PAMPA–Excipient Classification Gradient Map

Abstract

The effect of excipients on the artificial membrane permeability (Double-Sink PAMPA) properties of eight sparingly soluble drugs was studied. Quantities of excipient were selected to match the concentrations expected in the gastrointestinal fluid under clinically relevant conditions. Over 1,200 measurements were performed. To correct for the effects of the aqueous boundary layer and determine the intrinsic permeability, precisely measured ionization constants were used. The intrinsic permeability of weak acids was enhanced (up to 100 fold) but that of weak bases depressed (up to 270 fold) by the excipients: mefenamic acid > glybenclamide > progesterone > griseofulvin > clotrimazole > astemizole > dipyridamole > butacaine. Excipient enhancement ranked: 3 mM NaTC > 0.24% PEG400 > 0.2 M KCl > 0.24% NMP > 5% PEG400 > 0.24% PG > 1% PEG400 > 0.1M KCl > 1% PG > 1% NMP > 5% PG > 0.24% HP-β-CD > 1% HP-β-CD > 15 mM NaTC. The study clearly indicates that the method is suitable for use in preclinical development to assess the effect of excipients on the permeability of sparingly soluble drug candidates. The method is quick, cost-effective, and reasonably accurate. The self-rank-ordered PAMPA-Mapping may be a helpful visualization tool for delivery screening.

Appendix

The approximation in Eq. 4 is accurate when P _o > 10 P _ABL, but with mildly lipophilic molecules (cf., Fig. 3d—dipyridamole), the exact form is required to calculate ${\text{p}}K^{{{\text{flux}}}}_{{\text{a}}}$. The solid-line curve in Fig. 3d only approximately indicates the ${\text{p}}K^{{{\text{flux}}}}_{{\text{a}}}$ at the pH corresponding to the half-integral slope (that’s why no ${\text{p}}K^{{{\text{flux}}}}_{{\text{a}}}$ value is noted in Fig. 3d). The precise (more complicated) form of Eq. 4, which can describe the case of dipyridamole is given by

$$ \log P_{{\text{e}}} = \log P^{{\max }}_{{\text{e}}} - \log {\left( {10^{{ \pm {\left( {{\text{pH}}{\text{ - }}{\text{p}}K^{{{\text{flux,}}{\text{app}}}}_{{\text{a}}} } \right)}}} + 1} \right)} $$

(6)

where observed ${\text{p}}K^{{{\text{flux, app}}}}_{{\text{a}}}$ is an apparent value, not precisely equal to the true ${\text{p}}K^{{{\text{flux}}}}_{{\text{a}}}$. The difference between the two values becomes greater, the smaller the difference between the intrinsic permeability and the ABL permeability. It is tedious, but otherwise straight forward, to derived the explicit logarithmic form of Eq. 3. In the exact solution,

$$ {\text{p}}K^{{{\text{flux, app}}}}_{{\text{a}}} = {\text{p}}K^{{{\text{flux}}}}_{{\text{a}}} \mp \log {\left[ {{{\left( {P_{{\text{o}}} {\text{ }} - {\text{ }}P_{{{\text{ABL}}}} } \right)}} \mathord{\left/ {\vphantom {{{\left( {P_{{\text{o}}} {\text{ }} - {\text{ }}P_{{{\text{ABL}}}} } \right)}} {{\left( {P_{{\text{o}}} + P_{{{\text{ABL}}}} } \right)}}}} \right. \kern-\nulldelimiterspace} {{\left( {P_{{\text{o}}} + P_{{{\text{ABL}}}} } \right)}}} \right]} $$

(7)

where the ‘−’ refers to acids and the ‘+’ sign refers to bases. Figure 6 shows the relationships between the ionization constants as a function of the difference between log P _o and log P _ABL. Above one log unit of difference (highly lipophilic compounds), ${\text{p}}K^{{{\text{flux, app}}}}_{{\text{a}}} \approx {\text{p}}K^{{{\text{flux}}}}_{{\text{a}}}$. It should be further emphasized that Eqs. 4 and 6 are only valid when P_o > P_ABL, as we had noted previously (9,21). In practice, the approximate form (Eq. 3) has proven to be useful in visualizing the balance between membrane and water factors controlling the transport of molecules across biologically relevant membrane barriers.

Fig. 6

The exact Eq. 4 relationship (applied to a weak acid) between the true pK _a (dashed curve), the flux value, ${\text{p}}K_{{\text{a}}}^{{{\text{flux}}}}$ (solid line curve), and the apparent flux value, ${\text{p}}K_{{\text{a}}}^{{{\text{flux,app}}}}$ (dotted curve) constants as a function of the difference between the logarithmic intrinsic permeability (log P _o) and the aqueous boundary layer permeability (log P _ABL). The two flux values become nearly equivalent for log P _o − log P _ABL > 1. The thin straight line is the identity line.