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

Pharmacokinetics of mefloquine in combination with sulfadoxine-pyrimethamine and primaquine in male Thai patients with falciparum malaria J. Karbwang,1 D.J. Back,2 D. Bunnag,3 & A.M. Breckenridge4 The pharmacokinetics of mefloquine (M) were studied in 59 male Thai patients with falciparum malaria. Mefloquine was administered alone (750mg orally; group 1), or with primaquine (PQ, 45mg; group 2), or in combination with sulfadoxine (1.5g)+ pyrimethamine (75mg) (MSP; group 3), or as MSP+ P0 (group 4). All patients in groups 1, 2 and 4 initially responded to treatment, but two patients from group 1 had RI recru- descent infections. One patient in group 3 failed to respond to treatment and was considered to have RII resistance, while a furtherpatient from this group hadRI recrudescence. Thepharmacokineticparameters for group l andgroup 3 were not significantly different. Co-administration ofprimaquine alone had no significant effecton the pharmacokinetics ofmefloquine, but there was a statistically significantdecrease in the terminal elimination half-life of mefloquine for group 4 relative to that for group 3. Introduction Mefloquine is an effective treatment for multidrug- resistant malaria (1-5) however, despite its wide- spread use in Thailand since 1984 there are a number of aspects of its pharmacokinetics that are poorly understood. Looareesuwan et al. have shown that the peak mefloquine concentrations in Thai patients with acute falciparum malaria who received a 250-mg dose were approximately three times higher than in healthy Caucasian volunteers; also, the apparent volume of distribution was smaller and the terminal half-life was significantly shorter in the Thai patients (6). However, it was not possible to determine whether ethnic or disease-related factors were responsible for these dif- ferences. Recently Karbwang et al. studied the kinetics of a single oral dose of mefloquine (750 mg) in Thai patients with falciparum malaria and compared the results with those ofa previous study involving healthy Thai volunteers (7,8). For patients and controls there were no significant differences in the peak plasma concentrations of mefloquine, time to peak concentra- tion, area under the concentration-time curve, or apparent volume of distribution; however, the terminal half-life was significantly shorter in the patients. This Lecturer, Faculty of Tropical Medicine, Mahidol University, Bangkok, Thalland. I Reader, Department of Pharmacology and Therapeutics, Uni- versity of Liverpool, New Medical Building, Ashton Street, P.O. Box 147, Liverpool L69 3BX, England. Requests for reprints should be sent to this address. ' Professor, Faculty of Tropical Medicine, Mahidol University, Bangkok, Thailand. 4Professor, Department of Pharmacology and Therapeutics, University of Liverpool, Liverpool, England. Reprint No. 5118 study suggested therefore that malaria increased the rate of elimination of mefloquine, although the mech- anism of the changes produced was not clear. Mefloquine is currently marketed in combination with sulfadoxine-pyrimethamine, and Karbwang et al. have shown that in healthy Thai volunteers the com- bination mefloquine plus sulfadoxine-pyrimethamine has a slightly longer terminal half-life and mean residence time than mefloquine alone (8). It is important to ascertain whether this is also the case for patients with falciparum malaria. Finally, in malaria clinics in Thailand mefloquine is used in conjunction with primaquine and there is therefore the potential for pharmacokinetic interac- tion between these two antimalarials. This is especially significant in the light of reports that primaquine inhibits hepatic microsomal enzymes both in vitro and in vivo in animals (9-13) and humans (14-15). The present study reports on the pharmacokinetics of mefloquine when used in combination with