Angiostrongylus cantonensis: paralysis due to avermectin B1a and ivermectin

Susceptibility of Angiostrongylus cantonensis Larvae to Anthelmintic Drugs

Human helminthiasis affects approximately one in five people in the world and disproportionally affects the poorest and most deprived communities. Human angiostrongyliasis, caused by nematode Angiostrongylus cantonensis, is a neglected emerging disease with escalating importance worldwide. Chemotherapy is the main control method for helminthiasis, but the therapeutic arsenal is limited. This study aimed to evaluate the antiparasitic and molecular properties of the major available anthelmintic drugs against A. cantonensis in vitro. The first-stage larvae (L1), isolated from feces of an A. cantonensis-infected rat, were exposed to a set of 12 anthelmintic drugs in vitro. The larvae were monitored, and the concentration- and time-dependent viability alterations were determined. From 12 anthelmintic drugs, six (ivermectin, salamectin, moxidectin, pyrantel pamoate, albendazole and levamisole) were identified to affect the viability of A. cantonensis. The macrocyclic lactones (ivermectin, salamectin, moxidectin) and the imidazothiazole levamisole, were the most effective drugs, with IC50 ranging from 2.2 to 2.9 µM and a rapid onset of action. Albendazole, the most widely used anthelmintic in humans, had a slower onset of action, but an IC50 of 11.3 µM was achieved within 24 h. Molecular properties studies suggest that a less lipophilic character and low molecular weight could be favorable for the biological activity of the non-macrocyclic molecules. Collectively, our study revealed that macrocyclic lactones, levamisole, pyrantel pamoate, and albendazole are important anthelmintic agents against A. cantonensis. The results of this in vitro study also suggest that A. cantonensis L1 may be a particularly sensitive and useful model for anthelmintic studies.

In vitro efficacy of anthelmintics on Angiostrongylus cantonensis L3 larvae

Angiostrongylus cantonensis is the leading cause of eosinophilic meningitis worldwide, with life-threatening complications if not managed correctly. Previous in vitro studies have utilized change in motility patterns of adult female worms to assess the efficacy of anthelmintics qualitatively. However, it is the third stage larvae (L3) that are infectious to humans. With differential staining using propidium iodide penetration as the indicator of death, we can distinguish between dead and live larvae. This assay has enabled us to quantify the in vitro efficacy of nine clinically established anthelmintics on A. cantonensis L3. All drugs were tested at a 1 mm concentration. Piperazine and niclosamide were ineffective in inducing larval death; however, albendazole sulfoxide, pyrantel pamoate, diethylcarbamazine, levamisole and praziquantel were effective as compared to unexposed controls (P < 0.05). Ivermectin and moxidectin did not induce significant levels of mortality, but they considerably reduced larval motility almost immediately. This study indicates the need for further in vivo studies to determine the optimal dose and time frame for post-infection treatment with anthelmintics that demonstrated efficacy.

Drug trials for treatment of human angiostrongyliasis

Abdominal and cerebral angiostrongyliasis are two important infections produced by metastrongylid worms, the former occurring in Central and South America and the later in Asia and Pacific Islands. Drug treatment is a challenge since the worms and its evolving larvae live or migrate inside vessels and efficient killing of the parasites may produce more severe lesions. Larvicidal effect of certain drugs appears to be more easily accomplished but this outcome is not useful in abdominal angiostrongyliasis since clinical manifestations appear to result from sexual maturation of the worms. We review the drug trials in murine experimental models and conclude that most of them could not be considered good candidates for treatment of human infection, except for PF1022A, pyrantel and flubendazole.

Nematocidal activities of thiabendazole and ivermectin against the larvae of Strongyloides ratti and S. venezuelensis

With the aim of developing therapeutic agents for strongyloidosis, the disease caused by infection with Strongyloides stercoralis, we established a novel assay technique using S. ratti and S. venezuelensis as models for S. stercoralis. The newly developed assay technique was found to more accurately represent treatment-induced larval paralysis than existing assays. Our method uses paper disks impregnated with the test solution, which even allows materials that are sparingly soluble in water to be tested. An inverted microscope was used to observe the larval states, and these states were recorded using a digital camera. We observed the activities of ivermectin and thiabendazole against larvae and calculated larval motility and velocity. These two factors were then combined to determine the overall viability of larvae at selected concentrations. The activities of the anthelmintics were compared by calculating the concentrations at which 50% viability was demonstrated, or in other words, the concentration at which paralysis was caused in 50% of the individuals (50% paralysis concentration; PC(50)). Evaluations after 24h of exposure yielded the following reproducible PC(50) values for ivermectin and thiabendazole, respectively: S. ratti, 2.4 and 140 microM; and S. venezuelensis, 2.3 and 190 microM. After treatment with ivermectin, there was a tendency for larval motility to be greater than that of the controls at low concentrations, a result that might be associated with its mechanism of action.

