Enhanced efficacy of chloroquine-chlorpheniramine combination in acute uncomplicated falciparum malaria in children

Plasmodium falciparum and Plasmodium vivax Demonstrate Contrasting Chloroquine Resistance Reversal Phenotypes

High-grade chloroquine (CQ) resistance has emerged in both Plasmodium falciparum and P. vivax The aim of the present study was to investigate the phenotypic differences of CQ resistance in both of these species and the ability of known CQ resistance reversal agents (CQRRAs) to alter CQ susceptibility. Between April 2015 and April 2016, the potential of verapamil (VP), mibefradil (MF), L703,606 (L7), and primaquine (PQ) to reverse CQ resistance was assessed in 46 P. falciparum and 34 P. vivax clinical isolates in Papua, Indonesia, where CQ resistance is present in both species, using a modified schizont maturation assay. In P. falciparum, CQ 50% inhibitory concentrations (IC₅₀s) were reduced when CQ was combined with VP (1.4-fold), MF (1.2-fold), L7 (4.2-fold), or PQ (1.8-fold). The degree of CQ resistance reversal in P. falciparum was highly correlated with CQ susceptibility for all CQRRAs (R² = 0.951, 0.852, 0.962, and 0.901 for VP, MF, L7, and PQ, respectively), in line with observations in P. falciparum laboratory strains. In contrast, no reduction in the CQ IC₅₀s was observed with any of the CQRRAs in P. vivax, even in those isolates with high chloroquine IC₅₀s. The differential effect of CQRRAs in P. falciparum and P. vivax suggests significant differences in CQ kinetics and, potentially, the likely mechanism of CQ resistance between these two species.

Chlorpheniramine Analogues Reverse Chloroquine Resistance in Plasmodium falciparum by Inhibiting PfCRT

The emergence and spread of malaria parasites that are resistant to chloroquine (CQ) has been a disaster for world health. The antihistamine chlorpheniramine (CP) partially resensitizes CQ-resistant (CQR) parasites to CQ but possesses little intrinsic antiplasmodial activity. Mutations in the parasite's CQ resistance transporter (PfCRT) confer resistance to CQ by enabling the protein to transport the drug away from its site of action, and it is thought that resistance-reversers such as CP exert their effect by blocking this CQ transport activity. Here, a series of new structural analogues and homologues of CP have been synthesized. We show that these compounds (along with other in vitro CQ resistance-reversers) inhibit the transport of CQ via a resistance-conferring form of PfCRT expressed in Xenopus laevis oocytes. Furthermore, the level of PfCRT-inhibition was found to correlate well with both the restoration of CQ accumulation and the level of CQ resensitization in CQR parasites.

The effects of α1-acid glycoprotein on the reversal of chloroquine resistance inPlasmodium falciparum

An in-vitro model based on the semi-automated microdilution technique has been developed for selecting compounds that might be used clinically for the reversal of chloroquine resistance. This was used initially to test the susceptibility of Plasmodium falciparum clone W2 to chloroquine (CQ). The model was then employed to investigate the effects of each of four resistance-reversing agents (verapamil, desipramine, chlorpheniramine and promethazine, at 1 microM) on this parasite's susceptibility to CQ, with and without alpha(1)-acid glycoprotein (AGP), at a patho-physiological concentration (1.25 g/litre), in the culture medium. In the absence of AGP, each of the resistance-reversing agents reduced the median inhibitory concentrations of CQ by 82%-97%, from a baseline value of about 94 ng/ml. In the presence of AGP, however, most of the resistance-reversing agents had much less effect. There appears to be competitive interaction between CQ, the resistance-reversing agents and AGP. The binding kinetics between CQ, resistance-reversing agents, AGP and other plasma proteins will clearly need to elucidated if clinically effective resistance-reversing agents are to be selected in vitro.

Know your enemy: understanding the role of PfCRT in drug resistance could lead to new antimalarial tactics

The prevention and treatment of malaria is heavily dependent on antimalarial drugs. However, beginning with the emergence of chloroquine (CQ)-resistant Plasmodium falciparum parasites 50 years ago, efforts to control the disease have been thwarted by failed or failing drugs. Mutations in the parasite's 'chloroquine resistance transporter' (PfCRT) are the primary cause of CQ resistance. Furthermore, changes in PfCRT (and in several other transport proteins) are associated with decreases or increases in the parasite's susceptibility to a number of other antimalarial drugs. Here, we review recent advances in our understanding of CQ resistance and discuss these in the broader context of the parasite's susceptibilities to other quinolines and related drugs. We suggest that PfCRT can be viewed both as a 'multidrug-resistance carrier' and as a drug target, and that the quinoline-resistance mechanism is a potential 'Achilles' heel' of the parasite. We examine a number of the antimalarial strategies currently undergoing development that are designed to exploit the resistance mechanism, including relatively simple measures, such as alternative CQ dosages, as well as new drugs that either circumvent the resistance mechanism or target it directly.

Potential contribution of prescription practices to the emergence and spread of chloroquine resistance in south-west Nigeria: caution in the use of artemisinin combination therapy

Background

Prescription practices have been shown to influence the emergence of anti-malarial drug resistance. Thus efforts in this study were devoted to evaluating the prescribing practices prior to introduction of the artemisinin based combination therapy (ACT) in Nigeria and its potential contribution to emergence of chloroquine resistant malaria in south-west Nigeria, in order to forestall a similar situation with the ACT.

Methods

A retrospective quantitative study was designed to examine case records of patients treated for malaria in either a government or a private hospital in Ibadan, south-west Nigeria, over a 20-year period, cutting across three phases of resistance to chloroquine in Nigeria: pre-resistance, emerging resistance and dissemination of resistance. Patient prescriptions were examined for use of anti-malarial drugs, sub-therapeutic doses of chloroquine, co-administration of anti-histamines with chloroquine. Descriptive statistics of frequency and percentage were used to describe trends in the parameters assessed using EPI-info.

