Flashcards in 3 Medical Mathematics: Ionization of Drugs Deck (16):
Weak electrolytes (weak acids and weak bases) are present in solution in what forms?
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
Weak acids are more ionized in what kind of medium?
an alkaline medium
weak bases are more ionized in what kind of medium?
an acidic medium
Drugs cross cell membranes mainly by?
Weak acidic or weak basic drugs cross biological membranes in what form?
the uncharged (non-ionized) form while charged drugs can not cross biological membranes
Quaternary nitrogen compounds such as tubocurarine are mostly?
Diffusion depends on what?
the ratio between the uncharged to the charged drug
The ratio between non-ionized to ionized can be calculated from what?
the Henderson- Hasselbalch equation
If the pka of a weak acid is higher then N/I value is what?
If the pka of a weak base is lower then the N/I value is?
pka for an acid = pH of the medium + LogN/ I
pka for a base = pH of the medium + LogI/ N
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?
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
4.4 = 7.4 + Log N/I
Antilog N/I = -3
I = 1000
Drug is ion trapped in plasma
Total drug in plasma = 1 + 1000 = 1001
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%
d. About 90%
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.
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.