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Antihelmintic or anthelmintics Drugs:

Antihelmintic or Anthelmintics are drugs that either expel (vermifuge) or kill (vermicide) infesting helminths.

Terms:

Vermicidal: Antihelmintic drugs which kill the worms

Vermifuge: Antihelmintic drugs which affect the worm in such a way that help to expel the worms.

Classifications of Antihelmintic or Anthelmintics drugs:

Based on worm specific:

  •  For Roundworm, Hookworm, Pinworm:
    • Albendazole
    • Mebendazole
    • Pyrantel pamoate
    • Piperazine
    • Levamisole
  • For Threadworm:
    • Ivermectin
    • Albendazole
  • For whipworms, Trichinella spiralis:
    • Albendazole
    • Mebendazole
  • For Filariasis:
    • Diethylcarbamazine citrate
    • Ivermectin
    • Albendazole
  • For Tapeworms:
    • Praziquantel
    • Niclosamide
    • Albendazole
  • For Hydatid worm:
    • Albendazole
    • Mebendazole

Drug Therapy of worms:

Mebendazole:

  • It is a benzimidazole derivative
  • It is a broad spectrum of antihelmintic or anthelmintics activity
  • It is poorly(˂10%) absorbed
  • It has produced a nearly 100% cure rate/reduction in egg count in the roundworm, hookworm (both species), Enterobius, and Trichuris infestations, but is much less active on Strongyloides.
  • Up to 75% cure rate has been reported in tapeworms, but H. nana is relatively insensitive
  • Trichinella spiralis from intestines, but efficacy in killing larvae that have migrated to muscles is unsure.
  • Prolonged treatment has been  shown to cause regression of hydatid cyst in the liver

Mechanism of action:

The site of action: appears to be the microtubular protein ‘β-tubulin’ of the parasite.

  • It binds to the tubulin of susceptible worms with high affinity and inhibits its microtubule polymerization.
  • Intracellular microtubules in the cells of the worm are gradually lost.
  • In addition, it blocks glucose uptake in the parasite and depletes its glycogen stores.
  • The hatching of nematode eggs and their larvae is also inhibited.
  • Ascaris ova are killed.
  • Affected parasites are expelled in the feces

Slow acting drug: 2-3 days for parasitic clearance from the gut.

Adversely affects the ova  of the pinworm and the hookworm

Pharmacokinetics:

  • Mebendazole is administered orally
  • Poorly absorbed from the GI tract
  • Highly plasma proteins binding nature & metabolized in the liver
  • Most of the oral dose is excreted in feces (75–90%)
  • The fraction absorbed is excreted mainly as inactive metabolites in urine/feces.

Adverse effects:

  • The systemic toxicity of mebendazole is low because of its poor absorption.
  • It is well tolerated & rarely causes gastrointestinal adverse drugs effects:-
    • Diarrhea, nausea, anorexia, vomiting, and abdominal pain
  • Allergic reactions, loss of hair, and granulocytopenia (at high doses)
  • Embryotoxic and teratogenic in animals
  • Should be avoided in pregnancy and children below one year

Uses:

Highly effective against intestinal nematodes such as Roundworm, Hookworm, Whipworm, Pinworm, and mixed worm infestations.

  • Tapeworm (Taenia) infestation
  • Hydatid cysts of the liver: may cause bone marrow aplasia, hence Albendazole is preferred.
  • More effective than albendazole in trichuriasis

Dose and administration:

  • For roundworm, hookworm, whipworm:-100 mg orally twice a day for 3 consecutive days. It does not require fasting or purging, is well-tolerated, & is comparatively cheap.
  • Pinworm (Enterobius): 100 mg single dose, but repeatedly after 2–3 weeks to kill the ova that have developed later. Strictly hygienic measures & simultaneous treatment of all children in the family or class are advocated to cut down autoinfection & person-to-person infection or transmission. 

This holds true of enterobiasis, disregarding the drug used.

  • Trichinosis: 200 mg BD for 4 days; but it is a less effective drug than albendazole.
  • Hydatid disease: 200–400 mg BD or TDS for 3–4 weeks; but is a less effective drug than albendazole

 

Albendazole:

  • It is a benzimidazole derivative.
  • It is a broad spectrum of anthelmintics or antihelmintic activity and excellent tolerability of its predecessor.
  • It has the advantage of single-dose administration in many common intestinal worm infestations and is cost-effective.
  • It also has larvicidal actions in hydatid disease, roundworm, and hookworm
  • Ovicidal properties in roundworm, hookworm, and whipworm (Trichuris)

The mechanism of action similar to Mebendazole.

