Introduction and Background of the Project

This is a continuation project of 'Hit-to-Lead Discovery for New Anti-malarials in Collaboration between MMV and Eisai' invested in 2015.

Introduction

Malaria is still one of the most life-threating infectious diseases in the world, and WHO estimates that it caused 429,000 deaths in 2015, 70% of which were children under 5, mainly in African countries (1). To treat malaria, the artemisinin combination therapies (ACTs) are widely used as the first-line treatment. Artemisinin is an antimalarial drug identified through the work of Dr. Youyou Tu, for which she received the Nobel Prize for Medicine in 2015. However, recent studies have reported the emergence of artemisinin-resistant parasites (2, 3). Because no vaccines with sufficient prophylactic efficacy are yet available, chemotherapy is still the most effective tool for the treatment and prevention of malaria. As a common standard for new antimalarial drugs toward the eradication of malaria, Medicines for Malaria Venture (MMV) has defined the Target Product Profiles (TPPs) and associated Target Candidate Profiles (TCPs), which are characteristics of compounds necessary to deliver future antimalarials (4). On the other hand, Eisai had started multiple antimalarial projects collaborating with external partners, such as MMV, aiming to contribute to the eradication of malaria, and this project is one of them.  The aim of this project is the modification of our antimalarial compounds to obtain new lead compounds with a new mode-of-action (MOA). MMV and Eisai screened approximately 20,000 compounds from Eisai’s library to discover hit compounds with antimalarial activity in the GHIT Screening Platform (GHIT-SP) program and identified several hit series which show antimalarial activity. As a starting point for chemical modification, we have selected two hit series with different characteristics and chemical structures. Combining Eisai’s knowledge of medicinal chemistry and drug discovery with MMV’s expertise in malaria research, the project strives to find novel lead compounds which show in vivo activity in a murine model, that can then become the next generation of antimalarial drugs.

Project objective

We will conduct hit-to-lead (HTL) work on two of several hit series that we identified from a GHIT-SP program of Eisai’s library compounds and aim to identify lead compounds from each series able to enter the lead optimization phase. A lead is a compound that shows in vivo activity in animal models, or in vivo Proof-of-Concept in other words. If one of the two hit series mentioned above is discontinued, we will conduct the HTL work on another hit series. 

To identify the lead compound series, we will

1) Design new antimalarial compounds from each hit series by applying rational medicinal chemistry principles around potency/activity.

2) Understand the structure-activity relationships around each hit series and enhance the in vitro and in vivo anti-Plasmodium activity of the compounds.

3) Pursue the potential of compounds to act across the malaria life-cycle: sexual blood stage, asexual blood stage, and liver stage.

4) Discover compounds with no cross-resistance with existing antimalarials and with good DMPK and physicochemical properties.

5) Define antimalarial profiles and the potential to meet selected TCP.

6) And then identify the lead series and independent back-ups series..

Project design

The HTL work will starts by synthesizing compounds around two hit series to allow the identification of lead compounds for each series. To reach this goal, we have to understand the chemical moiety important for antimalarial activity. We will first modify the original hit series and synthesize new compounds. Newly synthesized compounds will be in vitro assayed for antimalarial activity and for cytotoxicity. Thus, we will comprehend the important chemical moieties required for high antimalarial activity and for low cytotoxicity. Next the water solubility of compounds and their stability to liver microsomes will be assayed because solubility is an important factor for oral absorption and liver stability is required for the maintenance of compound levels in the body. Compounds with good solubility and stability will be tested in animals, mainly rodents, for pharmacokinetics and for in vivo efficacy, and also to check they don’t inhibit the metabolizing enzymes CYPs in the liver.  Safety will initially be assessed using hERG (cardiovascular safety assessment) and acute toxicity assays. These studies will be conducted mainly at Eisai. Compounds that make progress to this stage will be tested in various assays at MMV partners’ labs to understand important characteristics of the compounds; killing activity against malaria parasites, in vivo activity in a humanized SCID mouse model, antimalarial activity at other stages of the malaria parasite lifecycle; liver stage and sexual stage, a cross-resistance assay against several strains resistant to recent drugs, MOA screening studies, and so on. Thus we will iterate the compound synthesis and screening in various assays, and then after one and a half years, we aim to finally identify a lead compound from each series with both enhanced in vitro/in vivo activity and the good physicochemical, drug metabolism, and pharmacokinetics properties required to meet the criteria of a lead compound.

How can your partnership (project) address global health challenges?

