Antihistamines

Question 1

Ethylenediamines

Answer 1

• ex/ Pyrilamine - injectable antihistamine for dogs, cats horses and ruminants
• also an oral formulation for horses in feed
• both given with ephedrine

Question 2

Ethanolamines

Answer 2

• ex/ Diphenhydramine (Benadryl), dimenhydrinate (Gravol) and clemastine (Tavist)
• have significant adverse anticholinergic effects and cause sedation
• adverse GI affects is relatively low
• low oral bioavailability (3%)
• diphenhydramine can be given to reestablish the ACh -dopamine balance after phenothiazine tranquillizers wrecked it

Question 3

Alkylamine

Answer 3

• ex/ Chlorpheniramine (Chlor-Trimeton) used in dogs and cats (has antidepressant activities because inhibits serotonin reuptake)
• extreme enantiomer specific activity - difference in activity relates to the positioning and fit of the molecule in the histamine H1 receptor binding site
• fewer sedative and GI adverse effects
• greater chance of paradoxical CNS stimulation ( doesn’t sedate)

Question 4

Piperazines

Answer 4

• ex/ Trimeprazine (Vanectyl P) in dogs; Hydroxyzine (Atarax) in dogs, cats and horses (not as good as Vanectyl P)
• significant anticholinergic adverse effects
• used for motion sickness, vertigo, nausea and vomiting

Question 5

Tricyclic and Tetracyclic

Answer 5

• ex/ Cyproheptadine (Periactin)
• structurally related to tricyclic antidepressants, which is why they have antihistamine adverse effects
• antihistaminic and antiserotonergic agent - acts as serotonin receptor antagonist and also blocks Ca channels
• used in asthmatic cats to block bronchoconstriction but because of long half live it takes several days to reach steady state and become effective. may also stimulate appetite, cause lethargy, and depression
• can be given to horses with photic head shaking because it blocks the seotonin so there is no longer pain sensation

Question 6

Second generation antihistamines (not used in vet med)

Answer 6

• ex/ astemizole, ketotifen, cetirizine, loratadine and terfenadine
• more selective for peripheral H1 receptors so there is less adverse drug reactions while still providing effective relief of allergic conditions
• not used much in vet med because can be toxic and since they decrease the amount of sedation we don’t want that, we want dogs to be sleepy

Question 7

Third generation antihistamines (not used in vet med)

Answer 7

• ex/ levocetirizine, desloratidine, fexofenadine
• active enantiomer or metabolite derivatives of second generation drugs
• increased efficacy with fewer adverse drug reactions
• keeps you awake!