Elevated levels of methaemoglobin in Tanzanian children with severe and uncomplicated malaria

Methemoglobin levels in malaria: a systematic review and meta-analysis of its association with Plasmodium falciparum and Plasmodium vivax infections and disease severity

Reports indicate that Plasmodium infections influence methemoglobin levels. However, findings have been inconclusive or have varied across different geographic and demographic contexts. This systematic review and meta-analysis aimed to consolidate existing data regarding the association between Plasmodium infections and alterations in methemoglobin levels related to the severity of the infection. A comprehensive literature search of several databases, including Ovid, ProQuest, Embase, Scopus, MEDLINE, and PubMed, was conducted to identify relevant studies that examined methemoglobin levels in patients with malaria. Qualitative synthesis and meta-analysis of the pooled standardized mean difference were conducted to synthesize the differences in methemoglobin levels between: (1) patients with malaria and those without malaria and (2) patients with severe malaria and those with uncomplicated malaria based on various themes including publication year, study design, study area, Plasmodium species, age group, symptomatic status, severity status, and method of malaria detection. Of the 1846 studies that were initially identified from the main databases and additional searches on Google Scholar, 10 studies met the eligibility criteria and were selected for this review. The systematic review distinctly highlighted an association between malaria and elevated methemoglobin levels, an observation consistent across diverse geographical regions and various Plasmodium species. Furthermore, the meta-analysis confirmed this by demonstrating increased methemoglobin levels in patients with malaria compared to those without malaria (P < 0.001, Hedges' g 2.32, 95% CI 1.36-3.29, I2 97.27, 8 studies). Moreover, the meta-analysis found elevated methemoglobin levels in patients with severe malaria compared to those with uncomplicated malaria (P < 0.001, Hedges' g 2.20, 95% CI 0.82-3.58, I2 96.20, 5 studies). This systematic review and meta-analysis revealed increased methemoglobin levels in patients with P. falciparum and P. vivax infections, with a notable association between elevated methemoglobin levels and severe malaria. Future research should focus on elucidating the specific mechanisms by which changes in methemoglobin levels are related to infections by P. falciparum and P. vivax, particularly in terms of severity, and how these alterations could potentially impact patient management and treatment outcomes.

Methylene Blue-Based Combination Therapy with Amodiaquine Prevents Severe Malaria in an Experimental Rodent Model

Untreated malaria can progress rapidly to severe forms (<24 h). Moreover, resistance to antimalarial drugs is a threat to global efforts to protect people from malaria. Given this, it is clear that new chemotherapy must be developed. We contribute new data about using methylene blue (MB) to cure malaria and cerebral malaria in a combined therapy with common antimalarial drugs, including mefloquine (MQ) and amodiaquine (AQ). A C57BL6/J mouse model was used in an experimental cerebral malaria model. Mice were infected with Plasmodium berghei ANKA on Day 0 (D0) and the treatment started on D3 (nearly 1% parasitaemia) with AQ, MQ or MB alone or in combination with AQ or MQ. AQ, MQ and MB alone were unable to prevent cerebral malaria as part of a late chemotherapy. MB-based combination therapies were efficient even if treatment began at a late stage. We found a significant difference in survival rate (p < 0.0001) between MBAQ and the untreated group, but also with the AQ (p = 0.0024) and MB groups (p < 0.0001). All the infected mice treated with MB in combination with AQ were protected from cerebral malaria. Partial protection was demonstrated with MB associated with MQ. In this group, a significant difference was found between MBMQ and the untreated group (p < 0.0001), MQ (p = 0.0079) and MB (p = 0.0039). MB associated with AQ would be a good candidate for preventing cerebral malaria.

Development of weight and age-based dosing of daily primaquine for radical cure of vivax malaria

BACKGROUND

In many endemic areas, Plasmodium vivax malaria is predominantly a disease of young adults and children. International recommendations for radical cure recommend fixed target doses of 0.25 or 0.5 mg/kg/day of primaquine for 14 days in glucose-6-phosphate dehydrogenase normal patients of all ages. However, for many anti-malarial drugs, including primaquine, there is evidence that children have lower exposures than adults for the same weight-adjusted dose. The aim of the study was to develop 14-day weight-based and age-based primaquine regimens against high-frequency relapsing tropical P. vivax.

METHODS

The recommended adult target dose of 0.5 mg/kg/day (30 mg in a 60 kg patient) is highly efficacious against tropical P. vivax and was assumed to produce optimal drug exposure. Primaquine doses were calculated using allometric scaling to derive a weight-based primaquine regimen over a weight range from 5 to 100 kg. Growth curves were constructed from an anthropometric database of 53,467 individuals from the Greater Mekong Subregion (GMS) to define weight-for-age relationships. The median age associated with each weight was used to derive an age-based dosing regimen from the weight-based regimen.

RESULTS

The proposed weight-based regimen has 5 dosing bands: (i) 5-7 kg, 5 mg, resulting in 0.71-1.0 mg/kg/day; (ii) 8-16 kg, 7.5 mg, 0.47-0.94 mg/kg/day; (iii) 17-40 kg, 15 mg, 0.38-0.88 mg/kg/day; (iv) 41-80 kg, 30 mg, 0.37-0.73 mg/kg/day; and (v) 81-100 kg, 45 mg, 0.45-0.56 mg/kg/day. The corresponding age-based regimen had 4 dosing bands: 6-11 months, 5 mg, 0.43-1.0 mg/kg/day; (ii) 1-5 years, 7.5 mg, 0.35-1.25 mg/kg/day; (iii) 6-14 years, 15 mg, 0.30-1.36 mg/kg/day; and (iv) ≥ 15 years, 30 mg, 0.35-1.07 mg/kg/day.

