3 Medical Mathematics: Ionization of Drugs

Question 1

Weak acids are more ionized in what kind of medium?

Answer 1

an alkaline medium

Question 2

weak bases are more ionized in what kind of medium?

Answer 2

an acidic medium

Question 3

Drugs cross cell membranes mainly by?

Answer 3

simple diffusion

Question 4

Weak acidic or weak basic drugs cross biological membranes in what form?

Answer 4

the uncharged (non-ionized) form while charged drugs can not cross biological membranes

Question 5

Quaternary nitrogen compounds such as tubocurarine are mostly?

Answer 5

Ionized

Question 6

Diffusion depends on what?

Answer 6

the ratio between the uncharged to the charged drug

Question 7

The ratio between non-ionized to ionized can be calculated from what?

Answer 7

the Henderson- Hasselbalch equation

Question 8

If the pka of a weak acid is higher then N/I value is what?

Answer 8

greater

Question 9

If the pka of a weak base is lower then the N/I value is?

Answer 9

Greater

Question 10

Henderson-Hasselbalch equation

Answer 10

pka for an acid = pH of the medium + LogN/ I

pka for a base = pH of the medium + LogI/ N

Question 11

Example 1:
Phenylbutazone is a weak acid with a pka 4.4 is introduced in the stomach (pH 1.4). The pH of plasma is 7.4. At equilibrium, how much of phenylbutazone is in the stomach and how much is in plasma?

Answer 11

Solution
pka of an acid = pH of the medium + LogN/I
4.4 = 1.4 + Log N/I
Antilog N/I = 3
N/I = 1000
N/I = 1000/1
This ratio favors absorption
When N = 1 then I = 0.001
The total drug in stomach = 1 + 0.001 = 1.001
pka =pH+LogN/I
4.4 = 7.4 + Log N/I
Antilog N/I = -3
N/I=1/1000
N=1
I = 1000
Drug is ion trapped in plasma
Total drug in plasma = 1 + 1000 = 1001

Question 12

Aspirin is a weak acid with a pka of 3.5. What percentage of a given dose will be in the lipid-soluble form at a stomach pH of 2.5?

a. About 1%
b. About 10%
c. About 50%
d. About 90%
e. About 99%

Answer 12

d. About 90%

Question 13

If a dog has just ingested a large overdose of promethazine, an antihistaminic drug that is a weak base with a pka of 9.1, then absorption of the drug would be faster from the stomach than from the small intestine.

a. True
b. False

Answer 13

b. False

Question 14

If a dog has just ingested a large overdose of promethazine, an antihistaminic drug that is a weak base with a pka of 9.1, then more of the drug would be ionized at blood pH than at stomach pH.

c. True
d. False

Answer 14

d. False

Question 15

All of the following statements are true EXCEPT

a. Aspirin (pka = 3.5) is 90% in its lipid-soluble, protonated form at pH = 2.5
b. The basic drug promethazine (pka = 9.1) is more ionized at pH = 7.4 than pH = 2
c. Absorption of a weakly basic drug is likely to occur faster from the intestine than from the stomach
d. Acidification of the urine accelerates the secretion of a weak base, pka = 8
e. Uncharged molecules more readily cross cell membranes than charged molecules

Answer 15

b. The basic drug promethazine (pka = 9.1) is more ionized at pH = 7.4 than pH = 2

Question 16

Weak electrolytes (weak acids and weak bases) are present in solution in what forms?

Answer 16

in ionized and non-ionized forms depending on if the drug is a weak acid or a weak base, the pka of the drug, and the pH of the medium