In-vitroactivity of primaquine against the asexual blood stages ofPlasmodium falciparum

Future antimalarials from Artemisia? A rationale for natural product mining against drug-refractory Plasmodium stages

Covering: up to 2023Infusions of the plants Artemisia annua and A. afra are gaining broad popularity to prevent or treat malaria. There is an urgent need to address this controversial public health question by providing solid scientific evidence in relation to these uses. Infusions of either species were shown to inhibit the asexual blood stages, the liver stages including the hypnozoites, but also the sexual stages, the gametocytes, of Plasmodium parasites. Elimination of hypnozoites and sterilization of mature gametocytes remain pivotal elements of the radical cure of P. vivax, and the blockage of P. vivax and P. falciparum transmission, respectively. Drugs active against these stages are restricted to the 8-aminoquinolines primaquine and tafenoquine, a paucity worsened by their double dependence on the host genetic to elicit clinical activity without severe toxicity. Besides artemisinin, these Artemisia spp. contain many natural products effective against Plasmodium asexual blood stages, but their activity against hypnozoites and gametocytes was never investigated. In the context of important therapeutic issues, we provide a review addressing (i) the role of artemisinin in the bioactivity of these Artemisia infusions against specific parasite stages, i.e., alone or in association with other phytochemicals; (ii) the mechanisms of action and biological targets in Plasmodium of ca. 60 infusion-specific Artemisia phytochemicals, with an emphasis on drug-refractory parasite stages (i.e., hypnozoites and gametocytes). Our objective is to guide the strategic prospecting of antiplasmodial natural products from these Artemisia spp., paving the way toward novel antimalarial "hit" compounds either naturally occurring or Artemisia-inspired.

Putative Contribution of 8-Aminoquinolines to Preventing Recrudescence of Malaria

Enhanced therapeutic efficacy achieved in treating Plasmodium vivax malaria with an 8-aminoquinoline (8-AQ) drug such as primaquine (PQ) together with a partner drug such as chloroquine (CQ) is usually explained as CQ inhibiting asexual parasites in the bloodstream and PQ acting against liver stages. However, PQ's contribution, if any, to inactivating non-circulating, extra-hepatic asexual forms, which make up the bulk of the parasite biomass in chronic P. vivax infections, remains unclear. In this opinion article, I suggest that, considering its newly described mode of action, PQ might be doing something of which we are currently unaware.

On the potential for discontinuing atovaquone-proguanil (AP) ad-hoc post-exposure and other abbreviated AP-regimens: Pharmacology, pharmacokinetics and perspectives

According to current guidelines, atovaquone-proguanil (AP) malaria chemoprophylaxis should be taken once daily starting one day before travel and continued for seven days post-exposure. However, drug-sparing regimens, including discontinuing AP after leaving malaria-endemic areas are cost-saving and probably more attractive to travelers, and may thus enhance adherence. AP has causal prophylactic effects, killing malaria parasites during the hepatic stage. If early hepatic stages were already targeted by AP, AP could possibly be discontinued upon return. Pharmacokinetic data and studies on drug-sparing AP regimens suggest this to be the case. Nevertheless, the evidence is weak and considered insufficient to modify current recommendations. Field trials require large numbers of travelers and inherently suffer from the lack of a control group. Safely-designed controlled human malaria infection trials could significantly reduce study participant numbers and safely establish an effective AP abbreviated regimen which we propose as the optimal trial design to test this concept.

Atovaquone, chloroquine, primaquine, quinine and tetracycline: antiproliferative effects of relevant antimalarials on Neospora caninum

Neospora caninum is an apicomplexan parasite that causes abortion in cattle, resulting in significant economic losses. There is no commercial treatment for neosporosis, and drug repositioning is a fast strategy to test possible candidates against N. caninum. In this article, we describe the effects of atovaquone, chloroquine, quinine, primaquine and tetracycline on N. caninum proliferation. The IC50 concentrations in N. caninum were compared to the current information based on previous studies for Plasmodium and Toxoplasma gondii, correlating to the described mechanisms of action of each tested drug. The inhibitory patterns indicate similarities and differences among N. caninum, Plasmodium and T. gondii. For example, atovaquone demonstrates high antiparasitic activity in all the analyzed models, while chloroquine does not inhibit N. caninum. On the other hand, tetracycline is effective against Plasmodium and N. caninum, despite its low activity in T. gondii models. The repurposing of antimalarial drugs in N. caninum is a fast and inexpensive way to develop novel formulations using well-established compounds.

