Pharmacokinetics of prophylactic mefloquine

The pharmacokinetics of the prophylactic dose of mefloquine (Lariam: 500 mg every 4 weeks, with a loading dose of 750 mg on the first week) was studied in six healthy Thai male volunteers. Mefloquine was well tolerated during the study period of 16 weeks. The only side-effects found were nausea and diarrhea in 2 volunteers after the first dose of mefloquine. The mean minimum concentration of mefloquine at steady state ranged from 290 to 460 ng/ml. The maximum concentration on week 16 after the last dose was 1558 +/- 48 ng/ml at the mean time of 38 +/- 19 hours. The other pharmacokinetic parameters obtained were: absorption half life = 6.6 +/- 3.0 hours; distribution = 5.1 +/- 3.1 days; terminal half life = 12.9 +/- 2.2 days; apparent volume of distribution = 10.5 +/- 2.3 l/kg; area under the concentration-time curve = 26.9 +/- 2.2 mg/dl. Although this prophylaxis regimen is ideal when considering the compliance, the minimum concentration obtained was much too low for optimum therapeutic concentration. We therefore suggest that weekly prophylaxis schedule should be a better regimen as the difference between minimum and maximum mefloquine concentration would be smaller.

Double blind randomised clinical trial of two different regimens of oral artesunate in falciparum malaria

A double blind randomized comparative trial of the efficacy of 7-day and 5-day courses of oral artesunate at 600 mg was studied in 89 Thai patients with uncomplicated falciparum malaria. Eighty patients completed the 28-day follow-up period. Artesunate was found to be well tolerated in either regimen. There was an increase of 7% in the cure rate obtained from a 7-day regimen. In 43 patients with a 7-day regimen, the cure rate was 92.5% and 15 patients showed P. vivax in their peripheral blood between days 12 and 34. The mean fever and parasite clearance times were 20 and 40 hours, respectively. In 46 patients with a 5-day regimen, the cure rate was 85% and 8 patients showed P. vivax during days 13 and 24. The mean fever and parasite clearance times were 29 and 40 hours, respectively. Although the cure rates of oral artesunate were high in both regimens, the efficacy was considered unsatisfactory since the aim of the treatment is to achieve 100% cure rate. We suggest however that the extension of the duration of treatment to 7 days together with the increase in total dose may improve therapeutic efficacy of artesunate in falciparum malaria.

Double blind randomised clinical trial of oral artesunate at once or twice daily dose in falciparum malaria

A double blind randomised comparative trial of the efficacy of daily dose (200 mg as an initial dose followed by 100 mg daily for another 4 days) and twice daily dose (100 mg 12 hourly for 2 doses on the first day, followed by 50 mg 12 hourly for another 8 doses) regimens of oral artesunate at 600 mg was studied in 59 Thai patients with uncomplicated falciparum malaria. Fifty patients had a complete 28-day follow-up period. Both regimens produced similar efficacy with no difference in adverse effects. The patients with the daily artesunate regimen had mean fever and parasite clearance times of 20 and 40 hours, respectively. The cure rate was 72%. Eight patients had recrudescence during days 15 to 28 while 8 showed P. vivax in their peripheral blood between days 12 and 21. The patients with the twice daily regimen had mean fever and parasite clearance time of 28 and 40 hours, respectively. The cure rate was 76%. Six patients had recrudescence during days 15 and 27 while 7 showed P. vivax during days 12 and 23. We suggest that the duration of the treatment may be a more important factor determining the efficacy of artesunate rather than the frequency of the doses. Further studies based on pharmacokinetics are therefore needed to improve the cure rate to 100% to prevent the spread of P. falciparum, particularly in areas where there are high numbers of multi-drug resistant strains.

Pharmacokinetics of mefloquine in treatment failure

The pharmacokinetics of mefloquine at the therapeutic dose of 750 mg single orally were compared between cured and recrudescent patients with acute uncomplicated falciparum malaria. Mefloquine was well-tolerated during the study. The side-effects found were nausea, vomiting and diarrhea. Five patients showed R-I and two showed R-II types of response. All recrudescent patients came from the eastern border of Thailand. The time taken to clear the parasites (PCT) was significantly longer in patients with recrudescence (99.6 +/- 36.9 and 63.0 +/- 8.9 hours); however, there was no difference regarding fever clearance time (FCT: 39.0 +/- 16.1 and 31.0 +/- 21.3 hours). The maximum concentration (Cmax) and the concentration on the first and second days in cured patients were significantly higher than those of treatment failure patients. Other pharmacokinetic parameters appeared to be similar in both groups. The present study indicates the existence of mefloquine-resistant falciparum malaria in the eastern border of Thailand. Inadequate mefloquine concentration may play an important role in this aspect. In addition, this study also suggests that Cmax or the concentrations on the first or second day of treatment may be used as guidelines to predict the outcome of treatment.

