The I'eteHnaU.flmrna11997, 154, 11-34 + Review Modes of Action of Anthelmintic Drugs R.J. MARTIN Depa,/men/ o/ l',e,li,i,al Vete,'i,m,y .Sciences. R. CDOS.V.S., Su,n,,,e,hall. University' of Edinbu,gh, Edinburgh Etq9 I QH, UK SUMMARY Modes of action of anthelmintic drugs are described. Some anthelmintic drugs act rapidly and selectively on neuronmscular transmission of nematodes. Levamisole, pyrantel and morantel are agonists at nicotinic acetylcholine receptors of nematode muscle and cause spastic paralysis. Dichlorvos and haloxon are organophosphorus cholinesterase antagonists. Piperazine is a GABA (T-amino-bu~ric acid) agonist at receptors on nematode muscles and causes flaccid paralysis. The avermectins increase the opening of glutanmte-gated chh)ride (GltlC1) channels and produce paralysis of phmTngeal pumping. Praziquantel has a selective efl'ect on the tegument of trematodes and increases permeability of calcium. Other anthehnintics have a biochemical mode of action. The benzimidazole drugs bind selectively to ~-tubulin of nematodes, cestodes and fluke, and inhibit nficrotubule formation. The salicylanilides: rafoxanide, oxych)zanide, brotianide and closantel and the substituted phenol, nitroxynil, are proton ionophores. Clorsuhm is a selective antagonist of fluke phosphoglycerate kinase and mutase. Diethylcarbamazine blocks host, and possibly parasite, enzymes involved in arachidonic acid metabolism, and enhances the innate, nonspecific immune system. Kx-~WOnl)S: Mode of action; anthehnintics; levamisole; dichlorvos; piperazine; avermectins; praziquantel; benzimidazoles; salicylanilides; clorsulon; diethylcarbamazine. INTRODUCTION NICOTINIC AGONISTS Parasitic nematodes affect animals and man caus- Levamisole, lmtamisole, pyrantel, morantel, ing considerable suffering and poor growth. oxantel, bephenium and thenium Fig. 1 shows the chemical structures of nicotinic Effective anthehnintic drugs, used to treat and control these infestations, must have selective anthelmintics. There are the imidazothiazoles toxic effects on these parasites. Unfortunately (levamisole and butamisole); the tetrahydropyrim- idines (pyrantel, morantel and oxantel); the quat- with the increased use of these compounds, anthelmintic resistance has appeared and ernatT ammonium salts (bephenium and theniuna) and tile pyrimidines (methyridine). increased in fl-equency (Prichard, 1994). If resist- These compounds act selectively as agonists at syn- ance to a particular anthehnintic has occurred, it aptic and extrasynaptic nicotinic acetTlcholine is likely that another anthelmintic with the same receptors on nematode muscle cells (Fig. 2) and mode of action will also be ineffective although produce contraction and spastic paralysis. The other anthehnintics with another mode of action, electrophysiological effects of levamisole, may still be effective. Clearly then, it is important pyrantel, morantel and oxantel have been studied to have an understanding of the mode of action of in greatest detail. anthehnintics in order to inform the selection of effective therapeutic agents. This rexqew describes modes of action of anthelmintic drugs (Table I). 1(190-0233/t)7/04(1011-24/S 12.00/0 © 1997 Bailli6re Tindall 12 THE VEH.:RIN..\RY .IOL'RNAI.. 154, I Table I. Summary table of the modes of action of anthelmintic drugs (;e,eric drueff name Iqxam/de,~ ,1 ,w,me (". K. & l '.&A. Trade .\'mm,.s Nicotinic agonists Icvanlisole Nilverm Illllanlisolt' Slyqllill pVlatltel Stl(ingid ii1( ii+~lll tcl P;u'alct'l I)cphcnhlili lht'llillni ( :anlll)ar nlcthvridinc Acetylcholinesterase inhibitors 11;llOXOll Mulitwttrma dichhwvos GABA agonist pipura/hic End.rid. GiuCI potentiators ivt'l+lllt'Clill [V(IIIR'C al)alllcclill d~WalllCClill l)t~('tOlll;tX inoxidcciin ( :v(It'ctill niill)cinvcin l) Calcium permeability increase praziqu,in tel l)l~mcit [~-tubulin binding thial)cnda]<dc Thibcnz.lc Callli)t,lld~iztilt + Equibcn, N~vil)cn ~lxil)cndaz<dc koditac all)cndazolc Valb,lzcn albcndazoic sulphoxidc Rvc<d)cn I{'nl)cndazoic Pilll[I('lll" tlxl{.lldazoh. Svstamcx lllt'l)('llda/ilit' Tchniu Ihd)cndazole Fhil)vn~l I~'bantcl ~alVVtTIII nclol)ilnin I Icpadcx lhioj)hanaic \V< wlnal:tc, Ncmal,tx triclabcnda/olc Fasincx Proton ionophores chls<tntcl Flukivcr i-alilyanidt+ Flukanidc. Ranidu oxvch izi.illidc Zanil I)r(>tianidc In Fhlkombin and Vermadcx nill~,xvnil Trodax, l)ov<.'n ix niclopholan BilevolL Dist<don ]lt'xachlorophcnt. (:oopaphcnc, l)ismdin dil)rllmosalan nich;samidc Inhibition of malate metabolism dialnphent'lhidc ( '.ol-il)an Inhibition of phosphoglycerate kinase C](II'SII](III In h'omec Super and mutase Inhibitor of arachidonic acid dicthvlcarl)amazine (:aricide,Filaricide metabolism and stimulation of innate immunity Electrophysiological effects of nicotinic ettes fl'mn Ascaris suum body muscles have shown anthelmintics in nematodes that bath application of tetramisole, the D/l. Intracelhllar recordings made with micropip- racemic mixture of levamisole, produces ntem- M()DES OF ACTION ()F ANTHELMINTIC DRU(;S 13 Butamisole Levamisole CH H 0 a / H N/H N CHa S Pyrantel Morantel Oxantel CHal CIita CI~:, N N N %) CHa Thenium CH:I Methyridine 0 -- CH 2 -- CH,2 -- N + -- CH._, I /CH 2- CH 2- O -- CH 2 CHa Bephenium CH a ~ O -- CH 2 -- CH 2 -- N + -- CH2 1 C> CHa' C? Fig. 1. Chmnical structures of nicotinic anthchnintics. brahe del)olarization, an increase in spike fl'e- inlents was: morantel=pyrantel>levamisole>acetyl- qnency and contraction (Aceves el al., 1970). choline (Harrow & Gration, 1985). Pvrantel and its analogues also produce depolariz- In addition to effects of tile anthehnintics on ation, increased spike actMtv and contl-action the membrane conductance of the muscle, when applied to Ascaris muscle (Aubr)' el al., 1970) Harrow and Gration (1985) described the conduc- suggesting that these compounds have a common tance dose-response curves for pyrantel and mor- mode of action. antel as being 'bell shaped'. The effect of this con- The two-microelectrode currelat-clamp and voh- centration-effect relationship is that the age-clamp teclmiques have been used to exalnine conductance-response increases then decreases as mttscle nlembrane conductance changes in Ascaris the concentration of the anthelmintics rises. One produced by acetylcholine, levamisole, pyrantel explanation tor this phenomena is that of open and morantel (Martin, 1982a; Harrow & Gration, channel block (Colquhoun & Sakmann, 1985) by 1985). These anthehnintics have I)een shown to the anthelmintic. Levamisole, pyrantel, morantel increase the meml)rane conductance and depolar- and oxantel are large organic cations, and could ize the membrane by opening non-selective cation enter the nicotinic ion-channel from the outside ion-channels that are permeable to both Na + and and tl-y to pass through the channel like Na + or K+ K+. Simultaneous application of acetylcholine and ions but produce the block at the narrow region pyrantel showed that both agonists acted on the of the channel, the selectivity filter. This block same nicotinic receptors, and the relative potency would be voltage-sensitive and increase with hyp- of the anthehnintics in bath application exper- erpolarization of the membrane and concen- 14 THE VETERINARY .JOURNAL 154, 1 (a) (c) ~ t~~ ~ Dorsal nelve cord ~ Syncylium eao ~'ry Arm Syncytium s cord I J 200 I.tm 1 mm Ventral nerve cord (b) Posterior Anterior Dorsal T ~ -YY Y V Y Y Y nerve .% cor DE3 Left ./ commissure J. ~kv .t ,k 2~ ,k v ., o w v v v Ventral YY YY YY YY YY YYYYY 7 nerve T cord Y g Y g 7 7 V T Fig. 2. Diagrams of tile neuronmscular organization of nematode somatic muscle. (a) Diagram of a s(mmlic muscle cell showing: the contractile spindle region; tile balloon-shaped bag that contains tile nucleus and glycogen granules; tile arm which is a process of tile muscle and reaches to one of tile nerve cords where i, divides into t]ngers Ihal I()lnl all electrically-coupled complex with the fingers of adjacent muscle cells and collectiveh' is known as the svncvtitml. The muscle cell possesses synaptic nicotinic acetylcholine receptors, and synaptic GABA receptors at tile svncvtial region and extrasynaptic acetylcholine and G,MIA receptors over tile surface of the muscle. (b) Diagram of the dorsal and ventral nerve cord showing the cell represented in a segment. All tile cell bodies of tile motor nettrones are contained in tile ventral nerve cord. Each segment has three right-hand commissures and one left-hand commissure. Each commissure is made up of one or two axons or dendrites of tile motor neurones that pass round the body to connect tile ventral and dorsal nerve cord. Within each segment there are 11 motor nero-ones that are divided into seven anatomical types (DEI, DE2, DE3: dorsal excitato~3.' motor neurones. DI: dorsal inhibitory motor neurones. VI: ventral inhibitory motor neurone. VI & V2: ventral excitatocv ntotor neurones. The remaining axons in the ventral nelwe cord are intersegmental neurones. Tile excitatory motor neurones are cholinergic and tile inhibitory motor neurones are GABAergic. (c) Diagrmn of a cross section of the body just caudal to the phau, ngeal muscle. It shows the relative locations of tile nerve cords, lateral lines, gut and muscle cells. tration of the anthelmintic. This property of pro- preparation (Robertson & Martin, 1993) using the ducing open channel block has been established patch-clamp technique. The ion-chalmel currents using single-channel recording techniques activated by low levamisole concentrations were (Robertson & Martin, 1993). shown to cart3 , cations and to have similar kinetics to acetylcholine-activated channels. The levami- Single-channel currents activated by nicotinic sole activated channels have a mean open time of anthelmintics 1.34 ms and a conductance in the range 20-45 pS. Initially, levamisole-activated channel currents At higher concentrations of levamisole, a flicker- [Fig. 3(a)] were recorded fi'om the muscle vesicle ing open channel block that increases on hyper- MODES OF ACTION OF ANTHELMINTI(" DRUGS 15 polarization is obsel-ved [Fig. 3(b)]. At -50 mV with oxantel, occupying an intermediate position the dissociation constant for the channel block between morantel and oxantel. The least potent was 123 ~,xl.