Parasitic Disease - A Perspective
It is estimated that well over one billion people are the victims of parasitic disease worldwide, and for some diseases in certain areas, more than 80% of the population are infected. One of the greatest problems in discovering new drugs to treat these infections arises from geography and economic considerations. Significant advancements in antiinfective therapy have improved the quality of life in highly developed nations. However, in underdeveloped countries, there exist major infectious diseases that account for a large portion of global morbidity. The vast majority of these diseases exist in tropical areas, but some of these diseases have the potential to become a threat to those living in North America. Tuberculosis claims an estimated 2 million lives each year, and drug-resistant strains originally found in New York and Russia are now being identified in other locations. Malaria is estimated to kill 1million people worldwide each year, and most of these victims are pregnant women and children under 5 years of age in Africa. A child dies of malaria every 30 seconds. African trypanosomiasis and leishmaniasis accounted for an additional 109,000 deaths in 2002, and estimates suggest that these numbers are rising rapidly. Current therapies for parasitic infection are inadequate, especially in light of the emergence of drug-resistant parasitic strains. Many of the drugs currently used are toxic or non-efficacious, and there are no effective treatments for some parasitic diseases. Drug discovery efforts against the diseases mentioned above are limited, either because infected persons in underdeveloped areas cannot afford even a single course of therapy, or because the infected population is too small to justify the required research expenditures. Efforts to fight parasitic diseases in Third World nations are often hampered by economic issues and political turmoil. These facts virtually assure that the world's most impoverished people will continue to bear the major burden of parasitic disease. Clearly, there is a need for new antiinfective agents that are potent, non-toxic and inexpensive to manufacture.
Diseases Caused by Protozoan Parasites
Amebiasis. Amebiasis is a parasitic infection caused by the protozoon Entamoeba histolytica. It is the third leading parasitic cause of death worldwide, surpassed only by malaria and schistosomiasis. On a global basis, amebiasis affects approximately 50 million persons each year, resulting in nearly 100,000 deaths.The parasite exists in 2 forms: a motile form, called the trophozoite, and a cyst form, responsible for the person-to-person transmission of infection. The trophozoite of E. histolytica infects the large intestine to produce lesions of amebic colitis. Invasion of the colonic mucosa leads to dissemination of the organism to extracolonic sites, predominantly the liver. Faced with an adverse colonic environment, the trophozoite changes to the cystic form, better adapted to survival. Symptoms range from intermittant diarrhea to enlargement of the liver to acute amoebic dysentery.
Amoebiasis is treated with a series of drugs related to the nitroimidazole analogue metranidazole. Metranidazole is basically a pro drug form which must be activated by anaerobic organisms to the corresponding hydroxylamine to have an antiparasitic effect. During this metabolic activation, reactive oxygen species (superoxide, hydroxyl radical) are produced, and the organism dies through oxidative stress. The drug tinidazole, which is also effective, also must be
activated by this pathway. The exact mechanism of action of metronidazole and tinidazole are unknown. Diloxanide furoate is approved for treatment of the asymptomatic form of amebiasis, but is not effective in the extraintestinal form of the disease. In the GI tract, diloxanide is cleaved to form dioxanide, which is the active form.
Giardiasis. Giardiasis is a diarrheal illness caused by the one-celled parasite Giardia lamblia. Once an animal or person has been infected, the parasite lives in the intestine and is passed in the stool. Because the parasite is protected by an outer shell, it can survive outside the body and in the environment for long periods of time. Giardia is now recognized as one of the most common causes of diarrhea in th US. Giardiasis tends to occur more often in people in institutional settings, people in daycare centers, foreign travelers and individuals who consume improperly treated surface water. The parasite is passed in the feces of an infected person or animal and may contaminate water or food. Person-to-person transmission may also occur in daycare centers or other settings where handwashing practices are poor.Infected patients experience mild or severe diarrhea, or in some instances no symptoms at all. Fever is rarely present. Occasionally, some will have chronic diarrhea over several weeks or months, with significant weight loss.Giardiasis is treated using the same drugs used for amebiasis, as described above.
