Introduction and Background of the Project

Introduction

Malaria, a protozoan disease transmitted by Anopheles mosquitoes, has plagued humankind for millennia and remains today the most important parasitic disease of human beings despite the availability of effective treatments and chemoprophylactic agents. The World Health Organization (WHO) estimates that 3.2 billion people, almost half of the world’s population, remain at risk of malaria. In 2015 alone, there were an estimated 214 million new cases of malaria and 438 000 deaths1. Novel classes of antimalarial medicines targeting different parasite stages are urgently needed to both provide effective alternatives when resistance to current therapies will inevitably progress and provide the ingredients needed to meet the malaria eradication agenda2.

This project builds on a Mitsubishi Tanabe Pharma Corporation (MTPC) 51,200 compound library that has been screened at MMV testing centres in USA (Prof. Winzeler – University of California, San Diego) and in Australia (Prof. Vicky Avery – Eskitis Institute for Drug Discovery, Griffith University) against the liver3, blood asexual4 and sexual5 stages of the malaria parasite. The library was made up of high quality, diverse compounds synthesized/purchased by MTPC. The project team is now working on three confirmed chemical hit series inhibiting the blood asexual and sexual (gametocyte) stages of the parasite’s life cycle. MTPC consider these series as “Dark Matter”6 because they were not identified as hits in high throughput screens against a variety of human enzymes, GPCR antagonists and ion channels.

Project objective

The objective of the hit-to-lead MTPC-MMV GHIT funded project is to identify at least one compound series that meets the GHIT/MMV criteria for progression to lead optimization within 18 months of the project initiating.

Project design

During the first phase of the project, frontrunner compounds from the three hit series selected for hit-to-lead progression will be profiled in the parasite life cycle assays to determine their Target Candidate Profile (TCP). In parallel, medicinal chemistry will be carried out on the three priority series and a structurally related analogues will be profiled in in vitro primary parasitology and cytotoxicity assays. The compound’s physical properties, stability and in vitro DMPK characteristics will also be determined. If necessary, work will be carried out on one or more additional compound series identified from the initial screening of the MTPC 51,200 compound library. The in vivo efficacy of the most promising compounds from the hit series will be evaluated in the mouse model of human malaria and in vivo DMPK properties determined in rodent species.

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

Malaria remains one of the most important diseases of the developing world, causing an estimated 214 million new cases of malaria and 438 000 deaths annually, mostly occurring in Africa in children under five years old. In the absence of a highly effective vaccine, malaria control and prevention depends greatly on chemotherapy and chemoprophylaxis. However, the success of both therapy and prevention is being threatened by the increasing prevalence of resistance to virtually all antimalarials, including the individual components of artemisinin-based combination therapies (ACTs), the current frontline treatment for malaria. Moreover, in the context of the malaria eradication agenda endorsed by the Bill and Melinda Gates Foundation and the WHO, new attributes to malaria medicines are required, such as the ability to block transmission between human and mosquitoes and to kill the dormant liver stages.

MMV has worked with the wider malaria community to establish five Target Candidate Profiles (TCPs) that defines the attributes of the next-generation antimalarials needed to not only control but also eradicate the disease7. This collaborative project between MTPC, MMV and GHIT is focused on delivering compounds that meet at least one of the five TCPs, namely: i) rapid clearance of blood-stage parasites to relieve disease symptoms quickly, ii) long-acting profile that, in a combination regimen, is capable of providing a total cure and post-treatment prophylaxis, iii) anti-relapse medicine for patients infected with Plasmodium vivax and Plasmodium ovale, iv) transmission blocking medicine targeting the host gametocyte population and preventing infection to the mosquito and v) chemoprotective agent capable of protecting vulnerable populations from reinfection.

What sort of innovation are you bringing in your project?

The confirmed actives from screening the MTPC library against asexual blood and liver, as well as sexual stage parasites have been clustered and prioritized based on novelty of chemical series (and, by inference, the mode of action), life-cycle fingerprint (in particular dual asexual blood and liver stage or asexual blood and sexual late stage gametocyte activity) relevant for eradication potential, and the extent to which the series fill strategic gaps in the MMV portfolio. Compound series will only be proposed for Lead Optimization if they fulfill the given criteria8 and also offer differentiation compared to existing series in the MMV portfolio.

Role and Responsibility of Each Partner

The project team consists of medicinal chemistry and biology experts from MTPC and MMV as well as parasitology and DMPK specialists from within the MMV network. 

MTPC provided analysis and clustering information on active chemical series, and were implicated in the process of series prioritization with MMV. MTPC's current role is to provide scientific advices on the medicinal chemistry and the selection of analogue compounds for further studies and profiling of the hit chemical series with scientists of MMV, the Eskitis Institute for Drug Discovery and the Centre for Drug Candidate Optimisation (CDCO). MTPC’s expertise in drug discovery and development and in-house knowledge on the chemical matter at the origin of the compounds that are proposed for hit-to-lead exploration will be key factors of success of the current project.

