Drug Absorption Continued

Question 1

Henderson-Hasselbach Equations
1. For a weak acid
2. For a weak base

Answer 1

Weak acid:
pH = pKa + log [A-] / [HA]

Weak base:
pH = pKa + log [B] / [BH+]

Question 2

Regarding Henderson-Hasselbach Equations
The Extent of ionization of a weak electrolyte depends on

Answer 2

pka of the drug
pH of the medium

Question 3

Physiologic pH of stomach

Answer 3

1 - 2.5

Question 4

Physiologic pH of small intestine sections
Duodenum
Terminal Ileum
Jejunum

Answer 4

6 - 7
7.4
6.8

Question 5

Physiologic pH of Large Intestine sections
Caecum
Colon

Answer 5

5.7
5 - 8

Question 6

Physiologic pH of rectum

Answer 6

6.7

Question 7

What is the pH-Partition Hypothesis?

Answer 7

The drug ionize at different degrees on either side of membrane.

So Total drug concentration on either side is unequal.

The side where drug is greater ionized will have greater concentration

Question 8

What is Drug Affinity?
How does this affect the drug movement across a membrane?

Answer 8

Drug affinity refers to how strongly a drug binds to its target.
A drug with a high affinity for a tissue will stay and accumulate
longer
Some drugs get trapped in tissues due to binding with proteins, ionization, or fat solubility.
If a drug is highly bound to tissue proteins, it may have difficulty crossing membranes and staying in circulation.

Question 9

What are the Factors Affecting Bioavailability ?

Answer 9

First pass hepatic metabolism
Solubility of the drug
Nature of the drug formulation

Question 10

What are the factors affecting Absorption of Drugs ?

Answer 10

Transport of drug from the GIT:

Effect of pH on drug absorption

Physical factors influencing absorption: Blood flow to site, Surface area of site, Contact time at the absorption surface

Question 11

What are the physical factors affecting Absorption of Drugs ?

Answer 11

Blood flow to site
Surface area of site
Contact time at the absorption surface