CONCLUSION

The proposed weight-based regimen showed less variability around the primaquine dose within each dosing band compared to the age-based regimen and is preferred. Increased dose accuracy could be achieved by additional dosing bands for both regimens. The age-based regimen might not be applicable to regions outside the GMS, which must be based on local anthropometric data. Pharmacokinetic data in small children are needed urgently to inform the proposed regimens.

Extracellular methemoglobin promotes cyto-adherence of uninfected RBC to endothelial cells: Insight into cerebral malaria pathology

The endothelial cell barrier is tightly regulated, and disruption or the leaky behavior of the barrier leads to pathology. Disturbance of blood-brain barrier is observed during viral infection, cerebral malaria, and acute hemorrhagic encephalitis. Red blood cells (RBCs) bind to the endothelial cells (ECs) and their affinity towards ECs enhances in the presence of Plasmodium falciparum infection. ECs stimulated with methemoglobin (MetHb; 20 µM) for 1 hour exhibit high levels of cyto-adherence receptors CD36 and ICAM-1 on their cell surface compared with unstimulated cells. These ECs have acquired affinity towards uninfected RBCs in flow at arterial shear stress. SEM analysis indicates that EC-RBC cyto-adherence involved multiple attachment points. Initially, ECs bind single layer of RBCs and the number of RBCs increases over time to give high-order cyto-adherence with more than 30 RBCs adhered to each endothelial cell. The cyto-adherence complexes are stable to high shear stress and can withstand shear stress up to 450 dyne/cm ² . MetHb-treated ECs exhibited high reactive oxygen species level, and preincubation of ECs with antioxidant (NAC or mannitol) abolished the formation of EC-RBC cyto-adherence complexes. In addition, gallic acid (present in red wine) and green tea extract has inhibited the formation of EC-RBC cyto-adherence complex. A better understanding of gallic acid and tea polyphenol targeting pathological cyto-adherence may allow us to develop a better adjuvant therapy for cerebral malaria and other noninfectious diseases.

Citrulline protects mice from experimental cerebral malaria by ameliorating hypoargininemia, urea cycle changes and vascular leak

Clinical and model studies indicate that low nitric oxide (NO) bioavailability due in part to profound hypoargininemia contributes to cerebral malaria (CM) pathogenesis. Protection against CM pathogenesis may be achieved by altering the diet before infection with Plasmodium falciparum infection (nutraceutical) or by administering adjunctive therapy that decreases CM mortality (adjunctive therapy). This hypothesis was tested by administering citrulline or arginine in experimental CM (eCM). We report that citrulline injected as prophylaxis immediately post infection (PI) protected virtually all mice by ameliorating (i) hypoargininemia, (ii) urea cycle impairment, and (iii) disruption of blood brain barrier. Citrulline prophylaxis inhibited plasma arginase activity. Parasitemia was similar in citrulline- and vehicle control-groups, indicating that protection from pathogenesis was not due to decreased parasitemia. Both citrulline and arginine administered from day 1 PI in the drinking water significantly protected mice from eCM. These observations collectively indicate that increasing dietary citrulline or arginine decreases eCM mortality. Citrulline injected ip on day 4 PI with quinine-injected ip on day 6 PI partially protected mice from eCM; citrulline plus scavenging of superoxide with pegylated superoxide dismutase and pegylated catalase protected all recipients from eCM. These findings indicate that ameliorating hypoargininemia with citrulline plus superoxide scavenging decreases eCM mortality.

Single low-dose primaquine for blocking transmission of Plasmodium falciparum malaria - a proposed model-derived age-based regimen for sub-Saharan Africa

BACKGROUND

In 2012, the World Health Organization recommended blocking the transmission of Plasmodium falciparum with single low-dose primaquine (SLDPQ, target dose 0.25 mg base/kg body weight), without testing for glucose-6-phosphate dehydrogenase deficiency (G6PDd), when treating patients with uncomplicated falciparum malaria. We sought to develop an age-based SLDPQ regimen that would be suitable for sub-Saharan Africa.

METHODS

Using data on the anti-infectivity efficacy and tolerability of primaquine (PQ), the epidemiology of anaemia, and the risks of PQ-induced acute haemolytic anaemia (AHA) and clinically significant anaemia (CSA), we prospectively defined therapeutic-dose ranges of 0.15-0.4 mg PQ base/kg for children aged 1-5 years and 0.15-0.5 mg PQ base/kg for individuals aged ≥6 years (therapeutic indices 2.7 and 3.3, respectively). We chose 1.25 mg PQ base for infants aged 6-11 months because they have the highest rate of baseline anaemia and the highest risks of AHA and CSA. We modelled an anthropometric database of 661,979 African individuals aged ≥6 months (549,127 healthy individuals, 28,466 malaria patients and 84,386 individuals with other infections/illnesses) by the Box-Cox transformation power exponential and tested PQ doses of 1-15 mg base, selecting dosing groups based on calculated mg/kg PQ doses.

RESULTS

From the Box-Cox transformation power exponential model, five age categories were selected: (i) 6-11 months (n = 39,886, 6.03%), (ii) 1-5 years (n = 261,036, 45.46%), (iii) 6-9 years (n = 20,770, 3.14%), (iv) 10-14 years (n = 12,155, 1.84%) and (v) ≥15 years (n = 328,132, 49.57%) to receive 1.25, 2.5, 5, 7.5 and 15 mg PQ base for corresponding median (1st and 99th centiles) mg/kg PQ base of: (i) 0.16 (0.12-0.25), (ii) 0.21 (0.13-0.37), (iii) 0.25 (0.16-0.38), (iv) 0.26 (0.15-0.38) and (v) 0.27 (0.17-0.40). The proportions of individuals predicted to receive optimal therapeutic PQ doses were: 73.2 (29,180/39,886), 93.7 (244,537/261,036), 99.6 (20,690/20,770), 99.4 (12,086/12,155) and 99.8% (327,620/328,132), respectively.

