PHARMACEUTICAL CHEMISTRY II (PHCM672) Lecture 2, Antihelmintics and drugs used against ectoparasitic infections Dr. Mohammad Abdel-Halim LICE

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1 PHARMACEUTICAL CHEMISTRY II (PHCM672) Lecture 2, Antihelmintics and drugs used against ectoparasitic infections Dr. Mohammad Abdel-Halim LICE

2 Part I Antihelmintics Benzimidazoles Thiabendazole Mebendazole Albendazole Piperazines Piperazine citrate Diethylcarbamazine citrate thers Niclosamide Pyrantel pamoate Ivermectin Praziquantel 2 xamniquine

3 Antihelmintics Drugs against worms (helminths) infections ne of the most serious public health problems, espacially in developing coutries, ¼ of the world population may be infected Helminths Platyhelminths Nematodes (roundworms) Cestodes (tapeworms) Trematodes (flukes) Hook worm (ancylostomiasis) Pinworm (enterobiasis) Whipworm (trichuriasis) Ascariasis Filariasis (Elephantiasis) nchocerciasis (River blindness) 3

4 Relative incidence of helminth infections worldwide 4

5 Hookworm infection (Ancylostomiasis)- intestinal nematode This disease is caused by American hookworm (Necator americanus) and ld World hookworm (Ancylostoma duodenale), Larvae found in the soil are transmitted by penetrating the skin or oral ingestion The worm attaches to intestinal mucosa causing anorexia, ulcer-like symtoms and chronic intestinal blood loss,leading to anmeia, living for 9-15 years Diagnosed by detection of characteristic eggs in the stool. fales usually 5 to 9 mm long and females about 1 cm long 5

6 Enterobiasis (Pinworm Disease)-intestinal nematode This disease is caused by Enterobius vermicularis. It is the most common helminthic infection in the United States. Pruritus ani occurs, with white worms visible in the stools or perianal region. The disease is transmitted by ingesting the organism s eggs. Humans are the only host. Mature worms reach 10 mm in size. The female migrates to the rectum to deposit her eggs (perianal itching) Pinworm disease is diagnosed by identifying eggs present around the perianal region. It is treated with mebendazole or pyrantel pamoate Pinworms leaving the anus of a five-year-old child 6

7 Ascariasis - intestinal nematode This disease is caused by Ascaris lumbricoides. It is second only to pinworms as the most prevalent multicellular parasite (USA). The disease is transmitted by ingestion of soil containing the organism s eggs. Humans are the sole host. Larva grow in the intestine, causing abdominal symptoms, including intestinal obstruction. The worm is reaching cm in length Roundworms may pass to the blood. Roundworm disease is diagnosed by detection of characteristic eggs in the stool. It is treated with pyrantel pamoate or mebendazole. 7

8 Trichuriasis (Whipworm Disease)- intestinal nematode This disease is caused by Trichuris trichiura. The infection is usually asymptomatic; however, abdominal pain, diarrhea, flatulence, and rectal prolapse can occur. The disease is transmitted by ingestion of soil containing the organism s eggs. Whipworm disease is diagnosed by identifying characteristic eggs in the stool. It is treated with mebendazole Female mm long male mm long 8

9 Filariasis (Elephantiasis) Tissue nematode infection The most frequent organisms causing this disease are Wuchereria bancrofti and Brugia malayi. These filarial worms block the flow of lymph, causing edematous arms, legs, and scrotum. The disease is transmitted by the bite of infected female Anopheles and Culex mosquitoes. Filariasis is diagnosed by detection of microfilariae in blood. It is treated with a combination of diethylcarbamazine and albendazole. Elephantiasis ther forms can affect the eye 9

10 Tapeworm infections- intestinal cestode Beef tapeworm infects people eating undercooked beef reaches > 5 m length containing ~ 100 segments Pork tapeworm found in uncooked pork attaches itself to the intestinal wall of the human host reaches ~ 5 m length Dwarf tapeworm reaches only 3-4 cm length transmitted directly from human to human children mostly affected Fish tapeworm reaches ~ 10 m length containing ~ 400 segments absorbs folic acid and vitamin B 12 from the host - Infection by eating raw or undercooked fish that contain cysts. Most infected individuals are asymptomatic. 10

11 Blood flukes (Schistosomiasis)- Trematode Bilharziosis, snail fever (endemic in Egypt) The disease is transmitted through penetration of skin by the free swimming form cercaria released from fresh water snails Schistosomiasis (ld World) Caused by Schistosoma haematobium. The primary sites of infection are veins of the urinary bladder, where the organism s eggs can induce fibrosis, granulomas, and hematuria. Diagnosed by identifying characteristic eggs in the urine or bladder wall. It is treated with praziquantel. Adult worms reach ~ 2 cm in length male Schistosomiasis (New World) Caused by Schistosoma mansoni and Schistosoma japonicum. The primary site of infection is the gastrointestinal tract. Damage to the intestinal wall is caused by the host s inflammatory response to eggs deposited at that site. The eggs also secrete proteolytic enzymes that further damage the tissue. Clinical presentation includes GI bleeding, diarrhea, and liver damage. Diagnosed by identifying characteristic eggs in stool. It is treated with praziquantel

