MICA: Defining the two-step relay mechanism of action of the 8-aminoquinolines: A precondition for optimal combination therapies for relapse malaria

There is a global commitment to the elimination/eradication of malaria but despite significant advances in the last decade, malaria morbidity and mortality remains unacceptably high. According to the latest World malaria report (2020), there were 229 million cases of malaria in 2019, with an estimated number of malaria deaths at 409,000. Malaria elimination programmes have been successful in some countries and the ambition is to roll these out to many more countries in the coming years. An important element in malaria elimination programmes is to have drugs that are able to cure relapse malaria (by killing the malaria parasite that infects and persists in the liver) as well as drugs that are able to block the transmission of the disease (by killing the stages that are transmitted to and live in the mosquito host).

There is only one class of antimalarial drugs (known an 8-aminoquinolines) that are registered with these desired properties, primaquine (PQ) and tafenoquine (TQ). PQ is potentially lethal to people with a genetic disorder (known as Glucose-6-phosphate dehydrogenase deficiency) that affects some 400 million people word-wide, especially people from malaria endemic countries. For reasons that we do not understand, TQ has been shown to be safer than PQ. Given that TQ has improved safety features, it was planned that TQ would replace PQ in the near future. Unfortunately however, in a recent clinical trial, TQ failed to prevent relapse malaria when administered in combination with current first-line antimalarials (known as artemisinin combination therapies, ACTs). This recent finding is a major public health concern and critically we do not understand the reason for this. This project sets out to gain a deeper understanding of the mode of action of TQ and related drugs so that we can rapidly identify suitable combination partners that are active against drug resistant malaria parasites and will not adversely affect the efficacy of TQ.

In a recent study we were able for the first time in the 70 year history of this drug class, to show how this drug class kills malaria parasites. The model that we proposed is a two-step model. Whilst we believe that all drugs from this class share the same mechanism of action in the second step of the model, we believe that there are differences among the drug class in the first step of the model.

This MRC grant application proposes to use the very latest biological techniques and experimental platforms to generate the definitive evidence that explains the mechanisms underlying the first step of our model. This information will then allow the community to deploy effective TQ combination therapies that prevent relapse malaria and that are effective against the majority of drug resistance malaria parasites. In addition, we hope that the information we generate from our studies will have the potential to inform the design of second generation drugs that have improved efficacy and safety profiles.

Towards achieving our stated goal, we have assembled an international and multidisciplinary team of researchers with extensive experience and expertise in malaria therapeutics, with specific knowledge of this antimalarial drug class. This will not only ensure the successful execution of the study, but it will also ensure that the basic science outputs have the potential to be translated to tangible benefits to patients and people living in malaria-endemic countries - the ultimate goal of all of our work

The 8-aminoquinolines (8AQs) that include primaquine (PQ) and tafenoquine (TQ), are the only licensed drugs capable of treating relapse malaria. TQ possesses improved pharmacokinetic properties over PQ and is likely to replace PQ in future treatment regimens.

However it is very significant, that a recent Phase III trial of TQ + dihydroartemisinin-piperaquine for the radical cure of P. vivax, failed to demonstrate the prevention of relapses. This observation raises a major public health issue for future deployment of TQ in countries where ACTs are standard therapy.

We have recently shown that the MoA of PQ operates via a two-step biochemical relay. Step 1 of the MoA relates to the metabolic generation of redox-active metabolites, whilst Step 2 describes the mechanism by which redox-active metabolites exert parasite killing.

In an expansion of this model to include TQ and other 8-AQs, we hypothesise that there is convergence amongst 8-AQs for step 2 and that parasite killing occurs as a result of H2O2 production, via the cycling of redox-active metabolites. However, we further hypothesise that PQ and TQ and other 8-AQ, display significant differences in Step 1 of the MoA, specifically the routes and rates of generation of redox-active metabolites. We present initial supporting evidence for this hypothesis including evidence that the choice of combination partner drug can significantly affect the generation H2O2 and in vitro malaria parasite liver-stage efficacy.

Significant knowledge gaps remain with regards to Step1 as we do not know the identity of TQ metabolites, we do not know the antimalarial efficacy and toxicity of the metabolites and critically we have no information of the routes and rates of metabolite generation. This information is central to defining the therapeutic window of this drug class, and central to understanding why malaria relapse prevention by TQ and PQ is affected by the choice of the drug combination partner.