CONCLUSIONS

We plan to test the safety of this age-based dosing regimen in a large randomised placebo-controlled trial (ISRCTN11594437) of uncomplicated falciparum malaria in G6PDd African children aged 0.5 - 11 years. If the regimen is safe and demonstrates adequate pharmacokinetics, it should be used to support malaria elimination.

Methemoglobin and nitric oxide therapy in Ugandan children hospitalized for febrile illness: results from a prospective cohort study and randomized double-blind placebo-controlled trial

BACKGROUND

Exposure of red blood cells to oxidants increases production of methemoglobin (MHb) resulting in impaired oxygen delivery to tissues. There are no reliable estimates of methemoglobinemia in low resource clinical settings. Our objectives were to: i) evaluate risk factors for methemoglobinemia in Ugandan children hospitalized with fever (study 1); and ii) investigate MHb responses in critically ill Ugandan children with severe malaria treated with inhaled nitric oxide (iNO), an oxidant that induces MHb in a dose-dependent manner (study 2).

METHODS

Two prospective studies were conducted at Jinja Regional Referral Hospital in Uganda between 2011 and 2013. Study 1, a prospective cohort study of children admitted to hospital with fever (fever cohort, n = 2089 children 2 months to 5 years). Study 2, a randomized double-blind placebo-controlled parallel arm trial of room air placebo vs. 80 ppm iNO as an adjunctive therapy for children with severe malaria (RCT, n = 180 children 1-10 years receiving intravenous artesunate and 72 h of study gas). The primary outcomes were: i) masimo pulse co-oximetry elevated MHb levels at admission (>2 %, fever cohort); ii) four hourly MHb levels in the RCT.

RESULTS

In the fever cohort, 34 % of children admitted with fever had elevated MHb at admission. Children with a history of vomiting, delayed capillary refill, elevated lactate, severe anemia, malaria, or hemoglobinopathies had increased odds of methemoglobinemia (p < 0.05 in a multivariate model). MHb levels at admission were higher in children who died (n = 89) compared to those who survived (n = 1964), p = 0.008. Among children enrolled in the iNO RCT, MHb levels typically plateaued within 12-24 h of starting study gas. MHb levels were higher in children receiving iNO compared to placebo, and MHb > 10 % occurred in 5.7 % of children receiving iNO. There were no differences in rates of study gas discontinuation between trial arms.

CONCLUSIONS

Hospitalized children with evidence of impaired oxygen delivery, metabolic acidosis, anemia, or malaria were at risk of methemoglobinemia. However, we demonstrated high-dose iNO could be safely administered to critically ill children with severe malaria with appropriate MHb monitoring.

TRIAL REGISTRATION

ClinicalTrials.gov Identifier: NCT01255215 (Date registered: December 5, 2010).

The immunological balance between host and parasite in malaria

Coevolution of humans and malaria parasites has generated an intricate balance between the immune system of the host and virulence factors of the parasite, equilibrating maximal parasite transmission with limited host damage. Focusing on the blood stage of the disease, we discuss how the balance between anti-parasite immunity versus immunomodulatory and evasion mechanisms of the parasite may result in parasite clearance or chronic infection without major symptoms, whereas imbalances characterized by excessive parasite growth, exaggerated immune reactions or a combination of both cause severe pathology and death, which is detrimental for both parasite and host. A thorough understanding of the immunological balance of malaria and its relation to other physiological balances in the body is of crucial importance for developing effective interventions to reduce malaria-related morbidity and to diminish fatal outcomes due to severe complications. Therefore, we discuss in this review the detailed mechanisms of anti-malarial immunity, parasite virulence factors including immune evasion mechanisms and pathogenesis. Furthermore, we propose a comprehensive classification of malaria complications according to the different types of imbalances.

Study of Methaemoglobin in Malaria Patients

To estimate the concentration of methemoglobin (MetHb) in malaria patients and correlate with severity of malaria infection. This prospective study included 30 untreated cases of malaria confirmed by Quantitative Buffy Coat (QBC) test and 30 age sex matched non-malarial cases taken as controls. All the patients underwent thorough clinical examination and routine biochemical investigation. Methemoglobin levels were estimated by spectrophotometric (co-oxymeter) method on day 1 and day 10 of diagnosis of all study group patients and correlated with clinical profile and severity. Out of 30 malaria patients 22 were males and eight were females. The clinical presentations in complicated malaria group (n = 21) were fever 21 (100 %), anemia 17 (80.95 %), renal failure 12 (52.38 %) and coma/convulsion 5 (23.8 %). The mean age of the study group was 41.66 years. Mean MetHb in complicated malaria on day 1 was 2.55 ± 1.75 % and day 10 was 10.69 ± 8.19 % (statistically significant). The overall mortality was 13 (43.33 %) among study group while 5 (16.66 %) was found among control group. Mean MetHb who died (n = 13) on day 1 was 3.144 ± 1.829 % and (n = 8) on day 10 it was 19.982 ± 8.406 %. Increase in level of methaemoglobin is detrimental to the body and is associated with increase in mortality. Routine MetHb estimation may be used as a prognostic indicator in the management of malaria patients. It is suggested that addition of drugs which reduce MetHb may be tried along with antimalarial drugs to decrease morbidity and mortality in malaria.

