Lecture 2: Ionization of Drugs Flashcards
Ionization state
-impacts solubility, absorption, binding to receptor
Charged molecules
more soluble, less likely to cross lipid membranes
pH
affects ionization
HA –> A- + H+
BH+ –> B + H+
Ionizable functional groups
-weak acids and bases
Ionizable groups with pKa < 7
-arylcarboxylic acid
-arylamine
-aromatic amines
-alkylcarboxylic acid
ionizable groups with pKa > 7
-alkyl amines
-phenol
-guanidine
nonionizable groups
-alkyl alcohol
-alkyl aldehyde
-ether
-ester
-ketone
-amide
-quarternary amine
Drug examples
SLIDE 6
Henderson-Hasselbach equation
pH= pKa + log [A-]/[HA]
when pH < pKa
[A-] < [HA]
-protonated
when pH > pKa
[A-] > [HA]
-deprotonated
when [A-]/[HA]
[HA] = [A-]
Acetaminophen
STRUCTURE
Water
-polar solvent that dissolves polar molecules
-H bond donors and acceptors
-ionic groups
Solubility
-like dissolves like
-can be improved by changing pH
1g Procaine
needs 200mL water
1g procaine-HCl
needs 1 mL water
At pH 7.4, what is the ratio of ibuprofen in the acid form (neutral) to
that in the base form (negatively-charged)? pKa= 4.4
1:1000
Membrane permeability
-GI tract, blood brain barrier, lipid membranes
-most drugs absorbed by passive diffusion
Absorption rate
related to the drugs liphophilicity
lipopholic drugs
absorbed more efficiently
Transport across membrane
-intercellular junctions
-lipid cell membranes
-transporters
-endo and exocytosis
Passive diffusion of ionizable drugs (cant cross lipid bilayer)
-more efficient at pH where drug is neutral
-even while mostly ionized it still moves through bilayer slowly
-equilibrium
diffusion rate
proportional to amount of neutral drug