Anthelmintic Drugs

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Chapter: Essential pharmacology : Antiamoebic And Other Antiprotozoal Drugs

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


ANTHELMINTIC DRUGS

 

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

 

Helminthiasis is prevalent globally (1/3rd of world’s population harbours them), but is more common in developing countries with poorer personal and environmental hygiene. Multiple infestations in the same individual are not infrequent. In the human body, g.i.t. is the abode of many helminths, but some also live in tissues, or their larvae migrate into tissues. They harm the host by depriving him of food, causing blood loss, injury to organs, intestinal or lymphatic obstruction and by secreting toxins. Helminthiasis is rarely fatal, but is a major cause of ill health.

 

Malefern and chenopodium had been used for worm infestations for centuries. Many drugs were discovered in the early part of the present century. However, over the past 4 decades many new, highly efficacious and well tolerated anthelmintics have been developed. These have largely replaced the older drugs. The choice of drug for each worm infestation is based not only on efficacy, but also on lack of side effects/toxicity, ease of administration (preferably single dose) and low cost.

 

Development of resistance has not been a problem in the clinical use of anthelmintics. The current choice of drugs for worm infestations common in the Indian subcontinent is given in Table 61.1.

 


 

Mebendazole

 

It is a benzimidazole introduced in 1972. This congener of thiabendazole became very popular because it retained the broad-spectrum anthelmintic activity but not the toxicity of its predecessor. It has produced nearly 100% cure rate/reduction in egg count in roundworm, hook worm (both species), Enterobius and Trichuris infestations, but is much less active on Strongyloides. Upto 75% cure has been reported in tapeworms, but H. nana is relatively insensitive. It expels Trichinella spiralis from intestines, but efficacy in killing larvae that have migrated to muscles is uncertain. Prolonged treatment has been shown to cause regression of hydatid cysts in the liver. Treatment after resection of the cyst may prevent its regrowth.

 

The immobilizing and lethal action of mebendazole on worms is rather slow: takes 2–3 days to develop. It acts probably by blocking glucose uptake in the parasite and depletion of its glycogen stores. Intracellular microtubules in the cells of the worm are gradually lost. The site of action of mebendazole appears to be the microtubular protein ‘βtubulin’ of the parasite. It binds to βtubulin of susceptible worms with high affinity and inhibits its polymerization.

 

Hatching of nematode eggs and their larvae are also inhibited. Ascaris ova are killed.

 

Pharmacokinetics

 

Absorption of mebendazole from intestines is minimal; 75–90% of an oral dose is passed in the faeces. The fraction absorbed is excreted mainly as inactive metabolites in urine/ faeces.

 

Adverse Effects

 

Mebendazole is well tolerated even by patients in poor health. Diarrhoea, nausea and abdominal pain have attended its use in heavy infestation. Incidents of expulsion of Ascaris from mouth or nose have occurred, probably due to starvation of the parasite and their slow death. Allergic reactions, loss of hair and granulocytopenia have been reported with high doses.

Safety of mebendazole during pregnancy is not known, but it is contraindicated on the basis of animal data.

 

Uses  And Administration: Mebendazole is available as: MEBEX, WORMIN 100 mg chewable tab and 100 mg/5 ml suspension. MEBAZOLE 100 mg tab

 

The dose and duration of treatment is the same for children above 2 years as for adults; ½ dose for 1–2 yr age.

 

Roundworm

Hookworm

Whipworm

*100 mg twice a day for 3 consecutive days. No fasting, purging or any other preparation of the patients is needed.

 

Enterobius: 100 mg single dose, repeated after 2–3 weeks (to kill the ova that have developed later). Strict hygienic measures and simultaneous treatment of all children in the family or class is advocated to cut down autoinfection and person to person infection. This holds true of enterobiasis, irrespective of drug used.

 

Trichinella spiralis: 200 mg BD for 4 days; less effective than albendazole.

 

Hydatid disease: 200–400 mg BD or TDS for 3–4 weeks; less effective than albendazole.

 

Mebendazole is one of the preferred drugs for treatment of multiple infestations and is more effective than albendazole in trichuriasis. It has also been used for mass treatment, but need for multiple doses is a drawback.

