In vitro susceptibility of Plasmodium falciparum to chloroquine, amodiaquine, quinine, mefloquine, and sulfadoxine/pyrimethamine in Equatorial Guinea

Temporal evolution of the resistance genotypes of Plasmodium falciparum in isolates from Equatorial Guinea during 20 years (1999 to 2019)

Background

Malaria is one of the deadliest diseases in the world, particularly in Africa. As such, resistance to anti-malarial drugs is one of the most important problems in terms of global malaria control. This study assesses the evolution of the different resistance markers over time and the possible influence of interventions and treatment changes that have been made in Equatorial Guinea.

Methods

A total of 1223 biological samples obtained in the period 1999 to 2019 were included in the study. Screening for mutations in the pfdhfr, pfdhps, pfmdr1, and pfcrt genes was carried out by nested PCR and restriction-fragment length polymorphisms (RFLPs), and the study of pfk13 genes was carried out by nested PCR, followed by sequencing to determine the presence of mutations.

Results

The partially and fully resistant haplotypes (pfdhfr + pfdhps) were found to increase over time. Moreover, in 2019, the fully resistant haplotype was found to be increasing, although its super-resistant counterpart remains much less prevalent. A continued decline in pfmdr1 and pfcrt gene mutations over time was also found. The number of mutations detected in pfk13 has increased since 2008, when artemisinin-based combination therapy (ACT) were first introduced, with more mutations being observed in 2019, with two synonymous and five non-synonymous mutations being detected, although these are not related to resistance to ACT. In addition, the non-synonymous A578S mutation, which is the most frequent on the African continent, was detected in 2013, although not in the following years.

Conclusions

Withdrawal of the use of chloroquine (CQ) as a treatment in Equatorial Guinea has been shown to be effective over time, as wild-type parasite populations outnumber mutant populations. The upward trend observed in sulfadoxine-pyrimethamine (SP) resistance markers suggest its misuse, either alone or in combination with artesunate (AS) or amodiaquine (AQ), in some areas of the country, as was found in a previous study conducted by this group, which allows selective pressure from SP to continue. Single nucleotide polymorphisms (SNPs) 540E and 581G do not exceed the limit of 50 and 10%, respectively, thus meaning that SP is still effective as an intermittent preventive treatment (IPT) in this country. As for the pfk13 gene, no mutations have been detected in relation to resistance to ACT. However, in 2019 there is a greater accumulation of non-synonymous mutations compared to years prior to 2008.

Graphical Abstract

Trends in Molecular Markers Associated with Resistance to Sulfadoxine-Pyrimethamine (SP) Among Plasmodium falciparum Isolates on Bioko Island, Equatorial Guinea: 2011-2017

PURPOSE

Antimalarial drug resistance is one of the major challenges in global efforts to control and eliminate malaria. In 2006, sulfadoxine-pyrimethamine (SP) replaced with artemisinin-based combination therapy (ACT) on Bioko Island, Equatorial Guinea, in response to increasing SP resistance, which is associated with mutations in the dihydrofolate reductase (Pfdhfr) and dihydropteroate synthase (Pfdhps) genes.

PATIENTS AND METHODS

To evaluate the trend of molecular markers associated with SP resistance on Bioko Island from 2011 to 2017, 179 samples collected during active case detection were analysed by PCR and DNA sequencing.

RESULTS

Pfdhfr and Pfdhps gene sequences were obtained for 90.5% (162/179) and 77.1% (138/179) of the samples, respectively. For Pfdhfr, 97.5% (158/162), 95.7% (155/162) and 98.1% (159/162) of the samples contained N51I, C59R and S108N mutant alleles, respectively. And Pfdhps S436A, A437G, K540E, A581G, and A613S mutations were observed in 25.4% (35/138), 88.4% (122/138), 5.1% (7/138), 1.4% (2/138), and 7.2% (10/138) of the samples, respectively. Two classes of previously described Pfdhfr-Pfdhps haplotypes associated with SP resistance and their frequencies were identified: partial (IRNI-SGKAA, 59.4%) and full (IRNI-SGEAA, 5.5%) resistance. Although no significant difference was observed in different time periods (p>0.05), our study confirmed a slowly increasing trend of the frequencies of these SP-resistance markers in Bioko parasites over the 7 years investigated.

