a value of the organic compound. Equation (34) (for acids) and Eq. (35) (for bases) describe the apparent partition coefficients log Papp of acids and bases at different pH values of the aqueous phase in an -octanol/buffer system ( Pu = partition coefficient of the neutral, uncharged form P = partition coefficient of the ionized,... 

Equations (34) and (35) describe sigmoidal curves with apparent partition coefficients log Papp = log Pu at pH < pATa (acids) and pH > pK (bases), and apparent partition coefficients log Papp - log P at pH3>pATa (acids) and pHwith constant Papp values for the neutral forms and steadily decreasing Papp values with increasing ionization, at pH > pKa for acids and pH < pATa for bases. 

Compound Acronym Molecular weight MW [g mole ] Dissociation constant pXa Partitioning coefficient log Kq/w Apparent partitioning coefficient log D ... 

Apparent partition coefficient (logZ)) at an ionic strength of / = 0.02M, log P value of the neutral microspecies and the acidic dissociation constant of 5 was calculated (97ANC4143). The distribution coefficient of 5 was determined between 1-octanol and universal buffer in the pH range 3-10 at a... 

The limitation of Eq. (2.1) is that measurement of concentrations of solute within different parts of the membrane is very inconvenient. However, since we can estimate (or possibly measure) the distribution coefficients between bulk water and the membrane, log Kj (the pH-dependent apparent partition coefficient), we can convert Eq. (2.1) into a more accessible form... 

Four neutral lipid models were explored at pH 7.4 (1) 2% wt/vol DOPC in dode-cane, (2) olive oil, (3) octanol, and (4) dodecane. Table 7.5 lists the effective permeabilities Pe, standard deviations (SDs), and membrane retentions of the 32 probe molecules (Table 7.4). The units of Pe and SD are 10 6 cm/s. Retentions are expressed as mole percentages. Figure 7.22a is a plot of log Pe versus log Kd (octanol-water apparent partition coefficients, pH 7.4) for filters loaded with 2% wt/vol DOPC in dodecane (model 1.0, hlled-circle symbols) and with phospholipid-free dodecane (model 4.0, open-circle symbols). The dashed line in the plot was calculated assuming a UWL permeability (see Section 7.7.6) Pu, 16 x 10-6 cm/s (a typical value in an unstirred 96-well microtiter plate assay), and Pe of 0.8 x 10-6 cm/s... 

Figure 7.22 Lipophilic nature of membrane retention, log(%R) versus octanol-water apparent partition coefficient, pH 7.4, neutral lipid models.

Figure 3.1 shows the appearance of dihydromethysticin in the acceptor well as a function of time [15], The solid curve is a least-squares fit of the data points to Eq. (1), with the parameters Pe = 32 x 10-6 cm s 1, R = 0.42, and t s = 35 min. The membrane retention, R, is often stated as a mole percentage (%R) of the sample (rather than a fraction). Its value can at times be very high - up to 90% for chlor-promazine and 70% for phenazopyridine, when 2% wt/vol DOPC in dodecane is used. Figure 3.2 shows a plot of log %R versus log Ka(7.4), the octanol/water apparent partition coefficient. It appears that retention is due to the lipophilicity of molecules this may be a good predictor of the pharmacokinetic volume of distribution or of protein binding. 

Tab. 2.2 Apparent partition coefficients of a-adrenoceptor agonists in liposome-buffer (log K, ) systems as indicated and in n-octanol buffer (log P) at 37°C. (Adapted from Table 1 of ref. 37)...

For slightly simpler amphoteric compounds, such as p-aminobenzoic acid and sulfonamides, the apparent partition coefficient is maximal at the isoelectric point. Figure 5.10 illustrates the variation with pH of log forp-aminobenzoic acid and for two sulfonamides. 

Figure 8.3. Empirical relationship between i siixi intestinal absorption rate constants ( abs) and apparent partition coefficients. The curve is described hy logi abs = O.lOSlogP - 0.09(log Pf - 0.833 (16).

Partition coefficients were measured by a conventional shake-flask technique at 37°C [10]. The concentrations of the compound in the aqueous phase before and after partitioning were determined spectrophotometrically. Birffer salts were used to control the pH of the aqueous phase. Log P is the partition coefficient of the neutral form of the compoimd. It is assumed that only the neutral form of the compound partitions into the organic phase, whence log P is related to the apparent partition coefficient (PJ of the compoimd, measured at a pH at which the neutral and monoprotonated forms are in equilibrium in the aqueous phase, and to the dissociation constant K ) of a proton from the most basic site in the molecule, by equation (1) ... 

However, as stated above, the partition coefficients measured by the shake-flask method or by potenhometric titration can be influenced by the potenhal difference between the two phases, and are therefore apparent values which depend on the experimental condihons (phase volume ratio, nature and concentrahons of all ions in the solutions). In particular, it has been shown that the difference between the apparent and the standard log Pi depends on the phase volume raho and that this relationship itself depends on the lipophilicity of the ion [80]. In theory, the most relevant case for in vivo extrapolation is when V /V 1 as it corresponds to the phase ratio encountered by a drug as it distributes within the body. The measurement of apparent log Pi values does not allow to differentiate between ion-pairing effect and partihoning of the ions due to the Galvani potential difference, and it has been shown that the apparent lipophilicity of a number of quaternary ion drugs is not due to ion-pair partitioning as inihally thought [80]. 

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 [371,377-379]. It is not as popular a method as it used to be, apparently due to instrumental challenges. Cyclic voltammetry (CV) has become the new method used to get access to very low log D values, with partition coefficients reported as low as —9.8 [261,269,362]. 

Fig. 3.2. Log of%mole fraction membrane retention versus log of apparent octanol/water partition coefficient at pH 7.4. Microfilters were impregnated with 2% wt/vol DOPC in dodecane.

The correlation between partition coefficient and chromatographic retention has been examined by many workers in an attempt to predict log F, the logarithm of water-octanol partition coefficient used in establishing structure-function relationships, from retention data or vice versa. The data (92, 203, 215) seem to imply changes in log k are linear in changes in log P but that log k changes more slowly. Some literature data suggest that the retention is directly proportional to the partition coefficient. This apparent equality disappears, however, if the data are recalculated to identical experimental conditions. 

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