Assessment of the pharmacodynamic properties of antimalarial drugs in vivo

Combination therapy for malaria: the way forward?

Unless new strategies are deployed to combat malaria, the already enormous health and economic burden related to the disease in tropical countries is bound to worsen. The main obstacle to malaria control is the emergence of drug resistant strains of Plasmodium falciparum. As for HIV/AIDS and tuberculosis, the use of combinations of antimalarial drugs reduces the risk of selecting for resistant mutants of the plasmodial parasites. In large field trials, the combination of an artemisinin derivative and a partner drug with an unrelated mode of action (in this case mefloquine), has shown a remarkable double effect: preventing the emergence and spread of drug resistance, and interrupting the transmission of P. falciparum. This has opened the way for a new approach to the deployment of antimalarial drugs. Coupled with early detection and confirmed diagnosis, this strategy represents the only way forward in the chemotherapy of malaria. Massive economic assistance will be needed to detect and treat adequately the estimated 500 million cases of malaria per year, but without radical action there is no prospect of 'Rolling Back' malaria.

Malaria parasites giving rise to recrudescence in vitro

Recrudescences were simulated in vitro with drug treatment to examine how drug-sensitive parasites survive the treatment. Various numbers of cultured parasites were treated with lethal doses of pyrimethamine or mefloquine for various lengths of time. Recrudescences were observed in parasite populations with larger initial numbers of parasites when the treatment duration was prolonged. Equal numbers of parasitized erythrocytes were treated with various concentrations of pyrimethamine or mefloquine. There was no clear linear relationship between the incidence of recrudescence and the drug concentration. Parasites that had recrudesced were continuously allowed to recrudesce in the succeeding recrudescence experiments. Both the duration from the cessation of treatment to the time at which the recrudescent parasitemia level reached 1% and the growth rate of recrudescent parasites were equal among these recrudescences. The recrudescent parasites in these experiments were as sensitive to the drugs as the parasites tested before treatment were. These results suggest that a parasite culture may contain parasites in some phases that are not killed by drug for up to 10 days, which explains the recrudescences that occur even after treatment.

Oral artesunate dose-response relationship in acute falciparum malaria

The combination of an oral artemisinin derivative (usually artesunate) and mefloquine has become standard treatment for multidrug-resistant falciparum malaria in several parts of Southeast Asia. The doses of artesunate used in monotherapy and combination treatment have largely been derived empirically. In order to characterize the in vivo dose-response relationship for artesunate and thus rationalize dosing, 47 adult patients with acute uncomplicated falciparum malaria and parasitemia > or = 1% were randomized to receive a single oral dose of artesunate varying between 0 and 250 mg together with a curative dose of oral mefloquine. Acceleration of parasite clearance was used as the pharmacodynamic variable. An inhibitory sigmoidal maximum effect (Emax) pharmacodynamic model typical of a dose-response curve was fitted to the relationship between dose and shortening of parasite clearance time (PCT). The Emax was estimated as 28.6 oral h, and the 50% effective concentration was 1.6 mg/kg of body weight. These results imply that there is no reduction in PCTs with the use of single doses of artesunate higher than 2 mg/kg, and this therefore reflects the average lower limit of the maximally effective dose.

Medical need, scientific opportunity and the drive for antimalarial drugs

Continued and sustainable improvements in antimalarial medicines through focused research and development are essential for the world's future ability to treat and control malaria. Unfortunately, malaria is a disease of poverty, and despite a wealth of scientific knowledge there is insufficient market incentive to generate the competitive, business-driven industrial antimalarial drug research and development that is normally needed to deliver new products. Mechanisms of partnering with industry have been established to overcome this obstacle and to open up and build on scientific opportunities for improved chemotherapy in the future.

Mechanisms of resistance of Plasmodium falciparum to antimalarial drugs

Chemotherapy and chemoprophylaxis are the principal means of combating malaria parasite infections in the human host. In the last 75 years, since the introduction of synthetic antimalarials, only a small number of compounds have been found suitable for clinical usage, and this limited armoury is now greatly compromised by the spread of drug-resistant parasite strains. Our current knowledge of the molecular mechanisms underlying resistance in the lethal species Plasmodium falciparum is reviewed here.

A comparison of oral artesunate and artemether antimalarial bioactivities in acute falciparum malaria

AIMS

Artesunate and artemether are the two most widely used artemisinin derivatives in the treatment of uncomplicated Plasmodium falciparum malaria, but there is little information on their comparative pharmacokinetics. The aim of this study was to examine the relative oral antimalarial bioavailability and pharmacokinetics of the two derivatives.

METHODS

The pharmacokinetic properties of oral artesunate and artemether (4 mg kg(-1)) were compared in a randomized cross-over study of 14 adult patients in western Thailand with acute uncomplicated Plasmodium falciparum malaria. Antimalarial activity was compared using a previously validated, sensitive bioassay.

