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3. Clin Pharm > Anthelmintics > Flashcards

Anthelmintics Flashcards

1
Q

Broad-spectrum anthelmintic
drug classes

A
  1. macrocyclic lactones
  2. benzimidazoles / pro-benzimidazoles
  3. pyrimidines
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2
Q

origin and subgroups of macrocyclic lactones

A
  • origin = natural fermentation products of streptomycetes
  • two subgroups:
    a) avermectins
    b) milbemycins
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3
Q

Macrocyclic lactones:
types of avermectins:

A
  • ivermectin
  • doramectin
  • eprinomectin
  • selamectin
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4
Q

Macrocyclic lactones:
types of milbemycins:

A
  • milbemycin
  • moxidectin
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5
Q

macrocyclic lactones spectrum of activity? what is it not effective against?

A
  • broad spectrum activity (“endectocides”):
    > nematodes (e.g. hookworms, Ostertagia)
    > arthropods
  • insects (e.g.lice,bots)
  • mites (e.g. Chorioptes, Otodectes)

No activity against cestodes, trematodes & protozoa

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6
Q

macrocyclic lactones effective at what dose? safety profile and withdrawl? cost?

A
  • highly effective at low dosages (mcg/kg body weight)
  • some have zero meat / milk withdrawal time
  • generally wide safety margin
  • BUT - expensive
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7
Q

mechanism of action of macrocyclic lactones

A
  1. bind with high affinity to glutamate-gated Cl- channels (invertebrate nerve and muscle cells)
    > increase membrane permeability to Cl- ions
    > hyperpolarization of nerve / muscle cell
  2. bind to GABA-gated Cl- channels:
    - nematodes + arthropods > synapses between nerves and nerve/muscle cells
    - stimulates GABA release
    - enhances postsynaptic binding of GABA
    (GABA = inhibitory neurotransmitter)
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8
Q

Low toxicity of macrocyclic lactones is due to:

A
  1. Mammals do not have glutamate-gated Cl- channels.
  2. GABA receptors in central nervous system of mammals
  3. Macrocyclic lactones do not readily cross the blood-brain barrier:
    > P-glycoprotein expressed by brain endothelial cells > drug efflux from CNS
  4. Macrocyclic lactones have a low affinity for other ligand-gated Cl- channels.
    (affinity for invertebrate receptors = 100x mammalian receptors)
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9
Q

ivermectin - history and origins

A
  • first macrocyclic lactone developed for commercial use
  • natural fermentation product of Streptomycetes avermitilis
  • chemically modified (“semisynthetic”)
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10
Q

Ivermectin formulations

A
  1. Oral
    - liquid / drench
    -chewable
    - feed premix
    - paste
  2. Injection - liquid
  3. topical - pour-on solution
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11
Q

Ivermectin toxicity; who is susceptible ?

A
  • multi-drug resistance gene (MDR1) - encodes P-glycoprotein efflux pump in blood-brain barrier
  • some dog breeds - mutation of MDR1: decreased ability to pump ivermectin out of CNS
  • breeds affected:
  • 3 out of every 4 Collies in USA
    also
  • Shetland Sheepdogs
  • Australian Shepherds
  • Old English Sheepdogs
  • German Shepherds
  • Long-haired Whippets
  • Silken Windhounds
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12
Q

ivermectin toxicity: symptoms

A

Dogs:
Acute toxic syndrome: mydriasis, depression, tremors, ataxia, stupor, emesis, salivation, coma

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13
Q

what dosage of ivermectin is toxic? vs the licensed dose

A

What dosage is toxic ?
* licensed dosage = 0.006 mg/kg (heartworm prevention)
* toxic dosage in Collies = 0.1 mg/kg
* Demodex dosage = 0.4-0.6 mg/kg/day

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14
Q

how do you know if a collie is susceptible to ivermectin toxicity?

A

cheek brush sample > genotype

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15
Q

macrocyclic lactones used in dogs and cats? which are licensed as safe in collie breeds?

A
  • ivermectin, milbemycin, moxidectin, selamectin
  • eprinomectin (cats only)
  • moxidectin & selamectin = licensed as safe in Collie breeds
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16
Q

macrocyclic lactones used in cattle

A
  • ivermectin, doramectin, eprinomectin
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17
Q

macrocyclic lactones used in pigs

A
  • ivermectin, doramectin
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18
Q

macrocyclic lactones used in horses

A
  • moxidectin > not licensed for foals <4 months due to low therapeutic index
  • ivermectin
19
Q

drug resistance to macrocyclic lactones is a problem for which parasites in dogs (and where)?