The drug nitazoxinide is also an effective treatment for giardiasis, and is also a pro drug that must be converted to tizoxinide (TIZ) to be active. A portion of TIZ is converted to the hydroxylamine or the glucuronide conjugate, both of which maintain activity against the organism. TIZ and its metabolites are though to kill the parasite by inhibiting the parasitic enzyme pyruvate:ferredoxin oxidoreductase.
Trichomoniasis. Trichomoniasis is caused by the trophozoite parasite Trichomonas vaginalis, and is usually confined to the vagina, urethra or prostate. Because of the site of infection, trichomoniasis is considered a sexually transmitted disease. Infections in the male are usually asymptomatic, but in women symptoms include vaginitis and a foul-smelling discharge. Tricomoniasis is generally treated with metronidazole or tinidazole.
From left to right: Pneumocystic carinii, the microsporidia Encephalitozoon cuniculi and Cryptosporidium parvum
Pneumocystis, Microsporidia and Cryptosporidia. These three organisms do not commonly cause infection in normal individuals, but are common opportunistic infections in patients who are immunocompromised. About 80% of AIDS patients contract Pneumocystis carinii pneumonia (PCP), and it is a major cause of death in this population, but the extrapulmonary form is rare. Pneumocystis has many characteristics of a protozoan parasite, but has rRNA and mitochondrial DNA sequences suggestive of a fungus. It is treated with a combination of sulfamethoxazole and trimethoprim. Sulfamethoxazole blocks the incorporation of p-aminobenzoic acid (PABA) into folic acid, and trimethoprim inhibits the conversion of dihydrofolic acid to tetrahydrofolic acid by dihydrofolate reductase (DHFR). Pentamidine is also effective orally or by inhalation, but has serious side effects when given by intramuscular injection. Difluoromethylornithine (DFMO), which is a mechanism-based inhibitor of the enzyme ornithine decarboxylase, has also been used. Atovaquone, which is described below, was synthesized as an antimalarial, but is also marginally useful for pneumocystis infection.
The microsporidia are obligate intracellular parasites that have been recognized in a variety of animals, including humans. The infective form of microsporidia is the resistant spore and it can survive for a long time in the environment. Typical sizes of the spores range from 1.5 to 2.0 micrometers in humans. Human microsporidiosis represents an important and rapidly emerging opportunistic disease, occurring mainly, but not exclusively, in severely immunocompromised patients with AIDS. Additionally, cases of microsporidiosis in immunocompromised persons not infected with HIV as well as in immunocompetent persons also have been reported. The clinical manifestations of microsporidiosis are very diverse, varying according to the causal species with diarrhea being the most common.The treatment of choice for microsporidiosis is oral albendazole, although this indication is considered investigational. In essence, there is no effective treatment for microsporidiosis.
Cryptosporidiosis is a diarrheal disease caused by microscopic parasites of the genus Cryptosporidium. Once an animal or person is infected, the parasite lives in the intestine and passes in the stool. The parasite is protected by an outer shell that allows it to survive outside the body for long periods of time and makes it very resistant to chlorine-based disinfectants. During the past two decades, cryptosporidiosis has become recognized as one of the most common causes of waterborne disease (recreational water and drinking water) in humans in the United States. The parasite is found in every region of the United States and throughout the world. The most common symptom of cryptosporidiosis is watery diarrhea. Other symptoms include stomach cramps or pain, dehydration, nausea, vomiting, fever and weight loss. Some patients are asymptomatic.The only treatment approved for the disease is nitazoxanide.
Diseases caused by trypanosomatids. The parasitic diseases human African trypanosomiasis (HAT), Chagas disease and leishmaniasis collectively are a daily threat to more than 550 million people in Mexico, Central America, South America, sub-Saharan Africa, the Middle East, Indonesia and India. Collectively, these three diseases cause nearly 150,000 deaths annually. These diseases are caused by unicellular trypanosomatid parasites of the genera Trypanosoma brucei, Trypanosoma cruzi and Leishmania sp. Unfortunately, drug therapy for these diseases has not changed significantly in the past 50 years. Currently used agents are far less than satisfactory due to extreme toxicity, or the emergence of resistant parasitic strains.