sulfa- doxine-pyrimethamine and/or primaquine. Materials and methods Patients Adult (> 15 years of age) male patients with acute falciparum malaria (asexual forms of Plasmodium falciparum evident in blood smears) were included in the study. The patients were admitted to the Hospital for Tropical Diseases, Bangkok, Thailand, and their written informed consent was obtained. The study was approved by the Ethics Committee of the Faculty of Tropical Medicine, Mahidol University. Patients were excluded if they had a history of recent antimalarial treatment; a history of gastro- intestinal disease with malabsorption or previous Bulletin of the World Health Organization, 66 (5): 633-638 (1990) © World Health Organization 1990 633 J. Karbwang et al. surgery to the upper gastrointestinal tract; asexual parasitaemia of >50%; or impaired consciousness, jaundice, oliguria, or vomiting that required parenteral treatment before starting antimalarial therapy. Patients were also excluded if chloroquine (Wilson and Dill- Glazko tests) or sulfadoxine (lignin test) was detected in their urine. Pretreatment blood samples that con- tained either mefloquine or quinine (as estimated by high-performance liquid chromatography (HPLC)) were excluded from the data analysis. The patients were examined clinically in the hospital prior to commencing therapy and the data obtained were recorded on standard forms; the exam- ination included body weight, height, and temperature. Baseline laboratory investigations included parasite counts, complete blood examination, determination of serum chemistry, as well as screening tests for plasma quinine and mefloquine. Treatment groups The study was an open, randomized trial involving recruitment into the groups outlined below. * Group 1: 750 mg mefloquine (M) (base tablets each containing 250 mg mefloquine). * Group 2: 750mg mefloquine+ 45 mg primaquine (PQ). * Group 3: MSP (750 mg mefloquine, 1500 mg sulfa- doxine (S), 75 mg pyrimethamine (P)). * Group 4: MSP (as above)+ 45 mg PQ. The drugs were administered as single doses. Study design Blood samples were collected using an indwelling intravenous Teflon catheter kept patent with hepari- nized saline. The samples were taken pre-dose, and at 1,2,4, 8, 12,24,48,72, and 96 hours, as well as at 7, 14, 21, 28, and 42 days after the dose. Samples were collected into heparinized tubes and the plasma sepa- rated within 30 minutes; this was stored in plastic tubes at -20 °C until analysed. Parasite counts were made twice daily until para- sitaemia had cleared, then daily until 28 days, and again on day 42. Patients' temperatures were measured every 4 hours. A full blood examination and determination of serum chemistry were carried out on days 1,4,7,14,28, and 42. The clinical examinations were performed daily for 7 days, then on days 14, 21, 28, and 42. Determination of metloquine concentration Mefloquine was determined by HPLC using the method described by Riviere et al. (16). The lower limit of detection of the assay, which was defined as the minimum concentration that could be determined with a precision of better than 10%, was 20 ng/ml. The inter-assay coefficient of variation was 4.1% at a con- centration of 100 ng/ml and 5.7% at 600 ng/ml. Mefloquine pharmacokinetic analysis The peak concentration of mefloquine (C,,,,) and the time to peak concentration (t,,,,) were obtained. The area under the plasma concentration-time curve (AUC) was calculated using the linear trapezoidal rule. The estimated area for the last sampling time to t0, the first-order elimination rate constant 4k and the half-life (t112) were calculated using conventional methods (17). The mean residence time of mefloquine in the body (MRT) was calculated using the expression: MRT= { tC dt/ f Cdt where t is time (in days) and C is the plasma concentration of mefloquine. The apparent volume of distribution (V.) was calculated