CONCLUSION

The findings reveal the general existence of SP-resistance markers on Bioko Island even after the replacement of SP as a first-line treatment for uncomplicated malaria. Continuous molecular monitoring and additional control efforts in the region are urgently needed.

An unusual distribution of the kdr gene among populations of Anopheles gambiae on the island of Bioko, Equatorial Guinea

In West Africa, Anopheles gambiae exists in discrete subpopulations known as the M and S molecular forms. Although these forms occur in sympatry, pyrethroid knock-down resistance (kdr) is strongly associated with the S molecular form. On the island of Bioko, Equatorial Guinea we found high frequencies of the kdr mutation in M form individuals (55.8%) and a complete absence of kdr in the S form. We also report the absence of the kdr allele in M and S specimens from the harbour town of Tiko in Cameroon, representing the nearest continental population to Bioko. The kdr allele had previously been reported as absent in populations of An. gambiae on Bioko. Contrary to earlier reports, sequencing of intron-1 of this sodium channel gene revealed no fixed differences between M form resistant and susceptible individuals. The mutation may have recently arisen independently in the M form on Bioko due to recent and intensive pyrethroid application.

Efficacies of chloroquine, sulfadoxine-pyrimethamine and quinine in the treatment of uncomplicated, Plasmodium falciparum malaria in eastern Sudan

The efficacies of several antimalarial drugs in the treatment of uncomplicated Plasmodium falciparum malaria were compared, during an open, randomized trial, in New Halfa, eastern Sudan. The 96 patients who completed the 28 days of follow-up were treated with chloroquine (N = 26), sulfadoxine-pyrimethamine (N = 38) or quinine (N = 32). No treatment failures were observed among the patients given sulfadoxine-pyrimethamine. Only 23.1% of the patients given chloroquine showed adequate clinical response, however, the rest showing early (15.4%) or, more frequently, late (61.5%) treatment failure. In terms of parasitological failure, 54.1% of the patients given chloroquine showed early RI resistance, 7.7% showed late RI, and 15.1% showed RIII. Most (90.6%) of the patients treated with quinine had adequate treatment responses, the rest having late treatment failures (and late RI). The frequency of treatment failure was significantly higher, however, among the patients given chloroquine than in the quinine-treatment arm. The present results and those of earlier investigations indicate that the problem of chloroquine resistance is worsening in eastern Sudan, and that the use of chloroquine as the first-line drug for the treatment of uncomplicated malaria in this area is now compromised. The response to quinine may also be faltering.

Synergism between amodiaquine and its major metabolite, desethylamodiaquine, against Plasmodium falciparum in vitro

The in vitro activity of the prodrug amodiaquine and its metabolite monodesethyl-amodiaquine has been studied for three strains of Plasmodium falciparum: LS-2, LS-3, and LS-1. Both compounds showed significant activity against all three strains; the activity of amodiaquine was slightly higher than that of the metabolite. By use of a checkerboard design, interaction studies with both compounds yielded evidence of significant synergism; means of the sums of the fractional inhibitory concentrations were 0.0392 to 0.0746 for strain LS-2, 0.1567 to 0.3102 for strain LS-3, and 0.025 to 0.3369 for strain LS-1. In further investigations, the interaction of amodiaquine with monodesethyl-amodiaquine was tested at clinically relevant concentrations of both compounds. In these studies, involving amodiaquine at picomolar and femtomolar concentrations, the compound was found to exert high potentiating activity on monodesethyl-amodiaquine. This interaction produced mean ratios of observed to expected activity of 0.0505 to 0.0642 for strain LS-2, 0.0882 to 0.3820 for strain LS-3, and 0.0752 to 0.2924 for strain LS-1. The synergistic activity was most marked at monodesethyl-amodiaquine/amodiaquine ratios up to 100,000:1 but was still evident at higher ratios.