RESULTS

Despite a 29% lower molar dose, oral artesunate administration resulted in significantly larger mean area under the plasma antimalarial activity time curve and median maximum plasma antimalarial activity than after oral artemether (P

CONCLUSIONS

The oral antimalarial bioavailability following artemether was significantly lower than that after artesunate. Artemether oral antimalarial bioavailability is reduced in acute malaria.

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Key Words

• artemether
• artesunate
• bioassay
• bioavailability
• combination
• malaria
• pharmacokinetics
• plasmodium falciparum
• thailand

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MESH Headings

• Acute Disease
• Administration, Oral
• Adolescent
• Adult
• Antimalarials/administration & dosage
• Antimalarials/pharmacokinetics
• Antimalarials/therapeutic use
• Area Under Curve
• Artemether
• Artemisinins
• Artesunate
• Biological Availability
• Cross-Over Studies
• Female
• Humans
• Malaria, Falciparum/drug therapy
• Malaria, Falciparum/metabolism
• Male
• Models, Biological
• Sesquiterpenes/administration & dosage
• Sesquiterpenes/pharmacokinetics
• Sesquiterpenes/therapeutic use
• Thailand
• Time Factors

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Grants

• Wellcome Trust

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Affiliation(s)

Y Suputtamongkol Department of Medicine, Siriraj Hospital, Bangkok, Thailand
P N Newton
B Angus
P Teja-Isavadharm
D Keeratithakul
M Rasameesoraj
S Pukrittayakamee
N J White

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Changing the first line drug for malaria treatment--cost-effectiveness analysis with highly uncertain inter-temporal trade-offs

Access to effective treatment would substantially reduce the burden of malaria in sub-Saharan Africa, but resistance to chloroquine, the most commonly used first line drug, is now widespread. There has been considerable debate over the level of chloroquine resistance at which a new first line drug should be adopted. Two issues make this an extremely complex decision: it involves trade-offs in costs and health outcomes over time; and many of the parameters are uncertain. A modelling approach was identified as appropriate for addressing these issues. The costs and effects of changing from chloroquine to sulphadoxine-pyrimethamine (SP) as the first line drug were modelled over 10 years, allowing for growth in drug resistance. Probabilistic sensitivity analysis was used to allow for the high levels of parameter uncertainty. The optimal year of switch was highly dependent on both empirical values, such as initial resistance and resistance growth rates, and on subjective values, such as the time preferences of policy-makers. It was not possible to provide policy-makers with a definitive threshold resistance level at which to switch, but the model can be used as an analytical tool to structure the problem, explore trade-offs, and identify areas for which data are lacking.

Pharmacokinetic interactions of antimalarial agents

Combination of antimalarial agents has been introduced as a response to widespread drug resistance. The higher number of mutations required to express complete resistance against combinations may retard the further development of resistance. Combination of drugs, especially with the artemisinin drugs, may also offer complete and rapid eradication of the parasite load in symptomatic patients and thus reduce the chance of survival of resistant strains. The advantages of combination therapy should be balanced against the increased chance of drug interactions. During the last decade, much of the pharmacokinetics and metabolic pathways of antimalarial drugs have been elucidated, including the role of the cytochrome P450 (CYP) enzyme complex. Change in protein binding is not a significant cause of interactions between antimalarial agents. CYP3A4 and CYP2C19 are frequently involved in the metabolism of antimalarial agents. Quinidine is a potent inhibitor of CYP2D6, but it appears that this enzyme does not mediate the metabolism of any other antimalarial agent. The new combinations proguanil-atovaquone and chlorproguanil-dapsone do not show significant interactions. CYP2B6 and CYP3A4 are involved in the metabolism of artemisinin and derivatives, but further studies may reveal involvement of more enzymes. Artemisinin may induce CYP2C19. Several artemisinin drugs suffer from auto-induction of the first-pass effect, resulting in a decline of bioavailability after repeated doses. The mechanism of this effect is not yet clear, but induction by other agents cannot be excluded. The combination of artemisinin drugs with mefloquine and the fixed combination artemether-lumefantrine have been studied widely, and no significant drug interactions have been found. The artemisinin drugs will be used at an increasing rate, particularly in combination with other agents. Although clinical studies have so far not shown any significant interactions, drug interactions should be given appropriate attention when other combinations are used.

Controlling malaria: challenges and solutions

Antimalarial drug resistance is a major public health challenge and the principal reason for the erosion of efficacious treatments. Cost and the limited number of antimalarial drugs in current use impose considerable constraints on malaria control, especially in sub-Saharan Africa. The paper describes a multilateral, multidisciplinary research project on artemisinin-based combination therapy, which offers a new and potentially highly effective way to prevent or retard the development of drug resistance.

