The chemotherapy of rodent malaria. XLVI. Reversal of mefloquine resistance in rodent Plasmodium

Multiple drug resistance in Plasmodium falciparum is already showing evidence of extending to mefloquine, which at present is one of the few alternative antimalarials for the prevention or treatment of infection with such parasites. Neither verapamil nor cyproheptadine, which reverse chloroquine (CQ) resistance in P. falciparum and in rodent malaria parasites, reverse resistance to mefloquine (MEF) in the MEF-resistant NS/1100 line of P. yoelii ssp. NS. On the other hand, such resistance is clearly reversed when mefloquine is administered to infected mice together with penfluridol.     

The chemotherapy of rodent malaria XXXVI. Part IV. The activity of a new 8-aminoquinoline, WR 225,448 against exo-erythrocytic schizonts of Plasmodium yoelii yoelii

Light microscope and ultrastructural studies show that WR 225,448, an 8-aminoquinoline related to primaquine, causes substantial damage to developing pre-erythrocytic (EE) schizonts of Plasmodium yoelii yoelii in the liver of young rats. In addition to lesions of the mitochondria and nuclear membranes, secretory granules in the parasite fail to reach the surrounding host cell cytoplasm across the parasite membrane. The effective dose of WR 225,448 is much lower than that of primaquine. However, hepatic damage is also seen in treated animals. This may be the summation of damage produced by the experimental inoculation of a large quantity of infective salivary gland tissue from mosquitoes (which is also seen in untreated animals) plus toxic effects of the drug itself. Particular attention would have to be directed to this aspect in preclinical toxicity testing of the compound.     

The chemotherapy of rodent malaria, XXVII. Studies on mefloquine (WR 142,490).

Mefloquine (WR 142,490) is a potent blood schizontocide active against drug-sensitive and drug-resistant lines of Plasmodium berghei. The ED50 and ED90 against the P. berghei N strain in albino mice are 1.5 and 3.8 mg/kg respectively. The highly chloroquine-resistant RC line is less sensitive and mefloquine is not fully effective at the maximum tolerated dose in the '4-day test'. Mefloquine has a similar mode of action to quinine both in vitro and (as demonstrated by the morphological changes it induces in P. berghei) in vivo, but is some 100 times more potent. Unlike quinine and WR 122,455 it appears not to interact with DNA. It has no causal prophylactic effect. Mixtures of mefloquine with pyrimethamine, sulphaphenazole or primaquine have an additive effect.     

Mouse model for exoerythrocytic stages of Plasmodium falciparum malaria parasite.

Research on the exoerythrocytic (EE) stages of human malaria parasites has been hindered because of the lack of an easily available suitable animal model. We report here an approach to produce mature EE-stage Plasmodium falciparum parasites by using severe combined immunodeficient (scid) mice with transplanted human hepatocytes. Transplantation of human hepatocytes into scid mice (scid hu-hep), their subsequent intravenous infection with P. falciparum sporozoites, and the development of mature liver-stage merozoites was achieved. Immunofluorescent staining of scid hu-hep kidney tissue sections demonstrated the presence of circumsporozoite protein (early during infection), merozoite surface antigen 1, and liver schizont antigen 1. The scid hu-hep model can serve as a source of human malaria liver-stage parasites, decreasing the need for nonhuman primates. Use of this model will facilitate characterization of EE-stage antigens and the assessment of stage-specific chemotherapeutic agents and candidate vaccines.     

The chemotherapy of rodent malaria, XXIX DNA relationships within the subgenus Plasmodium (Vinckeia).

The relationships between rodent malarias were examined by means of DNA buoyant density determinations and DNA--DNA hybridization data. Current views of the existence in this group of four species: Plasmodium berghei, P. yoelli, P. vinckei and P. chabaudi were confirmed. The identity of two chloroquine-resistant lines derived from P. berghei N strain was established. One of these, the NS line, was found to be a subspecies of P. yoelii; the implications of this finding are discussed.     