Phagocytic uptake of oxidized heme polymer is highly cytotoxic to macrophages

Apoptosis in macrophages is responsible for immune-depression and pathological effects during malaria. Phagocytosis of PRBC causes induction of apoptosis in macrophages through release of cytosolic factors from infected cells. Heme polymer or β-hematin causes dose-dependent death of macrophages with LC₅₀ of 132 µg/ml and 182 µg/ml respectively. The toxicity of hemin or heme polymer was amplified several folds in the presence of non-toxic concentration of methemoglobin. β-hematin uptake in macrophage through phagocytosis is crucial for enhanced toxicological effects in the presence of methemoglobin. Higher accumulation of β-hematin is observed in macrophages treated with β-hematin along with methemoglobin. Light and scanning electron microscopic observations further confirm accumulation of β-hematin with cellular toxicity. Toxicological potentiation of pro-oxidant molecules toward macrophages depends on generation of H₂O₂ and independent to release of free iron from pro-oxidant molecules. Methemoglobin oxidizes β-hematin to form oxidized β-hematin (βH*) through single electron transfer mechanism. Pre-treatment of reaction mixture with spin-trap Phenyl-N-t-butyl-nitrone dose-dependently reverses the β-hematin toxicity, indicates crucial role of βH* generation with the toxicological potentiation. Acridine orange/ethidium bromide staining and DNA fragmentation analysis indicate that macrophage follows an oxidative stress dependent apoptotic pathway to cause death. In summary, current work highlights mutual co-operation between methemoglobin and different pro-oxidant molecules to enhance toxicity towards macrophages. Hence, methemoglobin peroxidase activity can be probed for subduing cellular toxicity of pro-oxidant molecules and it may in-turn make up for host immune response against the malaria parasite.

Methaemoglobin and COHb in patients with malaria

Background

Haemolytic conditions may contribute to disease pathogenesis and severe clinical manifestations through the liberation of free haemoglobin (Hb) and production of toxic free haem. Thus, free Hb and haem should be associated with altered MetHb and COHb levels in malaria as in other conditions.

Methods

This study comprises data collected at three different sites: (i) a retrospective analysis of the first arterial blood gas result (ABGS) of any patient during 2010 at the University Hospital in Lisbon; (ii) a retrospective analysis of ABGS from patients with severe malaria admitted to the intensive care unit in Berlin, Germany; and (iii) a prospective study of non-invasive MetHb measurements in children with and without malaria in Lambaréné, Gabon.

Results

In Lisbon, the mean MetHb level was 1.4% (SD: 0.5) in a total of 17,834 ABGS. Only 11 of 98 samples with a MetHb level of >3.0 referred to infections. COHb levels showed no particular association with clinical conditions, including sepsis. In 13 patients with severe malaria in Berlin, the mean MetHb levels on admission was 1.29%, with 1.36% for cerebral malaria and 1.14% for non-cerebral malaria (P > 0.05). All COHb measurements were below 2.3%. In Lambaréné, Gabon, 132 healthy children had a mean MetHb level of 1.57%, as compared to 150 children with malaria, with a value of 1.77% and 2.05% in uncomplicated and complicated cases, respectively (P < 0.01).

Conclusions

The data appears consistent with the methaemoglobin/haem hypothesis in malaria and sepsis pathogenesis. However, although MetHb was significantly different between healthy controls and children with malaria in Africa, the difference was rather small, also when compared to previous studies. Still, non-invasive bedside MetHb testing may warrant further evaluation as it could be a simple adjuvant tool for prognosis in resource poor settings.

Decreased endothelial nitric oxide bioavailability, impaired microvascular function, and increased tissue oxygen consumption in children with falciparum malaria

Endothelial nitric oxide (NO) bioavailability, microvascular function, and host oxygen consumption have not been assessed in pediatric malaria. We measured NO-dependent endothelial function by using peripheral artery tonometry to determine the reactive hyperemia index (RHI), and microvascular function and oxygen consumption (VO2) using near infrared resonance spectroscopy in 13 Indonesian children with severe falciparum malaria and 15 with moderately severe falciparum malaria. Compared with 19 controls, children with severe malaria and those with moderately severe malaria had lower RHIs (P = .03); 12% and 8% lower microvascular function, respectively (P = .03); and 29% and 25% higher VO2, respectively. RHIs correlated with microvascular function in all children with malaria (P < .001) and all with severe malaria (P < .001). Children with malaria have decreased endothelial and microvascular function and increased oxygen consumption, likely contributing to the pathogenesis of the disease.

1,4-naphthoquinones and other NADPH-dependent glutathione reductase-catalyzed redox cyclers as antimalarial agents

The homodimeric flavoenzyme glutathione reductase catalyzes NADPH-dependent glutathione disulfide reduction. This reaction is important for keeping the redox homeostasis in human cells and in the human pathogen Plasmodium falciparum. Different types of NADPH-dependent disulfide reductase inhibitors were designed in various chemical series to evaluate the impact of each inhibition mode on the propagation of the parasites. Against malaria parasites in cultures the most potent and specific effects were observed for redox-active agents acting as subversive substrates for both glutathione reductases of the Plasmodium-infected red blood cells. In their oxidized form, these redox-active compounds are reduced by NADPH-dependent flavoenzyme-catalyzed reactions in the cytosol of infected erythrocytes. In their reduced forms, these compounds can reduce molecular oxygen to reactive oxygen species, or reduce oxidants like methemoglobin, the major nutrient of the parasite, to indigestible hemoglobin. Furthermore, studies on a fluorinated suicide-substrate of the human glutathione reductase indicate that the glutathione reductase-catalyzed bioactivation of 3-benzylnaphthoquinones to the corresponding reduced 3-benzoyl metabolites is essential for the observed antimalarial activity. In conclusion, the antimalarial lead naphthoquinones are suggested to perturb the major redox equilibria of the targeted cells. These effects result in developmental arrest of the parasite and contribute to the removal of the parasitized erythrocytes by macrophages.