In vitro effects of VD-99-11 on Angiostrongylus cantonensis and isolated frog rectus

In vitro effects of VD-99-11 were examined using adult Angiostrongylus cantonensis and isolated frog rectus. In A. cantonensis, paralysis was elicited by VD-99-11 at 10(-9)-10(-6) g/ml. The paralysis caused by VD-99-11 (10(-8) g/ml) was antagonized by picrotoxin or bicuculline but not by phentolamine. A relationship between VD-99-11 and gabergic antagonists was observed in worm preparations contracted by eserine or pyrantel: VD-99-11 at higher concentrations (3x10(-6) g/ml) caused a marked contraction. In worm preparations contracted with eserine or pyrantel, the only additional contraction induced by VD-99-11 (5x10(-6) g/ml) was antagonized by strychnine. In experiments on the guanidine (2.5x10(-3) M)-induced twitch responses in isolated frog rectus, marked stimulation was caused by VD-99-11 (3-5x10(-6) g/ml). The stimulated responses induced by VD-99-11 were antagonized by tetrodotoxin, D-tubocurarine, strychnine, and hemicholinium-3, respectively. These results suggest that VD-99-11 seems superior to milbemycin D, milbemycin oxime, and ivermectin in some aspects, such as in vitro potency, though this new substance is similar to these drugs in having two different actions on the gabergic mechanism at lower concentrations and on the cholinergic mechanism at higher concentrations.

Effects of gabergic anthelmintics at higher concentrations on the guanidine-induced twitch responses in isolated frog rectus preparations

Effects of various gabergic anthelmintics on the guanidine-induced twitch responses in isolated frog rectus preparations were examined. All gabergic anthelmintics such as milbemycin oxime, milbemycin D, avermectin B1a, ivermectin, and diethylcarbamazine (DEC) showed stimulatory effects on the guanidine-induced twitch responses at their higher concentrations. Only piperazine caused inhibitory effects on the twitch responses, even at higher concentrations. The stimulation of the twitch responses by the gabergic anthelmintics was antagonized with tetrodotoxin, hemicholinium-3, d-tubocurarine, and strychnine. These results suggest that all gabergic anthelmintics except piperazine stimulate the release of acetylcholine from the nerve endings and that all of them, including piperazine, have different effects on the gabergic mechanism at lower concentrations and on the cholinergic mechanism at higher concentrations.

Efficacy of ivermectin against Parastrongylus malaysiensis infection in rats

The efficacy of ivermectin on experimental infections of P. malaysiensis in rats was determined. Ivermectin was 99.4% and 97.9% effective at a dosage of 400 meg and 800 meg respectively at seven days post-infection. The same two dosages of ivermectin when given at 14 days post infection had an efficacy of 100%. However, as an adulticide it had only 40.7% efficacy. Ivermectin may therefore be useful for the treatment of parastrongyliasis due to the larval stages of the worm which can cause significant pathology in man and animals.

In vitro effects of milbemycin oxime: mechanism of action against Angiostrongylus cantonensis and Dirofilaria immitis

The neuropharmacological mechanisms underlying the action of milbemycin oxime on the motility of Angiostrongylus cantonensis and Dirofilaria immitis were examined in vitro. In A. cantonensis, milbemycin oxime caused inhibitory effects at low concentrations of > or = 10(-9) g/ml, and paralysis was elicited at 10(-8) - 10(-6) g/ml. The paralysis was antagonized by picrotoxin and bicuculline but not by dibenamine. In addition, stimulatory effects were observed when the antibiotic was used at higher concentrations of 3-5 x 10(-6) g/ml, and the action was antagonized by strychnine. Both effects were also observed in the preparation contracted by eserine or pyrantel. When D. immitis was treated with milbemycin oxime at concentrations of 10(-7) and 3-5 x 10(-6) g/ml, only slight inhibitory and stimulatory effects, respectively, were observed. These effects were partially antagonized by picrotoxin and strychnine, respectively. These results suggest that the inhibitory and stimulatory actions of milbemycin oxime are caused through gabergic and cholinergic mechanisms in A. cantonensis and D. immitis.