Results

Case record files of 2,529 patients were examined. Chloroquine was the main drug used in treatment of malaria throughout the periods studied, with frequency of prescription at both sites ranging from 91.4% to 98.3% during the pre-resistance years. It was administered as standard doses during the pre resistance years. Anti-histamines, especially promethazine, were routinely co-administered with chloroquine at this period too. However, the practice of prescribing sub-therapeutic doses of chloroquine at the private health care facility coincided with the latter phase of emerging resistance and phase of dissemination of resistance. Frequency of prescription of sub-therapeutic doses increased from 6.7% in 1983 (pre-resistance years) to 43.6% in 1997 (dissemination of resistance phase) at the private health care facility. Frequency of co-administration of anti-histamines with chloroquine also reduced during the period of dissemination of resistance.

Conclusion

The results from this study describe a lack of adherence to national treatment guidelines, especially in the private sector, and a relationship between prescription practices and dissemination of drug resistant falciparum malaria. As Nigeria adopts the use of ACT, there is an urgent need to improve malaria treatment practices in Nigeria in order to prolong the clinical shelf-life of the combination.

Chloroquine resistant Plasmodium falciparum malaria in Osogbo Nigeria: efficacy of amodiaquine + sulfadoxine-pyrimethamine and chloroquine + chlorpheniramine for treatment

Chloroquine (CQ) resistance in Plasmodium falciparum contributes to increasing malaria-attributable morbidity and mortality in Sub-Saharan Africa. Despite a change in drug policy, continued prescription of CQ did not abate. Therefore the therapeutic efficacy of CQ in uncomplicated falciparum malaria patients was assessed in a standard 28-day protocol in 116 children aged between six and 120 months in Osogbo, Southwest Nigeria. Parasitological and clinical assessments of response to treatment showed that 72 (62.1%) of the patients were cured and 44 (37.9%) failed the CQ treatment. High initial parasite density and young age were independent predictors for early treatment failure. Out of the 44 patients that failed CQ, 24 received amodiaquine + sulphadoxine/pyrimethamine (AQ+SP) and 20 received chlorpheniramine + chloroquine (CH+CQ) combinations. Mean fever clearance time in those treated with AQ+SP was not significantly different from those treated with CH+CQ (p = 0.05). There was no significant difference in the mean parasite density of the two groups. The cure rate for AQ+SP group was 92% while those of CH+CQ was 85%. There was a significant difference in parasite clearance time (p = 0.01) between the two groups. The 38% treatment failure for CQ reported in this study is higher than the 10% recommended by World Health Organization in other to effect change in antimalarial treatment policy. Hence we conclude that CQ can no more be solely relied upon for the treatment of falciparum malaria in Osogbo, Nigeria. AQ+SP and CH+CQ are effective in the treatment of acute uncomplicated malaria and may be considered as useful alternative drugs in the absence of artemisinin-based combination therapies.

Enhanced efficacy of amodiaquine and chlorpheniramine combination over amodiaquine alone in the treatment of acute uncomplicated Plasmodium falciparum malaria in children

OBJECTIVE

To evaluate the comparative efficacy of amodiaquine (AMQ) alone and the combination of AMQ and chlorpheniramine (CP) in the treatment of acute uncomplicated malaria in children.

SUBJECTS

Of the 110 children enrolled in the study, 103 with acute uncomplicated malaria, aged 6 months to 12 years, were evaluated using the 14-day modification of the WHO field test. The patients were randomized to 2 groups. Group 1 received supervised treatment with AMQ alone (10 mg AMQ base/kg daily for 3 days), while group 2 received supervised treatment with AMQ (same dose as group 1) plus CP (AMQCP) for 7 days.

RESULTS

Both treatment regimens were well tolerated and no patient was withdrawn as a result of recurrent vomiting or drug-related adverse events. There was no significant difference in mean fever and parasite clearance times. The cure rates at day 7 were 90.2 versus 100% (rho = 0.027) for AMQ versus AMQCP, while the day 14 cure rates were 85.9 versus 98.1% for AMQ versus AMQCP, respectively (rho = 0.016).

CONCLUSION

The combination of AMQ plus CP proved significantly more effective than AMQ alone in the treatment of acute uncomplicated falciparum malaria, most probably due to the enhancement of the antimalarial effect of AMQ by CP. The combination of AMQCP could be a better alternative to AMQ alone as a companion drug in artemisinin-based combination therapies.

Enhancement of the antimalarial efficacy of amodiaquine by chlorpheniramine in vivo

Resistance in Plasmodium falciparum to amodiaquine (AQ) can be reversed in vitro with with antihistaminic and tricyclic antidepressant compounds, but its significance in vivo is unclear. The present report presents the enhancement of the antimalarial efficacy of AQ by chlorpheniramine, an H1 receptor antagonist that reverses chloroquine (CQ) resistance in vitro and enhances its efficacy in vivo, in five children who failed CQ and/or AQ treatment, and who were subsequently retreated and cured with a combination of AQ plus CP, despite the fact that parasites infecting the children harboured mutant pfcrtT76 and pfmdr1Y86 alleles associated with AQ resistance. This suggests a potential clinical application of the reversal phenomenon.

Prospects for the treatment of drug-resistant malaria parasites

Widespread parasitic resistance has led to an urgent need for the development and implementation of new drugs for the treatment of Plasmodium falciparum malaria. Artemisinin and its derivatives are becoming increasingly important, used preferably in combination with a second antimalarial agent to increase the efficacy and slow the development of resistance. However, cost, production and pharmacological issues associated with artemisinin derivatives and potential partner drugs are hindering the implementation of combination therapies. This article reviews the molecular basis of the action of, and resistance to, different antimalarials and examines the prospects for the next generation of drugs to combat this potentially lethal human pathogen.