Clinical trial of artesunate and artemether on multidrug resistant falciparum malaria in Thailand. A preliminary report

A 5-day course of oral artesunate at total doses of 1200, 600, 650 mg and intramuscular artemether 480 mg proved effective (90-100% cured) in the treatment of multidrug resistant falciparum malaria in Thailand. Shorter courses yielded high recrudescence rates. The fever clearance and parasite clearance times were short. The side effects were mild and transient including occasional abnormal electrocardiograms and pain at the injection site. Slight reduction of neutrophil leucocytes and reticulocytes was observed. Further studies of artesunate and artemether should be carried out to find the optimum dosage regimen and to clarify the hematological effects.

Rosette formation of Plasmodium falciparum-infected erythrocytes from patients with acute malaria

Noninfected erythrocytes form rosettes around those infected with trophozoites and schizonts of Plasmodium falciparum in vitro. These rosettes are thought to contribute to the microvascular obstruction which underlies the pathophysiology of severe falciparum malaria. To determine whether the percentage of infected erythrocytes forming rosettes for a parasite isolates in vitro correlates with the in vivo severity of disease, we studied the rosette formation behavior of 35 isolates of P. falciparum from patients with uncomplicated, severe, and cerebral malaria. There was a wide variation in the degree of rosette formation (0 to 53%). Four parasite isolates formed rosettes well (30 to 53%), and seven isolates formed rosettes poorly or not at all (0 to 5%), while the majority of the isolates formed rosettes to various degrees between these two extremes. In this relatively small sample of patients, we were unable to demonstrate a significant association between in vitro rosette formation and patients with cerebral malaria or conscious patients with significant renal (serum creatinine greater than 200 mumol/liter) or hepatic dysfunction (serum bilirubin greater than 50 mumol/liter and aspartate aminotransferase greater than 50 Reitman-Frankel units). However, there was an inverse relationship between rosette formation and cytoadherence (r = -0.575, P less than 0.01) which could not be explained on the basis of steric hindrance. This finding suggests that cytoadherence and rosette formation properties are intrinsic to the parasites, with isolates having a greater propensity for one or the other but not both. Further studies are required to establish the occurrence and pathophysiological role of rosette formation in vivo.

Effect of ampicillin on mefloquine pharmacokinetics in Thai males

The kinetics of a single oral dose of mefloquine given either alone or with ampicillin has been studied in 8 healthy Thai male volunteers. There was a significantly higher maximum whole blood mefloquine concentration after coadministration with ampicillin (1648 vs 1228 ng.ml-1), as well as a significantly reduced terminal half life (15.3 vs 17.7 days), mean residence time (20.1 vs 23.4 days) and volume of distribution at steady state (14.1 vs 19.4 l.kg-1). Although there was no significant change in the AUC from zero time to infinity, the AUC from zero time to 5 days was significantly increased by ampicillin (4.86 vs 3.27 micrograms.ml-1 day). These changes in mefloquine disposition after antibiotic treatment may be due both to an increase in fractional bioavailability and a reduction in the enterohepatic recycling of mefloquine.

Pharmacokinetics of halofantrine in Thai patients with acute uncomplicated falciparum malaria

Twelve patients with acute uncomplicated falciparum malaria were admitted to the Hospital for Tropical Diseases for 42 days. The patients were treated with halofantrine 500 mg 6 hourly for three doses and halofantrine and its desbutyl metabolite were analysed in plasma by h.p.l.c. Cmax values of halofantrine and desbutylhalofantrine (n = 12) were 1192 +/- 410 (mean +/- s.d.) and 397 +/- 160 ng ml-1 with tmax values of 16 +/- 2 and 55 +/- 26 h, respectively. AUC was 60.6 +/- 23.9 and 48.5 +/- 22.2 mg l-1 h, respectively, for halofantrine and its metabolite. Halofantrine cured 83% of the patients but in two patients a reduction only in asexual parasitaemia was seen and no overall parasite clearance occurred. One of these, however had relatively low plasma concentrations of both halofantrine and its desbutyl metabolite and it appeared to be a case of inadequate treatment rather than true resistance. We suggest that the large intersubject variability in plasma drug concentrations may relate in part to its poor and inconsistent bioavailability and this rather than true resistance might be responsible for some of the treatment failures.