using the expression: Vz=f x Dose x t,12/(AUC x 0.693) The clearance (Cl) was calculated by dividing the dose by the AUC. Since the bioavailability (f) of mefloquine was not known, values for Vz/f and Cl/f were obtained. Statistkal analysis All the pharmacokinetic parameters were analysed using a two-factor analysis of variance. Since all patients received mefloquine, the factors were the two (additional) drug combinations (MSP and MSP + PQ) and their interaction. Most of the variables measured had skewed distributions and for these, log-transformed data were used. Significance levels were obtained using Student's t-tests and confirmed using an unpaired Wilcoxon's rank sum test. The results are presented as means + standard deviations. Results Clinical and parasitological responses A total of 59 male patients were studied. All reported a history of fever lasting 1-3 days and all but one patient was febrile (Table 1). Altogether, 57 patients were followed up for 42 days; one patient (from group 2) was followed up for 21 days; and one (from group 3) for 29 days. In group 1 (mefloquine alone) all patients re- sponded to treatment, with mean fever and parasite clearance times of 38.0+ 20.2 hours and 66.0+14.1 WHO Bulletin OMS. Vol. 68 1990.&U4 Pharmacokinetics of mefloquine In combination with sulfadoxine-pyrimethamine and primaquine Table 1: Results of the baseline laboratory Investigation of patlents In the various treatment groups on admission to the study Mean +S.D.' Mefloquine Mefloquine + PQ MSP MSP+ PO (group 1) (group 2) (group 3) (group 4) Weight (kg) 52.0+4.6 54.8+3.3 56.7+2.9 53.9+4.3 Temperature (°C) 38.3+0.9 38.8+0.9 38.4+0.8 37.8±0.7 Haematocrit (%) 35.6+7.3 36.2+5.4 37.6+6.7 33.5+6.0 White blood cell count (x 109/1) 5496±2038 6171 +1805 5649±1186 5476±2261 Parasitaemia (x 109/l) 19 076 (3850-185 850)b 24 223 (4900-188 160) 17 745 (3380-193 900) 15 330 (4470-36 680) Serum bilirubin (mg/dl) 1.1+0.4 1.1 +0.6 0.9+0.2 1.0+0.6 Serum creatinine (mg/dl) 1.2+0.7 1.0+0.2 1.1+0.2 1.0+0.1 ' M = mefloquine; MSP = mefloquine + sulfadoxine-pyrimethamine; M + PQ = mefloquine + primaquine; MSP + PQ= mefloquine + sulfa- doxine-pyrimethamine + primaquine. b Figures in parentheses are the range. hours, respectively. Two patients who had no history of vomiting after taking mefloquine, underwent re- crudescence on day 21 and day 32 (symptomatic cases); their plasma concentrations of mefloquine are shown in Table 2. All patients in group 2 (M + PQ) responded to treatment, with mean fever and parasite clearance times of 47.3 + 17.9 hours and 65.5 + 16.2 hours, respectively. In group 3 (MSP), all but one patient responded to treatment, with mean fever and parasite clearance times of 55.7 + 28.4 hours and 73.6 + 36.9 hours, res- pectively. The patient who failed to respond to treat- ment was considered to have type II resistance and data for this patient were excluded from the analysis of fever and parasite clearance times. This patient had asymptomatic parasitaemia until day 17, when a second dose of MSP was administered. Among patients who responded initially to treatment, one also had recru- descence on day 21. The plasma concentrations of mefloquine for these two patients are shown in Table 2. All patients in group 4 (MSP + PQ) responded to treatment, with mean fever and parasite clearance times of 54.4 + 34.6 hours and 60.1 + 7.0 hours, respectively. Adverse effects Adverse effects were monitored daily by administering a questionnaire for 1 week, and then weekly until day 42 (6 weeks). The clinical examinations, serum chemistry profiles, and blood counts were normal from day 7 onwards. Three patients from group 1, two from group 2, and four from group 3 vomited after taking the medication. The peak plasma concentrations ofmeflo- quine and time of vomiting are shown in Table 3. One patient from group 4 had diarrhoea. No other adverse effects were