 

Albendazole

 

It is a subsequently introduced congener of mebendazole: retains the broad-spectrum activity and excellent tolerability of its predecessor, and has the advantage of single dose administration in many cases. One dose treatment has produced cure rates in ascariasis, hookworm (both species) and enterobiasis which are comparable to 3 day treatment with mebendazole. Results in trichuriasis have been inferior to mebendazole. In strongyloidosis, it is more effective than mebendazole: a 3 day course has achieved nearly 50% cure, and a second course repeated after 3 weeks cured practically all patients. Three day treatment has been found necessary for tapeworms including H. nana. Results in hydatid disease and hookworm have been superior to mebendazole. Albendazole has weak microfilaricidal action, kills cysticerci, hydatid larvae, ova of ascaris/hookworm and is also effective in cutaneous larva migrans. The mechanism of action of albendazole is similar to that of mebendazole.

 

Absorption of albendazole after oral administration is moderate, but inconsistent. It is enhanced when the drug is taken with fatty meal (may help in treating neurocysticercosis and hydatid disease). The fraction absorbed is converted by first pass metabolism to its sulfoxide metabolite which is active in contrast to the metabolites of mebendazole and thiabendazole. Albendazole sulfoxide is widely distributed in the body, enters brain and is excreted in urine with a t½ of 8.5 hours. Thus, albendazole is able to exert antihelmintic activity in tissues as well.

 

Albendazole is well tolerated; only gastrointestinal side effects have been noted. Few patients have felt dizziness. Prolonged use, as in hydatid or in cysticercosis, has caused headache, fever, alopecia, jaundice and neutropenia.

 

ZENTEL, ALMINTH, ALBEZOLE, COMBANTRINA 400 mg tab, 200 mg/5 ml suspension.

 

No preparation or post-drug fasting/ purging is required. For intestinal worms it should be given on empty stomach, while for cysticercosis, hydatid and cutaneous larva migrans it should be given with a fatty meal.

 

Ascaris, hookworm, Enterobius and Trichuris: a single dose of 400 mg (for adults and children above 2 yrs, 200 mg for 1–2 yr age).

 

Tapeworms and strongyloidosis: 400 mg daily for 3 consecutive days.

 

Trichinosis: Three day treatment expels the adult worm from intestine, but has limited effect on larvae that have migrated to muscles. They are not killed but symptomatic relief occurs. Corticosteroids are added if systemic manifestations are severe.

 

Neurocysticercosis: Albendazole is the anthelmintic of choice for the treatment of neurocysticercosis (see later). Usually 8–15 days course of 400 mg BD (15 mg/kg/day) is employed. Cysticercosis of other tissues (muscles, subcutaneous area) also responds, but no drug should be given for ocular cysticercosis—blindness can occur due to the reaction.

 

Cutaneous larva migrans: Albendazole 400 mg daily for 3 days is the drug of choice; kills larvae and relieves symptoms.

 

Hydatid disease: 400 mg BD for 4 weeks, repeat after 2 weeks (if required), up to 3 courses. It is the preferred treatment given before and after surgery as well as to inoperable cases.

 

Filariasis: Added to diethylcarbamazine (DEC) or ivermectin, albendazole has adjuvant value in treating lymphatic filariasis. A single dose of its combination with either DEC or ivermectin given yearly has been used in mass programmes to suppress micro-filaraemia and disease transmission.

 

 

Because it has exhibited embryotoxicity in animals, use in pregnant women is contraindicated. It should be given with caution to patients with hepatic or renal disease.

 

Thiabendazole

 

It was the first benzimidazole poly-anthelmintic introduced in 1961, which covered practically all species of nematodes infesting the g.i.t.—roundworm, hookworm, Enterobius, Trichuris, Strongyloides and Trichinella spiralis. It also inhibits development of the eggs of worms and kills larvae. Thiabendazole affords symptomatic relief in cutaneous larva migrans and skeletal muscle symptoms produced by migration of Trichinella spiralis larvae to muscles. Symptomatic relief also occurs in guinea worm disease.