HAT is caused by two members of the kinetoplastid family Trypanosoma brucei brucei (see above) including T. brucei gambiense and Trypanosoma brucei rhodesiense. T. b. rhodesiense produces an acute form of the disease known as East African trypanosomiasis, while T. b. gambiense produces a chronic disease called West African trypanosomiasis. T. b. brucei does not infect humans, but is a threat to livestock, and it has been suggested that these animals can act as a reservoir for T. b. gambiense and T. b. rhodesiense. Sleeping sickness is a daily threat to over 60 million people in 36 countries of sub-Saharan Africa. Of that number, only 3 to 4 million people are regularly monitored by health centers that can provide screening. Because medical resources are lacking in rural areas of the Third World, most people with sleeping sickness die prior to diagnosis. Nearly 45,000 new cases are reported annually, although the actual number may be as much as 10 times higher. T. b. gambiense and T. b. rhodesiense are transmitted by the bite of the tsetse flies Glossina fuscipes, Glossina palpalis or Glossina morsitans, respectively.These organisms have an extremely complicated life cycle that is beyond the scope of these lectures. In stage I trypanosomiasis, symptoms include high fever that may last for several days. After a latent oeriod, the parasite enters the CNS to produce stage II trypanosomiasis. In this stage, the symptoms of the disease (somnolence, loss of spontaneity, loss of speech, extrapyramidal signs and coma) give the disease its name.
Polyamine metabolism in African trypanosomes is similar to mammalian polyamine metabolism, in that the organism synthesizes putrescine and spermidine from ornithine, and trypanosomal forms of ornithine decarboxylase and spermidine synthase have been identified. However, these organisms do not produce spermine, but instead convert two molecules of host-derived glutathione (1) and spermidine (2) into reduced trypanothione (4), which is used to protect the organism against oxidative stress. The formation of reduced trypanothione (4, Figure 3) is mediated by two ATP-dependent enzymes, glutathionylspermidine synthetase, which produces glutathionylspermidine (3), and trypanothione synthetase (TS), which produces 4.16 In the presence of oxidative stress, oxidized trypanothione (5) is formed, and must be recycled to the reduced form 4 by a third enzyme unique to the parasite, trypanothione reductase (TR).
At the present time, there are only 4 drugs that are in common use for the treatment of early and late stage HAT, as shown above. Unfortunately, diagnosis of early stage trypanosomiasis is difficult, especially in rural areas, and as such many patients progress to late-stage disease before seeking treatment. Early stage disease is usually treated effectively with suramin (6) or pentamidine (7). Suramin enters the trypanosome by endocytosis, and is more than 75% bound to serum proteins following administration, a fact which appears to be related to its mechanism of action. The drug may hamper the required parasitic uptake of LDL by receptor-mediated endocytosis, it may associate with cytosolic enzymes inside the parasite, or it may associate with the highly positively charged glycolytic enzymes in the glycosome (a trypanosomal organelle similar to the mitochondria). Pentamidine is thought to act by inhibiting the P2 adenosine uptake system in the parasite. Trypanosomal strains that are resistant to suramin and pentamidine have been detected. There are only two effective treatments for late-stage trypanosomiasis, and these agents must penetrate the CNS in order to be effective. End stage trypanosomiasis is treated with melarsoprol (Mel B, 8), which was discovered in 1949. Following administration, melarsoprol is converted in vivo to its active metabolite, melarsen oxide (Mel Ox, 9), which has a plasma half-life of about 30 minutes. This arsenical drug produces its antitrypanosomal effect by forming a covalent complex with trypanothione known as Mel T, thus inactivating it and exposing the organism to oxidative damage. The treatment regimen for human African trypanosomiasis with melarsoprol is outdated and complicated, and patients require hospitalization and monitoring during therapy. Patients are usually given three series of four intravenous injections with interval of 10 days between each series. This schedule may not be the most effective and could possibly account, in part, for frequently reported side effects, including a 10% incidence of reactive encephalopathy which is fatal in 3-5% of patients receiving the drug. Melarsoprol is only soluble in propylene glycol, and is marketed as a 3.6% solution. As soon as a vial is opened, the drug must be used immediately, as it begins to deteriorate. In addition, administration is painful, and thrombophlebitis at the injection site is common. Many patients who survive melarsoprol treatment suffer from serious neurological sequellae, and additional adverse effects arise following the covalent binding of melarsoprol to native biomolecules that subsequently become antigenic. The situation is further complicated by the emergence of arsenic-resistant strains of T. b. gambiense and T. b. rhodesiense, a group which is now comprised of 30% of all trypanosomes. It is likely that this resistance is mediated by mutations in the P2 transporter, which is necessary for the import of melarsoprol into the organism.The ornithine decarboxylase inhibitor DFMO (10) has been shown to be curative in end stage infections caused by T. b. gambiense, but it is ineffective against late stage T. b. rhodesiense infection. DFMO is the only new molecule approved for the treatment of HAT over the last 50 years, and is considered a second-line agent for the treatment of arsenic (and hence melarsoprol) resistant T. b. gambiense.