Association of genetic mutations in Plasmodium vivax dhfr with resistance to sulfadoxine-pyrimethamine: geographical and clinical correlates

Mutations in the Plasmodium falciparum gene (dhfr) encoding dihydrofolate reductase are associated with resistance to antifols. Plasmodium vivax, the more prevalent malaria parasite in Asia and the Americas, is considered antifol resistant. Functional polymorphisms in the dhfr gene of P. vivax (pvdhfr) were assessed by PCR-restriction fragment length polymorphism using blood samples taken from 125 patients with acute vivax malaria from three widely separated locations, Thailand (n = 100), India (n = 16), and Madagascar and the Comoros Islands (n = 9). Upon evaluation of the three important codons (encoding residues 57, 58, and 117) of P. vivax dhfr (pvdhfr), double- or triple-mutation genotypes were found in all but one case from Thailand (99%), in only three cases from India (19%) and in no cases from Madagascar or the Comoros Islands (P < 0.0001). The dhfr PCR products of P. vivax from 32 Thai patients treated with the antifolate sulfadoxine-pyrimethamine (S-P) were investigated. All samples showed either double (53%) or triple (47%) mutations. Following treatment, 34% of the patients had early treatment failures and only 10 (31%) of the patients cleared their parasitemias for 28 days. There were no significant differences in cure rates, but parasite reduction ratios at 48 h were significantly lower for patients whose samples showed triple mutations than for those whose samples showed double mutations (P = 0.01). The three mutations at the pvdhfr codons for residues 57, 58, and 117 are associated with high levels of S-P resistance in P. vivax. These mutations presumably arose from selection pressure.

Factors contributing to anemia after uncomplicated falciparum malaria

The factors contributing to anemia in falciparum malaria were characterized in 4,007 prospectively studied patients on the western border of Thailand. Of these, 727 patients (18%) presented with anemia (haematocrit < 30%), and 1% (55 of 5,253) required blood transfusion. The following were found to be independent risk factors for anemia at admission: age < 5 years, a palpable spleen, a palpable liver, recrudescent infections, being female, a prolonged history of illness (> 2 days) before admission, and pure Plasmodium falciparum infections rather than mixed P. falciparum and Plasmodium vivax infections. The mean maximum fractional fall in hematocrit after antimalarial treatment was 14.1% of the baseline value (95% confidence interval [CI], 13.6-14.6). This reduction was significantly greater in young children (aged < 5 years) and in patients with a prolonged illness, high parasitemia, or delayed parasite clearance. Loss of parasitized erythrocytes accounted for < 10% of overall red blood cell loss. Hematological recovery was usually complete within 6 weeks, but it was slower in patients who were anemic at admission (adjusted hazards ratio [AHR], 1.9, 95% CI, 1.5-2.3), and those whose infections recrudesced (AHR, 1.2, 95% CI, 1.01-1.5). Half the patients with treatment failure were anemic at 6 weeks compared with 19% of successfully treated patients (relative risk, 2.8, 95% CI, 2.0-3.8). Patients coinfected with P. vivax (16% of the total) were 1.8 (95% CI, 1.2-2.6) times less likely to become anemic and recovered 1.3 (95% CI, 1.0-1.5) times faster than those with P. falciparum only. Anemia is related to drug resistance and treatment failure in uncomplicated malaria. Children aged < 5 years of age were more likely than older children or adults to become anemic. Coinfection with P. vivax attenuates the anemia of falciparum malaria, presumably by modifying the severity of the infection.

A comparison of artesunate alone with combined artesunate and quinine in the parenteral treatment of acute falciparum malaria

In some areas clinicians have combined parenteral artesunate and quinine in the belief that the 2 drugs would be additive or synergistic in severe malaria. A randomized comparison of the effectiveness of intravenous (i.v.) artesunate versus i.v. artesunate and i.v. quinine together on parasite clearance was conducted in 1998/99 amongst 69 patients with uncomplicated and severe Plasmodium falciparum malaria in western Thailand. The parasite clearance time did not differ significantly between the 2 treatment groups (P = 0.12), but adverse events were significantly more frequent in the artesunate plus quinine group (P = 0.05). Quinine did not have a significant antipyretic effect and artesunate did not affect the electrocardiographic QTc interval. There is no benefit evident from combining parenteral administration of these 2 antimalarial drugs in the acute phase of treatment.

Drug resistant falciparum malaria: clinical consequences and strategies for prevention

The rising prevalence of multidrug resistant falciparum malaria is occurring at an alarming rate and has serious implications for the health of many of the world's poorest countries. The dangers of not changing treatment practices immediately are huge and irreversible, threatening to both exacerbate the scale and scope of the malaria pandemic, and deprive policymakers of future options against the disease. If a health care disaster is to be avoided then massive and long term funding is urgently required. Funds need to be applied in a cohesive manner, accountable to funding bodies and tailored to the specifics of each endemic region. The key elements of such an approach should be improving early diagnosis and treatment of infection and the deployment of combination regimens containing an artemisinin derivative. These short term measures will need to be accompanied by a longer term strategy to encourage antimalarial drug research and development.