 

The mechanism of action of thiabendazole is the same as described for mebendazole. Thiabendazole has anti-inflammatory, analgesic and antipyretic actions. These may contribute to its effect in cutaneous larva migrans and other inflammatory conditions produced by larvae or worms in tissues.

 

Since thiabendazole is well absorbed from g.i.t., systemic adverse effects are frequent and often interfere with normal activity.

 

Nausea, vomiting, loss of appetite, headache, giddiness are most common. It can impair alertness—driving and operation of machinery should be prohibited.

 

Itching, abdominal pain, diarrhoea and a variety of other symptoms are also produced.

 

Dose 25 mg/kg/day in two divided doses taken after meals. Tablets must be chewed;

 

Uses

 

Because of frequent side effects and poor patient acceptability, thiabendazole is used only when other better tolerated drugs are ineffective.

 

1. Strongyloidosis

2. Cutaneous larva migrans

3. Trichinosis—intestinal infestation and larvae in muscles

*Give a 2 day course. If inadequate, repeat after a gap of 2 days.

 

Trichinella larvae in muscles are often not killed, but symptomatic relief occurs quickly.

 

Pyrantel Pamoate

 

It was introduced in 1969 for threadworm infestation in children; use soon extended to roundworm and hookworm as well. Efficacy against Ascaris, Enterobius and Ancylostoma is high and comparable to that of mebendazole. Lower cure rates (about 60%) have been obtained in case of Necator infestation. It is less active against Strongyloides and inactive against Trichuris and other worms.

 

Pyrantel causes activation of nicotinic cholinergic receptors in the worms resulting in persistent depolarization slowly developing contracture and spastic paralysis. Worms are then expelled. An anticholinesterase action has also been demonstrated. Because piperazine causes hyperpolarization and flaccid paralysis, it antagonizes the action of pyrantel. Cholinergic receptors in mammalian skeletal muscle have very low affinity for pyrantel.

 

Only 10–15% of an oral dose of pyrantel pamoate is absorbed: this is partly metabolized and excreted in urine.

 

Adverse Effects

 

Pyrantel pamoate is remarkably free of side effects: occasional g.i. symptoms, headache and dizziness is reported. It is tasteless, nonirritant; abnormal migration of worms is not provoked. Its safety in pregnant women and in children below 2 years has not been established.

 

Use And Administration

 

For Ascaris, Ancylostoma and Enterobius: a single dose of 10 mg/kg is recommended. A 3 day course for Necator and for Strongyloides has been suggested.

 

No fasting, purging or other preparation of the patient is needed.

 

NEMOCID, ANTIMINTH, EXPENT 250 mg tab, 50 mg/ ml suspension (10 ml bottle).

 

Piperazine

 

Introduced in 1950, it is a highly active drug against Ascaris and Enterobius; achieves 90–100% cure rates. However, it is now considered a second choice drug even for these worms. Piperazine causes hyperpolarization of Ascaris muscle by a GABA agonistic action opening Cl¯ channels that causes relaxation and depresses responsiveness to contractile action of ACh. No. Flaccid paralysis occurs and worms are expelled alive. They recover if placed in piperazine free medium. Therefore, often a purgative (senna) is given with it, but is not necessary. No fasting or patient preparation is required. Piperazine does not excite Ascaris to abnormal migration. It does not affect neuromuscular transmission in man.

 

Pharmacokinetics

 

A considerable fraction of the oral dose of piperazine is absorbed. It is partly metabolized in liver and excreted in urine.

 

Adverse Effects

 

Piperazine is safe and well tolerated. Nausea, vomiting, abdominal discomfort and urticaria are occasional.

 

Dizziness and excitement occur at high doses; toxic doses produce convulsions; death is due to respiratory failure. It is contraindicated in renal insufficiency and in epileptics, but is safe in the pregnant.

 

Dose: For roundworm infestation 4 g once a day for 2 consecutive days; children 0.75 g/year of age (max. 4 g) is considered curative. Because of its capacity to relax ascarids, it is of particular value in intestinal obstruction due to roundworms. It can be used during pregnancy while other drugs cannot be used.

 

Enterobiasis—50 mg/kg (max. 2 g) once a day for 7 days or 75 mg/kg (max. 4 g) single dose, repeated after 3 weeks.