Prof. Vicky Avery (Eskitis Institute for Drug Discovery, Griffith University) and Prof. Susan Charman (CDCO, Monash University) are MMV partners on the team with the responsibilities to deliver in vitro blood stage parasitology data and in vitro/in vivo DMPK data, respectively, on compounds selected by the project team.

Synthetic chemistry will be performed at TCGLS, a CRO based in Kolkata, India.

MMV is leading the project, in close partnership with MTPC, providing drug discovery and malaria biology expertise as well as strategic input to the project.  MMV also has the responsibility to connect the project team with partners in the MMV network to generate the data necessary to profile the chemical series and aid decision making.

Others (including references if necessary)

1. World Health Organization (WHO). WORLD MALARIA REPORT 2015. (2015). doi:ISBN 978 92 4 156515 8
2. Wells, T. N. C., Huijsduijnen, R. H. Van, Voorhis, W. C. Van, van Huijsduijnen, R. H. & Van Voorhis, W. C. Malaria medicines : a glass half full ? Nat. Rev. Drug Discov. 14, 424–442 (2016).
3. Meister, S. et al. Imaging of Plasmodium liver stages to drive next-generation antimalarial drug discovery. Science 334, 1372–7 (2011).
4. Duffy, S. & Avery, V. M. Development and optimization of a novel 384-well anti-malarial imaging assay validated for high-throughput screening. Am. J. Trop. Med. Hyg. 86, 84–92 (2012).
5. Lucantoni, L. & Avery, V. Whole-cell in vitro screening for gametocytocidal compounds. Future Med. Chem. 4, 2337–2360 (2012).
6. Wassermann, A. M. et al. Dark chemical matter as a promising starting point for drug lead discovery. Nat. Chem. Biol. (2015). doi:10.1038/nchembio.1936
7. Burrows, J. N., van Huijsduijnen, R. H., Möhrle, J. J., Oeuvray, C. & Wells, T. N. C. Designing the next generation of medicines for malaria control and eradication. Malar. J. 12, 187 (2013).
8. Katsuno, K. et al. Hit and lead criteria in drug discovery for infectious diseases of the developing world. Nat. Rev. Drug Discov. 14, 751–8 (2015).

Final Report

1. Project objective

To profile three series of novel chemical structures identified from a screen of 51,200 small molecules from the Mitsubishi Tanabe Pharma Corporation (MTPC) library to determine if one or more of these series have the potential to satisfy the MMV Target Candidate Profile following optimization.  Furthermore, by the end of the Hit-to-Lead phase, deliver at least one series to a Lead Optimization project within 18 months of the start of the project.

2. Project design

The project started by characterization of hits identified from the screen of the MTPC library for in vitro antimalarial activity, physicochemical and pharmacokinetic properties. This identified three series of focus (Series 1, 2 and 3) and several “backup” series for additional analog synthesis and characterization. Development of structure-activity-relationships (SAR) within the series through systematic variation of key structural elements in each series and analysis of the effect of these changes on the biological and pharmacokinetic profile was used to select compounds to advance to in vivo studies.

3. Results, lessons learned

In the first year of the project, the three series of focus were progressed in parallel through a well-defined screening cascade that evaluated the antimalarial potency, physicochemical and pharmacokinetic properties of the initial hits. The series 1 was found to have a promising profile, with numerous analogs exhibiting good antimalarial potency, as well as good physicochemical and pharmacokinetic properties.  In the series 2 and 3, some progress was made to improve the overall profile of these series, but several areas of concern were noted.  In the series 2, an off-target activity that raised a potential safety concern was identified, and removal of the chemical functionality likely associated with that off target activity resulted in loss of antimalarial potency.  In the series 3, sub-optimal physicochemical properties (solubility, lipophilicity) and high rates of metabolism were identified as challenges.  Consequently, work on these series was de-prioritized relative to the series 1.

Starting in the second year, continued development of the series 1 was undertaken, with good results noted for additional analogs synthesized by the team.  In particular, refinement of the synthetic route to allow for preparation of compounds with defined stereochemistry resulted in the emergence of clear SAR for the series, and improvements in antimalarial potency and pharmacokinetics allowed for demonstration of activity in a mouse model of malaria.  Concurrent examination of lead compounds from the series 1 in an array of in vitro biochemical assays has resulted in identification of a putative biochemical target of this class of compounds, providing additional enthusiasm for the potential of this series.  In addition to the series 1, profiling of several of the backup series identified at the outset of the project has continued, with some encouraging results obtained for two additional classes of compounds.

The success of the series 1 has prompted the team to propose progression of this series to a Lead Optimization project that has the objective of delivering a pre-clinical development candidate within two years. This proposal is currently being evaluated and considered by GHIT.