Cytostatic versus cytocidal profiling of quinoline drug combinations via modified fixed-ratio isobologram analysis

Background

Drug combination therapy is the frontline of malaria treatment. There is an ever-accelerating need for new, efficacious combination therapies active against drug resistant malaria. Proven drugs already in the treatment pipeline, such as the quinolines, are important components of current combination therapy and also present an attractive test bank for rapid development of new concepts.

Methods

The efficacy of several drug combinations versus chloroquine-sensitive and chloroquine-resistant strains was measured using both cytostatic and cytocidal potency assays.

Conclusions

These screens identify quinoline and non-quinoline pairs that exhibit synergy, additivity, or antagonism using the fixed-ratio isobologram method and find tafenoquine – methylene blue combination to be the most synergistic. Also, interestingly, for selected pairs, additivity, synergy, or antagonism defined by quantifying IC₅₀ (cytostatic potency) does not necessarily predict similar behaviour when potency is defined by LD₅₀ (cytocidal potency). These data further support an evolving new model for quinoline anti-malarials, wherein haem and haemozoin are the principle target for cytostatic activity, but may not be the only target relevant for cytocidal activity.

Methylene blue as an antimalarial agent

Methylene blue has intrinsic antimalarial activity and it can act as a chloroquine sensitizer. In addition, methylene blue must be considered for preventing methemoglobinemia, a serious complication of malarial anemia. As an antiparasitic agent, methylene blue is pleiotropic: it interferes with hemoglobin and heme metabolism in digestive organelles, and it is a selective inhibitor of Plasmodium falciparum glutathione reductase. The latter effect results in glutathione depletion which sensitizes the parasite for chloroquine action. At the Centre de Recherche en Santé de Nouna in Burkina Faso, we study the combination of chloroquine with methylene blue (BlueCQ) as a possible medication for malaria in endemic regions. A pilot study with glucose-6-phosphate dehydrogenase-sufficient adult patients has been conducted recently.

Increased carboxyhemoglobin in adult falciparum malaria is associated with disease severity and mortality

Heme oxygenase 1 expression is increased in pediatric patients with malaria. The carboxyhemoglobin level (a measure of heme oxygenase 1 activity) has not been assessed in adult patients with malaria. Results of pulse co-oximetry revealed that the mean carboxyhemoglobin level was elevated in 29 Indonesian adults with severe falciparum malaria (10%; 95% confidence interval [CI], 8%-13%) and in 20 with severe sepsis (8%; 95% CI, 5%-12%), compared with the mean levels in 32 patients with moderately severe malaria (7%; 95% CI, 5%-8%) and 36 controls (3.6%; 95% CI, 3%-5%; P < .001). An increased carboxyhemoglobin level was associated with an increased odds of death among patients with severe malaria (odds ratio, 1.2 per percentage point increase; 95% CI, 1.02-1.5). While also associated with severity and fatality, methemoglobin was only modestly increased in patients with severe malaria. Increased carboxyhemoglobin levels during severe malaria and sepsis may exacerbate organ dysfunction by reducing oxygen carriage and cautions against the use of adjunctive CO therapy, which was proposed on the basis of mouse models.

Preconditioning with hemin decreases Plasmodium chabaudi adami parasitemia and inhibits erythropoiesis in BALB/c mice

Increased susceptibility to bacterial and viral infections and dysfunctional erythropoiesis are characteristic of malaria and other hemolytic hemoglobinopathies. High concentrations of free heme are common in these conditions but little is known about the effect of heme on adaptive immunity and erythropoiesis. Herein, we investigated the impact of heme (hemin) administration on immune parameters and steady state erythropoiesis in BALB/c mice, and on parasitemia and anemia during Plasmodium chabaudi adami infection. Intra-peritoneal injection of hemin (5 mg/Kg body weight) over three consecutive days decreased the numbers of splenic and bone marrow macrophages, IFN-γ responses to CD3 stimulation and T(h)1 differentiation. Our results show that the numbers of erythroid progenitors decreased in the bone marrow and spleen of mice treated with hemin, which correlated with reduced numbers of circulating reticulocytes, without affecting hemoglobin concentrations. Although blunted IFN-γ responses were measured in hemin-preconditioned mice, the mice developed lower parasitemia following P.c.adami infection. Importantly, anemia was exacerbated in hemin-preconditioned mice with malaria despite the reduced parasitemia. Altogether, our data indicate that free heme has dual effects on malaria pathology.

Extracellular methemoglobin primes red blood cell aggregation in malaria: an in vitro mechanistic study

Toxic byproducts from infected RBC cause rheological alteration and RBC aggregation. Malaria culture supernatant has the ability to exhibit RBC aggregation. Ammonium sulfate fractionation and immunodepletion of methemoglobin from culture supernatant confirms methemoglobin as a major aggregant. In vitro treatment of RBC with methemoglobin induces irreversible high order RBC aggregates, resistant to shear stress and physical forces. Methemoglobin-mediated ROS generation in the external micro-environment to develop oxidative stress close to RBC membrane seems to be responsible for initiating and forming high order RBC aggregates through phosphatidyl-serine externalization. Removal of oxidative stress through antioxidant treatment abolishes high order RBC aggregate formation. In conclusion, we discovered a novel pathway of methemoglobin-mediated RBC aggregation and its potential role in patho-physiological effects during malaria.