 

PIPERAZINE CITRATE 0.75 g/5 ml elixir in 30 ml, 115 ml bottle; 0.5 g (as phosphate) tablets; Combination of any other anthelmintic (except piperazine) with a purgative in the same formulation is banned in India.

 

 

Levamisole, Tetramisole

 

Tetramisole was developed in the late 1960s. It is recemic; its levo isomer (levamisole) was found to be more active and is preferred now. Both are active against many nematodes, but use is restricted to ascariasis and ancylostomiasis, because action on other worms is poor. Strongyloides larvae are killed, but adult worms are not sensitive. The ganglia in worms are stimulated causing tonic paralysis and expulsion of live worms. Interference with carbohydrate metabolism (inhibition of fumarate reductase) may also be contributing.

 

Uses

 

For Ascaris infestation a single dose of levamisole 50 mg for children 10–19 kg body weight, 100 mg for 20–39 kg and 150 mg for >40 kg and adults is advocated. It achieves >90% cure rate.

 

Levamisole is a second line drug for A. duodenale; 2 doses at 12 hour interval are suggested— achieves 70–90% cure. It is less efficacious against Necator.

 

Tetramisole: DECARIS 50, 150 mg tab.

 

Levamisole: DEWORMIS, VERMISOL 50, 150 mg tab, 50 mg/5 ml syr.

 

Levamisole is an immunomodulator as well: restores depressed T cell function. It was used as a disease modifying drug in rheumatoid arthritis and as an adjunct in malignancies, aphthous ulcers and recurrent herpes, but repeated doses produce severe reactions; not used now.

 

Adverse Effects

 

One or two doses used in helminthiasis are well tolerated. Incidence of side effects—nausea, abdominal pain, giddiness, fatigue, drowsiness or insomnia is low.

 

Diethyl Carbamazine Citrate (DEC)

 

Developed in 1948, it is the first drug for filariasis. DEC is absorbed after oral ingestion, distributed all over the body (V = 3–5 L/kg), metabolized in liver and excreted in urine. Excretion is faster in acidic urine. Plasma t½ of usual clinical doses is 4–12 hours, depending on urinary pH.

 

Diethylcarbamazine has a highly selective effect on microfilariae (Mf). A dose of 2 mg/kg TDS clears Mf of W. bancrofti and B. malayi from peripheral blood in 7 days. However, Mf present in nodules and transudates (hydrocoele) are not killed. The most important action of DEC appears to be alteration of Mf membranes so that they are readily phagocytosed by tissue fixed monocytes, but not by circulating phagocytes. Muscular activity of the Mf and adult worms is also affected causing hyperpolarization due to the piperazine moiety, so that they are dislodged. Prolonged treatment may kill adult B. malayi and probably W. bancrofti worms also.

 

DEC is active against Mf of Loa loa and Onchocerca volvulus. The adult worm of L. loa but not O. volvulus is killed. DEC reduces worm burden in ascariasis, but efficacy is low.

 

Uses

 

Filariasis: 2 mg/kg TDS produces rapid symptomatic relief; Mf disappear from blood and patient becomes noninfective to mosquitoes in 7 days. However, the adult worm survives in the lymphatics and gives rise to intermittent micro-filaraemia and symptoms. Prolonged treatment with different schedules has been found to achieve radical cure in most patients. A total dose of 72–126 mg/kg spread over 12 days to 3 weeks has been found satisfactory; more than one course may be needed with a gap of 3–4 weeks. Elephantiasis due to chronic lymphatic obstruction is not affected by DEC, because fibrosis of lymphatics is irreversible. Yearly treatment with a combination of DEC and albendazole on mass scale has brought down transmission of filariasis by reducing micro-filaraemia.

 

Tropical Eosinophilia: DEC (2–4 mg/kg TDS) for 2–3 weeks produces dramatic improvement in the signs and symptoms of eosinophilic lung or tropical eosinophilia. The benefit probably reflects anti-microfilarial action: the symptoms of the disease being presumably due to reaction to the Mf.

 

HETRAZAN, BANOCIDE 50, 100 mg tab, 120 mg/5 ml syr; 50 mg/ 5 ml pediatric syr; to be taken after meals.

