Distribution Coefficient

The partition coefficient refers to the intrinsic lipophilicity of the drug, in the context of the equilibrium of unionized drug between the aqueous and organic phases. If the drug has more than one ionization center, the distribution of species present will depend on the pH. The concentration of the ionized drug in the aqueous phase will therefore have an effect on the overall observed partition coefficient. This leads to the definition of the distribution coefficient (log D) of a compound, which takes into account the dissociation of weak acids and bases.

Since in the aqueous phase, the total concentration may comprise both ionized and unionized forms the distribution is given as:

Distribution Coefficient, D = [Unionised](o)/[Unionised](aq) + [Ionised](aq) (8)

Log D = log 10 (Distribution Coefficient) (9)

Log D is related to log P and the pKa by the following equations: Log D (pH) = log P - log[ 1 + 10(pH"piQ>] for acids

Log D profile

Log D profile

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