Transcellular Drug Absorption—Simple and Facilitated Diffusion

4 Transcellular Drug Absorption—Simple and Facilitated Diffusion 

Upon passage, a poorly water-soluble drug also accumulates in the membrane. Doluisio and coworkers have shown that a biexponential plot can describe the absorption kinetics of lipid-soluble drugs, such as prochlorperazine, whereas a single exponent slope characterizes the absorption of hydrophilic drugs, such as trimeprazine, which does not accumulate in the membrane [129]. 

An important step in the understanding of partitioning phenomena is the ability to link between a drug molecule s pKa and its pH-dependent solubility. The major tool for this estimation has been, for the past 80 years, the Henderson-Hasselbalch equation [133]. The ionized fraction of a drug can be calculated for any pH value if its pKa is known [134], as follows  

Assuming a dissociation of a weak acid drug according to the general reaction  

Taking the negative logarithm on both sides of the equations to enable the use of the terms pH and pK and rearranging, the Henderson-Hasselbalch equation becomes 

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