How worthwhile is methylene blue as a treatment of malaria?

KEYWORDS:

• Plasmodium falciparum
• methylene blue
• triple therapy
• prevention of artemisinin resistance
• malaria elimination

Malaria remains an important cause of morbidity and mortality, especially in sub-Saharan Africa (SSA) [1,2]. Globally, the malaria burden has been much reduced over the last 15 years, but progress has stalled since 2015 [3]. Previous malaria control and elimination success stories were mainly attributed to the high coverage rates with insecticide-treated mosquito nets (ITNs) and the introduction of artemisinin-based combination therapy (ACT) as standard first-line treatment for uncomplicated falciparum malaria in all endemic areas [1–3]. However, resistance against artemisinins and current ACT regimens has emerged in the Greater Mekong Subarea (GMS) of South-East Asia, and it would result in a major public health catastrophe if it would spread or develop de novo in SSA [4,5]. To prevent and delay the emergence and spread of artemisinin resistance, adding another anti-malarial drug (triple therapy) to standard ACT has been proposed [6,7]. Two potentially useful triple therapy regimens – dihydroartemisinin-piperaquine-mefloquine and artemether-lumifantrine-amodiaquine – are currently investigated in clinical trials in the GMS [5]. Moreover, the combination of an ACT with a single dose of the gametocytocidal drug primaquine (PQ) is well known to reduce the transmission of P. falciparum parasites, including drug-resistant parasites [1,4,8]. PQ is not widely used for this indication in endemic areas because of concerns on the risk of haemolytic anaemia in individuals with glucose-6-phosphate dehydrogenase (G6PD) deficiency [9]. Nevertheless, a single, low dose of PQ (0.25 mg base/kg) has been considered as safe and is now recommended by the World Health Organization (WHO) to be added to standard ACT treatment of uncomplicated falciparum malaria, even without G6PD testing [3,9]. However, there remain concerns regarding potential toxicology of PQ when used for this indication, thus alternative drugs need to be investigated [2].

Methylene blue (MB) is a dye which has been used for a long time in medicine [10,11]. When given orally, MB is well absorbed from the gastrointestinal tract and widely distributed throughout the body with maximal plasma concentrations being reached after two hours and a plasma half-life of around 20 hours; the drug is mainly excreted through the kidney [12–14]. MB is registered in many countries for a number of indications such as visualization of organ structures during surgery, treatment of methaemoglobinaemia, prevention of urogenital infections, treatment of septic shock, prevention of ifosfamid-induced encephalopathy, treatment of priapism, as well as sterilization of blood transfusions [10,11,15–20].

MB is also the oldest synthetic antimalarial drug and has first been used by Paul Ehrlich and colleagues against malaria in a German hospital in the year 1891 [21]. Thereafter it was applied on a large scale in many endemic areas and against all types of malaria – especially against malaria disease which was not responding to quinine treatment – until it was replaced by new synthetic antimalarials without staining properties [22]. The research on the treatment of malaria with MB got reactivated in three biochemical laboratories towards the end of the 1990s and at a time when the development of multi-drug resistance had become a serious threat to global malaria control efforts [2]. Especially the detection of the P. falciparum glutathione reductase as a new drug target paved the way to further research on the role of MB in malaria treatment, with key studies having been conducted at the Heidelberg University Biochemistry Centre and under the leadership of the late Heiner Schirmer [2,10]. In addition to its activity against this enzyme, the effects on the malaria parasites are caused by MB acting as a subversive redox-cycling substrate, by its interaction with the polymerization of haem to hemozoin, and most likely by a number of further and still unknown effects of the pluripotent MB, which makes resistance development rather unlikely [2,10]. Preclinical studies have shown the potency of MB in the inhibition of P. falciparum and P. vivax, including of drug-resistant isolates, it’s very strong effects as to the reduction of P. falciparum gametocytes, and its synergy with artemisinin derivates [23–28]. As a consequence, MB has already been considered a potentially useful partner drug for ACT in the year 2009 [6].