Methemoglobin exposure produces toxicological effects in macrophages due to multiple ROS spike induced apoptosis

Macrophages are an integral part of the immune system, required to produce a robust immune response against an infectious organism. Presence of methemoglobin in body fluids such as blood, cerebrospinal fluid and urine is associated with tissue damage. We tested cytotoxic effects of MetHb and underlying molecular events in mouse macrophage cell line J774A.1. MetHb exposure dose dependently reduced macrophage viability in an MTT assay. Light microscopy and scanning electron microscopic (SEM) observation of MetHb treated macrophage indicated death (less number of cells per field), severe damage to membrane structure and accumulation of particulate matter in the cytosol. The macrophage death during MetHb exposure was due to induction of apoptosis as indicated by annexin-V/FITC staining and DNA fragmentation analysis. MetHb treatment generated a periodic ROS spikes with time in the macrophage cytosol to develop oxidative stress. Scavenging ROS spikes with NAC, mannitol or PBN dose dependently protected macrophages against MetHb induced toxicity, apoptosis and cellular membrane damage. Our work highlighted the contributions of MetHb mediated toxicity toward macrophage and its potential role in tissue damage and immune depression during malaria and other hemolytic disorders.

Interactions of the antimalarial drug methylene blue with methemoglobin and heme targets in Plasmodium falciparum: a physico-biochemical study

AIMS

Resistance of Plasmodium falciparum to drugs has led to renewed interest of redox-active methylene blue (MB) for which no resistance has been reported so far. Moreover, MB displays unique interactions with glutathione reductase (GR). However, the mechanisms of action/interaction with potential targets of MB are yet to be elucidated. Our physico-biochemical study on MB and relevant hematin-containing targets was performed under quasi-physiological conditions.

RESULTS

The water deprotonation of the Fe(III)protoporphyrin dimer, the major building block of β-hematin, was studied. At pH 6, the predominant dimer possesses water coordinated to both metals. Below pH 6, spontaneous precipitation of β-hematin occurred reminiscent of hemozoin biomineralization at pH 5.0-5.5 in the food vacuole of the malarial parasite. MB also forms dimers (K(Dim)=6800 M(-1)) and firmly binds to hematin in a 2:1 hematin:MB sandwich complex (K(D)=3.16 μM). MB bioactivation catalyzed by GR induces efficient methemoglobin(Fe(III)) [metHb(Fe(III))] reduction to hemoglobin(Fe(II)). The reduction rate, mediated by leucomethylene blue (LMB), was determined (k(metHb)(red)=991 M(-1)·s(-1)) in an assay coupled to the GR/reduced form of nicotinamide adenine dinucleotide phosphate system.

INNOVATION AND CONCLUSION

Our work provides new insights into the understanding of (i) how MB interacts with hematin-containing targets, (ii) other relevant MB properties in corroboration with the distribution of the three major LMB species as a function of pH, and (iii) how this redox-active cycler induces efficient catalytic reduction of metHb(Fe(III)) to hemoglobin(Fe(II)) mediated by oxidoreductases. These physico-biochemical parameters of MB open promising perspectives for the interpretation of the pharmacology and pathophysiology of malaria and possibly new routes for antimalarial drug development.

Cerebral malaria and the hemolysis/methemoglobin/heme hypothesis: shedding new light on an old disease

Malaria causes more than 1 million deaths every year with cerebral malaria (CM) being a major cause of death in Sub-Saharan African children. The nature of the malaria-associated pathogenesis is complex and multi-factorial. A unified hypothesis involving sequestration of infected red blood cells, systemic host inflammatory response and hemostasis dysfunction has been proposed to explain the genesis of CM. In this review, we discuss the role of hemolysis, methemoglobin and free heme in CM, brought to light by our recent studies in mice as well as by other studies in humans.

A central role for free heme in the pathogenesis of severe malaria: the missing link?

Malaria, the disease caused by Plasmodium infection, is endemic to poverty in so-called underdeveloped countries. Plasmodium falciparum, the main infectious Plasmodium species in sub-Saharan countries, can trigger the development of severe malaria, including cerebral malaria, a neurological syndrome that claims the lives of more than one million children (<5 years old) per year. Attempts to eradicate Plasmodium infection, and in particular its lethal outcomes, have so far been unsuccessful. Using well-established rodent models of malaria infection, we found that survival of a Plasmodium-infected host is strictly dependent on the host's ability to up-regulate the expression of heme oxygenase-1 (HO-1 encoded by the gene Hmox1). HO-1 is a stress-responsive enzyme that catabolizes free heme into biliverdin, via a reaction that releases Fe and generates the gas carbon monoxide (CO). Generation of CO through heme catabolism by HO-1 prevents the onset of cerebral malaria. The protective effect of CO is mediated via its binding to cell-free hemoglobin (Hb) released from infected red blood cells during the blood stage of Plasmodium infection. Binding of CO to cell-free Hb prevents heme release and thus generation of free heme, which we found to play a central role in the pathogenesis of cerebral malaria. We will address hereby how defense mechanisms that prevent the deleterious effects of free heme, including the expression of HO-1, impact on the pathologic outcome of Plasmodium infection and how these may be used therapeutically to suppress its lethal outcomes.

[Association of methemoglobinemia and glucose-6-phosphate dehydrogenase deficiency in malaria patients treated with primaquine]

This study had the aim of investigating occurrences of methemoglobinemia among individuals with glucose-6-phosphate dehydrogenase deficiency during treatment for malaria infection using primaquine. Patients with a diagnosis of malaria caused by Plasmodium vivax or the V+F mixture (Plasmodium vivax + Plasmodium falciparum) were selected. Group 1 consisted of 74 individuals with a clinical diagnosis of methemoglobinemia and Group 2 consisted of 161 individuals without a clinical diagnosis of methemoglobinemia. The glucose-6-phosphate dehydrogenase deficiency rates (numbers of enzymopenic individuals) in Groups 1 and 2 were 51.3% (38) and 8.7% (14) respectively. These data demonstrated a statistically significant association with methemoglobinemia only among the individuals in Group 1 (p<0.05). Investigation of the relationship between methemoglobinemia and glucose-6-phosphate dehydrogenase deficiency showed that there was a possible association such that enzymopenic individuals may develop methemoglobinemia more frequently.