observed. All episodes ofvomiting occurred 1 hour or later after taking the drugs, with the exception ofone patient who vomited after 30 minutes. The latter patient had the lowest peak plasma concen- tration in the study. Pharmacokinetics of mefloqulne Selected pharmacokinetic parameters for mefloquine are shown in Table 4. There was a considerable variation in the peak plasma concentrations within each group (group 1, 1591-3904 ng/ml; group 2, 1095-3754 ng/ml; group 3, 766-4513 ng/ml; group 4, Table 2: Plasma concentrations of mefloquine In the four patients with recrudescent infectlons Peak mefloquine Mefloquine concentration at Treatment concentration Day of recrudescence the time of recrudescence group' (ng/ml) (days after treatment) (ng/ml) Mefloquine (RI) 2437 D21 361 Mefloquine (RI) 2663 D32 270 MSP (RII)b 1663 401 MSP (Rl)C 2369 D21 301 "The type of recrudescence is shown in parentheses; MSP=mefloquine+sulfadoxine-pyrimethamine. b Patient vomited 2.5 hours after receiving the therapy. I Patient vomited 6 hours after receiving the therapy. WHO Bulletin OMS. Vol. 68 1990. 635 J. Karbwang et al. Table 3: Peak plasma concentrations of mefloqulne In patients who vomited aftr recelving the treatment Peak plasma Treatment concentration Time of vomiting group' (ng/ml) (hours' post-dosing) Mefloquine 1592 1 Mefloquine 1856 1 Mefloquine 3084 3 Mefloquine+ PO 1095 7 Mefloquine + PQ 1534 30 MSP 1663b 2.5 MSP 1483 1.5 MSP 766 0.5 MSP 236C 6 a PQ= primaquine; MSP= mefloquine +sulfadoxine-pyrimethamine. Treatment failure (RII). CTreatment failure (RI). 1213-4282 ng/ml). All groups exhibited inter- individual variation in the time to peak concentration (4-48 hours). The differences between the peak plasma concentrations and times to peak concentration were not significantly different for the various groups. There were no significant differences in the elimination half-life (t,J2), elimination rate constant(A4), area under the curve (AUC) mean residence time (MRT), apparent oral clearance (Cl/f) and apparent volume ofdistribution (V/f ) for patients who received mefloquine alone versus those who received MSP alone. Co-administration of primaquine alone had no significant effect on the pharmacokinetics of meflo- quine but there was a statistically significant decrease in the elimination half-life (t,,2 = 10.4+ 1.9 days) for group 4 (MSP+ PQ) compared with that for group 3, who received MSP alone (12.7±2.1 days; P<0.005). However, the other pharmacokinetic parameters were not significantly different. Discussion Four patients showed resistance to mefloquine- two in group 1 (mefloquine alone) and two in group 4 (MSP). The peak concentrations of mefloquine in these patients lay within the range exhibited by those who responded to the drug. It can therefore reasonably be claimed that these four patients had mefloquine- resistant falciparum malaria. The cure rate found in the study was 93.2%, a good response to mefloquine. The patient who gave an RII response to treat- ment had a mefloquine plasma level of401 ng/ml prior to receiving a second dose, but the results from the in vitro sensitivity test for mefloquine estimated the minimum therapeutic concentration to be 0.1 pmole/pl (40 ng/ml). The minimum concentration of meflo- quine required to cure chloroquine-resistant falciparum malaria in Thailand remains uncertain. Indeed, it is difficult to define such a level in view of confounding factors, such as immunity; further studies, particularly Table 4: Selected pharmacokinetic parameters for mefloqulne for the study subjects In the various treatment groups Mean +S.D.a Mefloquine Mefloquine+PO MSP MSP + PQ (group 1) (group 2) (group 3) (group 4) (n= 15) (n= 14) (n= 16) (n= 14) Time to peak concentration (hours) 16.9±13.2 14.1 +8.1 19.0±13.3 23.4+14.7(6-48)b (4-24) (6-48) (8-48) Peak concentration (ng/ml) 2690±672 2303+854 2559±1107 2756+ 1047 (1591-3904) (1095-3754) (766-4513) (1213-4282) Half-life (days) 11.7±2.0 11.4+1.3 12.7+2.1 10.4+1.9c(8.1-15.8) (8.9-13.9) (9.5-16.9) (7.0-12.9) Elimination rate constant (day") 0.061±0.010 0.061+0.007 0.056±0.010 0.069±0.015 (0.044-0.085) (0.050-0.078) (0.041-0.073) (0.054-0.099) Area under the curve (ug/ml x days) 27.0 ± 8.2 24.9 + 9.9 24.3+ 8.7 25.6± 8.7 (14.3-43.7) (13.4-44.8) (12.9-38.1) (12.7-46.4) Mean residence time (days) 16.3±3.7 15.5+1.8 16.4±3.2 14.1 ±3.3 (11.6-23.3) (11.8-18.2) (12.3-22.6) (9.2-21.4) Vzlf (litres) 500±135 587+265 667±322 511+246 (322-791) (285-973) (288-1348) (238-1089) Cl/f (I.day-') 30.6+10.0 34.9+13.7 35.7+14.1 33.9±13.3 (17.1-52.4) (16.8-56.0) (19.7-58.1) (16.2-60.9) a MSP= mefloquine + sulfadoxine-pyrimethamine; PO = primaquine. Figures in parentheses are the range. c Significantly different from MSP alone; P< 0.005. WHO Bulletin OMS. Vol. 68 1990.en6 Pharmacekinetics of mefloqulne In combination with sulfadoxinepyrimethamine and primaqulne of all cases of therapeutic failure are required to elucidate this. Of the patients who vomited after taking the medication, all except two responded to treatment. It is interesting that only one patient had an atypically low peak plasma concentration of mefloquine after vomit- ing, which suggests that a second dose of the drug under such circumstances may not always be necessary. Since in most cases vomiting occurred within a few hours of taking the drugs, the most likely explanation is local gastric irritation. Further studies, specifically designed to study the relationship of vomiting to the absorption of mefloquine, are, however, needed to provide guidelines for deciding when repeat dosing is necessary. The finding that there were no significant differ- ences in the pharmacokinetic parameters for male patients who received mefloquine alone and MSP alone indicates that the presence of sulfadoxine and pyrimethamine do not significantly influence the pharmacokinetics of mefloquine. This contrasts with the results of a study of Thai male volunteers, for whom the group that received MSP exhibited a 24% increase in the half-life of mefloquine and a 27% increase in the MRT compared with the group given mefloquine alone; there is no clear explanation for this difference between the patient and volunteer studies. It is probably more important to establish first the basis of the clear difference in the elimination kinetics of mefloquine between patients and volunteers. The half-life of mefloquine for the male volunteers was 15.4+0.9 days (7), whereas for the patients in the present study it was 11.7 + 1.9 days- a 25% decrease for the patients. IfMSP was used instead ofmefloquine alone, the respective half-lives were 19.1+4.4 days and 12.7+ 2.1 days for volunteers and patients, respectively. An examination of the possible role of enterohepatic recycling in the elimination of mefloquine should be carried out, since this mechanism may be altered in malaria patients. It is very interesting to note that in dogs mefloquine does not undergo enterohepatic re- cycling and that in dogs the half-life is shorter than in other animals or humans (G. Friedrich, personal communication, 1988). Co-administration of primaquine caused a statis- tically significant decrease (P <0.05) in the elimina- tion half-life of mefloquine in group 4 (MSP+ PQ; t12=10.4+ 1.9 days) compared with that for group 3 (MSP alone; 12.7 + 2.1 days). This may have arisen because of the ability of primaquine to inhibit the metabolism of sulfadoxine, thus maintaining its con- centration for longer in the body. Sulfadoxine could then interfere with the normal composition of bacterial flora, thus interrupting the enterohepatic recycling of mefloquine. The concentrations of mefloquine metabolites in the different groups were not investigated. Recently Franssen et al. showed that the plasma concentrations of the carboxylic acid metabolite of