 

Loa loa and O. volvulus infections can also be treated with DEC, but it is imperative to give small (25–50 mg) test dose initially to avoid severe reaction to dying Mf. Ivermectin does not produce such severe reactions and is preferred for initial treatment.

 

Adverse Effects

 

These are common but generally not serious. Nausea, loss of appetite, headache, weakness and dizziness are the usual complaints.

 

A febrile reaction with rash, pruritus, enlargement of lymph nodes and fall in BP may occur due to mass destruction of Mf and adult worms. This is usually mild, but may be severe. The reaction can be minimized by starting with a low dose (0.5 mg/kg). When it occurs, DEC should be temporarily withheld and antihistaminics and/ or corticosteroids given. Subsequent administration of DEC does not cause such reaction. Leukocytosis and mild albuminuria are also noted.

 

Ivermectin

 

It is an extremely potent semisynthetic derivative of the anti-nematodal principle obtained from Streptomyces avermitilis, that has been used in other countries for long, but marketed in India only recently. Ivermectin is the drug of choice for single dose treatment of onchocerciasis and strongyloidosis, and is comparable to DEC for bancroftian and brugian filaria. It is also highly effective in cutaneous larva migrans and ascariasis, while efficacy against Enterobius and Trichuris is moderate. Certain insects, notably scabies and head lice are killed by ivermectin.

 

Nematodes develop tonic paralysis when exposed to ivermectin. It acts through a special type of glutamate gated Cl¯ channel found only in invertebrates. Such channels are not involved in the motor control of flukes and tapeworms which are unaffected by ivermectin. Potentiation of GABAergic transmission in the worm has also been observed. The lack of GABA-related actions in man could be due to its low affinity for mammalian GABA receptors and its exclusion from the brain, probably by P-glycoprotein mediated efflux at the blood brainbarrier.

 

A single 10–15 mg (0.2 mg/kg) oral dose of ivermectin, preferably with 400 mg albendazole, given annually for 5–6 years has been used for filariasis. Single 0.15–0.2 mg/kg dose has yielded highest cure rate in strongyloidosis and reduces burden of other intestinal nematodes as well.

 

Ivermectin has replaced DEC for onchocerciasis and has been used in the ‘river blindness’ control programme of WHO in Africa and Latin America. One dose of ivermectin is given at 6–12 month intervals—produces long lasting reduction of Mf counts in eye and skin, without affecting the adult worm. Though it does not cure O. volvulus infection, ocular inflammation/damage as well as lymphadenopathy are suppressed with only mild ocular or systemic reactions.

 

Ivermectin is the only drug effective orally in scabies and pediculosis. Single 0.2 mg/kg dose cures most patients.

 

IVERMECTOL, IVERMEC, VERMIN 3, 6 mg tabs; to be taken on empty stomaCh. No.

 

Ivermectin is well absorbed orally, widely distributed in the body, but does not enter CNS, sequestrated in liver and fat, and has a long terminal t½ of 48–60 hours. Side effects have been mild—pruritus, giddiness, nausea, abdominal pain, constipation, lethargy and transient ECG changes, but more important are the reactions due to degeneration products of the Mf, which are similar to those occurring after DEC.

 

Niclosamide

 

Niclosamide is a highly effective drug against cestodes infesting man—Taenia saginata, T. solium, Diphyllobothrium latum and Hymenolepis nana, as well as threadworm. The drug appears to act by inhibiting oxidative phosphorylation in mitochondria and interfering with anaerobic generation of ATP by the tapeworm. Injured by niclosamide, the tapeworms are partly digested in the intestine. In cases of T. solium, digestion of the dead segments can be hazardous, because the ova released from them may develop into larvae in the intestine, penetrate its wall and cause visceral cysticercosis. Many experts do not use niclosamide now for T. solium infestation.