[Role of primaquine in malaria control and elimination in French-speaking Africa]

Primaquine, an 8-aminoquinoline, is a relatively unknown and underutilized drug in French-speaking African countries. It acts against the liver stage parasites of all human malaria species, asexual blood stages of Plasmodium vivax and, to a lesser degree, Plasmodium falciparum; P. falciparum mature gametocytes, and P. vivax and Plasmodium ovale hypnozoites. Gastrointestinal disturbances are its most common side effects. The clinical utility of primaquine is limited due to its hematological side effects in individuals with glucose-6-phosphate dehydrogenase (G6PD) deficiency and other contraindications (pregnant woman, breastfeeding woman, infants less than 6 months old). In the light of the recent recommendations of the World Health Organization (WHO), we propose to examine how primaquine can be used in French-speaking Africa to improve malaria control and move towards malaria elimination. Two indications supported by the WHO are of relevance in Africa. First, artemisinin-based combination therapies and primaquine given as a single low dose (0.25 mg base/kg) are effective to kill asexual and sexual parasites of P. falciparum, are well-tolerated, and have very little risk even in mild to moderate G6PD-deficient patients. This strategy may be helpful to contain transmission in an area in Africa where P. falciparum malaria incidence has decreased considerably. There is an ethical concern in administering primaquine as a gametocytocide as it does not confer any direct benefit to the treated patient. However, the single low-dose primaquine is most likely associated with very low risk for adverse hematological effects, and WHO recommends its use even without prior G6PD testing. In our opinion, clinical studies including G6PD test should be conducted to assess the safety of low-dose primaquine in African patients. Second, primaquine is effective and necessary for radical treatment of P. vivax and P. ovale, but the standard 14-day treatment (0.25-0.5 mg base/kg/day) is not recommended in patients with G6PD deficiency. Prior G6PD testing is required before prescribing primaquine for radical treatment. The use of primaquine for radical treatment in patients without contraindications does not raise any major ethical problem since the probability of relapse in patients who do not receive anti-hypnozoite treatment can be relatively high and each relapse can cause or aggravate anemia, especially in children. In our opinion, patients with mild or moderate G6PD deficiency should not be treated with primaquine at present. Further clinical studies are necessary to define the role of this drug for radical treatment in G6PD-deficient African patients. Without primaquine, the eventual elimination of P. vivax and P. ovale malaria appears to be very difficult. Updated epidemiological data on G6PD, Duffy antigen, and the current distribution of and burden due to P. vivax and P. ovale are required for a rational use of primaquine in the African continent. Moreover, clinical studies on primaquine are required in Africa.

Review: Malaria chemoprophylaxis for travelers to Latin America

Because of recent declining malaria transmission in Latin America, some authorities have recommended against chemoprophylaxis for most travelers to this region. However, the predominant parasite species in Latin America, Plasmodium vivax, can form hypnozoites sequestered in the liver, causing malaria relapses. Additionally, new evidence shows the potential severity of vivax infections, warranting continued consideration of prophylaxis for travel to Latin America. Individualized travel risk assessments are recommended and should consider travel locations, type, length, and season, as well as probability of itinerary changes. Travel recommendations might include no precautions, mosquito avoidance only, or mosquito avoidance and chemoprophylaxis. There are a range of good options for chemoprophylaxis in Latin America, including atovaquone-proguanil, doxycycline, mefloquine, and--in selected areas--chloroquine. Primaquine should be strongly considered for nonpregnant, G6PD-nondeficient patients traveling to vivax-endemic areas of Latin America, and it has the added benefit of being the only drug to protect against malaria relapses.

A high-throughput screen targeting malaria transmission stages opens new avenues for drug development

A major goal of the worldwide malaria eradication program is the reduction and eventual elimination of malaria transmission. All currently available antimalarial compounds were discovered on the basis of their activity against the asexually reproducing red blood cell stages of the parasite, which are responsible for the morbidity and mortality of human malaria. Resistance against these compounds is widespread, and there is an urgent need for novel approaches to reduce the emergence of resistance to new antimalarials as they are introduced. We have established and validated the first high-throughput assay targeting the red blood cell parasite stage required for transmission, the sexually reproducing gametocyte. This assay will permit identification of compounds specifically targeting the transmission stages in addition to the asexual stage parasites. Such stage-specific compounds may be used in a combination therapy, reducing the emergence of resistance by blocking transmission of resistant parasites that may be selected in a patient.

Can primaquine therapy for vivax malaria be improved?

The incidence and range of endemic malaria caused by Plasmodium vivax has expanded during the past 30 years. This parasite forms hypnozoites in the liver, creating a persistent reservoir of infection. Primaquine (PQ), introduced 50 years ago, is the only drug available to eliminate hypnozoites. However, lengthy treatment courses and follow-up periods are not conducive to assessing the effectiveness of this drug in preventing relapses. Resistance to standard therapy could be widespread. Studies are urgently needed to gauge this problem and to determine the safety, tolerability and efficacy of shorter courses and higher doses of PQ.