# Drug Biotechnology Questions and Answers – Drugs Absorption – pH Partition Hypothesis

This set of Drug Biotechnology Multiple Choice Questions & Answers (MCQs) focuses on “Drugs Absorption – pH Partition Hypothesis”.

1. According to the pH-partition hypothesis which one of the given options doesn’t govern the absorption?
a) The molecular size of the drug
b) PH at the absorption site
c) Lipid solubility of unionized drugs
d) Dissociation constant

Explanation: The pH-partition hypothesis tells that for drug compounds with molecular weight more than 100, that are transported by passive diffusion across the cell membrane, the process is governed by the capability of lipid solubility of unionized drugs, the dissociation constant of the drug and pH at the absorption site.

2. If the pH of either side of the membrane is different, then the compartment whose pH favours greater ionization will have less amount of drug.
a) True
b) False

Explanation: If the pH of either side of the membrane is different, then the compartment whose pH favours greater ionization will have more amount of drug. And only the ionized or the undissociated fraction of drug.

3. Which of the following equations correct for Henderson-Hasselbach equation for weak acids?
a) PH = pKa – log (Unionized drug concentration /Ionized drug concentration)
b) PH = pKa – log (Ionized drug concentration/ Unionized drug concentration)
c) PH = pKa + log (Ionized drug concentration/ Unionized drug concentration)
d) PH = pKa + log (Unionized drug concentration /Ionized drug concentration)

Explanation: For weak acids, the Henderson-Hasselbach equation is pH = pKa + log (Ionized drug concentration/ Unionized drug concentration). And the % of ionized drug = 10pH-pHKa/1+10PH-pKa * 100.

4. Which of the following equations correct for Henderson-Hasselbach equation for weak bases?
a) PH = pKa – log (Unionized drug concentration /Ionized drug concentration)
b) PH = pKa – log (Ionized drug concentration/ Unionized drug concentration)
c) PH = pKa + log (Ionized drug concentration/ Unionized drug concentration)
d) PH = pKa + log (Unionized drug concentration /Ionized drug concentration)

Explanation: For weak bases, the Henderson-Hasselbach equation is pH = pKa + log (Unionized drug concentration /Ionized drug concentration). And the % of ionized drug = 10pKa-pH/1+10 pKa-pH * 100.

5. What is the pH range of the stomach?
a) 1-4
b) 5-8
c) 1-3
d) 1-8

Explanation: PH range of gut is from 1-8. PH range of stomach is from 1-3 making it the most acidic place in human body. PH range of the intestine (duodenum to the colon) is from 5-8 where most of the drugs get absorbed.
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6. What is the pH range of the intestine?
a) 4-5
b) 1-3
c) 1-8
d) 5-8

Explanation: The pH range of the intestine (duodenum to the colon) is anywhere between 5-8, this allows the drugs to get easily absorbed and digested. The small intestine specifically has a pH ranging from 6 to 7.5.

7. Acids in the pKa range 2.5-7.5 are greatly affected by changes in pH making their absorption pH dependent.
a) True
b) False

Explanation: Acids in the pKa range 2.5-7.5 are greatly affected by changes in pH making their absorption pH dependent. Example, NSAIDs like aspirin, ibuprofen and some penicillin analogues. Such drugs can be absorbed from acidic conditions of the stomach.

8. Which one of these is an example of a strong acid drug?
a) Diazepam
b) Ibuprofen
c) Cromolyn
d) Aspirin

Explanation: Diazepam is a very weak basic drug. Ibuprofen and aspirin have pH range 2.5-7.5. Cromolyn pKa is less than 2.5. Thus, making Cromolyn one of the most acidic drug.

9. Which one of these is not an example of a basic drug with pKa range 5-11?
a) Cromolyn
b) Imipramine
c) Amitriptyline
d) Chloroquine

Explanation: Cromolyn pKa is 2.5. Rest all the other drugs are bases of Pka range 5-11. These are morphine analogues. Such drugs are better absorbed from relatively alkaline conditions of the intestine where these molecules largely exist in unionized form.

10. Example of a drug which has PKa > 11 is ________
a) Cromolyn
b) Imipramine
c) Amitriptyline
d) Mecamylamine

Explanation: Cromolyn pKa is 2.5. Imipramine and amitriptyline have pKa range from 5 to 11. Mecamylamine has pKa range greater than 11. These drugs are ionized in the entire pH range of GIT and thus they are poorly absorbed.

11. What should be the range of oil/water partition coefficient of any drug?
a) 1-2
b) 3-4
c) 2-3
d) 1-3

Explanation: The octanol/pH 7.4 buffer partition coefficient value for a drug should be in the range of 1-2. This is sufficient for passive absorption across lipodial membranes.

12. Ibuprofen a weak acid, in the stomach will be present in which of the given form?
a) Ionized form mostly
b) Non-ionized form mostly
c) Half ionized and half no ionized
d) Will form aggregate

Explanation: The pH of the stomach is 1.5. Ibuprofen being a weak acid will be in a unionized form in the stomach. At pH 7.4 in plasma, it will be in ionized form and in the same format pH 5 in the intestine.

13. Nitrazepam a weak base, will be in which form in the stomach?
a) Ionized form mostly
b) Non-ionized form mostly
c) Half ionized and half no ionized
d) Will form aggregate

Explanation: The pH of the stomach is 1.5. Nitrazepam being a weak base will be in an ionized form in the stomach. In plasma and small intestine with pH 7.4 and 5 respectively, Nitrazepam will be in unionized form.

14. All drugs which are weak acids or acidic in nature will be in a unionized form in the plasma.
a) True
b) False

Explanation: Acidic or weak acidic drugs are always present in their ionized form in a basic or neutral environment. In plasma and small intestine with pH 7.4 and 5 respectively, rendering all acidic drug be present in their ionized form.

15. Heroin with pKa 7.8 will be in which form in intestinal pH?
a) Ionized form mostly
b) Unionized form mostly
c) Half ionized and half no ionized
d) Will form aggregate

Explanation: Drugs that are moderately weak bases with pKa 5-11 will get ionized at gastric pH, stay unionized at intestinal pH and get absorbed through the intestinal wall. Thus, heroin being a weak base will stay at unionized condition in the intestinal pH. It will not form any aggregate.

Sanfoundry Global Education & Learning Series – Drug and Pharmaceutical Biotechnology.

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