mefloquine were 2-3 times greater than those ofmefloquine itselfwithin 2 days in healthy Caucasian volunteers. However, it is unlikely that the metabolite contributes to therapeutic response (based on in vitro IC5o determinations with three strains of P. falciparum (18)); it is therefore of considerable importance to determine whether the metabolite contributes to the side-effects (19). A further consideration is whether the sulfa- doxine-pyrimethamine component confers any advantage over mefloquine alone, in view of the potential for serious sulfonamide toxicity (20). If co-administration of sulfadoxine-pyrimethamine delays the development of resistance, as has been shown in rodent malaria (21), its benefits may override its potential toxicity. Recently, however, White has argued that the prevention of mefloquine resistance in falciparum malaria by sulfadoxine-pyrimethamine is unlikely, since mefloquine needs to be protected when malaria parasites encounter sub-inhibitory blood con- centrations, i.e., many weeks after single-dose treat- ment (22). This does not happen because in humans sulfadoxine and pyrimethamine have much shorter half-lives than in rodents. Thus, a low concentration of mefloquine persists in the blood for long periods, unprotected by the other drugs. Serious consideration therefore has to be given to the benefit:risk ratio for patients who receive MSP rather than mefloquine alone. Acknowledgements We are grateful to Dr N.J. White for his comments on the manuscript. This study was supported by the UNDP/World Bank/WHO Special Programme for Research and Training in Tropical Diseases (Project number 860363). R6sum6 Pharmacocin6tique de Ia m6floquins en assoclation avec Ia sulfadoxinepyrlm6tha- mine et Ia primaquine chez des Thailan- dais atteints de paludisme A faiciparum La pharmacocin6tique de la m6floquine (M) a et6 6tudi6e chez 59 Thaflandais de sexe masculin atteints de paludisme & faiciparum. Elle etait administree soit seule (750mg par voie orale, groupe 1), soit avec de la primaquine (PQ, 45 mg, groupe 2), soit en association avec de la sulfado- xine (1,5 g) et de la pyrim6thamine (75 mg) (MSP, groupe 3), soit encore en association multiple WHO Bulletin OMS. Vol. 68 1990. on7 J. Karbwang et al. MSP+PQ (groupe 4). Tous les sujets des groupes 1, 2 et 4 r6pondaient au traitement, bien qu'on ait observe chez deux malades du groupe 1 une recrudescence, le 21' jour chez l'un et le 32' jour chez l'autre. L'absence de reponse au traitement chez un sujet du groupe 3 a ete attribu6e a une r6sistance de type 11; un autre malade de ce groupe a presente une recrudescence au 21' jour. Bien que neuf malades aient vomi apr6s avoir pris I'antipaludique, leur pic plasmatique de meflo- quine ne diff6rait pas sensiblement de celui des autres malades. On n'a observ6 aucune dif- ference significative de la demi-vie d'elimination, de la constante d'elimination (A.), de l'aire sous la courbe, du temps de residence moyen, de la clairance orale apparente, du volume apparent de distribution, du pic de concentration, ni du delai d'apparition du pic de concentration chez les sujets ayant re,u la mefloquine+PQ ou I'associa- tion MSP. L'administration simultan6e de prima- quine etait sans effet significatif sur la phar- macocin6tique de la mefloquine; en revanche, on observait une baisse statistiquement significative de la demi-vie d'elimination chez les sujets du groupe MSP+PQ (tl,,=10,4+1,9 jours) par rapport au groupe MSP sans PQ (t112=12,7+2,1 jours; PO0,005); les autres parametres cinetiques ne differaient pas notablement entre ces deux groupes. Cette difference pourrait s'expliquer par I'aptitude de la primaquine a inhiber le m6tabo- lisme de la sulfadoxine, ce qui permettrait le main- tien de concentrations 6lev6es de cette derniere, et par suite freinerait le recyclage enterohepatique de la mefloquine. References 1. Rozman, R.S. & Canfleld, C.S. New experimental antimalarial drugs. Advances in pharmacology and chemotherapy, 16: 1-43 (1979). 