Effects of pyrimethamine-sulphadoxine, chloroquine plus chlorpheniramine, and amodiaquine plus pyrimethamine-sulphadoxine on gametocytes during and after treatment of acute, uncomplicated malaria in children

The effects of pyrimethamine-sulphadoxine (PS), chloroquine plus chlorpheniramine, a H1 receptor antagonist that reverses chloroquine resistance in Plasmodium falciparum in vitro and in vivo (CQCP), and amodiaquine plus pyrimethamine-sulphadoxine (AQPS) on gametocyte production were evaluated in 157 children with acute, symptomatic, uncomplicated falciparum malaria who were treated with these drugs. PS was significantly less effective than CQCP or AQPS at clearing asexual parasitaemia or other symptoms of malaria. Gametocyte carriage on days 3, 7, and 14 were significantly higher in those treated with PS. The ratio of the density (per microl blood) of peripheral young gametocyte (PYG), that is, < or = stage III to peripheral mature gametocyte (PMG), that is, stage IV and V, an index of continuing generation of gametocytes, rose to 1 by day 7 of treatment in those treated with PS, but remained consistently below 1 in the other treatment groups. PYG-PMG density ratio increased significantly from day 0-14 in those treated with PS and CQCP (chi2 = 76, P = 0.000001 and chi2 = 42.2, P = 0.00001, respectively) but decreased significantly in those treated with AQPS (chi2 = 53.2, P = 0.000001). Both PS-sensitive and -resistant infections generated PYG (18 of 29 vs 13 of 20, chi2 = 0.04, P = 0.93) but PYG was present only in those with resistant response to CQCP. Combination of PS with amodiaquine (AQ), that is, (AQPS) resulted in less production of PYG, but in this setting, PYG was not indicative of response to AQPS. These data indicate that PS enhanced production or release of young gametocytes when used alone, but generated less young gametocytes when used in combination with AQ. PYG may be used as an indicator of response to CQCP but not PS or PS-based combination drugs.

Potentiation of antimalarial drug action by chlorpheniramine against multidrug-resistant Plasmodium falciparum in vitro

Chlorpheniramine, a histamine H1 receptor antagonist, was assayed for in vitro antimalarial activity against multidrug-resistant Plasmodium falciparum K1 strain and chloroquine-resistant P. falciparum T9/94 clone, by measuring the 3H-hypoxanthine incorporation. Chlorphenirame inhibited P. falciparum K1 and T9/94 growth with IC50 values of 136.0+/-40.2 microM and 102.0+/-22.6 microM respectively. A combination of antimalarial drug and chlorpheniramine was tested against resistant P. falciparum in vitro. Isobologram analysis showed that chlorpheniramine exerts marked synergistic action on chloroquine against P. falciparum K1 and T9/94. Chlorpheniramine also potentiated antimalarial action of mefloquine, quinine or pyronaridine against both of the resistant strains of P. falciparum. However, chlorpheniramine antagonism with artesunate was obtained in both P. falciparum K1 and T9/94. The results in this study indicate that antihistaminic drugs may be promising candidates for potentiating antimalarial drug action against drug resistant malarial parasites.

Predictors of the failure of treatment with pyrimethamine-sulfadoxine in children with uncomplicated falciparum malaria

The prevalence of pyrimethamine-sulfadoxine (PS)-resistant Plasmodium falciparum malaria has been increasing in sub-Saharan Africa or other parts of the world in the last one or two decades. The factors that identify children at risk of treatment failure after being given PS were evaluated in 291 children with acute, symptomatic, uncomplicated, P. falciparum malaria. The children took part in four antimalarial drug trials between July 1996 and July 2004 in a hyperendemic area of southwestern Nigeria. Following treatment, 64 (22%) of 291 children failed treatment by day 7 or 14. In a multivariate analysis, an age < or = 1.5 years (AOR=2.9, 95% CI 1.3-6.4, P = 0.009) and presence of fever (AOR = 3.3, 95% CI 1.28-7.14, P = 0.01) were independent predictors of the failure of treatment with PS at presentation. Following treatment, delay in parasite clearance >3 days (AOR = 2.56, CI 1.19-5.56, P = 0.016) was an independent predictor of the failure of treatment with PS. In addition, compared with the children who had no fever then, children with fever three or more days after starting treatment were more likely to be treatment failures. These findings may have implications for malaria control efforts in some sub-Saharan African countries where treatment of malaria disease depends almost entirely on PS monotherapy, and for programmes employing PS or PS-based combination therapy.

Primaquine synergises the activity of chloroquine against chloroquine-resistant P. falciparum

In recent years, resistance to the antimalarial drug, chloroquine, has become widespread. It is, therefore, imperative to find compounds that could replace chloroquine or work synergistically with this drug to overcome chloroquine resistance. We have examined the interaction between chloroquine, a 4-aminoquinoline, and a number of 8-aminoquinolines, including primaquine, a drug that is widely used to treat Plasmodium vivax infections. We find that primaquine is a potent synergiser of the activity of chloroquine against chloroquine-resistant Plasmodium falciparum. Analysis of matched transfectants expressing mutant and wild-type alleles of the P. falciparum chloroquine resistance transporter (PfCRT) indicate that primaquine exerts its activity by blocking PfCRT, and thus enhancing chloroquine accumulation. Our data suggest that a novel formulation of two antimalarial drugs already licensed for use in humans could be used to treat chloroquine-resistant parasites.

Predictors of the failure of treatment with chloroquine in children with acute, uncomplicated, Plasmodium falciparum malaria, in an area with high and increasing incidences of chloroquine resistance

Resistance to chloroquine (CQ) in Plasmodium falciparum has reached unacceptably high levels in many endemic countries. The pre-treatment factors that identify the children who are at risk of treatment failure after being given CQ were evaluated in 385 children with acute, uncomplicated, Plasmodium falciparum malaria. These children each took part in one of six antimalarial drug trials conducted, between July 1996 and July 2004, in a hyper-endemic area of south-western Nigeria. Following treatment with CQ, 149 (39%) of the children failed treatment by day 7 or 14. In a multivariate analysis, an age of < or =7 years [giving an adjusted odds ratio (AOR) of 2.17, with a 95% confidence interval (CI) of 1.19-3.85; P = 0.01], an asexual parasitaemia of > or =100,000/microl (AOR = 2.17; CI = 1.08-4.35; P = 0.03), the presence of gametocytaemia (AOR = 2.08; CI = 1.14-3.85; P = 0.02) and enrolment >4 years after commencement of the study (i.e. after 2000; AOR = 2.13; CI = 1.3-4.0; P = 0.003) were found to be independent predictors at presentation of the subsequent failure of treatment with CQ. Compared with the other children, those who failed to clear their parasitaemias within 3 days and those who still had fever 1-2 days after commencing treatment were more likely to be treatment failures. Together, these findings may have implications for malaria-control efforts in all areas of sub-Saharan Africa where treatment of malaria depends almost entirely on antimalarial monotherapy.