American trypanosomiasis, known as Chagas' disease, was first characterized by Carlos Chagas in Brazil in 1909, and continues to be among the most important diseases in tropical and subtropical Mexico, Central America and South America. Chagas Disease is caused by the trypanosomatid species Trypanosoma cruzi (above), which is similar but distinct from the species that cause HAT. According to the World Health Organization, 25% of the total population in Central and South America is at risk. Currently, there are between 16-18 million people infected, with 6 million cases advancing to clinically significant disease and more than 45,000 deaths annually. The disease is transmitted through the bite of several species of triatomine bugs (also referred to as reduviid bugs, assassin bugs or “kissing bugs”), including Triatoma infestans, Triatoma dimidiata and Rhodnius prolixa, all of which live in dry, forested areas. These insects hide during the day in dark crevices or behind objects that are abundant in the type of housing used in endemic areas, as well as in animal nests and thatched roofs. At night, the insects emerge and feed on the blood of a variety of mammals, including humans. During a blood meal, the disease is not transmitted by the bite of the insect, but rather through its feces, which contains the organism. The parasite enters the host when the wound is scratched, or through the conjunctiva of the eye or the mucosa of the nose or mouth, and invades a variety of cell types including macrophages, smooth and striated muscle and fibroblasts. When the site of infection is near the eye, acute swelling of one eyelid is observed, a phenomenon known as Romaña’s sign.
After a 1-2 week incubation period, the disease progresses through three phases, termed acute, indeterminate and chronic. In the acute phase patients have high levels of the parasite in blood and tissues, and symptoms are generally mild (high fever and edema). Immunosuppressed patients and children can develop a more severe form of infection, with cardiac involvement and encephalomyelitis. Following the acute phase, the parasite burden in blood and tissues decreases dramatically, although low levels of the parasite are still detectable in certain tissues. Patients then experience an asymptomatic lag period of anywhere from 10 to 30 years that is known as the indeterminate form of Chagas disease. A percentage of these patients advance to chronic Chagas disease, and develop moderate to severe clinical symptoms including cardiomyopathy, heart failure and digestive tract abnormalities such as megacolon and megaoesophagus. If severe enough, these manifestations of the disease are the main causes of death.
T. cruzi is much more difficult to treat than HAT, since this trypanosomatid parasite is intracellular, and drugs used for the disease must pass through mammalian and parasite cell membranes to be effective. Laboratory and clinical studies conducted since 1969 have demonstrated that nifurtimox (11) and benznidazole (12) are the best agents for treating human T. cruzi infection, although they are far from being ideal drugs. Nifurtimox and benznidazole are indicated in the acute phase of the infection, the congenital form, reactivation of disease associated with immunosuppression, and in transfusions and organ transplants involving infected individuals. Both drugs are taken orally and must be given divided into 2-3 fractions after meals. They are generally well tolerated by children, particularly in the acute phase of the disease, but relatively frequent and severe gastrointestinal or dermatological adverse reactions may be observed. Recurrance of the disease is a significant problem, and as such these drugs are considered generally ineffective. The main limitations of both drugs are their long courses of administration and the occurrence of adverse side effects. The related compound megazol (13) has also been used for Chagas disease, but its use was discontinued because of severe mutagenic and cytotoxic effects.