Pharmacokinetics:

  • It is given orally
  • Better bioavailability than mebendazole
  • It is enhanced when the drug is taken with a fatty meal (this may help in treating neurocysticercosis and hydatid disease).
  • The fraction absorbed is converted by the first-pass metabolism to its sulfoxide metabolites which have potent anthelmintic action.
  • Will distributed in the tissues, bile, CSF, and hydatid cyst.
  • T1/2: 8-12 hours

Side effects:

  • Mild, mainly GI disturbances
  • Few patients have felt dizziness
  • Prolonged use, as in hydatid or in cysticercosis, has caused headache, fever, alopecia, jaundice, liver damage, bone marrow depression, and neutropenia.

Uses:

  • Ascaris, hookworm, Enterobius, and Trichuris: a single dose of a drug is 400 mg for in case of adults and children above 2 yrs, 200 mg for 1–2 yr age. Three-day treatment may be needed in heavy trichuriasis conditions.
  • Weak microfilaricidal
  • Tapeworms and strongyloidiasis: dose is 400 mg daily for 3 consecutive days. The efficacy in strongyloidiasis is low, and it is the second choice drug to ivermectin.
  • Trichinosis:
  • Neurocysticercosis: choice for the treatment of neurocysticercosis(Usually 8–15 days course of 400 mg BD (15 mg/kg/day) is employed.
  • Cutaneous larva migrans: Albendazole is 400 mg daily for three days
  • Hydatid disease: 400 mg BD for four weeks, repeat after two weeks if required, up to 3 courses treatment.

It is the preferred treatment given before and after surgery as well as inoperable cases.

  • Filariasis: Added to diethylcarbamazine (DEC) or ivermectin, albendazole has adjuvant value in treating lymphatic filariasis
  • Cysticercosis:
    • Drug of choice because of a short course
    • Less toxicity than praziquantel
    • Better penetration into CSF and cost-effective
    • Given 400mg bid for 21 days
    • Glucocorticoids are started prior to albendazole to reduce the intensity of the inflammatory reaction to the dead parasites.

Pyrantel pamoate:

  • Efficacy against Ascaris, Enterobius, and Ancylostoma is high and equivalent to that of mebendazole.
  • Lower cure rates of about 60 percent have been obtained in the case of Necator infestation.
  • It is less active against Strongyloides conditions and inactive against Trichuris and other worms.

Mechanism of action:

  • Pyrantel causes activation of nicotinic cholinergic receptors in the intestinal as well as extra-intestinal worms resulting in persistent depolarization → slowly developing contracture and spastic paralysis.
  • An anticholinesterase action has also been demonstrated.
  • Only 10–15 percent of an oral dose of pyrantel pamoate is absorbed in the body: this is partly or partially metabolized and excreted in the urine.

Adverse effects:

  • Mild and include GI disturbances
  • Abdominal pain
  • Headache
  • Drowsiness
  • Skin rashes

Uses:

  • Roundworm, Pinworm, Hookworm infestation
  • It can be used during pregnancy and in children below 2 years

Piperazine:

Roundworm and pinworm

Mechanism of action:

  • Acts as a GABA agonist and causes hyperpolarization of Ascaris muscle resulting in flaccid paralysis of the worms which are then easily expelled by peristaltic movements. (N.B. Opening of Cl¯ channels causes relaxation and depresses responsiveness to contractile action of ACh= Hperpolariztion)
  • This eliminates the danger of worm migration

Adverse effects:

  • Wide margin of safety, common:
    • Nausea, vomiting, diarrhea, and urticarial
  • Rare :
    • Muscular incoordination, hypotonia, ataxia
    • Worm wabble: the dropping of objects, clumsiness, and gait abnormalities
  • The drug is safe during pregnancy

Levamisole:

  • It causes sustained contracture of the somatic muscle of the worm by an irreversible, non-competitive depolarization type of neuromuscular block
  • Interference with carbohydrate metabolism

Adverse effects:

  • Nausea, vomiting, abdominal pain, diarrhea, giddiness, and drowsiness
  • Not active against larvae and hence follow-up re-evaluation of treated patients.

 

 

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