Should chloroquine be laid to rest?

Chloroquine (CQ) has been the front line antimalarial drug due to its efficacy, low cost and scanty side effects, until resistance has evolved. Although its use has been officially discontinued in most malaria-affected countries, it is still widely used. Practical and pharmacological considerations indicate that it could be still used in semi-immune adults and that more efficient treatment protocols could be devised to treat even patients infected with CQ-resistant parasite strains. Since its antimalarial activity is pleiotropic, drug resistance may be due to different mechanisms, each amenable to reversal by drug combination.

Predictors of the failure of treatment with chloroquine plus chlorpheniramine, in children with acute, uncomplicated, Plasmodium falciparum malaria

Resistance to chloroquine in Plasmodium falciparum can be reversed, both in vitro and in vivo, by chlorpheniramine, a histamine H(1) receptor antagonist. This reversal raises the possibility of using chlorpheniramine to prolong the clinical usefulness of chloroquine in resource-poor communities. The factors that identify children at risk of treatment failure after being given chloroquine plus chlorpheniramine have now been evaluated in 281 children with uncomplicated, P. falciparum malaria. The children, who had taken part in six trials of antimalarial drugs between February 1996 and September 1999, in a hyper-endemic area of south-western Nigeria, were enrolled prospectively for the present study. Following treatment with chloroquine plus chlorpheniramine, 13 (5%) of the children failed treatment by day 7 or 14. In a multivariate analysis, an age of < or =3 years (adjusted odds ratio = 11.1; 95% confidence interval = 2.2-55.3; P = 0.003) and a parasitaemia that took >3 days to clear (adjusted odds ratio=7.9; 95% confidence interval = 1.3-49.4; P = 0.027) were found to be independent predictors of treatment failure. In addition, compared with the children who had a lower axillary temperature then, the children who had an axillary temperature of > or =38 degrees C 2 days after commencing treatment were significantly more likely to be treatment failures. In resource-poor communities using chloroquine plus chlorpheniramine, the easily identifiable predictors of treatment failure might be used to identify children requiring alternative antimalarial drugs.

Efficacy of chloroquine plus chlorpheniramine in chloroquine-resistant falciparum malaria during pregnancy in Nigerian women: a preliminary study

The efficacy of chloroquine plus chlorpheniramine, a histamine H receptor antagonist, which reverses chloroquine 1 insensitivity in Plasmodium falciparum in vitro and in vivo , was evaluated in 30 pregnant women with recrudescent chloroquine-resistant Plasmodium falciparum malaria. All patients had at least one or more treatment failures with one or more courses of chloroquine or pyrimethamine-sulphadoxine. There was a prompt response to treatment with parasitaemia and fever clearing in all patients within 48 and 96 hours respectively of commencement of therapy with the combination. The cure rate on day 14 was 77%. Parasitaemia recurred in seven patients after day 14 and was successfully treated with oral mefloquine. The combination was well tolerated; pruritus and drowsiness were the only noticeable adverse effects. The progress of pregnancy and its outcome were not adversely affected by treatment with the combination. When fully developed, the combination of chloroquine plus chlorpheniramine may be an alternative in the treatment of chloroquine-resistant malaria during pregnancy in Nigerian women.

Evidence for increased metabolism of chloroquine during the early third trimester of human pregnancy

OBJECTIVE

To examine the possibility of a different extent of chloroquine (CQ) metabolism in human pregnancy by determining blood level profiles of the drug and its major metabolite, desethylchloroquine (CQM).

METHODS

Five women in the early third trimester of pregnancy and five non-pregnant women received each a single 600 mg oral dose of CQ and blood samples were collected at pre-determined intervals following drug administration. Plasma concentrations of CQ and CQM were analysed by an established HPLC method.

RESULTS

The C(max) and AUC(0-48 h) of CQM were significantly higher in the pregnant than the non-pregnant group (P = 0.009). The ratio AUC(CQ)/AUC(CQM) ranged from 0.09 to 0.35 among pregnant women, and from 1.70 to 4.81 among non-pregnant women.

CONCLUSION

Results from this preliminary study indicate an occurrence of induction of metabolism of CQ in the early third trimester of pregnancy. In view of toxicological importance of CQ metabolites, it is suggested that caution should be exercised in evaluation of higher dosage regimen of CQ in pregnant women.

Open randomized study of pyrimethamine-sulphadoxine vs. pyrimethamine-sulphadoxine plus probenecid for the treatment of uncomplicated Plasmodium falciparum malaria in children

BACKGROUND

Increasing drug resistance in Plasmodium falciparum has necessitated renewed search for cheap, effective alternatives to commonly available antimalarials, chloroquine and pyrimethamine-sulphadoxine, for the treatment of malaria in Africa. Probenecid, an inhibitor of organic anion transporters and multiresistance-associated proteins, can chemosensitize P. falciparum to pyrimethamine and sulphadoxine in vitro, but the clinical significance is unclear. We assessed the safety, treatment efficacy, and effects on gametocyte carriage of adding probenecid to pyrimethamine-sulphadoxine.

METHODS

We evaluated 151 children aged 12 years or younger who had uncomplicated P. falciparum malaria. Patients were randomly assigned pyrimethamine-sulphadoxine (25 mg/kg of the sulphadoxine component) or pyrimethamine-sulphadoxine as above plus probenecid 20-25 mg/kg of bodyweight in two divided doses daily for 3 days. The primary endpoints were parasitological cure rates on days 14 and 28.

RESULTS

Both regimens were well tolerated; no child was withdrawn because of drug intolerance. Fever (1.9 +/- 1.1 vs. 2.4 +/- 1.2 days, P = 0.02) and parasite clearance (2.3 +/- 0.9 vs. 2.7 +/- 1.1 days, P = 0.04) were significantly shorter, and the parasitological cure rate on day 14 (96.2%vs. 83.5%, P = 0.02) but not day 28 (79.4%vs. 72.6%, P = 0.4), was significantly higher in children treated with pyrimethamine-sulphadoxine-probenecid than in those treated with pyrimethamine-sulphadoxine. Gametocyte carriage was similar with both treatment regimens.