Leishmaniasis is caused by one of 20 strains of the trypanosomatid parasite Leishmania, and currently threatens 350 million men, women and children in 88 countries around the world. Health statistics are only maintained in 32 of the 88 countries affected by leishmaniasis, and as such, a substantial number of cases are never reported. It is estimated by WHO that 2 million new cases (1.5 million cutaneous and 500,000 visceral, see below) occur annually, with an estimated 12 million people infected worldwide. It is transmitted by a female sandfly from one of 30 species of the genus Phlebotomus. The disease can be divided into three categories (cutaneous, mucocutaneous and visceral leishmaniasis), and is further categorized as Old World or New World leishmaniasis, depending on the geographic location of the various parasite species causing the disease. Cutaneous and mucocutaneous leishmaniasis are diseases that are generally not life threatening, but the resulting disfugurement can cause social stigma that has a significant impact on lifestyle. Visceral leishmaniasis, also called kala-azar, produces life threatening systemic infection if left untreated. Although leishmaniasis is endemic to rural South America and Africa, it is encroaching on urban areas with substandard hygienic practices, and is also a risk for travelers in affected areas (e.g. Gulf War veterans). In addition, leishmania as an opportunistic infection in HIV patients has become a significant health threat in some areas.
Cutaneous leishmaniasis manifests itself in several forms based on the species of parasite involved in the infection. Old World cutaneous leishmaniasis is primarily caused by the species L. major, L. aethiopica and L. tropica, and is transmitted by phlebotomine sandflies P. papatasi and P. sergentii. The vast majority of cases of Old World cutaneous leishmaniasis occur in the Sudan, Afghanistan, Iran, Iraq, Saudi Arabia and Syria, and normally produce skin ulcers on the exposed parts of the body such as the face, arms and legs. This is the cause of the "Baghdad Boil" that some US trrops contract while serving in the Middle East. The disease can produce a as many as 200 lesions, causing serious disability and permanent scarring. In most cases, the disease is self-curing, although the healing process can take a year or more. New World cutaneous leishmaniasis extends from Texas to central South America, and has been found in the Carribean, but occurs primarily in Brazil and Peru. The disease is transmitted by female sandflies of the genus Lutzomyia, and infections are caused by a variety of strains, including L. mexicana, L. garnhami, L. lainsoni, L. venezuelensis, L. peruviana, L. colombiensis, L. guyanensis, L. amazonensis, L. panamensis and L. pifanoi. Unlike Old World disease, New World cutaneous leishmaniasis is passed between various mammals (dogs, rodents, opposum, anteaters), as well as among human hosts. The symptoms of New World cutaneous leishmaniasis are similar to those of Old World disease, but there a wide variation in the degree of tissue destruction occurs, depending on the strain of the infection.Treatment with ketoconazole or benznidazole is generally given, especially if the infection involves the soft cartilage of the ear.
Left to right: Visceral, mucocutaneous and cutaneous leishmaniasis
The trypanosomatid parasite L. braziliensis is the main causative organism for mucocutaneous leishmaniasis, and more than 90% of this form of leishmaniasis occurs in Bolivia, Brazil and Peru. The initial lesions are similar to those seen in both forms of cutaneous leishmaniasis, but develop into more severe lesions of the mucosa in about 80% of untreated cases. These secondary lesions can lead to partial or total destruction of the mucous membranes of the nose, mouth and throat, and in severe cases the resulting disfigurement can result in victims being humiliated and cast out from society.