Comparison of effects of pyronaridine, amodiaquine, mefloquine and qinghaosu on rodent malaria.

Blood schizontocidal effect of antimalarials were compared by 4-day suppressive test with an extended observation period of 31 days. On a drug-sensitive Plasmodium berghei ANKA strain, pyronaridine (PND) exhibited the best effect, followed by amodiaquine (ADQ), mefloquine (MFQ), and qinghaosu (QHS). On a moderately chloroquine-resistant P. berghei NS line, the order of effects was the same, PND greater than ADQ greater than MFQ greater than QHS. On a highly pyronaridine-resistant P. berghei RP line, ADQ, MFQ and QHS showed cross resistance with PND.     

The chemotherapy of rodent malaria. XLII. Halofantrine and halofantrine resistance

As a blood schizontocide, halofantrine is about three times as active against a drug-sensitive line of Plasmodium berghei (N strain) as chloroquine, but it lacks any causal prophylactic effect. This activity is retained against parasites highly resistant to primaquine, cycloguanil, pyrimethamine, sulphaphenazole and menoctone. A marked resistance to halofantrine is shown by parasites resistant to mefloquine, quinine, chloroquine and amodiaquine, although the moderately chloroquine-resistant 'NS line' is sensitive. The artemisinin-resistant line shows a moderately reduced response. A high level of resistance was rapidly developed in the 'NS line' to halofantrine (NS/HAL line) but not in the N strain (N/HAL). The NS/HAL line was cross-resistant to mefloquine, amodiaquine and, to a lesser degree, artemisinin, but remained sensitive to chloroquine and quinine. The implications to a lesser degree, artemisinin, but remained sensitive to chloroquine and quinine. The implications of these observations for the possible future clinical deployment of halofantrine are discussed, and emphasis is laid on the need to find means of impeding the resistance to it of Plasmodium falciparum.     

Protective immunity to malaria: studies with cloned lines of rodent malaria in CBA/Ca mice. IV. The specificity of mechanisms resulting in crisis and resolution of the primary acute phase parasitaemia of. Plasmodium chabaudi chabaudi and P. yoelii yoelii

Low numbers of parasites from cloned lines of the rodent malaria parasites, Plasmodium chabaudi chabaudi AS and P. yoelii yoelii A, injected into CBA/Ca mice produce acute but usually self-limiting infections. During crisis, i.e. 1-2 days after peak parasitaemia, 'pre-immune' mice experiencing such 'background' infections were reinfected intravenously with homologous parasites or parasites of heterologous strains or species. P. c. chabaudi AS pre-immune mice controlled an AS challenge with essentially the same kinetics as the background infection. Reinfection of AS pre-immune mice with the heterologous (CB and IP-PCI) P. c. chabaudi strains or P. chabaudi adami DS had little effect on the initial growth of these parasites, although eventually the parasitaemia was controlled. In contrast, a partial inhibitory effect on the growth of P. vinckei lentum DS was evident. Challenge with the non-lethal (A) or lethal (YM) variants of P. y. yoelii resulted in an increase in both the growth and virulence of these parasites. P. y. yoelii A pre-immune mice controlled a homologous challenge, but were less effective at controlling the YM variant. In addition, they were unable to clear rapidly a P. c. chabaudi AS or P. v. lentum DS challenge. Both the multiplication and virulence of P. berghei ANKA were enhanced. These findings demonstrate that resolution of the primary acute parasitaemia in P. c. chabaudi AS- and P. y. yoelii A-infected mice is predominantly mediated by species- and strain-specific mechanisms.     