Effect of single-dose ivermectin therapy on human Onchocerca volvulus infection with onchocercal ocular involvement

Ivermectin has shown promise as a potentially safe and effective microfilaricidal drug for the treatment of onchocerciasis. Several limited studies have shown it to have fewer side effects, especially ocular complications, than the currently available drug, diethylcarbamazine. The detailed ocular findings in 200 moderately to heavily infected Liberians who were enrolled in a safety and dose-finding study are presented. They received either 0, 100, 150, or 200 micrograms/kg of ivermectin and were followed up for 12 months. In clinical studies so far carried out ivermectin in a dose of 100, 150, or 200 micrograms/kg has not been associated with any major adverse reactions nor were there any sight-threatening effects even in the presence of severe ocular disease. Each of these doses significantly reduced the ocular microfilaria load for at least 12 months when compared with either the placebo (p less than 0.05) or pretreatment values (p less than 0.001). However, the 100 and 150 micrograms/kg doses caused fewer minor side effects than the higher dose. These results confirm that ivermectin in a single oral dose may be a safe and effective microfilaricidal drug for the treatment of onchocerciasis and that it appears to be free of major ocular side effects.

Effects of milbemycin D on adult Angiostrongylus cantonensis in rats

Effects of milbemycin D against adult Angiostrongylus cantonensis in rats were examined. The first-stage larval counts in rat faeces (larvae per gram of faeces per female worm recovered, LPG/female) were most conspicuously reduced in the group treated with nine consecutive weekly doses of 5.0 mg/kg. The effect was more marked in the group treated with five or ten successive daily doses of 5.0 mg/kg than the group treated with a single dose of 25.0 or 5.0 mg/kg. Host lung-body weight ratio and number of recovered worms were reduced significantly only in the group treated with five or ten successive daily doses of 5.0 mg/kg. These results suggest that the action of milbemycin D on the reproductive system of the worms might be differentiated from its killing action. The in vitro motility of females recovered from rats medicated with nine consecutive weekly doses of 5.0 mg/kg was inhibited, and almost all females and males were semitransparent and colourless. Results obtained from sectioned worms showed little content in their digestive tracts and uteri. In addition, there were few eggs and first-stage larvae in the lung tissues of treated rats. These suggest that milbemycin D affects the reproductive functions of the worms through an indirect mode of action including paralysis and inhibition in food intake and energy and/or synthetic metabolism.

A double-blind comparison of the efficacy and safety of ivermectin and diethylcarbamazine in a placebo controlled study of Senegalese patients with onchocerciasis

Ivermectin (MK-933) has been compared with diethylcarbamazine (DEC) and placebo in a double-blind study in 30 adult male Senegalese patients with Onchocerca volvulus infection. 10 patients were randomly assigned to each treatment group. Ivermectin was administered as a single oral dose of 12 mg and DEC as 50 mg daily for two days and 100 mg twice daily for the following six days, total 1.3 g in eight days. Skin O. volvulus microfilaria densities remained near pre-study values in the placebo patients, but decreased rapidly with both active drugs to mean values about 2% of pretreatment (Day 8) and then increased slowly, reaching in 12 months about 4% of pre-treatment (ivermectin) and 18% (DEC). This difference is statistically significant. Clinical adverse reactions were recorded in four ivermectin, ten DEC and three placebo patients. One ivermectin and six DEC patients received steroid treatment for relief of these reactions. Serious adverse ocular changes were not seen in any patients, possibly because of the steroid therapy in the DEC patients. Adult O. volvulus from onchocercal nodules one and six months after treatment showed no effect of either drug on viability. Intra-uterine developing forms of the microfilariae appeared normal in all three treatment groups at the one month examination but deformed and degenerated forms were evident at six months in the ivermectin group but not in the DEC and placebo patients. Ivermectin as a single oral dose appears to be a safer and more effective microfilaricidal drug in human onchocerciasis than DEC in the standard multi-dose regimen.