CONCLUSIONS

The combination of pyrimethamine-sulphadoxine, and probenecid, at a relatively moderate dose, improved treatment efficacy but had no effect on gametocyte carriage. The pyrimethamine-sulphadoxine-probenecid combination merits further evaluation as a potential treatment for use in Nigeria.

Antimalarial drug discovery: old and new approaches

New drugs against malaria are greatly needed. Many approaches to antimalarial drug discovery are available. These approaches must take into account specific concerns, in particular the requirement for very inexpensive and simple to use new therapies and the need to limit the cost of drug discovery. Among important efforts that are currently ongoing are the optimization of therapy with available drugs, including the use of combination therapy, the development of analogs of existing agents, the discovery of natural products, the use of compounds that were originally developed against other diseases, the evaluation of drug resistance reversers, and the consideration of new chemotherapeutic targets. The last category benefits from recent advances in malaria research technologies and genomics and is most likely to identify new classes of drugs. A number of new antimalarial therapies will likely be needed over the coming years, so it is important to pursue multiple strategies for drug discovery.

Plasmodium falciparum gametocytaemia in Nigerian children: before, during and after treatment with antimalarial drugs

We evaluated gametocyte carriage and intensities of gametocytaemia in 710 children presenting with acute, symptomatic, uncomplicated Plasmodium falciparum malaria who were treated with various antimalarial drug regimens: chloroquine (CQ); chloroquine plus chlorpheniramine, a histamine H1 receptor antagonist that reverses CQ resistance in P. falciparum in vitro and in vivo (CQCP); chloroquine plus ketotifen, a histamine H1 receptor antagonist that reverses CQ resistance in P. falciparum in vitro but not in vivo in the present study (CQK); chloroquine plus pyrimethamine-sulphadoxine (CQPS); amodiaquine (AQ); amodiaquine plus pyrimethamine-sulphadoxine (AQPS); and pyrimethamine-sulphadoxine (PS). On presentation, gametocyte carriage was significantly higher in CQ-resistant (CQ-R) than in CQ-sensitive (CQ-S) infections. Following CQ treatment, gametocyte carriage was significantly higher at all times after treatment and gametocyte density significantly higher on day 7 of follow-up in children with CQ-R than CQ-S infections. CQ treatment of CQ-R infections resulted in significantly higher density of gametocytaemia on day 7 compared with pre-treatment (day 0), but similar treatment of CQ-S infections resulted in significantly lower density of gametocytaemia on day 14 compared with day 0. Among children with CQ-R infections, those with mild (RI) resistance carried gametocytes significantly more often than those with moderate (RII) resistance on days 5 and 7 of follow-up (P = 0.04 and 0.01, respectively). Disposition kinetics of gametocytaemia using a non-compartmental method showed that the half life of gametocytaemia was longer and the clearance slower in children with CQ-R than in those with CQ-S infections. PS treatment was associated with significantly higher gametocyte carriage at all times between days 1 and 14, and significantly higher gametocytaemias on days 7 and 14 than in the other treatment regimens. Combination of AQ with PS significantly decreased gametocyte carriage at all times between days 1 and 14 of follow-up. Continuing use of CQ in CQ-R infections may encourage transmission of CQ-R infections; SP monotherapy is associated with significant gametocyte carriage and gametocytaemia and may encourage transmission of SP resistant infections as resistance to the drug increases.

Asymptomatic, recrudescent, chloroquine-resistant Plasmodium falciparum infections in Nigerian children: clinical and parasitological characteristics and implications for the transmission of drug-resistant parasites

The clinical features of their primary Plasmodium falciparum infections and of the recrudescent infections that emerged after chloroquine (CQ) treatment were evaluated in 48 children. Compared with the primary infections, the recrudescent infections were accompanied by significantly fewer symptoms and lower densities of parasitaemia but a much higher gametocytaemia:parasitaemia ratio (0.051 v. 0.00097; P = 0.0000000). There was a negative correlation between the recrudescent parasitaemia and the time elapsing between its detection and the subsequent onset of symptoms (r = -0.44; P = 0.006). The recrudescent parasitaemia was significantly lower [with geometric means and (ranges) of 117 (40-9756) v. 1112 (30-25,592) asexual forms/microl; P = 0.009] and the recrudescent gametocytaemia:parasitaemia ratio significantly higher (0.59 v. 0.024; P = 0.0000002) in those who were asymptomatic for at least 5 days after detection of the recrudescent parasitaemia than in those who were symptomatic at recrudescence. The disposition kinetics of the gametocytaemias in 18 primary infections--from nine children who failed CQ treatment and nine other children, matched for age and gender with the failures, who were CQ-treatment successes--were analysed. The results showed that the maximum gametocytaemia, time taken to reach the maximum gametocytaemia, the half-life of the gametocytaemia and the area inscribed by the plot of gametocytaemia against time were significantly higher, and the clearance of gametocytaemia significantly slower, in the children with primary infections that recrudesced. It therefore appears that continuing the use of CQ in areas where some parasites are resistant to the drug may confer survival and propagation advantages on the resistant parasites.

