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Partition coefficient

The partition coefficient is the ratio of concentrations of a compound between two immiscible solvents at equilibrium. For drug delivery, the lipophilic/hydrophilic balance affects the rate and extent of drug absorption, as biological membranes are lipoidal in nature and passively absorbed drugs' rate of transfer is directly related to their lipophilicity. Drug classes are determined based on factors like pKa and LogP values, with class affecting absorption. Parameters like pH, pKa, and lipid solubility of un-ionized drugs influence the gastrointestinal absorption of weakly acidic or basic drugs.

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Prepared by- Hasan Hussain
 In the physical sciences, a partition coefficient (P) is the ratio of concentrations of a compound in a mixture of two immiscible solvents at equilibrium. The partition coefficient generally refers to the concentration ratio of un-ionized species of compound, whereas the distribution coefficient refers to the concentration ratio of all species of the compound (ionized plus un- ionized).
 For drug delivery, the lipophilic/hydrophilic balance has been shown to be a contributing factor for the rate and extent of drug absorption.  Since biological membrane is lipoidal in nature the rate of drug transfer for passively absorbed drugs is directly related the lipophilicity of the molecule.
 Class I ◦ Example: metoprolol, paracetamol  Class II ◦ Example: glibenclamide, bicalutamide, ezetimibe, aceclofenac  Class III ◦ Example: cimetidine  Class IV ◦ Example: Bifonazole, amphotericin B, furosemide, acetazolamide, ritonavir, paclitaxe.
 pKa < 3 is for a strong acid  3<pKa<7 is for a weak acid  7<pKa<11 is for a weak base  pKa>11 is for a strong base
 Paracetamol- It has a pKa value of 9.5 and is therefore largely unionized over the physiological range of pH.  LogP - 0.46
 Aceclofenac LogP - 2.17  pKa. : 4.7
 Cimetidine is weakly basic with the pKa values reported as 6.93.  LogP- 0.4
 Acetazolamide pKa- 7.93  LogP -0.26
 Since majority of the drugs are either weakly acidic or basic, their solution in water contain ionised and unionised species.  The drug undergoes dissociation in their aqueous solutions. The extent of ionisation depends upon the pH of the solution. For example a weak acid will ionise mostly in alkaline pH while a weakly basic will ionise in acidic medium. The unionised drug is more lipid soluble and thus readily absorbed.
 Hence gastrointestinal absorption of a drug can be improved if the extent of ionisation is reduced. The absorption of weakly acidic or basic can be influenced by the following: 1. pH at the site of absorption. 2. Dissociation constant 3. Lipid solubility of unionised drugs
 The contents of the stomach have pH ranging from 1-3.  Weakly acidic drugs having pKa value less than 4.3 were absorbed faster. Strongly acidic drugs were almost not absorbed.  pH of intestinal fluid ranges from 5-8.  In case of basic drugs absorption is faster for those having pKa values less than 8.5 than those having pKa values between 9-12
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