12 Antihelmintics Benzimidazoles Thiabendazole Mebendazole Albendazole Piperazines Piperazine citrate Diethylcarbamazine thers Niclosamide Pyrantel pamoate Ivermectin Praziquantel xamniquine 12

13 Benzimidazoles Broad-spectrum agents against many GI helminths Discovered in the 60s, several thousand analogs Mechanism of action: - Inhibition of fumarate reductase (helminth-specific enzyme involved in NADH NAD) - They bind to tubulin and prevent its polymerization to microtubules (cellular components involved in mitosis cell division inhibitors) Poor water solubility - poor absorption from GI tract, increased by fatty meals - beneficial because the primary indications are intestinal helminths 13

14 1) Thiabendazole (Mintezol ) 2-(4-Thiazolyl)benzimidazole Soluble in mineral acids (pka 4.7) Forms complexes with metal ions Used to treat diverse hookworm and whipworm infections Used in veterinary medicine to control intestinal helminths in livestock N N H N S thiazole Toxicity: Steven-Johnson syndrome, hepatotoxicity, crystallurea (less used than other benzimidazoles) Rapid metabolism in the liver 14

15 2) Mebendazole (Vermox ) Methyl (5-benzoyl-benzimidazol-2-yl) carbamate Effective against many nematodes (whipworm, pinworm, roundworm, hookworm) Teratogenic in rats, should not be given during pregnancy Metabolism H H N N H N N H NH 2 N H CH 3 Conjuga tes Conjugates 15

16 3) Albendazole (Zental ) Methyl [5-(propylthio)-benzimidazol-2-yl] carbamate Widely used against intestinal nematode and cestode infections (less effective) Teratogenic in rats, should not be given during pregnancy H 3 C thioether S N N H NH Me Unique metabolism: rapid first-pass metabolism to sulfoxide (active metabolite, half-life h), further oxidation to inactive sulfone Minor metabolites: carbamate hydrolysis, propyl side-chain oxidation sulfoxide (active) Sulfone (inactive) 16

17 Piperazine citrate Piperazine analogs Diethylcarbamazine citrate Highly soluble in water and rapidly absorbed Treatment of pinworm and roundworm infections Syrup and tablet forms Mode of action : Causes paralysis of the worm by blocking the neuromuscular transmission Especially effective against filariasis Discovered during WWII in search for treatment of American troops Inactive in vitro, unknown mech. of action Metabolites 50% 23% 17

18 Niclosamide (Yomesan ) Cl H N H Cl N Salicylanilide drevative Water insoluble Little systemic absorption after oral administration Drug of choice against tapeworm infections (not effective against pinworms and round worms) Causes rapid desintegration of the worm segments 18

19 Pyrantel pamoate (USP, Antiminth ) C Salt S H H HN N CH 3 CH 2 H H pyrantel pamoate CH Pamoate causes reduced water solubility reduced absorption and longer residence time in GI tract Used against pinworms and roundworms Mode of action : Causes spastic paralysis in susceptible helminths (depolarizing neuromuscular blocking agent) 19

20 16-membered macro-cyclic lactones glycosidically linked to a disaccharide Ivermectin Ivermectin is Semi-synthetic, prepared by catalytic hydrogenation of the natural Avermectins R= Me or Et Avermectins are members of a family of structurally complex antibiotics produced by fermentation with a strain of Streptomyces avermitilis. Mechanism of action :blocks interneuron motor neuron transmission in nematodes by stimulating the release of the inhibitory neurotransmitter GABA. The drug is used primarily in the treatment of African river blindness 20 caused by nchocerca volvulus and lymphatic filariasis.

21 Half of the2015 Nobel Prize in Physiology or Medicine was awarded jointly to Campbell and Ōmura for discovering Avermectin William C. Campbell Satoshi Ōmura

22 Always do your best. What you plant now, you will harvest later. Ōmura identified avermectin from Streptomyces avermitilis. Campbell purified avermectin from cultures obtained from Ōmura and led efforts leading to the discovery of ivermectin, a derivative of greater potency and lower toxicity. Ivermectin was introduced in The Nobel prize is awarded in

23 Praziquantel (Biltricide ) riginally developed as antipsychotic drug Levo enantiomer more active Broad-spectrum agent against trematodes (flukes; intestinal, liver, and lung forms) Drug of choice against schistosomiasis (blood flukes) Mode of action Increases cell-permeability for Ca 2+ muscle contraction and paralysis of the worms followed by phagocytosis 80% oral bioavailability, Rapid first-pass metabolism 23

