Synthesis and biological activity of anthelmintic thiadiazoles using an AF-2 receptor binding assay

Receptor-based discovery strategies for insecticides and parasiticides: a review

Drug discovery is an iterative process with high risks and low chance of success. New genomics technologies allow veterinary medicine and agrochemical companies to validate and functionally screen new receptor-based targets, including neuropeptide G-protein coupled receptors, which were previously not amenable to high throughput screening. However this is just the first step in a long process to translate a mechanistic assay hit into a drug on the market. In addition to effectively eradicating pests on crops and parasites on their host, the molecules must also be safe, cheap to synthesise, formulatable and patentable. This is a costly process in which early attrition of unsuitable molecules is key to any successful program. Although first principle discovery is risky the ultimate benefits are considerable and future genomics resources will help to generate higher quality hits to strengthen the discovery pipeline.

Nematode neuropeptide receptors and their development as anthelmintic screens

This review addresses the potential use of neuropeptide receptors for the discovery of anthelmintic agents, and particularly for the identification of non-peptide ligands. It outlines which nematode neuropeptides are known and have been characterized, the published information on drug discovery around these targets, information about existing high- and low-throughput screening systems and finally the likely safety of neuropeptide mimetics.

The nematode neuropeptide, AF2 (KHEYLRF-NH2), increases voltage-activated calcium currents in Ascaris suum muscle

BACKGROUND AND PURPOSE

Resistance to all the classes of anti-nematodal drugs like the benzimidazoles, cholinergic agonists and avermectins, has now been recorded in parasites of animals and/or humans. The development of novel anthelmintics is an urgent and imperative need. Receptors of nematode neuropeptides have been suggested to be suitable target sites for novel anthelmintic drugs.

EXPERIMENTAL APPROACH

To investigate the effect of AF2 on calcium-currents in Ascaris suum somatic muscle cells we employed the two-micropipette current-clamp and voltage-clamp techniques and a brief application of AF2.

KEY RESULTS

Here we report the isolation of voltage-activated, transient, inward calcium currents. These currents are similar in characteristics to Caenorhabditis elegans UNC-2 type currents, non-L-type calcium currents. Following a 2-minute application of 1 microM AF2 , there was a significant long-lasting increase in the transient inward calcium current; AF2 increased the maximum current (from -84 nA to -158 nA) by shifting the threshold in the hyperpolarising direction (V (50) changed from -7.2 to -12.8 mV) and increasing the maximum conductance change from 1.91 to 2.94 microS.

CONCLUSION AND IMPLICATIONS

These studies demonstrate a mechanism by which AF2 increased the excitability of the neuromuscular system by modulating calcium currents in nematodes. A selective small molecule agonist of the AF2 receptor is predicted to increase the contraction and act synergistically with cholinergic anthelmintics and could counter resistance to these compounds.

Brief application of AF2 produces long lasting potentiation of nAChR responses in Ascaris suum

Resistance of parasitic nematodes to the cholinergic anthelmintic levamisole is associated with a reduction in the proportion of time that acetylcholine receptor ion-channels are in the open state decreasing the response of nematode parasites to the drug. Here we examine electrophysiological and contractile responses to acetylcholine and the cholinergic agonist, levamisole, in Ascaris suum muscle looking for a pharmacological approach that may be developed to increase the response to cholinergic agonists. We found that short application of the FMRFamide, AF2, produced modulation (long lasting potentiation) of the peak membrane potential response to acetylcholine but not to levamisole. Since levamisole preferentially activates L-type acetylcholine receptors, we also tested the effect of nicotine (selective activator of N-type acetylcholine receptors) and bephenium (selective activator of B-type acetylcholine receptors) and found again no effect of AF2 on peak membrane potential responses. We then tested atropine on the AF2 potentiation of acetylcholine and found it to inhibit the peak potentiation suggesting that AF2 receptors interact with muscarinic receptors to produce the potentiation of acetylcholine. We saw similar atropine sensitive potentiation of acetylcholine responses in our muscle contraction experiments. The potentiation of the acetylcholine responses shows that nematode acetylcholine receptors are capable of a level of plasticity. A model involving calcium release from the sarcoplasmic reticulum, CaM Kinase, calcineurin, muscarinic receptors and AF2 receptors is proposed to explain our observations. These observations are important because they point to a pharmacological approach that may be developed to counter resistance to cholinergic anthelmintics.

Effects of KHEYLRFamide and KNEFIRFamide on cyclic adenosine monophosphate levels in Ascaris suum somatic muscle

KHEYLRF-NH(2) (AF2) is a FMRFamide-related peptide (FaRP) present in parasitic and free-living nematodes. At concentrations as low as 10 pM, AF2 induces a biphasic tension response, consisting of a transient relaxation followed by profound excitation, in neuromuscular strips prepared from Ascaris suum. In the present study, the effects of AF2 on cyclic adenosine monophosphate (cAMP), cyclic guanosine monophosphate (cGMP) and inositol-1,4,5-triphosphate (IP(3)) levels were measured following muscle tension recordings from 2 cm neuromuscular strips prepared from adult A. suum. AF2 induced a concentration- and time-dependent increase in cAMP, beginning at 1 nM; cAMP levels increased by 84-fold following 1 h exposure to 1 microM AF2. cGMP and IP(3) levels were unaffected by AF2 at concentrations </=1 microM. AF2-induced stimulation of cAMP was unaffected by removal of the dorsal or ventral nerve cord, even though this form of denervation abolished the excitatory phase of the tension response. The effects of 0.1 and 1 microM AF2 on cAMP were also unaffected by 10 microM SDPNFLRF-NH(2) (PF1, an inhibitory FaRP) and 10 microM PF1022A (an inhibitory cyclodepsipeptide), even though each of these peptides abolished the excitatory phase of the tension response induced by AF2. Within an alanine-scan series of AF2 analogues, only KHAYLRF-NH(2) stimulated cAMP production with equipotency to AF2; the effects of this peptide on muscle tension also mimicked AF2. Another excitatory FaRP present in nematodes, KNEFIRF-NH(2) (AF1), also stimulated cAMP production, but was 100-fold less potent than AF2. The stimulatory effects of AF1 on tension and cAMP levels were blocked by an alanine-substituted analogue of this peptide (Ala(6)-AF1, KNEFIAF-NH(2)), while the stimulatory effects of AF2 on tension and cAMP were not affected by this analogue. AF2 and AF1 increase A. suum somatic muscle cAMP by targeting different receptors. Increases in cAMP stimulated by AF2 can be decoupled from the excitatory response caused by this peptide, and it is not possible to establish a causal linkage between the contractile response elicited by this peptide and its effects on cAMP accumulation.