Visceral leishmaniasis, also known as kala-azar in India or dumdum fever in Africa, is caused by organisms of the L. donovani complex (L. donovani, L. infantum and L. chagasi). More than 90% of visceral leishmaniasis cases occur in Bangladesh, Brazil, India, Nepal and Sudan. L. donovani is the primary cause of visceral leishmaniasis in India and East Africa. The disease is also caused by L. infantum in Mediterranean countries and L. chagasi in the New World. Human beings are the only known reservoir of L. donovani, but canines, especially domestic and stray dogs, provide a reservoir for L. infantum and L. chagasi. These differences have a major impact on the control of the disease and the emergence of drug resistance. Typically, patients with visceral leishmaniasis present with irregular bouts of fever, cough, abdominal pain, diarrhea, epistaxis, splenomegaly, hepatomegaly, substantial weight loss and moderate to severe anemia. If left untreated, the fatality rate in developing countries can be as high as 100% within 2 years. As the organism develops in the skin cells, spleen, liver or bone marrow of the host, patients experience anemia and cachexia, and ultimately succumb to the parasite. There were more than 41,000 recorded deaths due to visceral leishmaniasis in 2000.
Because leishmania are biochemically similar to trypanosomes, many of the antitrypanosomals mentioned above are also effective against L. donovani. This is not universally true, however, due to the differences in tissue distribution between the parasites. In general, the intracellular amastigote form is considered the target for chemotherapy. The standard treatment for visceral leishmaniasis is sodium stibogluconate 14 or meglumine antimoniate 15, moderately toxic antimonials for which resistance is increasing. These essentially equivalent compounds are actually complex mixtures of carbohydrate/antimony complexes which are dosed based on their antimony(V) content. The mechanism of the antileishmanial activity of antimony remains to be elucidated, but it is known that 14 and 15 suppress glycolysis and fatty acid metabolism in the parasitic glycosome. They may also interact with the leishmanial form of trypanothione. In addition to high cost and a difficult administration schedule, there are numerous resistant strains, and antimonial toxicity is frequent when HIV coinfection is present. In addition, there are quality control problems and batch-to-batch variability for both branded and generic antimonial preparations. The poor quality of some generic formulations of the drug in India has led to serious toxicity. Pentamidine is also marginally effective in the visceral form of leishmaniasis when given at 2-4 mg/kg by intramuscular injection, either daily or every other day, for a course of 12-15 injections. Due to toxicity and resistance, especially in India, pentamidine is being abandoned as a second line treatment for leishmaniasis. The macrocyclic antibiotic amphotericin B is used as a second line treatment, and is administered at 7-20 mg/kg total dose intravenously for up to 20 days. Cure rates have been reported as high as 97%, and there have been no reports of resistance, but the drug is non-ideal due to dose-limiting toxicity. Lipid-associated formulations of amphotericin B are highly effective against visceral leishmaniasis and better tolerated than the conventional preparation. However, such products are prohibitively expensive, and as such their utility in the Third World is severely restricted.
Malaria. Malaria is still one of the world's most deadly diseases that threatens 40% of the world’s population, and infects approximately 300 million people worldwide. In Africa alone, more than one million children under the age of 5 die from malaria each year, translating to one death from malaria every 30 seconds. The World Health Organization has estimated that funding for malaria control alone, including only existing methods for vector control, will need to increase to $2.5 billion annually by 2007, and $3.1 billion annually by 2015. Malaria-carrying mosquitoes have recently been found in the United States, causing infection in individuals who had not traveled to areas where the disease is endemic. In 2002, two pools of malarial mosquitoes were discovered near the Potomac River, one 4 miles and the other 6 miles from the homes of the two teenagers who were diagnosed with malaria. Authorities say it is the first case in at least two decades in which malaria has been detected in mosquitoes and humans in an American community. Outbreaks of malaria in the US could become more common due to global warming.