A randomized comparison of chloroquine and chloroquine plus ketotifen in the treatment of acute, uncomplicated, Plasmodium falciparum malaria in children

The increasing resistance of Plasmodium falciparum to chloroquine (CQ) has created urgent needs for the evaluation of alternative antimalarial drugs that are effective, safe, readily available and affordable. Ketotifen, a histamine H(1)-receptor antagonist that reverses CQ resistance in P. falciparum in vitro, may potentially enhance the effects of CQ in vivo. The effects of oral treatment with CQ alone (30 mg/kg base given over 3 days) were compared with those of this CQ regimen combined with ketotifen fumarate (0.25 mg at presentation followed by 0.125 mg/kg every 8 h for 5 days). The subjects were 145 children aged 1-10 years who were suffering from acute, symptomatic, uncomplicated, P. falciparum malaria: 74 given CQ alone and 71 given CQ plus ketotifen (CQK). Although the mean fever-clearance time was significantly shorter following treatment with CQK than after treatment with CQ alone, all other therapeutic responses were similar in the two treatment groups. Among siblings in whom there was clustering of infections, the likelihood of cure was also similar in the two treatment groups. Retreatment of 17 CQ-treatment failures with CQK produced a cure in six children, and retreatment of 22 CQK-treatment failures with CQK produced a cure in eight children. Retreatment of all drug failures with a combination of amodiaquine plus pyrimethamine-sulfadoxine resulted in complete clearance of parasitaemia and symptoms within 2-3 days and a cure 'rate' of 100% on day 28. The prevalences and intensities of gametocytaemias on day 3 or days 3, 7 and/or 14 combined were similar in the two groups. Adverse drug reactions were always tolerable, and limited to pruritus, gastro-intestinal disturbances, drowsiness and weight gain; the latter two adverse effects were significantly more frequent in those treated with CQK than in those given CQ alone. Haematological and biochemical parameters were not adversely affected by either treatment regimen. The findings indicate that - at least at the dosing regimen used in the present study and among children with acute, uncomplicated, P. falciparum malaria from Ibadan - the addition of ketotifen to CQ produced little or no significant enhancement of the antimalarial effect of CQ.

The effects of chloroquine, amodiaquine and chloroquine plus chlorpheniramine on the disposition kinetics of the hepatomegaly associated with acute, uncomplicated, Plasmodium falciparum malaria in children

The effects of chloroquine (CQ), amodiaquine (AQ) and CQ plus chlorpheniramine (a histamine H(1) antagonist that reverses CQ resistance in vitro and in vivo) on the disposition of the enlarged liver associated with acute, symptomatic, uncomplicated, Plasmodium falciparum malaria were evaluated. The subjects, 131 children aged 0.6-12 years who lived in an endemic area of Nigeria, were randomly allotted to the three treatment groups. The cumulative proportions of the children with complete resolution of their enlarged livers at 48, 96, 168 or 336 and 504 h after commencement of treatment were significantly higher in those treated with CQ plus chlorpheniramine (CQCP) than in the other two treatment groups (with P-values of 0.02, 0.001, 0.00000 and 0.00002, respectively). Among those with complete resolution, however, the times to resolution of 50% (HR50) or 90% (HR90) of the liver enlargement were similar in all the treatment groups. Complete resolution of the enlarged liver within 168 h was associated with a sensitive response to each treatment. Overall, in children with complete or partial resolution of their enlarged livers, the area produced by plotting liver size against time (i.e. the area under the curve of hepatomegaly v. time, or AUC(hp)) and the half-life of the hepatomegaly (t(1/2hp)) were significantly lower in the CQCP group than in the other two groups. The volume of blood completely cleared of the 'hepatic pathological processes' which led to the hepatomegaly (CL(Bhp)) and the fractional reduction of AUC(hp) at 48 and 96 h (i.e. AUC(hpFr148) and AUC(hpFr96)) were significantly higher in the CQCP group than in the other treatment groups. When the children with complete resolution of their liver enlargement were considered separately, t(1/2hp) (P=0.0008) but not AUC(hp) was found to be significantly lower, and AUC(hpFr196) (P=0.01) and CL(Bhl) (P=0.002) were found to be significantly higher in the CQCP group than in the other groups. Among the children with only partial resolution of their enlarged livers, the indices of resolution and the kinetic parameters of disposition were similar in all three groups. The data indicate that the addition of chlorpheniramine to chloroquine had a beneficial effect on both the early and late stages of the resolution of the liver enlargement associated with acute, symptomatic, uncomplicated, P. falciparum malaria.

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.

Reversal of chloroquine resistance in Plasmodium falciparum using combinations of chemosensitizers

Research into chloroquine resistance reversal in Plasmodium falciparum has revealed a widespread range of functionally and structurally diverse chemosensitizers. However, nearly all of these chemosensitizers reverse resistance optimally only at concentrations that are toxic to humans. Verapamil, desipramine, and trifluoperazine were shown to potentiate chloroquine accumulation in a chloroquine-resistant (CQ(r)) strain of P. falciparum, while progesterone, ivermectin, and cyclosporin A were not shown to potentiate chloroquine accumulation. The simultaneous use of two or even three of these chemosensitizers at concentrations within their therapeutic ranges in humans displayed an additive effect in potentiating chloroquine accumulation in the CQ(r) strain. The levels of resistance reversal achieved with these multiple combinations were comparable to those achieved with high concentrations of the single agents used to enhance the activity of chloroquine. No chemosensitizer, whether used singly or in combination, potentiated any change in chloroquine accumulation or a shift in the 50% inhibitory concentration for the chloroquine-sensitive strain. The use of combinations of chemosensitizers at concentrations not toxic to humans could effectively reverse chloroquine resistance without the marked toxicity from the use of a single agent at high concentrations. This cocktail of chemosensitizers may serve as a viable treatment to restore the efficacy of chloroquine in patients with malaria.

Structural analysis of chloroquine resistance reversal by imipramine analogs

For imipramine, desipramine, and eight analogs of these well-known drugs, an N-5-aminoalkyl substitution was a minimum but insufficient structural feature associated with chloroquine resistance reversal. Although a second distal aliphatic nitrogen atom was unnecessary for resistance reversal, the direction of the dipole moment vector was critical.