2-tert-butyl-8-quinolinamines exhibit potent blood schizontocidal antimalarial activity via inhibition of heme crystallization

We have recently reported that the attachment of a bulky metabolically stable tert-butyl group at the C-2 position of a quinoline ring in primaquine results in a tremendous improvement in the blood schizontocidal antimalarial activity of 8-quinolinamine. Because free heme released from hemoglobin catabolism in a malarial parasite is highly toxic, the parasite protects itself mainly by crystallization of heme into insoluble nontoxic hemozoin. We now demonstrate the ability of 2-tert-butylprimaquine to inhibit in vitro beta-hematin formation, to form a complex with heme with a stoichiometry of 1:1, and to enhance heme-induced hemolysis. The results described herein indicate that a major improvement in the blood-schizontocidal antimalarial activity of 2-tert-butylprimaquine might be due to a disturbance of heme catabolism pathway in the malarial parasite.

Low nitric oxide bioavailability contributes to the genesis of experimental cerebral malaria

The role of nitric oxide (NO) in the genesis of cerebral malaria is controversial. Most investigators propose that the unfortunate consequence of the high concentrations of NO produced to kill the parasite is the development of cerebral malaria. Here we have tested this high NO bioavailability hypothesis in the setting of experimental cerebral malaria (ECM), but find instead that low NO bioavailability contributes to the genesis of ECM. Specifically, mice deficient in vascular NO synthase showed parasitemia and mortality similar to that observed in control mice. Exogenous NO did not affect parasitemia but provided marked protection against ECM; in fact, mice treated with exogenous NO were clinically indistinguishable from uninfected mice at a stage when control infected mice were moribund. Administration of exogenous NO restored NO-mediated signaling in the brain, decreased proinflammatory biomarkers in the blood, and markedly reduced vascular leak and petechial hemorrhage into the brain. Low NO bioavailability in the vasculature during ECM was caused in part by an increase in NO-scavenging free hemoglobin in the blood, by hypoargininemia, and by low blood and erythrocyte nitrite concentrations. Exogenous NO inactivated NO-scavenging free hemoglobin in the plasma and restored nitrite to concentrations observed in uninfected mice. We therefore conclude that low rather than high NO bioavailability contributes to the genesis of ECM.

In vitro assessment of methylene blue on chloroquine-sensitive and -resistant Plasmodium falciparum strains reveals synergistic action with artemisinins

Methylene blue (MB) represents a promising antimalarial drug candidate for combination therapies against drug-resistant parasite strains. To support and facilitate the application of MB in future field trials, we studied its antiparasitic effects in vitro. MB is active against all blood stages of both chloroquine (CQ)-sensitive and CQ-resistant P. falciparum strains with 50% inhibitory concentration (IC50) values in the lower nanomolar range. Ring stages showed the highest susceptibility. As demonstrated by high-performance liquid chromatography-tandem mass spectrometry on different cell culture compartments, MB is accumulated in malarial parasites. In drug combination assays, MB was found to be antagonistic with CQ and other quinoline antimalarials like piperaquine and amodiaquine; with mefloquine and quinine, MB showed additive effects. In contrast, we observed synergistic effects of MB with artemisinin, artesunate, and artemether for all tested parasite strains. Artemisinin/MB combination concentration ratios of 3:1 were found to be advantageous, demonstrating that the combination of artemisinin with a smaller amount of MB can be recommended for reaching maximal therapeutic effects. Our in vitro data indicate that combinations of MB with artemisinin and related endoperoxides might be a promising option for treating drug-resistant malaria and should be studied in future field trials. Resistance development under this drug combination is unlikely to occur.

Glutathione reductase of the malarial parasite Plasmodium falciparum: crystal structure and inhibitor development

The malarial parasite Plasmodium falciparum is known to be sensitive to oxidative stress, and thus the antioxidant enzyme glutathione reductase (GR; NADPH+GSSG+H(+) <==> NADP(+)+2 GSH) has become an attractive drug target for antimalarial drug development. Here, we report the 2.6A resolution crystal structure of P.falciparum GR. The homodimeric flavoenzyme is compared to the related human GR with focus on structural aspects relevant for drug design. The most pronounced differences between the two enzymes concern the shape and electrostatics of a large (450A(3)) cavity at the dimer interface. This cavity binds numerous non-competitive inhibitors and is a target for selective drug design. A 34-residue insertion specific for the GRs of malarial parasites shows no density, implying that it is disordered. The precise location of this insertion along the sequence allows us to explain the deleterious effects of a mutant in this region and suggests new functional studies. To complement the structural comparisons, we report the relative susceptibility of human and plasmodial GRs to a series of tricyclic inhibitors as well as to peptides designed to interfere with protein folding and dimerization. Enzyme-kinetic studies on GRs from chloroquine-resistant and chloroquine-sensitive parasite strains were performed and indicate that the structure reported here represents GR of P.falciparum strains in general and thus is a highly relevant target for drug development.

Changes in rodent-erythrocyte methemoglobin reductase system produced by two malaria parasites, viz. Plasmodium yoelii nigeriensis and Plasmodium berghei

The methemoglobin reductase system plays a vital role in maintaining the equilibrium between hemoglobin and methemoglobin in blood. Exposure of red blood cells to oxidative stress (pathological/physiological) may cause impairment to this equilibrium. We studied the status of erythrocytic methemoglobin and the related reductase system during Plasmodium yoelii nigeriensis infection in mice and P. berghei infection in mastomys. Malaria infection was induced by intraperitoneal inoculation with 10(6) infected erythrocytes. The present investigation revealed a significant decrease in the activity of methemoglobin reductase, with a concomitant rise in methemoglobin content during P. yoelii nigeriensis infection in mice erythrocytes. This was accompanied with a significant increase in reduced glutathione and ascorbate levels. The activity of lactate dehydrogenase, glucose 6-phosphate dehydrogenase and glutathione reductase increased with a progressive rise in parasitemia. However, no methemoglobin or associated reductase activity was detected in normal and P. berghei-infected mastomys. P. berghei infection in mastomys resulted in an increase in the level of reduced glutathione and ascorbate in erythrocytes, and also in the activity of lactate dehydrogenase, glucose 6-phosphate dehydrogenase and glutathione reductase. These results suggest that antioxidants/antioxidant enzymes may prevent or reduce the formation of methemoglobin in the host and thereby protect the host from methemoglobinemia.