 

Regimen For Tapeworm: Niclosamide is available as 0.5 g tab (NICLOSAN). After a light breakfast, 2 tablets are to be chewed and swallowed with water, followed by another 2 tablets after 1 hr (total 2 g); total dose for children 2–6 years is 1 g. A saline purge is given 2 hours after the later dose to wash off the worm. The scolex should be searched in the stools to be sure that the worm will not grow again. Cure rate of 85–95% has been obtained by one day treatment. A thorough purge is essential in the cases of T. solium so that all segments are passed out and cysticercosis does not occur. Because praziquantel does not lead to digestion of the worm and kills encysted larvae as well, it is the drug of choice for T. solium.

 

For H. nana, the 2 g dose is repeated daily for 5 days. This is needed because cysticerci of H. nana (which are not affected by niclosamide) develop in the jejunal villi of the same host and worms appear in the intestinal lumen after 4 days. However, no purgative is required. In some cases treatment  may  have  to  be  repeated  after  10  days.

 

Praziquantel is now preferred due to single dose treatment.

 

Adverse Effects

 

Niclosamide is tasteless and nonirritating. It is minimally absorbed from g.i.t.— no systemic toxicity occurs. It is well tolerated; minor abdominal symptoms are produced occasionally. Malaise, pruritus and light headedness are rare. Niclosamide is safe during pregnancy and in patients with poor health.

 

Praziquantel

 

This novel anthelmintic has wide ranging activity against Schistosomes, other trematodes, cestodes and their larval forms but not nematodes. It is rapidly taken up by susceptible worms and appears to act by causing leakage of intracellular calcium from the membranes contracture and paralysis. The tapeworms lose grip of the intestinal mucosa and are expelled. Flukes and schistosomes are also dislodged in tissues and veins. Praziquantel is active against adult as well as juvenile and larval stages of tapeworms.

 

At relatively higher concentrations, it causes vacuolization of the tegument and release of the contents of tapeworms and flukes followed by their destruction by the host. This action appears to be more important in cases of schistosomes and flukes.

 

Pharmacokinetics

 

Praziquantel is rapidly absorbed from intestines and absorption is enhanced by ingesting it with food. It undergoes high first pass metabolism in liver which limits its systemic bioavailability. Phenytoin, carbamazepine and possibly dexamethasone induce praziquantel metabolism and further reduce its bioavailability. Patients of neurocysticercosis are often receiving these drugs—may contribute to therapeutic failure of praziquantel. It crosses bloodbrain barrier and attains therapeutic concentrations in the brain and CSF. The plasma t½ is short (1.5 hours). Metabolites are excreted chiefly in urine.

 

Adverse Effects

 

Despite systemic absorption, praziquantel has exhibited no systemic toxicity.

 

It tastes bitter: can produce nausea and abdominal pain. Other side effects are headache, dizziness and sedation. When used for schistosomes and visceral flukes, symptoms like itching, urticaria, rashes, fever and bodyache occur as a reaction to the destroyed parasites.

 

No interaction with food, alcohol or tobacco has been noted.

 

Uses

 

1. Tapeworms: Praziquantel administered as a single dose has achieved 90–100% cure rate in all human tapeworms. This level of activity is similar to that of niclosamide and even better in case of H. nana.

 

T. saginata, T. solium: 10 mg/kg single dose in morning. It is especially valuable in case of T. solium, because it kills the tapeworm larvae within the cysts and there are no chances of systemic cysticercosis developing.

 

H. nana, D. latum: 15–25 mg/kg single dose in morning. This is much simpler compared to 5 day treatment needed with niclosamide for eradication of H. nana. In case of heavy infestation, retreatment after one week is desirable.

 

2. Neurocysticercosis: Praziquantel was the first drug found to be effective in neurocysticercosis: 50–100 mg/kg daily in 3 divided doses for 15 days kills the larvae lodged in brain and other tissues. However, it is now the 2nd choice drug to albendazole (see below).

 

Praziquantel or albendazole are contraindicated in ocular cysticercosis.

 

3. Schistosomes: All 3 species can be treated with 40–75 mg/kg given once or in instalments in one day.

 

4. Other flukes: Praziquantel is the drug of choice for all schistosome and fluke infestations except Fasciola hepatica. The flukes respond to 75 mg/ kg/day given for one day in most and two days in some cases.

 

CYSTICIDE 500 mg tab, DISTOCIDE 600 mg tab.

 

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