Antihistamines

Question 1

H1-blockers PK

Answer 1

limited oral bioavailability
lipid soluble
high volume of distribution
hepatic metabolism = active metabolite

Question 2

pyrilamine

Answer 2

gen: 1st
mechanism: H1 blocker
effect: anticholinergic - sedation at a low dose, CNS excitement at high dose, can also inhibit muscarinic
admin: injectable and oral

Question 3

diphenhydramine (benadryl)

Answer 3

gen: 1st
mechanism: H1 blocker
effect: anticholinergic - sedation at low doses CN excitement at high doses and inhibit muscarinic receptors
admin: extra label, human formulations only

Question 4

Dimenhydrinate (gravol, dramamine)

Answer 4

Gen: 1st

mechanism: H1 blocker, diphenhydramine + 8-chlorotheophylline
effect: anticholinergic, sedation low dose, high dose CNS excitement
admin: extra label human drug

Question 5

Trimeprazine

Answer 5

Gen: 1st piperazine
mechanism: H1 blocker
effect: anti-pruritic
found in vanectyl-P (steroid +antihistamine)

Question 6

2nd generation H1 blockers

Answer 6

more selective for peripheral H1 receptors (doesn’t cross into CNS)
generally longer half life and dosing intervals
Zwiterrions, more Pgp substrates = less CNS effect = less antihistamine and anticholinergic CNS effect

Question 7

cetirizine

Answer 7

2nd gen H1 blocker

Question 8

loratadine (Claritin)

Answer 8

2nd gen H1 blocker

Question 9

3rd generation H1 blocker

Answer 9

block H1 receptor and serotonin receptors
contains only active enantiomer or metabolite
generally less adverse effect and greater efficacy

Question 10

cyproheptadine (periactin)

Answer 10

3rd gen H1 blocker

used in photic headshaking in horses + appetite stim in cats (not great)