24 xamniquine (Mansil, Vansil ) Tetrahydroquinoline analog Indicated for the treatment of intestinal schistosomiasis caused by S. mansoni Good oral bioavailability Metabolism inactive metabolites H N 2 N HN H N CH 2 H HN 2 N CH 24

25 xamniquine (Mansil, Vansil ) Mode of action: activated via esterification to phosphate or sulfate inside the parasite, that dissociate to an electrophile alkylating the helminth DNA electrophile the helminth DNA is destroyed

26 Antiscabious and Antipedicular agents Part II Drugs against ectoparasitic infections Benzyl Benzoate Lindane Crotamiton Pyrethrins 26

27 Scabies Ectoparasitic infections Scabies Lice Caused by the itch mite (Sarcoptes scarbei) Spread by person-to-person contact Parasite can live 2-3 days in clothing, bedding, house dust Mites burrow into the epidermis in the folds of the skin of fingers, ellbows, breasts, penis, scrotum, buttocks Females tunnel into the dead, outermost layer (stratum corneum) of a host's skin and deposit eggs in the shallow burrows. Mites live for days Common in children and adults in institutional settings Symptoms: severe itching 27

28 Lice Pediculiosis Caused by Pediculas humans capitis (head lice), P. h. corporis (body lice), P. h. pubis (crab lice, genital area) Bloodsucking insects living days Females lay eggs which are attached to the hair White nits hatch in 8-10 days Insects suck blood through punctures in the skin itching, scratching, secondary infections Spread by person-to-person contact Head lice common among children head lice 28

29 Benzyl Benzoate Natural product from Perubalsam Clear colorless liquid insoluble in water Used topically against scabies Immediate relief from itching Can be skin irritating in pure form Nonirritating, odorless preparation: emulsion of benzyl benzoate in oleic acid stabilized with ethanolamine Synthesis: 29

30 Lindane (Kwell, Scabene ) γ-1,2,3,4,5,6-hexachlorocyclohexane (γ-isomer has the highest insecticidal activity) Synthesized by exaustive chlorination of benzene, benzene hydrogenation, and separation of the γ-isomer) Mechanism of insecticidal action: neurotoxic by interfering with GABA receptors in CNS Readily absorbed through skin (especially the scalp) neurotoxic side effects (convultions, dizziness ) Highly lipophilic drug accumulating in the body Widely used in agriculture since 1940s Stockholm Convention of Persistant rganic Pollutants banned its use (USA did not ratified, but banned its agricultural use) Second-line agent against scabies and lice Available as lotion or shampoo Application should avoid longer contact with the skin Cl Cl Cl Cl 30 Cl Cl

31 alfa beta

32 Crotamiton (Eurax ) trans N-ethyl amide of o-toluidine and crotonic acid Indicated for the topical treatment of scabies Available as a lotion or cream in 10% concentration No systemic neurotixicity Most common side-effect: skin irritation Less effective than lindane or permethrin succesful treatment may require repeated application 32

33 Pyrethrines and Pyrethroids Pyrethrum: old insecticide made from dried flower heads of Chrysanthemum lants used since 1800 (Persian insect powder) Active ingredients: Pyrethrines: esters, e,g, pyrethrin I : is an ester of hrysanthemic acid and the alcohol pyrethrolone chrysanthemic pyrethrolone H H H 3 C H H 3 C Natural H Pyrethrin I Pyrethrins are expensive and undergo easy hydrolysis and oxidation 33 this is why pyrethroides are prpared (synthetic analogs of pyrethrines)

34 Pyrethrins (A 200, RID ) Mode of action :They act on the nerve cell membranes disturbing the sodium channel conductunce High costs, rapid metabolic degradation inside the insect Rapid metabolism not toxic for humans xidation Epoxidation HC H 3 C H H 3 C H H CH 2 H 1,2-diol metabolite Natural H 3 C H H CH 2 H 1,4-diol metabolite 34

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36 Permethrin (Nix, Elimite ) Synthetic analog (pyrethroid) with increased stability and availability Not used with a synergist (see next slide) Used against head lice (1% lotion as a single application ) and scabies (5% creme) Metabolism xidation xidation Hydrolysis Synthetic 36

37 Piperonyl butoxide piperonyl butoxide : inhibits the P450 enzyme of the insect preventing the oxidative inactivation of the drug by the insect, used in combination with Pyrethrins or metabolically labile pyrethroides (but not with Permethrin as it is considred relatively stable in insects) Licid TM is a combination of piperonyl butoxide and a metabolically labile pyrethroide drevative 37

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39 Thank You 39