pH, pKa and drugs

Mostly drug exists as a weak acid or a weak base in the body. Since the body compartments are already having their own nature which is either acidic or basic, it will certainly have an impact on the drug reaches it.

pKa of a drug is defined as the pH concentration at which they exist in 50% ionized and 50% unionized forms.

The relationship between pH and pKa can be given by the Henderson- Hasselbach equation as follows,

 

ION TRAPPING 

  At a steady state concentration an acidic drug will be located more on the basic side of the membrane and a basic drug will be on the acidic side of the membrane.

  

IN DIFFERENT MEDIUMS

    Acidic drugs are absorbed in acidic medium

Eg: Ibuprofen is an acidic drug with pKa 4.4 ,after oral administration it reaches the

 Gastric juice where pH is 2.5 ,according to the formulae 79.43%  is unionized where absorption of the drug takes place, from where it enters the blood where pH is 7.4  and gets distributed through the body and finally excreted via urine where pH is 8

    Basic drugs are better absorbed in basic medium 

Eg : Atropine is a basic drug with pKa 9.3, which is usually given via an intravenous route where pH is 7.4  according to the formulae 79.43% is unionized and absorption of the drug takes places and it reaches different tissues of the body to perform its anti-cholinergic effects after which it undergoes enzymatic degradation in liver whereas some are excreted unchanged in urine where pH is 8.

The relationship between the pKa of an acidic (AH) and basic (B) drug and the pH of the biological medium (sr: dentalcare.com)