Possible modes of action of the artemisinin-type compounds

Artemisinin-type compounds are used for the treatment of uncomplicated and severe forms of malaria. They reduce parasitaemia more rapidly than any other antimalarial compound known, and are effective against multidrug-resistant parasites. However, uncertainties remain as to how they act on the parasite and cause toxicity. In this review, we summarize current ideas.

Can treatment of P. vivax lead to a unexpected appearance of falciparum malaria?

Of 994 patients admitted to the Bangkok Hospital for Tropical Diseases for P. vivax malaria, 104 (10.5%) experienced appearance of Plasmodiumfalciparum following drug treatment for P. vivax . In all patients, P. falciparum parasites were not found by microscopic examination upon admission. The mean time for P. falciparum appearance was 12.6 days after the commencement of chloroquine treatment. Patients experiencing appearance of P. falciparum had significantly lower hematocrit, and greater initial P. vivax parasite counts. We use a mathematical model to explore the consequences of chloroquine treatment of such mixed infections. Both clinical results and features of the model suggest that such "hidden infections" may be quite common, and that the appearance of P. falciparum may be stimulated by treatment of P. vivax.

Mefloquine pharmacokinetic-pharmacodynamic models: implications for dosing and resistance

Antimalarial resistance develops and spreads when spontaneously occurring mutant malaria parasites are selected by concentrations of antimalarial drug which are sufficient to eradicate the more sensitive parasites but not those with the resistance mutation(s). Mefloquine, a slowly eliminated quinoline-methanol compound, is the most widely used drug for the treatment of multidrug-resistant falciparum malaria. It has been used at doses ranging between 15 and 25 mg of base/kg of body weight. Resistance to mefloquine has developed rapidly on the borders of Thailand, where the drug has been deployed since 1984. Mathematical modeling with population pharmacokinetic and in vivo and in vitro pharmacodynamic data from this region confirms that, early in the evolution of resistance, conventional assessments of the therapeutic response </=28 days after treatment underestimate considerably the level of resistance. Longer follow-up is required. The model indicates that initial deployment of a lower (15-mg/kg) dose of mefloquine provides a greater opportunity for the selection of resistant mutants and would be expected to lead more rapidly to resistance than de novo use of the higher (25-mg/kg) dose.

Artemisinin drugs: novel antimalarial agents

Artemisinin and its derivatives, artesunate and artemether, represent a new class of antimicrobial drug with potent activity against Plasmodium falciparum. Although they show excellent efficacy in both severe and uncomplicated malaria, dosage regimens still need to be optimised and pharmacokinetic profiles defined. In the treatment of uncomplicated malaria, the artemisinin drugs should be used in combination with a long acting antimalarial to protect both drugs against the emergence of resistance. In the treatment of severe malaria, parenteral artemether is at least as effective as quinine and is simpler to use. The use of rectal preparations of artesunate and artemisinin at the rural health level will facilitate early initiation of the treatment of falciparum malaria and this may reduce the proportion of patients progressing to severe disease. All of the artemisinin drugs have comparable efficacy; the choice of derivative should be based upon availability, cost and quality of the preparation. Artemisinin, artesunate and artemether are well-tolerated in both adults and children, with no evidence to date of serious clinical toxicity.

Therapeutic responses to different antimalarial drugs in vivax malaria

The therapeutic responses to the eight most widely used antimalarial drugs were assessed in 207 adult patients with Plasmodium vivax malaria. This parasite does not cause marked sequestration, so parasite clearance can be used as a direct measure of antimalarial activity. The activities of these drugs in descending order were artesunate, artemether, chloroquine, mefloquine, quinine, halofantrine, primaquine, and pyrimethamine-sulfadoxine (PS). Therapeutic responses to PS were poor; parasitemias did not clear in 5 of the 12 PS-treated patients, whereas all the other patients made an initial recovery. Of 166 patients monitored for > or =28 days, 35% had reappearance of vivax malaria 11 to 65 days later and 7% developed falciparum malaria 5 to 21 days after the start of treatment. There were no significant differences in the times taken for vivax malaria reappearance among the different groups except for those given mefloquine and chloroquine, in which all vivax malaria reappearances developed >28 days after treatment, suggesting suppression of the first relapse by these slowly eliminated drugs. There was no evidence of chloroquine resistance. The antimalarial drugs vary considerably in their intrinsic activities and stage specificities of action.

