PTN - Antihistamines (Rosenzweig)

Question 1

How is histamine formed?

Answer 1

Histidine is decarboxylated via the enzyme histidine decarboxylase to form histamine

Question 2

What is the effect of histamine on nerve endings?

Answer 2

Produces itching and pain

Question 3

What enzyme is targeted by cimetidine? What drug can be given in place of cimetidine to block H2 receptors without blocking this enzyme?

Answer 3

Cimetidine inhibits cytochrome P450 enzymes, which can lead to drug toxicity if taken with other drugs that are metabolized by this enzyme

Famotidine can be given because it does the same thing without as big of an effect on CYP450.

Question 4

Which antihistamine can be used as a local anesthetic?

Answer 4

Diphenhydramine

Question 5

What functional group is consistently found in antihistamines?

Answer 5

Two benzene rings

Question 6

What is histamine bound to when it is stored in mast cell/basophil granules?

Answer 6

Heparin

Question 7

What is the primary type of histamine receptor in bronchial smooth muscle?

Answer 7

H1 receptors

Question 8

What is an agonist of H1 receptors?

Answer 8

2-methylhistamine

Question 9

Which H1 receptor blocker is used in sunburn preparations?

Answer 9

Diphenhydramine

CAUTION: can lead to an allergic reaction in sunburned/denuded skin

Question 10

What are two ways to reduce sedation effects of antihistamines?

Answer 10

1. Take at bedtime
2. Give half-strength with a decongestant

Question 11

The enzyme N-methyltransferase is used to convert histamine into what?

Answer 11

N-methylhistamine (a byproduct that can be excreted by body)

Question 12

How does cromolyn sodium work?

Answer 12

Cromolyn sodium stabilizes the mast cell membrane. This prevents influx of calcium needed for vesicles containting granules to travel and dock on the cell membrane, thus inhibiting release of histamine.

Question 13

Is Cromolyn sodium a fast-acing drug or a slow-acting drug?

Answer 13

Slow-acting

Must be taking medication before exposure to prevent allergic reaction

Question 14

What are the two enzymes which function directly in the degradation of histamine?

Answer 14

1. N-methyltransferase - via pathway 1
   
   1. Converts histamine into N-methylhistamine, which can be excreted
2. Histaminase - via pathway 2
   
   1. Converts histamine into imadizone acetic acid (either free or attached to riboside)

Question 15

What is the primary mechanism of action of Omalizumab and why is it not prescribed as often as other antihistamines?

Answer 15

Omalizumab binds Fc region of IgE and prevents it from binding to mast cells.

Not as often prescribed because it is very expensive

Question 16

What is the difference between a classic and a second generation H1 receptor antagonist?

Answer 16

Classic H1 antagonists are able to cross the blood brain barrier, making sedation and anti-cholinergic activity possible side effects.

Second generation H1 antagonists have less CNS side effects, and are considered “non-drowsy” antihistamines.

Question 17

Antihistamine poisoning in children can lead to a paradoxical effect that is characterized by what symptoms?

Answer 17

1. Fever
2. Flushing
3. CNS excitement

In adults, CNS depression is observed. This is why it is paradoxical in children

Eventually this leads to coma and respiratory collapse, followed by death.

Question 18

What is the primary difference between H1 and H2 receptor activation via histamine?

Answer 18

H1 receptors are more concentrated in the respiratory tract and cause vasodilation and bronchoconstriction

H2 receptors are more concentrated in the gut and cause HCl secretion in stomach, arteriolar and vasodilation

Question 19

What is the main reason that Fexofenadine is prescribed over diphenhydramine and chlorpheniramine?

Answer 19

Fexofenadine is a second generation H1 blocker, so it has less sedative side effects

Question 20

Chloramphenirmine has a similar mechanism of action, therapeutic use, and side effects as what other antihistamine?

Answer 20

Diphenhydramine