Pharmacokinetic aspects of chloroquine-induced pruritus: influence of dose and evidence for varied extent of metabolism of the drug

The significance of a pharmacokinetics basis in chloroquine (CQ)-induced pruritus was investigated by determining the disposition of the drug in two groups of volunteers; pruritus positive and pruritus negative. Single oral dose of 600 mg CQ was administered to each of 36 volunteers, 18 for each of the two groups. After a washout period of 9 months, 150 mg single oral dose of the drug was given to 12 of the same volunteers, six each from the two groups. Blood and urine samples were collected at predetermined times following administration of each dose. Concentrations of CQ and its major metabolite, desethylchloroquine (CQM), were measured in plasma and urine using an established HPLC method. Results showed that the ratio, AUC (CQ)/AUC (CQM), as well as AUC(0-48 h) and 24-h urinary CQ excretion were all significantly higher (P<0.05) in pruritus-positive compared to pruritus-negative volunteers, following administration of the 600-mg CQ dose. Also, urinary drug-metabolite ratios monitored over 0-48 h postdose were markedly higher in the pruritus positive group. However, after administration of the 150-mg dose, 24-h urinary CQ collection and urinary drug-metabolite ratios were highly comparable between the two groups (P>0.1). This study indicates that there might be a decreased metabolism of CQ in subjects susceptible to CQ-induced pruritus following ingestion of a therapeutic dose. It also suggests that the extent of metabolism of CQ in this group may be influenced by the dose of the drug. Comparatively higher CQ levels in pruritus susceptible subjects may possibly be responsible for the pruritus experienced by such individuals when given therapeutic regimen.

The chemotherapy of rodent malaria. LIX. Drug combinations to impede the selection of drug resistance, Part 3: Observations on cyproheptadine, an antihistaminic agent, with chloroquine

Several compounds in current clinical use as antihistaminic agents, among them cyproheptadine (CYP), have been shown, in experimental models, to reverse resistance of the asexual, intra-erythrocytic stages of rodent or human malarial parasites to chloroquine (CQ). Although preliminary clinical trials with CYP failed to confirm such activity in subjects with naturally acquired infection with Plasmodium falciparum, Nigerian investigators have reported that another antihistaminic, chlorpheniramine, significantly restores the blood schizontocidal action of CQ in semi-immune patients with CQ-resistant P. falciparum, when the two compounds are administered together. The rates at which resistance to CYP can be produced, in mice infected either with CQ-resistant P. yoelii ssp. NS or CQ-resistant P. yoelii nigeriensis, when drug-selection pressure is exerted with this compound alone have now been compared with the rate and extent to which resistance develops in infected animals that are treated with various combinations of CYP and CQ. The data indicate that, in both parasites, stable resistance develops slowly to CYP alone and that exposure to a combination of CYP plus CQ significantly impedes the selection of resistance to CYP. Although the antimalarial action of CYP is reported to extend to the pre-erythrocytic hepatic stages, there was no evidence of gametocytocidal activity in the present study. The future implications of these observations are discussed in relation to the clinical potential of CYP + CQ and similar combinations and possible future research.

Effects of absorption enhancers in chloroquine suppository formulations: I. In vitro release characteristics

The need to develop chloroquine suppository formulations that yield optimal bioavailability of the drug has been emphasized. This study demonstrates the effects of incorporation of known absorption-enhancing agents (nonionic surfactants and sodium salicylate) on the in vitro release characteristics of chloroquine from polyethylene glycol (1000:4000, 75:25%, w/w) suppositories. The release rates were determined using a modification of the continuous flow bead-bed dissolution apparatus for suppositories. Results showed that the extent of drug release from suppositories containing any of three surfactants (Tween 20, Tween 80 and Brij 35) was 100%, whereas 88% release was obtained with control formulation (without enhancer) (P<0.05). However, Tween 20 was more effective than Brij 35 and Tween 80 in improving the drug release rate. There was a concentration-dependent effect with Tween 20, and 4% (w/w) of this surfactant was associated with the highest increase in the rate of drug release from the suppositories. Sodium salicylate at a concentration of 25% (w/w) also significantly enhanced the drug release rate, but a higher concentration of the adjuvant markedly reduced both the rate and extent of drug release. Combined incorporation of Tween 20 and sodium salicylate did not significantly modify (P0.05) the rate of drug release when compared to the effect of the more effective single agent. Due to their effects in improving the drug release profiles coupled with their intrinsic absorption-promoting properties, it is suggested that incorporation of 4% (w/w) Tween 20 and/or 25% (w/w) sodium salicylate in the composite polyethylene glycol chloroquine suppository formulations, may result in enhancement of rectal absorption of the drug. This necessitates an in vivo validation.

Effect of chlorpheniramine on the pharmacokinetics of and response to chloroquine of Nigerian children with falciparum malaria

Chlorpheniramine (CP), a histamine H1-receptor antagonist, enhances the efficacy of chloroquine (CQ) in acute uncomplicated falciparum malaria. The effects of this combination therapy on the pharmacokinetic disposition of CQ is, however, unpredictable. A standard treatment with 25 mg CQ base per kg bodyweight was orally administered over 3 days, alone or in combination with CP, to 17 semi-immune Nigerian children with Plasmodium falciparum parasitaemia attending hospital in Lagos, Nigeria, and observed for 28 days. Whole-blood CQ concentrations were monitored 14 times during the follow-up by high-performance liquid chromatography analysis of blood dried on filter paper. Parasitaemia was determined on thick blood films stained with Giemsa, and treatment failures were established following the WHO classification for CQ resistance. Our pharmacokinetic data showed that the peak whole-blood CQ concentration was significantly increased (P < 0.05) by CP administration, and the time to achieve the peak was reduced in the presence of CP. The area under the first-moment drug-concentration-time curve was also significantly increased (P < 0.05) by CP administration. Treatment with CQ-CP combination resulted in a shorter parasite clearance time (2.0 +/- 0.5 days) and a higher cure rate (87.5%) compared to treatment with CQ alone (3.5 +/- 0.5 days; 66.7%). Our data suggest that CP enhanced the efficacy of CQ against resistant P. falciparum in acute uncomplicated malaria by increasing the uptake/concentration of CQ in resistant parasites.