A systematic review has been conducted on the efficacy and safety of MB in the treatment of human malaria, which considered all studies ever published until early 2017 [22]. Out of 474 reports identified, 21 studies reporting data from a total of 1,504 malaria patients were included in the analysis. These studies were conducted in Africa (n = 7), Asia (n = 3), the Americas (n = 3), and in Europe (n = 8), using different study designs and MB formulations. While the older studies were mainly case reports or case series which documented the effects of MB monotherapy applied over longer treatment periods in all types of malaria and in all endemic areas, the studies implemented in recent years were frequently randomized controlled trials which compared different combination regimens usually given for three days in P. falciparum patients in West Africa [22]. Although oral drug regimens were very heterogeneous ranging from low doses of MB given in several doses per day and over prolonged periods of time to high dose MB given ones a day for three days in combination with other antimalarials, the doses found to be effective were rather similar in historical studies compared to the more recent studies [22]. Cure rates with MB monotherapy were already around 90% in the historical monotherapy studies and varied between 80% and 100% in the more recent studies depending on the target population and respective combination regimens [22]. However, when given as monotherapy, P. falciparum parasite clearance was rather slow which leads to the conclusion that MB should always be given together with a rapidly acting partner drug [29]. The reviewed studies also confirmed the high efficacy of MB against the gametocytes of P. falciparum [30,31]. Regarding the safety of MB in the treatment of malaria, the review confirmed the known side effects of an unavoidable green-blue discoloration of the urine, of frequent gastro-intestinal side effects which largely depended on the formulation of the drug, and of mild and self-limiting urethritis symptoms [22]. No serious adverse events were associated with MB treatment, and a meta-analysis of all recently conducted trials in patients treated with MB-based combination therapy showed no clinically relevant hemolysis in a large population of G6PD-normal and G6PD deficient African patients with falciparum malaria [32].

The strong effect of MB on the gametocytes of P. falciparum has recently been confirmed in two clinical phase II studies in Mali and in Burkina Faso. In the Mali study, male patients with asymptomatic P. falciparum infection were randomized to receive either sulfadoxine-pyrimethamine plus amodiaquine, sulfadoxine-pyrimethamine plus amodiaquine plus a single dose of 0 · 25 mg/kg PQ, dihydroartemisinin-piperaquine, or dihydroartemisinin-piperaquine plus 15 mg/kg per day MB for 3 days [33]. The main outcome measure was the infectivity to Anopheles mosquitoes measured two and seven days after treatment through membrane feeding method. The results showed that the PQ and MB regimens were both highly effective in preventing P. falciparum transmission. In a recent randomized and controlled study carried out in children with uncomplicated falciparum malaria in Burkina Faso, MB-ACT was shown to be superior to PQ-ACT in its effects on the asexual parasites and the gametocytes of P. falciparum (unpublished).

In conclusion, the findings from the existing studies clearly demonstrate the efficacy and safety of MB in the treatment of malaria. While there is a need to conduct more clinical studies on the effects of MB in P. vivax malaria, there is already high evidence for strong effects of the drug against the parasites of P. falciparum, in particular against the gametocytes. MB can thus be considered as an alternative to PQ in the combination therapy of falciparum malaria, in particular where the goal of malaria programmes is elimination. Moreover, adding MB to existing ACT regimens is potentially useful to reduce P. falciparum transmission intensity, to increase treatment efficacy, and to reduce the risk for development and spread of malaria parasites resistant against artemisinins and ACT. However, there is a need to further modify the MB formulation to improve its taste and acceptability and to conduct a multi-country phase III study in different endemic areas and populations to confirm current findings on the efficacy and safety of MB-ACT combination schemes. This is in particular important as there are very different types of G6PD deficiency in malaria endemic areas, and MB has so far only been shown to be safe in the A- type of SSA. Finally, given the time needed and the high costs for the development of new antimalarial medication, the further development of MB-based combination therapy should be considered as highly cost-effective by the Global Health community.

Declaration of interest

The authors have no relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript. This includes employment, consultancies, honoraria, stock ownership or options, expert testimony, grants or patents received or pending, or royalties.

Reviewer disclosures

Peer reviewers on this manuscript have no relevant financial or other relationships to disclose.

Additional information

Funding

The project was funded by BASF (Germany).