^ research compounds (2% DOPC/dodecane)

Pe (dodecane), 10-6 cm/s units

Figure 7.50 Hydrogen bonding/electrostatics scale based on phospholipid-alkane permeability differences: A log Pe versus Pe (dodecane).

indicated in Fig. 7.50. Both H-bonding and ionic interactions may be encoded in the A log Pe parameter. This topic is the subject of further investigation at pION, with the aim of developing BBB PAMPA models.

7.7.9 Effects of Cosolvent in Donor Wells

Many research compounds are poorly soluble in water. When very lipophilic molecules precipitate in the donor wells, it is possible to filter the donor solution before the PAMPA sandwich is prepared. On occasion, the filtered donor solution contains such small amounts of the compound that determination of concentrations by UV spectrophotometry becomes impractical. One strategy to overcome the precipitation of the sample molecules in the donor wells is to add a cosolvent to the solutions (Section 7.4.4). It is a strategy of compromise and practicality. Although the cosol-vent may solubilize the lipophilic solute molecule, the effect on transport may be subtle and not easy to predict. At least three mechanisms may cause Pe and membrane retention (%R) values to alter as a result of the cosolvent addition. To a varying extent, all three mechanisms may simultaneously contribute to the observed transport:

1. The cosolvent will lower the dielectric constant of the mixed solvent, independent of the properties of the solute molecule. The ionization constant of acids will increase and that of bases will decrease (see Sections 3.3.3 and 3.3.4), the result of which is to increase the fraction of uncharged substance in solution (fu in Table 7.4). With an increased concentration of the uncharged species in the donor solution, both Pe and %R are expected to increase. Generally, this effect is minimal for cosolvent amounts less than 10% vol/vol [119,172].

2. The cosolvent may increase the aqueous solubility of the sparingly soluble molecules, which would lower the membrane-donor solution partition coefficient. According to Eq. (2.3), Pe will decrease. Since %R is related to lipophilicity (Section 7.7.2), the retention is also expected to decrease.

3. Sparingly soluble surface-active molecules, such as chlorpromazine, may form water-soluble high-molecular-weight (HMW) aggregates (Sections 6.2, 6.5.2, 6.5.3). Their diffusion in the unstirred water layer will decrease according to Eq. (7.60). Cosolvent may break up these aggregates, resulting in increased Pe, and to a lesser extent, an increased %R.

Table 7.19 summarizes the PAMPA (2% DOPC in dodecane) transport properties of several molecules, with and without 10% 1-propanol in the donor wells. This particular cosolvent was selected for its low UV absorbance and low volatility.

The most dramatic effects are with the bases. The first seven bases in Table 7.19 are the most lipophilic. Cosolvent causes their %R to decrease, consistent with effect (2) listed above. For the three least-lipophilic bases, %R increases with cosolvent, consistent with effect (1). Chlorpromazine and verapamil experience

TABLE 7.19 Effect of 10% 1-Propanol, pH 7.4a
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