Epidemiological models for the spread of anti-malarial resistance

BACKGROUND

The spread of drug resistance is making malaria control increasingly difficult. Mathematical models for the transmission dynamics of drug sensitive and resistant strains can be a useful tool to help to understand the factors that influence the spread of drug resistance, and they can therefore help in the design of rational strategies for the control of drug resistance.

METHODS

We present an epidemiological framework to investigate the spread of anti-malarial resistance. Several mathematical models, based on the familiar Macdonald-Ross model of malaria transmission, enable us to examine the processes and parameters that are critical in determining the spread of resistance.

RESULTS

In our simplest model, resistance does not spread if the fraction of infected individuals treated is less than a threshold value; if drug treatment exceeds this threshold, resistance will eventually become fixed in the population. The threshold value is determined only by the rates of infection and the infectious periods of resistant and sensitive parasites in untreated and treated hosts, whereas the intensity of transmission has no influence on the threshold value. In more complex models, where hosts can be infected by multiple parasite strains or where treatment varies spatially, resistance is generally not fixed, but rather some level of sensitivity is often maintained in the population.

CONCLUSIONS

The models developed in this paper are a first step in understanding the epidemiology of anti-malarial resistance and evaluating strategies to reduce the spread of resistance. However, specific recommendations for the management of resistance need to wait until we have more data on the critical parameters underlying the spread of resistance: drug use, spatial variability of treatment and parasite migration among areas, and perhaps most importantly, cost of resistance.

Epidemiology of drug-resistant malaria

Since the first reports of chloroquine-resistant falciparum malaria in southeast Asia and South America almost half a century ago, drug-resistant malaria has posed a major problem in malaria control. By the late 1980s, resistance to sulfadoxine-pyrimethamine and to mefloquine was also prevalent on the Thai-Cambodian and Thai-Myanmar (Thai-Burmese) borders, rendering them established multidrug-resistant (MDR) areas. Chloroquine resistance spread across Africa during the 1980s, and severe resistance is especially found in east Africa. As a result, more than ten African countries have switched their first-line drug to sulfadoxine-pyrimethamine. Of great concern is the fact that the efficacy of this drug in Africa is progressively deteriorating, especially in foci in east Africa, which are classified as emerging MDR areas. Urgent efforts are needed to lengthen the lifespan of sulfadoxine-pyrimethamine and to identify effective, affordable, alternative antimalarial regimens. Molecular markers for antimalarial resistance have been identified, including pfcrt polymorphisms associated with chloroquine resistance and dhfr and dhps polymorphisms associated with sulfadoxine-pyrimethamine resistance. Polymorphisms in pfmdr1 may also be associated with resistance to chloroquine, mefloquine, quinine, and artemisinin. Use of such genetic information for the early detection of resistance foci and future monitoring of drug-resistant malaria is a potentially useful epidemiological tool, in conjunction with the conventional in-vivo and in-vitro drug-sensitivity assessments. This review describes the various features of drug resistance in Plasmodium falciparum, including its determinants, current status in diverse geographical areas, molecular markers, and their implications.

Role of cytochrome P450 3A in the metabolism of mefloquine in human and animal hepatocytes