Combinations of artemisinin and quinine for uncomplicated falciparum malaria: efficacy and pharmacodynamics

Combinations of artemisinin and quinine for uncomplicated falciparum malaria were studied. A total of 268 patients were randomized to 7 days of quinine at 10 mg/kg of body weight three times a day (Q) or to artemisinin at 20 mg/kg of body weight followed by 3 (AQ3) or 5 (AQ5) days of quinine. Recrudescence rates were 16, 38, and 15% for the Q, AQ3, and AQ5 groups, respectively (P < 0.001). Recrudescence was associated with shorter parasite clearance time (PCT) and longer treatment after the blood smear had become negative (eradication time). However, classification of patients to outcome-recrudescence or radical cure-was correct in only 77% of patients. The population kinetics of the parasitemia was estimated with nonlinear mixed-effect models. Several models were tested, but the best model was a monoexponential decline of the parasitemia in which the mean parasite elimination half-life was shorter after artemisinin (5.1 h; 95% confidence interval [CI], 4.9 to 5.2 h) than after quinine (8.0 h [95% CI, 7.5 to 8.3 h]). Attempts to simulate the initial increase of the parasitemia did not result in better models with a biologically plausible interpretation. Recrudescence was associated with slower parasite clearance and a higher simulated terminal parasitemia (P(term)). The classification of patients to outcome groups based on P(term) was correct in 78% of patients. The data suggest that parasite strains with reduced sensitivity to quinine are prevalent in Vietnam, with slower parasite clearance and consequent recrudescence. A single dose of artemisinin induces rapid parasite reduction and lowers the value of P(term), but to prevent recrudescence, this should be followed by quinine for at least 3 days after parasite clearance, or 5 days in total.

Antimalarial bioavailability and disposition of artesunate in acute falciparum malaria

The pharmacokinetic properties of oral and intravenous artesunate (2 mg/kg of body weight) were studied in 19 adult patients with acute uncomplicated Plasmodium falciparum malaria by using a randomized crossover design. A sensitive bioassay was used to measure the antimalarial activity in plasma which results from artesunate and its principal metabolite, dihydroartemisinin. The oral study was repeated with 15 patients during convalescence. The mean absolute oral bioavailability of the antimalarial agent in patients with acute malaria was 61% (95% confidence interval [CI], 52 to 70%). The absorption and elimination of oral artesunate were rapid, with a mean elimination half-life of antimalarial activity of 43 min (95% CI, 33 to 53 min). Following oral administration to patients with acute falciparum malaria, peak antimalarial activity in plasma and the area under the plasma concentration-time curve were approximately double those during convalescence and the apparent volume of distribution and clearance were approximately half those during convalescence (P < or = 0.005). Acute malaria is associated with a significant reduction in the clearance of artesunate-associated antimalarial activity.

Pharmacokinetics and pharmacodynamics of lumefantrine (benflumetol) in acute falciparum malaria

The objective of this study was to conduct a prospective population pharmacokinetic and pharmacodynamic evaluation of lumefantrine during blinded comparisons of artemether-lumefantrine treatment regimens in uncomplicated multidrug-resistant falciparum malaria. Three combination regimens containing an average adult lumefantrine dose of 1,920 mg over 3 days (four doses) (regimen A) or 2,780 mg over 3 or 5 days (six doses) (regimen B or C, respectively) were given to 266 Thai patients. Detailed observations were obtained for 51 hospitalized adults, and sparse data were collected for 215 patients of all ages in a community setting. The population absorption half-life of lumefantrine was 4.5 h. The model-based median (5th and 95th percentiles) peak plasma lumefantrine concentrations were 6.2 (0.25 and 14.8) microgram/ml after regimen A, 9. 0 (1.1 and 19.8) microgram/ml after regimen B, and 8 (1.4 and 17.4) microgram/ml after regimen C. During acute malaria, there was marked variability in the fraction of drug absorbed by patients (coefficient of variation, 150%). The fraction increased considerably and variability fell with clinical recovery, largely because food intake was resumed; taking a normal meal close to drug administration increased oral bioavailability by 108% (90% confidence interval, 64 to 164) (P, 0.0001). The higher-dose regimens (B and C) gave 60 and 100% higher areas under the concentration-time curves (AUC), respectively, and thus longer durations for which plasma lumefantrine concentrations exceeded the putative in vivo MIC of 280 microgram/ml (median for regimen B, 252 h; that for regimen C, 298 h; that for regimen A, 204 h [P, 0.0001]) and higher cure rates. Lumefantrine oral bioavailability is very dependent on food and is consequently poor in acute malaria but improves markedly with recovery. The high cure rates with the two six-dose regimens resulted from increased AUC and increased time at which lumefantrine concentrations were above the in vivo MIC.