Possible involvement of platelets in Plasmodium yoelii nigeriensis malaria infection in mice

The possible involvement of platelets in the pathogenesis of malaria was examined by monitoring the changes in platelet function (bleeding time [BT] and thrombin time [TT]) concomitantly with changes in packed cell volume (PCV) and erythrocyte infection rate (EIR) in Plasmodium yoelii nigeriensis infection in mice. In untreated plasmodium-infected mice, there was a progressive reduction (day 1-7) in BT from 120.0 +/- 12.6 s to 77.5 +/- 5.9 s (p less than 0.005), a reduction in TT from 26.8 +/- 1.2 s to 17.8 +/- 1.2 s (p less than 0.005), an increase in EIR from 0 to 64.6 +/- 6.3% and a reduction in PCV from 44.9 +/- 1.9% to 21.5 +/- 3.9% (p less than 0.001). Mortality on day 9 was 100%. Antiplatelet serum treatment of plasmodium-infected mice protected against malaria and there was a blunting of the malaria-induced changes in platelet function, EIR and PCV. The values obtained in these rats were significantly higher than those of the untreated malarious mice. The respective values obtained on day 1 were comparable to those of control mice. Data obtained on day 7 were: BT, 106.4 +/- 7.4 s (p less than 0.05), TT, 22.7 +/- 1.1 s (p less than 0.05), EIR, 45.7 +/- 3.2% (p less than 0.01) and PCV, 39.5 +/- 2.0% (p less than 0.01). Mortality on day 9 of infection was 60%. The protection by antiserum was not due to its thrombocytopenic response as busulphan-induced thrombocytopenia did not have the same effect. These results suggest that platelets play an important role in malaria infection as well as in the attendant coagulopathic complications.     

Effect of mefloquine on electrocardiographic changes in uncomplicated falciparum malaria patients.

Asymptomatic sinus bradycardia and sinus arrhythmia have been noted in malarial patients receiving mefloquine. The present study was designed to assess the effect of mefloquine on electrocardiogram in malarial patients. The study was carried out in 102 acute uncomplicated falciparum malarial patients who were treated with mefloquine 750 mg and 1,250 mg regimens and 18 healthy male volunteers receiving 750 mg of mefloquine. Electrocardiogram was performed at intervals after mefloquine administration for a period of 42 days. PR, QRS, QT, QTc, RR interval and cardiac arrhythmia were measured and read by EKG-analyser with confirmation by a cardiologist. Sinus bradycardia and sinus arrhythmia were found in 61.8% and 45.1%, respectively during the first week after treatment. The patients' heart rate decreased significantly on day 6 after mefloquine administration when compared to day 0. The occurrence of sinus bradycardia and sinus arrhythmia were significantly higher in malarial patients when compared with findings in healthy subjects of the same age group. It is unlikely that these changes are associated with mefloquine concentration as mefloquine peaks around 12-24 hours after administration. There were no significant changes in PR-interval, QRS-interval and QTc interval on the electrocardiogram. The findings in this study suggest that cardiotoxicity from mefloquine is unlikely. Bradycardia may be the result of autonomic control modulation after resolution of high fever.     

Failure of falciparum malaria prophylaxis by mefloquine in travelers from West Africa

Due to the spread of chloroquine-resistant strains of Plasmodium falciparum in French speaking parts of Africa, we have found it necessary to prescribe mefloquine for antimalaria prophylaxis to travelers to this area. Weekly doses of 125 or 250 mg have been recommended for short journeys. In spite of this regimen, 16 documented cases of falciparum malaria in travelers have been recorded in the Bordeaux hospital center since October, 1988. Fifteen of these patients were tourists returning from West African countries, and one was an Ivorian student who had been on vacation to his home country. Nine of these patients were evaluated and found to have high plasma mefloquine levels. This report strongly supports the existence of mefloquine-resistant falciparum malaria in West Africa, especially in Sierra Leone, Burkina Faso, and Cote d'Ivoire.     

A clinical trial of mefloquine in the treatment of Plasmodium vivax malaria

A clinical field trial was conducted to determine if mefloquine is effective in the treatment of malaria due to Plasmodium vivax. Forty patients with P. vivax malaria were treated with either mefloquine, chloroquine or chloroquine plus primaquine and followed for 28 days. All patients responded rapidly and were cured. There were no significant side effects.