Effects of diethylcarbamazine on the motility of Angiostrongylus cantonensis and Dirofilaria immitis

Effects of piperazine derivatives, especially of diethylcarbamazine (DEC) on adult Angiostrongylus cantonensis and Dirofilaria immitis were examined. Piperazine (3 X 10(-5)-10(-4) M) paralyzed A. cantonensis and the action was antagonized by picrotoxin. 1,1-dimethyl-4-phenylpiperazinium iodide (DMPP) (10(-5)-10(-4) M) caused contraction but little effect was produced by strychnine. An inhibitory effect on untreated preparations was caused by lower concentrations (3 X 10(-6)-10(-5) M) of diethylcarbamazine citrate (DEC) and also on the preparations contracted by eserine. A stimulatory effect was also seen when higher concentrations (10(-4)-3 X 10(-4) M) of this drug were applied to both preparations. The inhibitory action of DEC was antagonized by gabergic antagonists such as picrotoxin and bicuculline, but not by alpha-adrenergic antagonists like dibenamine and phentolamine. When the worm preparation was paralyzed by strychnine or hexylresorcinol (inhibitors of the release of acetylcholine in this worm), the stimulatory effect of DEC was blocked, but pyrantel (a nicotinic cholinergic agonist) contracted the paralyzed preparation. However, the effect of DEC on D. immitis (10(-7)-3 X 10(-4) M) was inhibitory, and this action was also antagonized by picrotoxin. These results suggest that the DEC inhibitory and stimulatory action is through the gabergic and cholinergic mechanisms in adult A. cantonensis and D. immitis.

Double-blind study of ivermectin and diethylcarbamazine in African onchocerciasis patients with ocular involvement

In a randomised double-blind study, ivermectin was compared with diethylcarbamazine (DEC) and placebo in the treatment of onchocerciasis in 30 male patients from Mali with moderate to heavy Onchocerca volvulus infections and ocular involvement. 10 patients received a single oral dose of ivermectin, 12 mg, 10 received DEC daily for eight days (total dose 1.3 g), and 10 received matching placebo. Patients were examined periodically for twelve months. Punctate keratitis disappeared in 6 of 7 ivermectin patients but increased in DEC patients. Numbers of O volvulus microfilariae (mf) in the anterior chamber decreased slowly and eventually disappeared in most ivermectin patients during the six months following treatment; anterior chamber mf disappeared more rapidly in some patients after DEC, but reappeared within six months of stopping treatment. Both ivermectin and DEC caused a prompt decrease in mean skin mf density; density then increased in both groups over the twelve month observation period, reaching 9% of pretreatment values in ivermectin patients and 45% in the DEC group. Analysis of adult O volvulus from nodules excised at three and twelve months post treatment showed no effect of either drug on viability; however, there was evidence of degeneration of intra-uterine developing mf in the ivermectin group. Side-effects were less frequent and less severe in ivermectin patients than in DEC patients. Ivermectin as a single oral dose appears to be a more effective microfilaricidal drug than DEC in onchocerciasis.

Comparison of ivermectin and diethylcarbamazine in the treatment of onchocerciasis

We compared ivermectin with diethylcarbamazine for the treatment of onchocerciasis in a double-blind, placebo-controlled trial. Thirty men with moderate to heavy infection and ocular involvement were randomly assigned to receive ivermectin in a single oral dose (200 micrograms per kilogram of body weight), diethylcarbamazine daily for eight days, or placebo. Diethylcarbamazine caused a significantly more severe systemic reaction than ivermectin (P less than 0.001), whereas the reaction to ivermectin did not differ from the reaction to placebo. Diethylcarbamazine markedly increased the number of punctate opacities in the eye (P less than 0.001), as well as the number of dead and living microfilariae in the cornea over the first week of therapy. Ivermectin had no such effect. Both ivermectin and diethylcarbamazine promptly reduced skin microfilaria counts, but only in the ivermectin group did counts remain significantly lower (P less than 0.005) than in the placebo group at the end of six months of observation. Analysis of adult worms isolated from nodules obtained two months after the start of therapy showed no effect of either drug on viability. Ivermectin appears to be a better tolerated, safer, and more effective microfilaricidal agent than diethylcarbamazine for the treatment of onchocerciasis.