Synthesis and structure–activity-relationship studies of thiazolidinediones as antiplasmodial inhibitors of the Plasmodium falciparum cysteine protease falcipain-2

doi:10.1016/j.ejmech.2014.11.061

European Journal of Medicinal Chemistry

Volume 90, 27 January 2015, Pages 507-518

https://doi.org/10.1016/j.ejmech.2014.11.061 Get rights and content

Highlights

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SAR studies show key structural features for optimal antiplasmodial activity.
•
In silico docking with falcipain-2 provides insight into requirements for activity.
•
Microsomal metabolic stability was identified as a major liability.
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Metabolite identification studies revealed metabolic hot spots.

Abstract

Following a structure-based virtual screening, a series of 2,4 thiazolidinediones was synthesized in order to explore structure activity relationships for inhibition of the Plasmodium falciparum cysteine protease falcipain-2 (FP-2) and of whole cell antiparasitic activity. Most compounds exhibited low micromolar antiplasmodial activities against the P. falciparum drug resistant W2 strain. The most active compounds of the series were tested for in vitro microsomal metabolic stability and found to be susceptible to hepatic metabolism. Subsequent metabolite identification studies highlighted the metabolic hot spots. Molecular docking studies of a frontrunner inhibitor were carried out to determine the probable binding mode of this class of inhibitors in the active site of FP-2.

Graphical abstract

Introduction

Malaria is a major cause of morbidity and mortality. According to the 2012 World Health Organization (WHO) report, malaria is responsible for an estimated 500,000–900,000 deaths each year, especially among children and pregnant women [1]. Plasmodium falciparum, the most virulent human malaria parasite and Plasmodium vivax are responsible for more than 95% of malaria cases in the world. In light of established and continuing development of resistance of P. falciparum to most antimalarial drugs, the identification and characterization of novel antimalarial chemotypes is an important priority [2], [3], [4].

The P. falciparum cysteine protease falcipain-2 (FP-2), is a key enzyme in the life cycle of P. falciparum since it is involved in the digestion of host haemoglobin in the digestive vacuole of the parasite, providing amino acids for its survival and development [5]. This makes FP-2 an attractive target for antimalarial drug design. Hence, to date, many chemotypes have been identified as FP-2 inhibitors [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20]. In an effort to identify potential antimalarial inhibitors of FP-2, structure-based virtual screening of the ZINC database, which contains more than 13 million drug like compounds, was previously carried out. A total of 32 high ranked compounds were purchased from the SPECs chemical database (www.specs.net), and 14 compounds with activity against the drug resistant P. falciparum W2 strain and FP-2 were identified [21]. Amongst these structures is a thiazolidine-2,4-dione (TZD) derivative, TZD 4 (Fig. 1), which exhibited modest activity against FP-2 (IC₅₀ = 25.5 μM) and P. falciparum (EC₅₀ = 0.45 μM). Compound TZD 4 was selected for subsequent hit validation through resynthesis and biological retesting. Although thiazolidinedione derivatives have been reported to have antimicrobial, anti-proliferative, anti-viral, anti-convulsant, anti-inflammatory and anti-cancer activities [22], [23], [24], [25], [26], [27], [28], to our knowledge their antiplasmodial activity has not previously been studied.

In this paper we report the synthesis and structure–activity-relationship studies around the thiazolidinedione core. In addition, we report microsomal metabolic stability profiling and molecular docking studies of frontrunner compounds.

Section snippets

Chemistry

The target compounds TZD 4, 4a–t were synthesized using an efficient 3-step synthetic route (Scheme 1; a). The synthesis commenced with commercially available 1-phenyl piperazine 1, which was reacted with chloroacetyl chloride in the presence of triethylamine at 0 °C for 1 h to give 2-chloro-1-(4-phenylpiperazin-1-yl)ethanone 2. This intermediate and 2,4-thiazolidinedione 5, in acetone under basic conditions, were heated for 24 h to yield the key intermediate 3, which was reacted with

Conclusions

Analogues of substituted thiazolidine-2,4-diones based on a hit identified from virtual screening of the commercial ZINC database for falcipain-2 inhibition were synthesized using relatively facile synthetic routes. The in vitro biological evaluation of these compounds against FP-2 and P. falciparum revealed several that had IC₅₀ values in the low micromolar range. In particular, the presence of a trimethoxy-substituted phenyl ring attached to C-5 of the core 2,4-thiazolidinedione was found to

General

¹H NMR spectra were recorded on a Varian Mercury Spectrometer at 300 MHz or a Varian Unity Spectrometer at 400 MHz with Me₄Si as internal standard. ¹³C NMR spectra were recorded at 75 and 100 MHz on a Varian Mercury Spectrometer or at 100 MHz on Varian Unity Spectrometer with Me₄Si as internal standard. High resolution mass spectra were recorded on a VG70 SEQ micromass spectrometer. Melting points were determined using a Reichert-Jung Thermovar hot-stage microscope and are uncorrected.

Acknowledgements

The University of Cape Town, South African Medical Research Council, and South African Research Chairs Initiative of the Department of Science and Technology, administered through the South African National Research Foundation are gratefully acknowledged for support (K.C.).

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Original articleSynthesis and structure–activity-relationship studies of thiazolidinediones as antiplasmodial inhibitors of the Plasmodium falciparum cysteine protease falcipain-2

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Chemistry

Conclusions

General

Acknowledgements

Bioorg. Med. Chem.

Eur. J. Med. Chem.

Bioorg. Med. Chem.

Eur. J. Med. Chem.

Carbohydr. Res.

Bioorg. Med. Chem. Lett.

Bioorg. Med. Chem. Lett.

Eur. J. Med. Chem.

Protein Expr. Purif.

J. Biol. Chem.

Structure

J. Immunol. Methods

World Malaria Report 2012

The state of the art in anti-malarial drug discovery and development

Curr. Top. Med. Chem.

Thousands of chemical starting points for antimalarial lead identification

Nature

The global pipeline of new medicines for the control and elimination of malaria

Malar. J.

Cysteine proteases of malaria parasites: targets for chemotherapy

Curr. Pharm. Des.

Tetraoxane-pyrimidine nitrile hybrids as dual stage antimalarials

J. Med. Chem.

Synthesis and biological activities of novel artemisinin derivatives as cysteine protease falcipain-2 inhibitors

Arch. Pharm. Res.

Design, synthesis and biological evaluation of novel benzothiazole and triazole analogs as falcipain inhibitors

MedChemComm

Original article
Synthesis and structure–activity-relationship studies of thiazolidinediones as antiplasmodial inhibitors of the Plasmodium falciparum cysteine protease falcipain-2