Advances in the discovery and development of anthelmintics by harnessing natural product scaffolds

Abstract

Widespread resistance to currently-used anthelmintics represents a major obstacle to controlling parasitic nematodes of livestock animals. Given the reliance on anthelmintics in many control regimens, there is a need for the continued discovery and development of new nematocides. Enabling such a focus are: (i) the major chemical diversity of natural products; (ii) the availability of curated, drug-like extract-, fraction- and/or compound-libraries from natural sources; (iii) the utility and practicality of well-established whole-worm bioassays for Haemonchus contortus—an important parasitic nematodes of livestock—to screen natural product libraries; and (iv) the availability of advanced chromatographic (HPLC), spectroscopic (NMR) and spectrometric (MS) techniques for bioassay-guided fractionation and structural elucidation. This context provides a sound basis for the identification and characterisation of anthelmintic candidates from natural sources. This chapter provides a background on the importance and impact of helminth infections/diseases, parasite control and aspects of drug discovery, and reviews recent work focused on (i) screening well-defined compound libraries to establish the methods needed for large-scale screening of natural extract libraries; (ii) discovering plant and marine extracts with nematocidal or nematostatic activity, and purifying bioactive compounds and assessing their potential for further development; and (iii) synthesising analogues of selected purified natural compounds for the identification of possible ‘lead’ candidates. The chapter describes some lessons learned from this work and proposes future areas of focus for drug discovery. Collectively, the findings from this recent work show potential for selected natural product scaffolds as candidates for future development. Developing such candidates via future chemical optimisation, efficacy and safety evaluations, broad spectrum activity assessments, and target identification represents an exciting prospect and, if successful, could pave the way to subsequent pre-clinical and clinical evaluations.

Introduction

Diseases caused by gastrointestinal roundworms (nematodes) have a substantial adverse impact on animal health worldwide (Fenwick, 2012; Fitzpatrick, 2013). In livestock animals, such worms result in reduced productivity, leading to major economic losses in agriculture and the food industries (Charlier et al., 2014, Charlier et al., 2015; Lane et al., 2015). The direct production losses associated with helminth infections relate to adverse effects on weight gain, milk, meat and wool production, infertility, carcass quality and composition, and feed intake (Albers et al., 1989; Charlier et al., 2014). The economic impact of parasitic diseases is not only due to direct production losses, but also arises from controlling these parasites, which includes chemotherapy, vaccination and/or parasitic management programs (Corwin, 1997).

A sound practice for controlling gastrointestinal nematodes of small ruminants (including sheep and goats) involves the use of an integrated management approach, in which multifaceted strategies, such as pasture and grazing management, nutritional management and/or vaccination are used, in combination with the strategic treatment of animals with anthelmintics (Kearney et al., 2016). The classes of anthelmintics mostly used for this purpose are benzimidazoles, imidazothiazoles, macrocyclic lactones, aminoacetonitrile derivatives, tetrahydropyrimidines, salicylanilides and organophosphates (Table 1). The drugs from each of these chemical groups have unique targets in the parasites, and represent either β-tubulin binders, nicotinic agonists, glutamate-gated chloride channel potentiators, acetylcholinesterase inhibitors or proton ionophores (Besier et al., 2016; Lanusse et al., 2016; Martin, 1997). However, parasitic nematodes, particularly those of the order Strongylida, have developed anthelmintic resistance to each of the currently available drugs (e.g. Bartley et al., 2019; Drudge et al., 1964; Kaplan and Vidyashankar, 2012; Papadopoulos et al., 2012; Sangster et al., 2018; Scott et al., 2013; Van den Brom et al., 2015; Wolstenholme et al., 2004). Resistance against a drug class can develop due to a change in the drug target or a gene encoding the target, drug distribution and/or alterations in parasite metabolism over the time (Wolstenholme et al., 2004). Such resistance can develop relatively quickly in small ruminants (resistance developed within 4–9 years to all current anthelmintics), as chemicals are usually over-relied upon by farmers, and animals are often under-dosed (Van Wyk, 2001). The slow rate of resistance development in cattle compared with sheep and goats suggests that genetic changes in worms are not the only factor in the process of resistance selection. Several other factors, such as host physiology and biochemistry, host–parasite relationship, treatment regimen, epidemiology and biology of the worms (or a combination thereof), can be responsible for resistance development (Kaplan, 2004; Kotze and Prichard, 2016). Furthermore, numerous studies have reported the development of resistance against multiple drugs (Bartley et al., 2019; Cazajous et al., 2018; Cezar et al., 2010; Green et al., 1981; Ploeger and Everts, 2018; Thomaz-Soccol et al., 2004); such ‘multidrug resistance’ seriously threatens the control of parasitic nematodes in animals. Therefore, new drugs with novel modes of action are required to combat widespread resistances to single and multiple drugs.

In the last decades, there has been a focus on discovering and developing synthetic drugs as anthelmintics (Fig. 1). However, there has been limited progress to date, with only few examples of successful developments of anthelmintics, including tribendimidine (Xiao et al., 2005), monepantel (Kaminsky et al., 2008a, Kaminsky et al., 2008b) and derquantel (Little et al., 2010). The recent Nobel Prize-winning discoveries of the anti-parasitic drugs artemisinin and avermectin, and disappointing results from large synthetic compound library screens, encourage the reconsideration of natural products as candidates for screening in drug discovery programs (Harvey et al., 2015). The extraordinary biodiversity in both the terrestrial and marine environments makes these systems reservoirs of diverse arrays of bioactive compounds (Harvey, 2000; Li and Lou, 2018). In addition, there are thousands of natural extracts that have been used in traditional medicines, of which the active constituents have not been studied systematically (Li, 2016; Penido et al., 2016; Tulp and Bohlin, 2004). Compounds from natural sources are naturally aligned with Nature's need to develop a ‘chemical warfare’ arsenal (Baell, 2016). This and several other advantageous properties of natural product libraries, particularly substantial chemical diversity compared with synthetic compound libraries, enhance their potential for drug discovery (Harvey et al., 2015; Koehn and Carter, 2005).

However, natural product drug discovery has some inherent challenges, including restrictions and difficulties relating to sample collection, recollection and identification of source species (David et al., 2015; Harvey, 2000), low compatibility of crude samples for advanced high throughput screening systems as well as challenges relating to traditional bioassay-guided fractionation (Gaudencio and Pereira, 2015; Harvey et al., 2015). However, the attempts made by selected institutes to maintain drug-like extract and pre-fractionated extract libraries, and/or different types of natural compound libraries (Harvey et al., 2015; Li and Vederas, 2009), together with advances in chromatography, spectrometry and spectroscopy techniques can assist in overcoming these limitations (Harvey et al., 2015; Li and Vederas, 2009). These avenues and advances thus encourage the exploration of natural sources for anthelmintic drug candidates.