Human infection can be caused by four distinct species of the protozoon genus Plasmodium, but P. vivax and P. falciparum account for more than 95% of malaria cases. Nearly all deaths caused by malaria are due to infection by P. falciparum. Malaria is transmitted by the bite of the female anopheles mosquito, at which time sporozoites of P. falciparum are discharged into the puncture wound. Sporozoites are then carried to the liver, where they enter hepatic mesenchymal cells and begin to grow. Lysis of the hepatocyte then releases the merozoite form of P. falciparum, which invades host red blood cells (RBC's), feeding on hemoglobin during the erythrocytic portion of its life cycle. In the RBC, the parasite expresses a number of polypeptide products that are exported to the surface of the RBC, rendering it antigenic. In order to escape the host immune system, the parasite regularly exchanges these peptides in a process called antigenic variation. Malaria has become more difficult to treat, due to an increase in multi-drug resistant strains. Indeed, the reemergence of malaria as a worldwide epidemic can be largely attributed to the rapid development of parasite resistance. Current major areas of antimalarial research include prevention and vector control, identification of new targets, development of an effective vaccine and design and synthesis of new antimalarial agents. When travelling to malaria endemic areas, region-dependent prophylactic treatment consisting of chloroquine, mefloquine (Larium)or a combination of atovaquone and proguanil (see below) are required. It should be noted that Larium prophylaxis has been shown to cause mental disturbances in some patients.
Synthetic analogues related to quinine and chloroquine - The cinchona alkaloid quinine was the first compound to exhibit significant antimalarial activity. Subsequent studies produced the synthetic analogue chloroquine, 3, which was initially an excellent treatment for malaria. However, the evolution of chloroquine-resistant strains of P. falciparum have rendered this drug virtually useless in certain areas of the world. To address this resistance, a number of Mannich bases of chloroquine were synthesized, producing the active quinoline analogues amodiaquine, 4 and tebuquine, 5. These analogues are significantly more active against Plasmodia, but unfortunately, chronic toxicity limits their use. Other quinolines of interest include tafenoquine 8, which is a second generation agent related to primaquine, 9 and mefloquine, 10. Tafenoquine, also known as WR 238605, appears to be an effective prophylactic agent for the prevention of P. falciparum malaria. The mechanism of all 4-quinoline antimalarials appears to be the same, although several theories have been put forth to explain the activity of these analogues. During the degradation of hemoglobin, plasmodium must eliminate free heme, which is a toxic by-product. The parasite forms a polymer of heme known as hemazoin to alleviate the toxicity of heme. It is thought that the 4-quinolines cap the growing hemazoin polymer, and the parasite is killed by heme toxicity.
Artemisinin. Perhaps the most promising advance in the treatment of malaria is the discovery of artemisinin, which is also known by the Chinese name qinghaosu, and the related compound arteether. These analogues are 1,2,4-trioxosesquiterpenes that produce oxidative stress in P. falciparum, and they are reduced by the organism in an Fe(II)-dependent process to produce cytotoxic radical intermediates. Artemisinin itself is a potent antimalarial, with an IC50 of 7.3 nM against P. falciparum. Artemisinin and its derivatives have limited oral bioavailability and are hydrolytically unstable, problems which have been addressed by the synthesis of analogues. Considerable research has been done aimed at elucidating the potential mechanism of artemisinin and similar compounds, and there are multiple theories as to the fate of the initial adduct produced by iron-mediated opening of the characteristic endoperoxide moiety. The promising antimalarial activity of artemisinin and its derivatives prompted the evaluation of dispiro-1,2,4,5-tetraoxanes such as WR 148999. WR 148999 possesses antimalarial activity comparable to artemisinin, but also shares the characteristics of being hydrolytically unstable and poorly bioavailable by oral administration.
Miscellaneous antimalarials. A number of synthetic agents and natural product derivatives have been shown to possess antimalarial activity. The herbal natural product (+)-febrifugine (Chinese name: chang shan) and its derivatives possess significant antimalarial activity. Promising cure rates in humans have been attained by using atovaquone and proguanil in combination. The recently discovered phenanthrene halofantrine has also shown significant antimalarial activity in vitro. Screening of known (bis)phosphonates has shown that these compounds can exhibit good antimalarial activity, and also are effective against other parasitic organisms such as trypanosomes and leishmania. Finally, a novel series of phenylalanine-statin-based peptidomimetic analogues have been described that are potent, specific inhibitors of the aspartyl protease plasmepsin II, the initial step in the parasite-mediated degradation of hemoglobin. Unfortunately, these compounds do not traverse cell membranes effectively, and thus have relatively low IC50 values against P. falciparum. Second generation inhibitors have now been developed that are still effective plasmepsin II inhibitors, but show greater efficacy in an infected erythrocyte in vitro assay. Additional studies are required to optimize the structure of plasmepsin II inhibitors.