2. Sweeney, J.R. The present status of malarial chemo- therapy: mefloquine, a novel antimalarial. Medical research review, 1: 281-301 (1981). 3. Harlnasuta, T. et al. A phase 11 clinical trial of meflo- quine in patients with chloroquine-resistant falci- parum malaria in Thailand. Bulletin of the World Health Organization, 61: 299-305 (1983). 4. De Souza, J.M. A phase I clinical trial of mefloquine in Brazilian male subjects. Bulletin ofthe World Health Organization, 61: 809-814 (1983). 5. De Souza, J.M. A phase I clinical trial of mefloquine in Brazilian male subjects. Bulletin ofthe World Health Organization, 61: 815-820 (1983). 6. Looareesuwan, S. et al. Studies of mefloquine bio- availability and kinetics using a stable isotope tech- nique: a comparison of Thai patients with falciparum malaria and healthy Caucasian volunteers. British journal of clinical pharmacology, 24: 37-42 (1987). 7. Karbwang, J. et al. A comparison of the pharmaco- kinetics of mefloquine in Thai healthy volunteers and in patients with falciparum malaria. European journal of clinical pharmacology, 35: 677-680 (1988). 8. Karbwang, J. et al. The pharmacokinetics of meflo- quine when given alone or in combination with sulpha- doxine and pyrimethamine in Thai male and female subjects. European journal of clinical pharmacology, 32: 173-177 (1987). 9. Back, D.J. et al. Inhibition of drug metabolism by the antimalarial drugs chloroquine and primaquine in the rat. Biochemical pharmacology, 32: 257-263 (1983). 10. Murray, M. et al. In vitro effects of quinoline derivatives on cytochrome PA450 and aminopyrine N-demethylase activity in rat hepatic microsomes. Biochemical phar- macology, 33: 3277-3283 (1984). 11. Mihaly, G.W. et al. The effects of primaquine stereo- isomers and metabolites on drug metabolism in the isolated perfused rat liver and in vitro rat liver micro- somes. Biochemical pharmacology, 34: 331-336 (1985). 12. Rlvlere, J.H. & Back, D.J. Effect of mefloquine on hepatic drug metabolism in the rat: comparative study with primaquine. Biochemical pharmacology, 34: 567- 571 (1985). 13. RIvIere, J.H. & Back, D.J. Inhibition of ethinyloestradiol and tolbutamide metabolism by quinoline derivatives in vitro. Chemical and biological interactions, 59: 301-308 (1986). 14. Back, D.J. et al. Effect of chloroquine and primaquine on antipyrine metabolism. British journal of clinical phar- macology, 16: 497-502 (1983). 15. Back, D.J. et al. In vitro inhibition studies of tolbutamide hydroxylase activity of human liver microsomes by azoles, sulphonamides and quinolines. British journal of clinical pharmacology, 26: 23-29 (1988). 16. RIvIere, J.H. et al. The pharmacokinetics of mefloquine in man. Lack of effect of mefloquine on antipyrine metabolism. British journal of clinical pharmacology, 20: 469-474 (1985). 17. GIbaWdI, M. & Perrler, D. Pharmacokinetics. New York, Marcel Dekker, 1982. 18. Franasn, G. et al. Divided-dose kinetics of mefloquine in man. British journal of clinical pharmacology, 28: 179-184 (1989). 19. Rouvelx, B. et al. Mefloquine-induced acute brain syndrome. Annals of internal medicine, 110: 577-578 (1989). 20. Miller, K.D. et al. Severe cutaneous reactions among American travellers using pyrimethamine-sulfadoxine (Fansidar) for malaria prophylaxis. American journal of tropical medicine and hygiene, 35: 451-458 (1986). 21. Peters, W. & Robinson, B.L. The chemotherapy of rodent malaria xxxv: further studies on the retardation of drug resistance by the use of a triple combination of mefloquine, pyrimethamine and sulfadoxine in mice infected with P. berghei and P. berghei NS. Annals of tropical medicine and parasitology, 78: 459-466 (1984). 22. WhIe, N.J. Combination treatment for falciparum pro- phylaxis. Lancet, 1: 680-681 (1987). 638 WHO Bulletin OMS. Vol. 68 1990.