Pharmacokinetics of prophylactic mefloquine in Thai healthy volunteers

Malaria constitutes one of the most serious public health problems in Thailand. The disease undermines the health of the people and threatens the economy and security of the country as it is most prevalent in the rural region in forested mountain areas along the border where government officials (border police) have to perform their duties. A safe and effective prophylactic drug for use by these government officials is needed. Nine healthy border police volunteers who were working on the Thai-Cambodia border, aged between 22 to 50 years, and whose weight ranged between 48 and 61 kg, with no history of liver or kidney disease were recruited into the study. 375 mg of mefloquine (as Fansimef tablets) was given as a loading dose, followed by 250 mg every 4 weeks for 4 doses. Whole blood concentrations of mefloquine were measured by high performance liquid chromatography at intervals for 19 weeks. None of the volunteers developed malaria during study period. Seven volunteers had mild adverse effects which required no specific treatment. No changes in liver or renal function or in blood profiles occurred during 19 weeks of observation. Pharmacokinetic analysis revealed a mean maximum concentration of 420 +/- 141 ng/ml a time to peak concentration of 12 +/- 8 hours, terminal half-life was 14.93 +/- 4.43 days, apparent volume of distribution (Vd/f) was 16.5 +/- 5.6 l/kg and total clearance was 0.99 +/- 0.62 ml/min/kg. The mean minimum whole blood mefloquine concentration derived from this study was approximately 100 ng/ml which is considered to be low for treatment.(ABSTRACT TRUNCATED AT 250 WORDS)

Tumour markers in the detection of opisthorchiasis-associated cholangiocarcinoma

Opisthorchiasis is a major public health problem in north-east Thailand, where over 7 million inhabitants are reported to be infected. A significant percentage of infected individuals develops cholangiocarcinoma terminally, which is rapidly fatal. To determine whether certain tumour markers are elevated in Thai patients with cholangiocarcinoma, and thus might be useful in the diagnosis of cholangiocarcinoma associated with opisthorchiasis in Thailand, the tumour markers CA125 and CA19-9 were measured by radioimmunoassay in 94 serum samples. The subjects consisted of 52 patients admitted for non-gastroenterological diseases, 7 patients with histologically proven cholangiocarcinoma, 7 patients with clinically suspected cholangiocarcinoma, and 28 patients with uncomplicated opisthorchiasis. The mean levels (+/- standard deviation [SD]) of CA19-9 and CA125 in the controls were 12.5 +/- 10.2 and 24.7 +/- 11.1 units/ml respectively. Using the mean + 3SD as the cut-off level, 57.1% of patients with confirmed cholangiocarcinoma had elevated CA19-9 and 28.6% had elevated CA125. In patients with clinically suspected cholangiocarcinoma, 71.4% had elevated CA19-9 and 28.6% had elevated CA125. Among opisthorchiasis patients, 3.6% had elevated CA125 and none had elevated CA19-9. 1.9% of controls had elevated CA19-9 and none had elevated CA125. When positivity of either CA125 or CA19-9 was used as an indicator of malignancy, the sensitivity and specificity of the combined tests reached 85.7% and 98.1% respectively. These preliminary results suggest that the measurement of CA125 and CA19-9 may be useful in the early detection of opisthorchiasis-associated cholangiocarcinoma.

Plasma quinine levels in patients with falciparum malaria when given alone or in combination with tetracycline with or without primaquine