Potent antimalarial activity of clotrimazole in in vitro cultures of Plasmodium falciparum

The increasing resistance of the malaria parasite Plasmodium falciparum to currently available drugs demands a continuous effort to develop new antimalarial agents. In this quest, the identification of antimalarial effects of drugs already in use for other therapies represents an attractive approach with potentially rapid clinical application. We have found that the extensively used antimycotic drug clotrimazole (CLT) effectively and rapidly inhibited parasite growth in five different strains of P. falciparum, in vitro, irrespective of their chloroquine sensitivity. The concentrations for 50% inhibition (IC(50)), assessed by parasite incorporation of [(3)H]hypoxanthine, were between 0.2 and 1.1 microM. CLT concentrations of 2 microM and above caused a sharp decline in parasitemia, complete inhibition of parasite replication, and destruction of parasites and host cells within a single intraerythrocytic asexual cycle (approximately 48 hr). These concentrations are within the plasma levels known to be attained in humans after oral administration of the drug. The effects were associated with distinct morphological changes. Transient exposure of ring-stage parasites to 2.5 microM CLT for a period of 12 hr caused a delay in development in a fraction of parasites that reverted to normal after drug removal; 24-hr exposure to the same concentration caused total destruction of parasites and parasitized cells. Chloroquine antagonized the effects of CLT whereas mefloquine was synergistic. The present study suggests that CLT holds much promise as an antimalarial agent and that it is suitable for a clinical study in P. falciparum malaria.

Comparison of a rapid field immunochromatographic test to expert microscopy for the detection of Plasmodium falciparum asexual parasitemia in Thailand

We assessed a rapid, Plasmodium falciparum histidine rich protein 2 (PfHRP2)-based immunochromatographic test (ICT Malaria Pf Test), for detection of asexual P. falciparum parasitemia in 551 subjects in three groups: (1) symptomatic patients self-referring for diagnosis, (2) villagers in a screening survey, and (3) patients recently treated for P. falciparum malaria. Expert light microscopy was the reference standard. ICT test performance was similar for diagnostic and screening modes. Four findings emerged: (1) test sensitivity correlated directly with parasite density, (2) test band intensity correlated directly with parasite density, (3) persistent test positivity after parasite clearance precludes its use for monitoring early therapeutic responses, and (4) a false negative test at 18,000 parasites/microl is unexplained. We conclude that a strong positive ICT test is highly predictive of falciparum asexual parasitemia for the diagnosis of new cases of falciparum malaria in Thailand, but a negative test result is inadequate to exclude parasitemia < 300/microl, and in some instances, even a higher parasitemia.

Rapid absorption and clinical effectiveness of intragastric mefloquine in the treatment of cerebral malaria in African children

To obviate the lack of injectable quinine in a hospital in rural Burundi, mefloquine, only available as an oral formulation, was administered (25 mg/kg bodyweight) as a single dose by nasogastric tube to four small children with cerebral malaria. All patients recovered uneventfully after a mean coma duration of 20.5 h. Mefloquine was rapidly absorbed and therapeutic serum concentrations were achieved within a few hours in all subjects, with parasite reduction ratios after 48 h within the expected range for drug-sensitive parasites. These findings suggest that intragastric mefloquine deserves consideration whenever parenteral drugs are not available for the treatment of cerebral malaria.

Blood-stage dynamics and clinical implications of mixed Plasmodium vivax-Plasmodium falciparum infections

We present a mathematical model of the blood-stage dynamics of mixed Plasmodium vivax-Plasmodium falciparum malaria infections in humans. The model reproduces features of such infections found in nature and suggests several phenomena that may merit clinical attention, including the potential recrudescence of a long-standing, low-level P. falciparum infection following a P. vivax infection or relapse and the capacity of an existing P. vivax infection to reduce the peak parasitemia of a P. falciparum superinfection. We simulate the administration of antimalarial drugs, and illustrate some potential complications in treating mixed-species malaria infections. Notably, our model indicates that when a mixed-species infection is misdiagnosed as a single-species P. vivax infection, treatment for P. vivax can lead to a surge in P. falciparum parasitemia.

Clinical pharmacokinetics and pharmacodynamics and pharmacodynamics of artemether-lumefantrine

The combination of artemether and lumefantrine (benflumetol) is a new and very well tolerated oral antimalarial drug effective even against multidrug-resistant falciparum malaria. The artemether component is absorbed rapidly and biotransformed to dihydroartemisinin, and both are eliminated with terminal half-lives of around 1 hour. These are very active antimalarials which give a rapid reduction in parasite biomass and consequent rapid resolution of symptoms. The lumefantrine component is absorbed variably in malaria, and is eliminated more slowly (half-life of 3 to 6 days). Absorption is very dependent on coadministration with fat, and so improves markedly with recovery from malaria. Thus artemether clears most of the infection, and the lumefantrine concentrations that remain at the end of the 3- to 5-day treatment course are responsible for eliminating the residual 100 to 10 000 parasites. The area under the curve of plasma lumefantrine concentrations versus time, or its correlate the plasma concentration on day 7. has proved an important determinant of therapeutic response. Characterisation of these pharmacokinetic-pharmacodynamic relationships provided the basis for dosage optimisation, an approach that could be applied to other antimalarial drugs.

