This document contains information about the
● Drug dissociation:
- pH-partition hypothesis
- Henderson-Hasselbalch Equation
- Limitations of the pH-partition hypothesis
● Lipid solubility
- Partition coefficient
● Molecular size & hydrogen bonding.
Excipients and Vehicles in Extemporaneous Preparations
All for this textbook (3)
Written for
University of the Western Cape (UWC)
Biopharmaceutics (PHA426)
All documents for this subject (3)
Seller
Follow
Boicha
Content preview
Drug absorption
Learning outcomes
Understand the concepts of :
● Drug dissociation:
- pH-partition hypothesis
- Henderson-Hasselbalch Equation
- Limitations of the pH-partition hypothesis
● Lipid solubility
- Partition coefficient
● Molecular size & hydrogen bonding.
Drug Dissociation and lipid solubility
1. Influencing Factors:
○ Dissociation Constant: Affects the degree of ionization of a drug.
○ Lipid Solubility: Impacts how well a drug can pass through cell
membranes.
○ pH at Absorption Site: Influences the ionization state of the drug and
thus its absorption.
2. pH-Partition Hypothesis (Overton, 1899):
○ Concept: Drug absorption is influenced by its degree of ionization, which
is affected by its dissociation constant and the pH at the absorption site.
○ Mechanism: Drugs pass through cell membranes more effectively if they
are in their non-ionized (lipid-soluble) form.
3. Transcellular Passive Route:
○ Preference: Most drugs are absorbed via this route, where they diffuse
through cell membranes.
4. Framework Usefulness:
○ Simplification: Though oversimplified, the pH-partition hypothesis helps
in understanding the basic principles of drug absorption
, pH-Partition hypothesis
According to the pH-partition hypothesis:
1. Lipid Barrier:
○ The gastrointestinal epithelium acts as a lipid barrier.
○ Drugs absorbed by passive diffusion must be lipid-soluble.
2. Weak Electrolytes:
○ Most drugs are weak electrolytes.
○ Unionized forms (lipid-soluble) pass through the epithelium more easily.
3. pH-Partition Hypothesis:
○ Absorption of weak electrolytes depends on the extent of their unionized
form at the absorption site.
4. Ionization Calculation:
○ Use the Henderson-Hasselbalch equation to determine ionization.
○ For weakly acidic drugs with one ionizable group:
−
[𝐴 ]
𝑙𝑜𝑔 ( [𝐻𝐴]
= 𝑝𝐻 − 𝑝𝐾𝑎
Where:
−
● [𝐴 ]= concentration of ionized form
● [HA] = concentration of unionized form
● pH = pH of the solution
● pKa = acid dissociation constant of the drug
5. Example Drugs: Weakly acidic drugs include aspirin, phenobarbital, and ascorbic
acid (vitamin C).
The benefits of buying summaries with Stuvia:
Guaranteed quality through customer reviews
Stuvia customers have reviewed more than 700,000 summaries. This how you know that you are buying the best documents.
Quick and easy check-out
You can quickly pay through credit card for the summaries. There is no membership needed.
Focus on what matters
Your fellow students write the study notes themselves, which is why the documents are always reliable and up-to-date. This ensures you quickly get to the core!
Frequently asked questions
What do I get when I buy this document?
You get a PDF, available immediately after your purchase. The purchased document is accessible anytime, anywhere and indefinitely through your profile.
Satisfaction guarantee: how does it work?
Our satisfaction guarantee ensures that you always find a study document that suits you well. You fill out a form, and our customer service team takes care of the rest.
Who am I buying these notes from?
Stuvia is a marketplace, so you are not buying this document from us, but from seller Boicha. Stuvia facilitates payment to the seller.
Will I be stuck with a subscription?
No, you only buy these notes for £4.78. You're not tied to anything after your purchase.