Quinine has been an effective drug for severe chloroquine-resistant falciparum malaria. However, there has been a decline in the sensitivity of Plasmodium falciparum to quinine. In 1978-1979 the cure rate was 94% compared to 86% in 1979-1980 and 76% in 1980-1981. The combination of quinine and tetracycline has improved the cure rate to 95-100%. However, the mechanism responsible for this has not been identified. We have compared plasma quinine levels on day 2, day 5 and day 7 (before and at 2 hours after dosing) in twenty-one patients with acute falciparum malaria who were treated with quinine alone (8 patients) or quinine in combination with tetracycline (8 patients) or quinine with tetracycline and primaquine (5 patients). All patients who received combination of quinine and tetracycline with or without primaquine responded well to the treatment with no recrudescence. Two patients who were treated with quinine alone had RI responses. Plasma quinine concentrations from the quinine alone group were significantly lower than those obtained from combination groups on days 2, 5 and 7. The minimal plasma quinine levels from quinine alone group were all lower than MIC, ie below 10 micrograms/ml while those obtained from the combination group were higher than MIC for 7 days. The results from the present study suggest that tetracycline has influence on the maintenance of plasma quinine levels above MIC throughout the treatment period. Therefore, this must be one possible explanation for the better cure rate.

Pharmacokinetics and pharmacodynamics of mefloquine in Thai patients with acute falciparum malaria

A double-blind randomized comparative study of the pharmacokinetics and pharmacodynamics of a single oral dose of 750 mg or 1250 mg of mefloquine was carried out on 20 Thai male patients with acute uncomplicated falciparum malaria. In the 750-mg group, one patient exhibited an RII response, while the others responded to the treatment with a mean fever clearance time of 50.2 +/- 28.2 hours and a mean parasite clearance time of 70.2 +/- 17.3 hours. The main adverse effects were dizziness, nausea, vomiting, abdominal pain, and diarrhoea. Electrocardiogram monitoring detected sinus bradycardia in three patients and sinus arrhythmia in three others. In the 1250-mg group, one patient exhibited an RII response, while the others responded to the treatment with a mean fever clearance time of 43.4 +/- 36.6 hours and a mean parasite clearance time of 73.4 +/- 25.2 hours. However, during the follow-up period, two patients recrudesced on day 23 and on day 31 (RI response). Dizziness, nausea, vomiting, abdominal pain, and diarrhoea were the major adverse effects, with dizziness being more frequent compared with the 750-mg group. Sinus bradycardia occurred in four patients and sinus arrhythmia in four others. The pharmacokinetics of the two regimens were similar, with the absorption of mefloquine increasing linearly with the dose; however, vomiting within an hour of taking the drug reduced the whole blood mefloquine concentrations. The results do not indicate that there is any advantage in using a single dose of 1250 mg of mefloquine rather than 750 mg.

Quinine resistant falciparum malaria treated with mefloquine

Twenty eight adult male patients with acute uncomplicated falciparum malaria which showed RI or RII responses to quinine sulfate at the dosage of 600 mg 8 hourly for 7, 10 or 14 days were treated with a single dose of mefloquine (Lariam); 25 patients received 1000 mg, 2 received 750 mg and 1 received 500 mg. The initial response was good; there was no RII or RIII response. Three patients were lost to followup. Of 25 patients who stayed in the Bangkok Hospital for Tropical Diseases where there was no malaria transmission for 28-65 days, only one patient in the 1000 mg group had recrudescence on day 21. The cure rate was 96%. Our prospective study suggests that mefloquine was effective in the treatment of quinine resistant falciparum malaria and the risk of cross-resistance between quinine and mefloquine in P. falciparum in vivo is very low.

Synergy of antithrombin III concentrate and antivenom in preventing coagulopathy in a rat model of Malayan pit viper envenoming

The effects of unrefined equine antivenom and antithrombin III (AT-III) concentrate on the coagulopathy induced by systemic envenomation by Malayan pit viper (Calloselasma rhodostoma; MPV) venom were investigated in a rat model. 37 rats received an intramuscular injection of MPV venom and serial blood samples were taken from the femoral vein for simple whole blood clotting tests and measurement of AT-III activity. 30 min after venom injection, treatment (antivenom, AT-III or both) was given intravenously. 6 rats were untreated and all developed uncoagulable blood and AT-III depletion 90-210 (median 180) min after venom injection. A combination of high dose AT-III concentrate (0.5 units/g) and antivenom (20 micrograms/g) prevented abnormal clotting (P less than 0.001), whereas AT-III alone, antivenom alone, or a combination of low dose AT-III (0.25 units/g) and antivenom did not (P less than 0.05). These results suggest that the coagulation abnormality in MPV envenomation is secondary to activation of the coagulation cascade at several levels, and that treatment with antivenom alone may not be sufficient to reverse or prevent this phenomenon.