Antimalarial drug resistance and combination chemotherapy

Antimarial drug resistance develops when spontaneously occurring parasite mutants with reduced susceptibility are selected, and are then transmitted. Drugs for which a single point mutation confers a marked reduction in susceptibility are particularly vulnerable. Low clearance and a shallow concentration-effect relationship increase the chance of selection. Use of combinations of antimalarials that do not share the same resistance mechanisms will reduce the chance of selection because the chance of a resistant mutant surviving is the product of the per parasite mutation rates for the individual drugs, multiplied by the number of parasites in an infection that are exposed to the drugs. Artemisinin derivatives are particularly effective combination partners because (i) they are very active antimalarials, producing up to 10,000-fold reductions in parasite biomass per asexual cycle; (ii) they reduce malaria transmissibility; and (iii) no resistance to these drugs has been reported yet. There are good arguments for no longer using antimalarial drugs alone in treatment, and instead always using a combination with artemisinin or one of its derivatives.

Malaria: new developments in treatment and prevention

Malaria still kills some 0.5-2.5 million people per year in the tropics. Resistance to the cheap, most commonly used antimalarials continues to spread alarmingly and could outpace drug development. The artemisinin derivatives have had an important clinical impact both on the treatment of resistant falciparum malaria and on the incidence of disease in low-transmission areas. A few promising new antimalarials are being tested clinically but there is an imperative need for cheap, well-tolerated drugs that can be used in short courses, and for strategies to delay the onset of drug resistance. Bed nets have been shown to reduce the incidence of severe malaria in many areas but an effective vaccine is urgently needed.

Pharmacokinetics of mefloquine combined with artesunate in children with acute falciparum malaria

Combining artemisinin or a derivative with mefloquine increases cure rates in falciparum malaria patients, reduces transmission, and may slow the development of resistance. The combination of artesunate, given for 3 days, and mefloquine is now the treatment of choice for uncomplicated multidrug-resistant falciparum malaria acquired on the western or eastern borders of Thailand. To optimize mefloquine administration in this combination, a prospective study of mefloquine pharmacokinetics was conducted with 120 children (4 to 15 years old) with acute uncomplicated falciparum malaria, who were divided into four age- and sex-matched groups. The patients all received artesunate (4 mg/kg of body weight/day orally for 3 days and mefloquine as either (i) a single dose (25 mg/kg) on day 2 with food, (ii) a split dose (15 mg/kg on day 2 and 10 mg/kg on day 3) with food, (iii) a single dose (25 mg/kg) on day 0 without food, or (iv) a single dose (25 mg/kg) on day 2 without food. Delaying administration of mefloquine until day 2 was associated with a mean (95% confidence interval) increase in estimated oral bioavailability of 72% (36 to 109%). On day 2 coadministration with food did not increase mefloquine absorption significantly, and there were no significant differences between patients receiving split- and single-dose administration. In combination with artesunate, mefloquine administration should be delayed until the second or third day after presentation.

Recent developments in the management of malaria

The last decade has seen in increase in our understanding of the pathophysiology of severe malaria in both adults and children. However, this increased knowledge has yet to be matched by progress in the clinical management of this medical emergency. In the last few years a wide variety of measures have been advocated to lessen the mortality due to severe malaria, but none have shown a significant benefit in terms of mortality or morbidity. In this review we concentrate on aspects of the pathophysiology of malaria which are amenable to intervention at present or in the near future. In the field of uncomplicated malaria problems remain mostly related to the continued spread of drug-resistance and the limited array of available drugs. Outlined are a number of important advances in antimalarial pharmacology and parasite biology that may lead to future improvement in the care of patients with malaria.

Population pharmacokinetics and therapeutic response of CGP 56697 (artemether + benflumetol) in malaria patients

AIMS

To investigate the pharmacokinetic and pharmacodynamic properties of artemether and benflumetol in a fixed combination tablet (CGP 56697) and to offer an explanation for the lower than expected cure rate in a Thai clinical trial.

METHODS

Two hundred and sixty patients were enrolled into a randomized, double-blind, parallel group, dose-finding trial. CGP 56697 was given orally, either as: A, 4 x 4 tablets over 48 h; B, 4 x 2 tablets over 48 h or C, 3 x 4 tablets over 24 h. Each tablet contained artemether 20 mg amd benflumetol 120 mg. The pharmacokinetics were determined using a population-based approach combining full profiles (42 patients) and sparse data (218 patients). Parasite clearance time and 28 day cure rate were correlated with the derived pharmacokinetic parameters.