Effect of beta-arteether treatment on erythrocytic methemoglobin reductase system in Plasmodium yoelii nigeriensis infected mice

BACKGROUND

The methemoglobin reductase system plays a vital role in maintaining the equilibrium between hemoglobin (Hb) and methemoglobin (MetHb) in blood. Exposure of red blood cells to an oxidative stress (pathological/physiological) may cause impairment in this equilibrium.

OBJECTIVE

The status of MetHb and the related reductase system was studied during Plasmodium yoelii nigeriensis (P. y. nigeriensis) infection and beta-arteether treatment in mice.

METHODS

Mice were divided into four groups. Normal group, normal mice treated with beta-arteether, P. y. nigeriensis infected mice and P. y. nigeriensis infected mice treated with beta-arteether.

RESULTS

The present investigation revealed a marked decrease in the activity of MetHb reductase, with concomitant rise in MetHb levels during P. y. nigeriensis infection in mice erythrocytes (P < 0.001) as compared to normal mice. However, the activities of the associated enzymes viz., lactate dehydrogenase, glucose 6-phosphate dehydrogenase and glutathione reductase were found to be increased with progressive rise in parasitemia. beta-Arteether treatment (12.5 mg/kg body weight) of infected mice (parasitemia 20-25%) from day 5 of post infection resulted in complete clearance of parasitemia on day 7 of post infection, which was accompanied by restoration of all the altered above mentioned indices to near normal levels as compared to infected mice (P < 0.001).

CONCLUSION

These results suggest that there is a marked impairment of methemoglobin and methemoglobin reductase system during P. y. nigeriensis infection in mice. beta-Arteether treatment of infected mice resulted in complete clearance of parasitemia which also caused the restoration of methemoglobin and methemoglobin reductase system to near normal levels.

No evidence for cerebral hypoperfusion during cerebral malaria

OBJECTIVE

Among the mechanisms suggested for altered consciousness during cerebral malaria is the hypothesis of cerebral ischemia, which remains controversial, with little supportive O2 conflicting hemodynamic data. The purpose of this study was to test the hypothesis that cerebral ischemia is a main mechanism for altered consciousness during cerebral malaria.

SETTING

University hospital pediatric ward in a region with endemic cerebral malaria.

DESIGN

Prospective evaluation of cerebral hemodynamics and cerebral oxygenation during cerebral malaria compared with severe malaria anemia without altered consciousness.

PATIENTS

During a 2-wk period, we evaluated all patients who were admitted for cerebral malaria (n = 5). Age-matched patients admitted for severe malaria anemia without altered consciousness (n = 3) and outpatients (n = 3) were investigated for comparison.

INTERVENTIONS

All patients received the usual treatment according to their needs, which was determined by the physician in charge. Repeated neurologic evaluations were performed during the early management period in patients with cerebral malaria.

METHODS AND MAIN RESULTS

We repeatedly measured cerebral blood flow velocity (transcranial Doppler) and simultaneous systemic determinants of cerebral blood flow (arterial pressure, arterial oxygen saturation, PaCO2, rectal temperature, and hemoglobin concentration). The adequacy of cerebral blood flow to oxygen demands during cerebral malaria was assessed by continuous recording of jugular bulb venous oxygen saturation (using a fiberoptic device). Marked cerebral vasodilation was observed during cerebral malaria (systolic velocity, 1.45 +/- 0.09 m/s; diastolic velocity, 0.75 +/- 0.08 m/s; n = 4) and during severe malaria anemia (systolic velocity, 1.18 +/- 0.14 m/s; diastolic velocity, 0.55 +/- 0.05 m/s; n = 3) compared with control children (systolic velocity, 0.84 +/- 0.13 m/s; diastolic velocity, 0.35 +/- 0.06 m/s; n = 3; p < .05). During cerebral malaria, jugular bulb venous oxygen saturation remained stable, including during neurologic recovery, with initial values of 67.5 +/- 4.3%.

CONCLUSIONS

Because jugular bulb venous oxygen saturation remained within the normal range, cerebral hyperemia seems to be an adaptive response to altered systemic determinants, which argues against a hemodynamic mechanism for altered consciousness during cerebral malaria.

8-Aminoquinolines active against blood stage Plasmodium falciparum in vitro inhibit hematin polymerization

From the Walter Reed Army Institute of Research (WRAIR) inventory, thirteen 8-aminoquinoline analogs of primaquine were selected for screening against a panel of seven Plasmodium falciparum clones and isolates. Six of the 13 8-aminoquinolines had average 50% inhibitory concentrations between 50 and 100 nM against these P. falciparum clones and were thus an order of magnitude more potent than primaquine. However, excluding chloroquine-resistant clones and isolates, these 8-aminoquinolines were all an order of magnitude less potent than chloroquine. None of the 8-aminoquinolines was cross resistant with either chloroquine or mefloquine. In contrast to the inactive primaquine prototype, 8 of the 13 8-aminoquinolines inhibited hematin polymerization more efficiently than did chloroquine. Although alkoxy or aryloxy substituents at position 5 uniquely endowed these 13 8-aminoquinolines with impressive schizontocidal activity, the structural specificity of inhibition of both parasite growth and hematin polymerization was low.