Comparative cardiac effects of halofantrine and chloroquine plus chlorpheniramine in children with acute uncomplicated falciparum malaria

The cardiac effects of halofantrine (HF) and chloroquine plus chlorpheniramine (CQ-CP), a histamine H1 antagonist which reverses chloroquine insensitivity in Plasmodium falciparum in vitro and in vivo, were assessed in 41 children with acute symptomatic uncomplicated P. falciparum malaria by electrocardiographic and clinical monitoring over a period of 14 days. In addition, the cardiac effects of chloroquine (CQ) alone and CQ-CP were compared in 10 age- and sex-matched children. HF produced a significantly higher proportion of abnormally prolonged P-R interval (8 abnormally prolonged out of 132 total P-R events) than CQ-CP (1 of 133 P-R events) (P = 0.03), but a similar proportion of prolonged Q-Tc interval to that of CQ-CP (46 of 149 versus 29 of 134 events, P = 0.07). Compared with pre-treatment Q-Tc, HF significantly prolonged this interval from 6 to 96 h post treatment with a maximum effect at 24 h after commencing HF treatment. CQ-CP by contrast produced significant changes in Q-Tc values from 6 to only 48 h with a maximum effect at 48 h. HF-induced Q-Tc prolongations were significantly higher than those of CQ-CP only at 24 h. The cardiac effects of CQ-CP were similar to those of CQ alone. Despite the electrocardiogram abnormalities, rhythm disturbance was rare and there was no clinical symptom in any of the treatment groups. Compared with HF, CQ-CP produced cardiac effects that were less severe and in fewer children with acute falciparum malaria. The addition of CP to CQ does not significantly amplify the cardiac effects of CQ in children with acute uncomplicated falciparum malaria.

Comparative efficacy of chloroquine plus chlorpheniramine and halofantrine in acute uncomplicated falciparum malaria in Nigerian children

In the face of growing chloroquine resistance of Plasmodium falciparum, efforts to prolong the clinical usefulness of the drug have partly concentrated on its combination with potential resistance-reversing compounds. However, clinical studies on such combinations have been limited. We have compared the efficacy of halofantrine, an arylaminoalcohol effective in chloroquine resistant malaria, and a combination of chloroquine plus chlorpheniramine, a histamine H1 receptor antagonist which reverses chloroquine resistance of P. falciparum in vitro and in vivo, in 100 children with acute symptomatic uncomplicated falciparum malaria in an area in Nigeria where the rate of chloroquine resistance is 35-45%. Both chloroquine plus chlorpheniramine and halofantrine produced similar parasite and fever clearance times and cure rates (96%). Both treatment regimens were relatively well tolerated. Pruritus was commoner in patients treated with chloroquine plus chlorpheniramine than in those treated with halofantrine. Intravascular haemolysis occurred in one patient, and abdominal pain with or without diarrhoea occurred in 4 patients, treated with halofantrine. In vitro, the chloroquine resistance of P. falciparum isolates obtained from the patients was reversed by verapamil. All patients with isolates which were chloroquine-resistant in vitro were cured by either therapy. These results indicate that chloroquine plus chlorpheniramine is as effective as halofantrine and is without overt deleterious effect in treating acute uncomplicated chloroquine-resistant falciparum malaria in children, and may be a clinically useful alternative for this purpose in Nigeria.

Enhancement of the antimalarial effect of chloroquine by chloropheniramine in vivo

The efficacy of chloroquine and chloroquine plus chloropheniramine, a histamine H1 receptor blocker which reverses chloroquine insensitivity in Plasmodium falciparum in vitro, was studied in 96 children with acute symptomatic uncomplicated falciparum malaria. The chloroquine/chloropheniramine combination produced a significantly higher cure rate than chloroquine alone and cured 77% of children with chloroquine treatment failures. Children with chloroquine treatment failure had mean plasma chloroquine concentrations above the minimum therapeutic concentration for the area. Chloroquine concentrations in plasma and red blood cells and ratio of red cell to plasma chloroquine concentrations on days 3 and 7 after initiation of therapy were not significantly different in the two groups. Chloroquine/chloropheniramine produces a higher cure rate than chloroquine alone and reverses chloroquine insensitivity in Plasmodium falciparum in vivo. It may be a useful way of optimising the antimalarial effect of chloroquine.

Comparative efficacy of chloroquine plus chlorpheniramine and pyrimethamine/sulfadoxine in acute uncomplicated falciparum malaria in Nigerian children

The efficacy of pyrimethamine/sulfadoxine (PS) and chloroquine plus chlorpheniramine, a histamine H1 receptor blocker which reverses chloroquine insensitivity in Plasmodium falciparum in vitro, was evaluated in 100 consecutive children with acute symptomatic uncomplicated falciparum malaria. Parasitaemia on day 3 following initiation of treatment, fever and symptom clearance times were significantly lower in the chloroquine/chlorpheniramine (CQ/CP) combination group than in the PS group. The cure rate was also significantly higher in the combination group. The combination cured all children who had failed PS treatment. Gametocytaemia and the gametocyte carrier rate following therapy were significantly lower in the combination group than in those receiving PS. Both treatments were well tolerated but adverse drug reactions were commoner in the children given PS. CQ/CP is effective in PS treatment failure in Nigerian children and may be useful for this condition in African children in general.

Comparative efficacy of chloroquine/chlorpheniramine combination and mefloquine for the treatment of chloroquine-resistant Plasmodium falciparum malaria in Nigerian children

The efficacy of chloroquine for treating uncomplicated Plasmodium falciparum malaria was evaluated in 98 children in Nigeria. Forty-three children failed chloroquine treatment (21 RI, 20 RII, 2 RIII) and were allocated at random to receive multiple doses of a combination of chloroquine and chlorpheniramine or a single dose of mefloquine orally. The parasite and fever clearance times were 2.7 +/- 1.0 d and 1.6 +/- 0.6 d, respectively, in children treated with the combination and 1.6 +/- 0.5 d and 1.1 +/- 0.3 d, respectively, for mefloquine treatment. The cure rate on day 14 was 81% among children receiving chloroquine/chlorpheniramine and 100% on days 14 and 28 with mefloquine. Three children who failed treatment with the combination responded promptly to mefloquine, with clearance of parasitaemia and fever within 48 h. Adverse effects following therapy were minimal, comprising drowsiness and pruritus in the combination group and abdominal discomfort in the mefloquine group. Isolates obtained from children who failed initial treatment with chloroquine were resistant to chloroquine but sensitive to mefloquine in vitro. The efficacy of this combination of chloroquine and chlorpheniramine confirmed previous reports of enhanced activity and it was effective in the management of mild to moderate chloroquine-resistant malaria. Although mefloquine is more effective in this respect, the combination, when developed, will be a valuable addition to the list of drugs for the management of chloroquine-resistant malaria.