Malaria is still one of the most life-threating diseases with the majority of the victims being young children, especially in African countries. The global community is working together toward a common goal, the eradication of malaria. To control and eradicate malaria, vector control, the use of vaccines to prevent infection, and treatment with drug are equally important. Antimalarial drugs have shown memorable improvements in last century, but recent studies suggest the potential devastating impact of new parasites with resistance to the existing therapies. We have proposed to contribute to the treatment and/or chemoprotection of malaria, including malaria caused by drug-resistant parasites, by providing lead compounds with a new and unique MOA. The two hit series for our HTL work have novel chemical pharmacophores and they have a low risk of cross-resistance with existing antimalarial drugs, such as the artemesinins. Actually in the preliminary evaluation they have shown no cross-resistance to the latest anti-malarial agents in development. They also display a desirable fast killing of malaria parasites. Bringing lead compounds with a novel MOA and potent activities against multiple stages of the parasites lifecycle is highly expected to deliver the new antimalarial drugs needed to help save the lives of many children in endemic areas.

What sort of innovation are you bringing in your project?

It is to be emphasized that the expected lead compounds from this project will have a novel MOA and activities against multiple stages of the parasites lifecycle. To achieve these objectives, two hit series were brought into this project from a GHIT Screening Platform. The two hit series have new chemotypes, different from currently available or known antimalarial drugs, suggesting that they have new MOAs and also show no cross-resistance to existing antimalarial drugs. The hit series actually showed no cross-resistance to existing antimalarial drugs, and in addition showed a fast killing profile against malaria parasites. In the GHIT-SP, the two hit series also showed antimalarial activities against parasites at two stages of the parasite lifecycle. In addition to drug resistance, another major problem in malaria is that primaquine is the only anti-relapse drug against vivax malaria among currently available therapeutics. The selected two new hit series have the potential to block the growth of liver schizonts and will also be tested for ability to prevent the relapse of vivax malaria in the liver. Based on these discussions, our new antimalarials could provide a new option for the treatment and/or chemoprotection of malaria.

Role and Responsibility of Each Partner

Eisai proceeds with the project in collaboration with MMV to provide the world with new drug candidates for the treatment of malaria. Leveraging its strength in medicinal chemistry, Eisai is responsible for the synthesis of new compounds, and then Eisai conducts primary anti-malarial assays, physicochemical assays, DMPK assays, and primary safety studies, so that Eisai can identify lead compound series, along with malaria data generated by partners of MMV. MMV will work in partnership with Eisai, providing drug discovery expertise and strategic input into the project. MMV is also responsible for connecting the team with partners in the MMV network, so that more detailed malaria screening data on selected compounds are available to help understand the compound properties and aid decision making.

Others (including references if necessary)

1. WHO World Malaria Report 2016. http://www.who.int/malaria/publications/world-malaria-report-2016/report/en/

2. Yeung S, Socheat D, Moorthy VS et al. Artemisinin resistance on the Thai-Cambodian border. Lancet 2009; 374: 1418-9.

3. Hawkes M, Conroy AL, Kain KC. Spread of artemisinin resistance in malaria. The New England journal of medicine 2014; 371: 1944-5.

4. Burrows JN, Duparc S, Winston E. Gutteridge WE et al. New developments in anti‑malarial target candidate and product profiles. Malaria journal 2017; 16: 16.

Final Report

1. Project objective

Our aims were to conduct hit-to-lead (HTL) work on our two hit series, Series-2 and Series-3, to identify a new lead(s) and to take the series forwards to lead optimization, in order to provide new drug(s) for the treatment of malaria. Our two hit series were discovered in the GHIT Screening Platform by testing approx. 20,000 compounds from Eisai’s library.

2. Project design

We designed new anti-malarial compounds from the hit series by applying rational medicinal chemistry principles around potency/activity, in order to understand the structure activity relationships around the improved hits and to enhance the in vitro anti-Plasmodium activity, solubility and metabolic stability of compounds. We also investigated the potential of the compounds to show activity across the malaria life-cycle: sexual blood stage, asexual blood stage, and liver stage, discovered compounds with no cross-resistance with existing anti-malarials, and identified early leads that met the criteria of each TCP. In addition, we generated mutants resistant to the early lead and analyze their mutation in order to identify the biological target of early leads.

3. Results, lessons learned

We conducted HTL works on two hit series, Series-2 and Series-3, discovered from the GHIT Screening Platform. In the course of the HTL works, we prioritized Series-2 because Series-3 showed no improved activity. In the HTL works on Series-2, we enhanced antimalarial activity approximately 1,000-fold from the original hit (IC₅₀, 600 nM) to the most active compound (IC₅₀, 0.4 nM), and identified early leads that showed potent in vitro activity against Plasmodium falciparum, Plasmodium vivax and/or Plasmodium berghei at the blood and liver stages. In addition, the early lead showed no cross-resistance to existing antimalarials and showed in vivo activity with ED90s of

We identified early leads in Series-2 and successfully achieved the project target by the completion of this HTL project at the end of March 2019. We will further investigate the possibility of lead optimization initiation.

Furthermore, we generated mutants resistant to the early lead of Series-2 and have analyzed them to identify its biological target. From mutation analysis, two enzymes were suggested as the biological target candidate of Series-2 and we will carry out further work to confirm the biological target of Series-2.