Mefloquine antimalarial prophylaxis in pregnancy: dose finding and pharmacokinetic study

1. A dose finding pharmacokinetic study was performed in 20 Karen women in the third trimester of pregnancy receiving antimalarial prophylaxis with mefloquine. Ten received 250 mg mefloquine base weekly and ten received identical tablets of 125 mg base/week. 2. Both dose regimens were well tolerated. Malaria was prevented effectively, there were no serious adverse effects, all pregnancies proceeded normally, and there were no abnormalities in the babies followed up to 2 years. 3. The median time from dose administration to peak whole blood mefloquine concentration was 6 (range 3-24) h. Mean (+/- s.d.) peak and trough concentrations in the seventh week were 722 +/- 279 and 488 +/- 155 ng ml-1 with the 250 mg/week dose, and 390 +/- 81 and 185 +/- 53 ng ml-1 with the 125 mg/week dose regimens respectively. These blood concentration values are lower than those reported previously in non-pregnant adults. 4. One and two compartmental models were fitted to the whole blood concentration-time data. Mean (+/- s.d.) clearance (CL/F) was 0.78 +/- 0.27 ml min-1 kg-1, and the apparent terminal elimination half-life (t1/2) was 11.6 +/- 7.9 days. 5. Further studies to determine the oral bioavailability of mefloquine are needed, but these results suggest that clearance may be increased in late pregnancy. These preliminary results of good efficacy without significant toxicity are encouraging, and a more extensive evaluation of mefloquine antimalarial prophylaxis in pregnancy is now warranted.

Immunological analysis of Opisthorchis and Clonorchis antigens

Immunoreactive components of Opisthorchis viverrini and Clonorchis sinensis were analysed by enzyme-linked immunosorbent assay (ELISA), radioimmunoprecipitation and immunoblotting. Somatic extracts from these two liver flukes as well as from other related parasites, together with the metabolic products, were tested for their reactivities with sera from patients with opisthorchiasis and clonorchiasis. A significant cross-reactivity in the ELISA was noted between Opisthorchis and Clonorchis. Immunoblotting and radioimmunoprecipitation analyses showed that the 89-kD protein which was previously shown to be a predominant metabolic product of O. viverrini reacted with sera from both groups of patients. However, an antigen with a molecular weight of 16 kD, apparently a predominant somatic component, appeared to be specific for O. viverrini.

Characterization of a monoclonal antibody that neutralizes the hyaluronidase activity of Russell's viper venom

Three monoclonal antibodies (WPN1, WPN2 and WPN3) raised against a partially purified fraction of Russell's viper venom (RVV) were characterized. All three monoclonal antibodies reacted with crude RVV when tested by ELISA, but only two (WPN1, WPN2) neutralized its hyaluronidase activity. WPN1 was the more potent and was effective at an antigen: antibody ratio of 1:3. Furthermore, WPN1 was shown to recognize only the 14,000 MW component of crude RVV. This has been identified in a previous study to be hyaluronidase. This antibody was also found to recognize some components of Calloselasma rhodostoma venom which also possesses potent hyaluronidase activity. The potential therapeutic role of antibodies that neutralize the hyaluronidase component of snake venoms should be investigated further.

Fecal egg output in relation to worm burden and clinical features in human opisthorchiasis

A study of the relation of fecal egg excretion to worm burden and clinical features was carried out in 45 opisthorchiasis patients who had no signs of biliary obstruction. The fecal egg excretion was consistent and correlated with the worm burden. Although there was no definite association between clinical signs and intensity of infection, mild hepatomegaly and thickened wall or dilatation of the gallbladder were found more commonly in heavily infected patients. Eosinophilia was observed more often than previous reports. Concomitant parasitic infections were found in 82% of the patients. After praziquantel treatment, egg counts increased greatly during the first few days then decreased to very low levels in 7 days.

Fatal Plasmodium falciparum malaria after an inadequate response to quinine treatment

A 24-year-old man with severe Plasmodium falciparum malaria died after 77 h of treatment with full parenteral doses of quinine. His peripheral parasitemia at death exceeded the level on admission. Plasma concentrations of quinine were abnormally low throughout. This case emphasizes the importance of pharmacokinetic factors in determining the therapeutic response in severe P. falciparum malaria.