We studied mefloquine metabolism in cells and microsomes isolated from human and animal (monkey, dog, rat) livers. In both hepatocytes and microsomes, mefloquine underwent conversion to two major metabolites, carboxymefloquine and hydroxymefloquine. In human cells and microsomes these metabolites only were formed, as already demonstrated in vivo, while in other species several unidentified metabolites were also detected. After a 48 hr incubation with human and rat hepatocytes, metabolites accounted for 55-65% of the initial drug concentration, whereas in monkey and dog hepatocytes, mefloquine was entirely metabolized after 15 and 39 hrs, respectively. The consumption of mefloquine was less extensive in microsomes, and unchanged drug represented 60% (monkey) to 85-100% (human, dog, rat) of the total radioactivity after 5 hr incubations. The involvement of the cytochrome P450 3A subfamily in mefloquine biotransformation was suggested by several lines of evidence. Firstly, mefloquine metabolism was strongly increased in hepatic microsomes from dexamethasone-pretreated rats, and also in human and rat hepatocytes after prior treatment with a cytochrome P450 3A inducer. Secondly, mefloquine biotransformation in rifampycin-induced human hepatocytes was inhibited in a concentration-dependent manner by the cytochrome P450 3A inhibitor ketoconazole and thirdly, a strong correlation was found between erythromycin-N-demethylase activity (mediated by cytochrome P450 3A) and mefloquine metabolism in human microsomes (r=0.81, P < 0.05, N=13). Collectively, these findings concerning the role of cytochrome P450 3A in mefloquine metabolism may have important in vivo consequences especially with regard to the choice of agents used in multidrug antimalarial regimens.

Atovaquone and proguanil versus pyrimethamine/sulfadoxine for the treatment of acute falciparum malaria in Zambia

Atovaquone and proguanil hydrochloride are blood schizonticides that demonstrate in vitro synergy against drug-resistant strains of Plasmodium falciparum. When coadministered, they may therefore be effective for the treatment of malaria in regions where there is known or suspected drug resistance. In an open-label, randomized, parallel-group, clinical trial conducted in Zambia, 163 patients (age range, 14 to 54 years) with acute P falciparum malaria were randomly assigned to receive treatment with atovaquone and proguanil hydrochloride (1000 and 400 mg, respectively, administered orally at 24-hour intervals for 3 doses; n = 82) or pyrimethamine/sulfadoxine (75/1500 mg administered orally as a single dose; n = 81). Efficacy was assessed by cure rate (the percentage of patients in whom parasitemia was eliminated and did not recur during 28 days of follow-up), parasite clearance time (PCT), and fever clearance time (FCT). Safety was determined by sequential clinical and laboratory assessments over 28 days. Cure rates did not differ significantly between patients treated with atovaquone and proguanil (100%) and those treated with pyrimethamine/sulfadoxine (98.8%). Patients in the atovaquone and proguanil group had a significantly shorter FCT than patients in the pyrimethamine/sulfadoxine group (mean, 30.4 vs 44.9 hours; P < 0.05) but a longer PCT (mean, 64.0 vs 51.4 hours; P < 0.05). Both treatments were well tolerated; adverse events and laboratory abnormalities were typical of those normally observed in patients with malaria. In this study, the combination of atovaquone and proguanil was equally effective and as well tolerated as pyrimethamine/sulfadoxine for the treatment of acute, uncomplicated, drug-resistant falciparum malaria in Zambia.

In vitro growth inhibition of Plasmodium falciparum by retinol at concentrations present in normal human serum

To assess the in vitro effect of retinol on Plasmodium falciparum, the standard isolates 3D7, D10, W2 and K1 in continuous culture were exposed to retinol added in concentrations ranging from 10(-7) to 0.1 mumol/l. Parasite growth inhibition was assessed from 3H-hypoxanthine incorporation. Triplicate experiments were performed at physiological pH and in the case of D10, additional experiments were performed at pH 7.2 and 7.6. Final media retinol concentrations were assayed using high performance liquid chromatography. Retinol inhibited growth of both asynchronous and synchronous cultures of 3D7 and D10 and asynchronous cultures of W2 and K1. IC50 values determined from assayed media concentrations ranged from 0.2 to 3.9 mumol/l and were comparable to concentrations in normal human serum (1.0-3.0 mumol/l). IC50 values for asynchronous D10 cultures at pH 7.2 were lower than at pH 7.4 or 7.6 (0.5, 3.9 and 5.0 mumol/l, respectively); results from synchronous cultures were similar. These data suggest that P. falciparum is a retinol-sensitive parasite, especially at pH levels equivalent to those in an acidotic patient. Adjunctive retinol therapy may have a role in clinical management of malaria.