RESULTS

The median absorption half-life of benflumetol was 5.3 h, with a tmax of 10 h and terminal elimination half-life of 4.5 days. For artemether (and its metabolite, dihydroartemisinin), the corresponding values were 1.9 (1.9) h, 1.8 (1.2) h, and 0.84 (0.43) h. The variability in bioavailability of artemether and dihydroartemisinin was large both between doses and between patients, but was less pronounced for benflumetol. Compared with the first dose, benflumetol bioavailability was estimated to increase three-fold by the third and fourth doses. Higher artemether or dihydroartemisinin AUC was found to decrease parasite clearance time. Higher benflumetol AUC was found to significantly increase the chance of cure.

CONCLUSIONS

Using a population-based approach it was confirmed that the pharmacokinetic and pharmacodynamic properties of benflumetol and artemether differ markedly. Benflumetol AUC is associated with cure and the effect of benflumetol when coadministered with artemether is to prevent recrudescence. The mode of action of benflumetol is consistent with its longer elimination half-life. A short course of low-dose artemether, which is rapidly absorbed and has a short elimination half-life, produced effective parasite clearance. The complementary pharmacokinetic and pharmacodynamic properties of benflumetol and artemether was the main rationale for developing a fixed-dose combination. While the 4 x 4 dose regimen is very effective in most endemic areas, the poorer absorption (2.5 fold lower than in China) and the more resistant parasites in Thailand require higher doses of this drug.

Preventing antimalarial drug resistance through combinations

Throughout the tropical world antimalarial drug resistance is increasing, particularly in the potentially lethal malaria parasite Plasmodium falciparum. In some parts of Southeast Asia, parasites which are resistant to chloroquine, pyrimethamine-sulfadoxine, and mefloquine are prevalent. The characteristics of a drug that make it vulnerable to the development of resistance are a long terminal elimination half-life, a shallow concentration-effect relationship, and that one or two base-pair mutations confer a marked reduction in susceptibility. The development of resistance can be delayed or prevented by drug combinations. The artemisinin derivatives are the most potent of all antimalarial drugs. They reduce the infecting parasite biomass by approximately 10 000-fold per asexual life cycle. There are good arguments for combining, de novo, an artemisinin derivative with all newly introduced antimalarial drugs.

Artesunate and mefloquine in the treatment of uncomplicated multidrug-resistant hyperparasitaemic falciparum malaria

Oral artesunate is the most effective treatment for uncomplicated hyperparasitaemia in falciparum malaria. To assess the contribution of mefloquine to therapeutic efficacy in an area endemic for mefloquine-resistant Plasmodium falciparum, an open randomized comparison of a 5 d course of oral artesunate (total dose 12 mg/kg) with and without a single dose of mefloquine (25 base mg/kg) was conducted in 100 adults and children with uncomplicated hyperparasitaemia (> 4% parasitized red blood cells). Both regimens were well tolerated and gave equally rapid clinical responses (84% of patients were aparasitaemic and 96% were afebrile within 48 h), but the recrudescence rate assessed at day 42 was 6% in those receiving artesunate with mefloquine compared to 36% in those receiving artesunate alone (adjusted hazard ratio 7, 95% confidence interval [95% CI] 2-32; P < 0.01). In addition, the efficacy of a 7 d course of artesunate, with and without the addition of mefloquine, was monitored in 178 patients who were not part of the randomized comparison. The failure rate was again lower in those receiving artesunate and mefloquine--7% (95% CI 2-13) compared with 26% (95% CI 8-44) in patients treated with artesunate alone. An oral regimen of 5 d or more of artesunate, together with mefloquine (25 mg/kg) given on day 2, is an effective treatment for uncomplicated hyperparasitaemic falciparum malaria in this area of high level multidrug resistance.

Randomized comparison of artemether-benflumetol and artesunate-mefloquine in treatment of multidrug-resistant falciparum malaria

An open, randomized comparison of artemether-benflumetol (CGP 56 697; Novartis) with artesunate-mefloquine was conducted in 617 patients with acute uncomplicated multidrug-resistant falciparum malaria on the western border of Thailand. Both treatments rapidly and reliably cleared fever and parasitemia, and there was no significant difference in the initial therapeutic response parameters. Parasite genotyping was used to distinguish recrudescences from new infections. The 63-day cure rate for artesunate-mefloquine (94%) was significantly higher than the cure rate for artemether-benflumetol (81%) (P < 0.001). Both regimens were well tolerated. Nausea, vomiting, dizziness, sleep disorders, and other neurological side effects were between two and four times more common in the artesunate-mefloquine group than in the artemether-benflumetol group (P < 0.001). Artemether-benflumetol is effective and very well tolerated in the treatment of multidrug-resistant falciparum malaria. A higher dose than that used in the present study may improve efficacy.