Synthesis and structure-activity relationship study of pyrrolidine-oxadiazoles as anthelmintics against Haemonchus contortus

Highlights

• Parasitic diseases present a considerable socio-economic impact to society.

• A new class of anthelmintic oxadiazol-pyrrolidine derivatives is reported.

• Compounds display lowmicromolar activity against Haemonchus contortus larvae.

• Compounds exhibit good selectivity against mammalian epithelial cell.

• Oxadiazol-pyrrolidine compounds are promising agents against Haemonchus contortus.

Abstract

Parasitic roundworms (nematodes) are significant pathogens of humans and animals and cause substantive socioeconomic losses due to the diseases that they cause. The control of nematodes in livestock animals relies heavily on the use of anthelmintic drugs. However, their extensive use has led to a widespread problem of drug resistance in these worms. Thus, the discovery and development of novel chemical entities for the treatment of parasitic worms of humans and animals is needed. Herein, we describe our medicinal chemistry optimization efforts of a phenotypic hit against Haemonchus contortus based on a pyrrolidine-oxadiazole scaffold. This led to the identification of compounds with potent inhibitory activities (IC₅₀ = 0.78–22.4 μM) on the motility and development of parasitic stages of H. contortus, and which were found to be highly selective in a mammalian cell counter-screen. These compounds could be used as suitable chemical tools for drug target identification or as lead compounds for further optimization.

Introduction

Helminth infestations of humans and animals pose a public health threat and result in economic losses around the globe [1,2]. In humans, helminths cause significant morbidity and mortality [3]. In animals, their economic impact on worldwide livestock production is substantial and often devastating [4]. The ubiquitous blood-feeding nematode, Haemonchus contortus – also known as the barber’s pole worm – is one of the most important pathogens of ruminant livestock (including sheep and goats). H. contortus worms attach to abomasal mucosa and feed on the blood, causing anaemia, oedema, and often death of infected animals [5,6]. The control of H. contortus and numerous other related nematodes relies heavily on the use of anthelmintic drugs. However, the extensive use of anthelmintics, particularly in livestock animals, has led to a widespread problem of drug resistance in nematode populations [7].

Efforts towards developing novel anthelmintics are relatively limited, despite the increased need to reduce the subclinical, clinical and economic burden associated with helminth infections [8,9]. Existing anthelmintic drugs include the chemical classes benzimidazoles, imidazothiazoles and macrocyclic lactones [10]. Over the past decades, only few new classes of anthelmintics have been discovered and approved, and only three new scaffolds have made it into the market since 2007 [10]: cyclodepsipeptides (e.g. emodepside), which are currently restricted to use in companion animals; amino-acetonitrile derivatives (e.g. monepantel) and spiroindoles (e.g. derquantel) (Fig. 1). Unfortunately, resistance has arisen to most of these drugs too [7,8], further driving the urgency for the discovery of new chemical entities to control nematode infections of livestock animals.

In a previous high-throughput screening (HTS) campaign on the ‘Open Scaffolds’ collection from Compounds Australia [11], we identified SN00797439 (4) as a ‘hit’ compound with broad-spectrum anthelmintic activity against biologically and genetically distinct nematodes (Fig. 2, unknown absolute stereochemical configuration). In particular, SN00797439 (4) induced a characteristic “coiled” exsheathed third-stage larvae (xL3) phenotype (IC₅₀ = 3.46–5.93 μM) in H. contortus, inhibited motility of fourth-stage larvae (L4; IC₅₀ = 0.31–12.5 μM) and caused considerable cuticular damage to L4s in vitro. In the absence of prior literature describing the anthelmintic activity of 1,2,4-oxadiazol-5-pyrrolidine-1-carboxamide derivatives, we undertook further development of SN00797439 (4) by means of a hit-to-lead and a lead optimization campaign. We aimed at improving the in vitro potency, selectivity and absorption, distribution, metabolism, excretion and toxicity (ADMET) properties, so as to identify a compound suitable for anthelmintic efficacy and safety evaluations in animals.

Here, we describe the synthesis and optimization efforts toward the discovery of a novel class of anthelmintics. The initial hit SN00797439 (4) was progressed through systematic structure-activity relationship (SAR) explorations around the northeast (NE) and southeast (SE) phenyl rings (Fig. 2). In vitro cytotoxicity evaluation of the active compounds, along with ADME studies, performed on a lead compound are also reported.