Splenic Fc receptor function in host defense and anemia in acute Plasmodium falciparum malaria

To determine splenic Fc receptor function in patients with acute Plasmodium falciparum malaria, the clearance of IgG-coated autologous 51Cr-labeled erythrocytes in 20 patients and 10 normal controls was studied. Clearance half-times were directly correlated with both the absolute parasite count (r = .635, P less than .005) and hematocrit (r = .791, P less than .001). Clearance half-times in patients varied from 1.0 to 96.3 h (median, 14.8 h) while those of controls ranged from 8.0 to 80.3 h (median, 23.1 h) (P = .10). Nine of the 20 patients had clearance half-times shorter than the lower 95% confidence limit of controls (less than 12.4 h). The clearance of IgG-coated erythrocytes was accelerated after parasites were eliminated from the circulation (P less than .05) and returned toward normal 6-8 weeks after the acute infection. Although circulating immune complexes were detectable, there was no correlation between immune complex levels and clearance half-times (P greater than .05). The failure to increase Fc receptor-mediated red cell clearance in patients with high parasitemias suggests inadequate splenic phagocytic activity in the face of considerable antigenic challenge. These findings indicate that splenic Fc receptor function may be important both in the control of infection and the development of anemia in P. falciparum malaria.

Pharmacokinetics of mefloquine in combination with sulfadoxine-pyrimethamine and primaquine in male Thai patients with falciparum malaria

The pharmacokinetics of mefloquine (M) were studied in 59 male Thai patients with falciparum malaria. Mefloquine was administered alone (750 mg orally; group 1), or with primaquine (PQ, 45 mg; group 2), or in combination with sulfadoxine (1.5 g) + pyrimethamine (75 mg) (MSP; group 3), or as MSP + PQ (group 4). All patients in groups 1, 2 and 4 initially responded to treatment, but two patients from group 1 had RI recrudescent infections. One patient in group 3 failed to respond to treatment and was considered to have RII resistance, while a further patient from this group had RI recrudescence. The pharmacokinetic parameters for group 1 and group 3 were not significantly different. Co-administration of primaquine alone had no significant effect on the pharmacokinetics of mefloquine, but there was a statistically significant decrease in the terminal elimination half-life of mefloquine for group 4 relative to that for group 3.

Activation of the coagulation cascade in falciparum malaria

The incidence and progression of coagulation abnormalities were studied in 52 patients with acute falciparum malaria. The patients were prospectively divided into 3 groups; severe (parasitaemia greater than or equal to 5% or vital organ dysfunction), 12 patients; moderate (parasitaemia 1%- less than 5% without complications), 16 patients; and mild (parasitaemia less than 1%), 24 patients. No case died or developed clinical evidence of disseminated intravascular coagulation. Conventional indices of coagulation (prothrombin time, partial thromboplastin time, fibrinogen, fibrin degradation products) were usually within the normal range but reduced plasma concentrations of antithrombin III (AT-III) levels were noted in all groups, and the incidence was significantly higher in patients with severe and moderate malaria (83% and 81%) compared with the mild group (37%; P less than 0.005). Depletion of AT-III was associated with thrombocytopenia, decreased AT-III activity and elevated plasma concentrations of thrombin-antithrombin III complexes (P less than 0.01), confirming activation of the coagulation cascade and increased clotting factor consumption. AT-III levels returned to normal coincident with clinical improvement. Activation of coagulation is a common and sensitive measure of disease activity in acute falciparum malaria. It is not a specific feature, nor is there evidence to suggest it has a primary pathological role in severe infections.

Electrophoretic variants of enzymes in isolates of Plasmodium falciparum, P. malariae and P. vivax from Thailand

A new electrophoretic variant of glucose phosphate isomerase (GPI), which we now denote GPI-3, has been found in isolates of Plasmodium falciparum from 6 patients, all of whom acquired the infection in the same region (in or near Prachinburi province) of Thailand. In other regions, from which 453 isolates have been tested, only GPI-1 and/or GPI-2 have been found. Two isolates of P. malariae from patients at Kanchanaburi showed a band of GPI activity on cellulose acetate gels at a cathodal position quite distinct from that of any previously known GPI variants in other human malaria parasites. Thirty-nine isolates of P. vivax from 3 regions of Thailand have been examined for variants of GPI and lactate dehydrogenase (LDH). Three forms of GPI were found, corresponding approximately in band positions to GPI-1, 2 and 3 of P. falciparum. The position of the band of LDH activity in P. vivax was the same in all the isolates examined, and different from that of LDH-1 in P. falciparum.