A
  • Dirofilaria immitis - USA
  • Ancylostoma caninum - USA/Canada?
20
Q

drug resistance to macrocyclic lactones is a problem for which parasites in sheep (and where)?

A
  • Haemonchus sp.
  • teladorsagia sp.

> USA, Canada, and globally

21
Q

drug resistance to macrocyclic lactones is a problem for which parasites in horses (and where)?

A
  • Parascaris equorum > USA, Canada & globally
  • cyathostomins > Brazil, Europe, (& USA)
22
Q

drug resistance to macrocyclic lactones is a problem for which parasites in cattle (and where)?

A
  • Cooperia sp. > USA, Canada, and globally
  • Ostertagia sp. > USA, Canada, and globally
23
Q

types of benzimidazoles

A
  • fenbendazole
  • albendazole
  • febantel = pro-benzimidazole
24
Q

activity of benzimidazoles

A

Active against large number of nematodes, flukes, some tapeworms and some protozoa (e.g. Giardia).

25
Q

mechainism of benzimidazoles

A
  • bind to beta-tubulin
    > inhibit polymerisation to microtubules:
    > inhibit cell division, glucose uptake, parasite motility
26
Q

benzimidazoles administration strategy

A
  • administered orally > poor absorption
    > absorption increases if ingested with a fatty meal
  • optimal efficacy normally only observed after repeated doses
27
Q

benzimidazoles toxicity profile

A
  • relatively non-toxic <-> poor systemic availability
  • rapidly dividing cells potentially at risk:
  • haematopoetic stem cells (eg. albendazole in dogs)
  • some have teratogenic potential in early stages of pregnancy (eg. albendazole in cattle and sheep)
28
Q

what animals in fenbendazole used in? what is its off-label activity?

A
  • dogs, horses, cattle, pigs, poultry (sheep)
  • common off-label activity = giardia
29
Q

drug of choice for foals with large burden of Parascaris equorum

A

fenbendazole

30
Q

albendazole used in which species? spectrum of activity vs others? issues with its use?

A
  • cattle, (sheep)
  • benzimidazole with broadest spectrum of activity
  • potentially teratogenic – don’t use in first trimester
  • uncommonly associated with bone-marrow toxicosis in dogs
31
Q

febantel is used in what animals?

A

dogs only (Note: toxic for cats)

32
Q

what happens to febantel in the liver?

A

biotransformed in liver > fenbendazole + oxfendazole

33
Q

febantel is given with what? why?

A
  • co-administration with pyrantel (Drontal Plus)
  • synergizes activity of pyrantel > Trichuris vulpis
34
Q

benzimidazoles resistance issues in sheep? where?

A

Sheep – Haemonchus +Teladorsagia - globally

35
Q

benzimidazoles resistance issues in horses? where?

A

Horses – cyathostomins - globally

36
Q

benzimidazoles resistance issues in cattle? where?

A

Cattle – Cooperia + Ostertagia – globally

37
Q

benzimidazoles resistance issues in dogs? where?

A

Dogs – Ancylostoma caninum – USA/Canada?

38
Q

what pyrimidines are used, and in what animals?

A
  • pyrantel pamoate (dogs, cats, horses)
  • pyrantel tartrate (pigs)
39
Q

what are pyrimidines effective against?

A

Active against large number of nematodes (e.g. Toxocara, cyathostomins,Ascaris) & horse tapeworm

40
Q

mechanism of action of pyrimidines

A
  • “nicotinic anthelmintic”
  • act selectively on parasite nicotinic acetylcholine receptors:
  • acetylcholine agonist + inhibition of acetylcholinesterase > depolarisation > paralysis > expulsion
41
Q

pyrantel administration and consequences of its properties

A
  • only administered orally
  • cat dosage = 4 x dog dosage
  • pyrantel pamoate:
  • low aqueous solubility > low systematic availability > increased margin of safety > increased efficacy against gut parasites
42
Q

contraindications for pyrantel

A

efficacy decreased if administered to animals with diarrhea

43
Q

resistances to pyrantel for horse and dog parasites, and geographic range

A
  • Horses – cyathostomins – globally, especially USA
    (availability of daily in-feed product = Strongid C)
  • Dogs – Ancylostoma caninum – Australia, USA/Canada?
44
Q

parasites with drug resistances of greatest concern for macrocyclic lactones, benzimidazole, and pyrimidine

A

Dirofilaria immitis - macrocyclic lactones

Ancylostoma caninum - macrocyclic lactones, benzimidazole, pyrimidine

Cyathostomins - (macrocyclic lactones), benzimidazole, pyrimidine

3. Clin Pharm (11 decks)

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