Helminth Infections
Parasitic worms remain one of the largest public health concerns around the world. It is estimated that more than one billion human infections by helminths currently exist (up to one fourth of the world's population), and travel to endemic areas complicates the problem. Many of these infections are chronic, even life-long, and can cause serious sequellae.
Nematodes. These worms are found throughout the world, and are the only kind of helminthic infection common in the US.
Ancylostomiasis (hookworm) - intestinal infestation caused by the American hookworm (Necator americanus) or the old World hookworm (Ancylostoma duodenale). Can live 9-15 years in the host digestive tract. Common in tropical and subtropical areas where there is well drained, sandy soil and abundant rainfall.
Enterobiasis (pinworm) - common household infestation (Enterobius vermicularis) in temperate zone areas. Can be transmitted by inhalation, then migrate to the cecum. Females exit the rectum at night and lay eggs, producing perianal pruritis.
Ascaris lumbricoides (roundworm) - Common in developing countries with unsanitary conditions and poor hygeine. Infection may be asymptomatic, or may cause abdominal discomfort in severe cases. Roundworm eggs reside in the soil (viable for up to 6 years), and when ingested, they penetrate the intestinal walls and are carried through the bloodstream to the lungs. The pulmonary phase lasts up to 10 days, after which larvae pass through the bronchioles, bronchi and trachea, where they are swallowed to return to the small intestine.
Trichuriasis (whipworm) - Eggs, which can live in the soil for many years, are swallowed from contaminated food and beverages. Eggs hatch in the small intestine, and larvae embed in the intestinal wall. Juvenile worms migrate to the large intestine, where they mature. Symptoms include irrittion and inflammation of the colonic mucosa, abdominal pain, diarrhea and distension.
Trichinosis (Trichinella spiralis) - ingested from infected muscle (meat, especially pork), where larvae live in an encysted form. Larvae penetrate the gut mucosa and are distributed to skeletal muscle. Symptoms include the usual GI problems, plus muscle pain and weakness and edema in the systemic phase.
Filariasis (Wuchereria bancrofti, Brugia timori, Loaloa (eyeworm) and Onchocerca volvulus (River blindness) - infection with any of these members of the genus Filaroidea. Elephantitis and eye infection are common features.Spread by mosquitoes or the River blackfly, and then the parasite enters through the lymphatic system.
Cestodes and Trematodes.
Tapeworm (Cestode)- Many types (Taenia saginata, Taenia solium, Hymenolepis nana, Diphyllobothrium latum), grow in segments called proglottids, each of which can be shed, and contains a complete reproductive system. Worms can reach a length of 5-10 meters! Most come from ingesting uncooked meat or fish.
Schistosomiasis (Trematode Blood flukes) - these are free-swimming larvae that penetrate skin of the host from a water source, mature in lung or skin tissue, and then migrate throughout the body. Eggs are then released in the feces, and they inhabit snails as intermediate hosts. There are approximately 200 million cases of schistosomiasis worldwide. Clinical features of the disease include headache, fatigue, fever and GI disturbances. Hepatic fibrosis and ascites follow in the later stages of the disease.
The bendazoles are a broad spectrum class of antihelminthic agent discovered in the 60's. They uncouple oxidative phosphorylation by inhibiting parasitic fumarate reductase (aha, Krebs Cycle!!), and they also bind to tubulin and prevent its polymerization, thus interrupting cell division.
Ivermectin, extracted from the soil actinomycete Streptomyces avermitilis, occurs as an 80:20 mixture of dihydroivermectin B1α and B1 ß. The compound reduces motility , allowing host immune cells to clear the parasite. It also may act as a GABA agonist or a chloride ion channel inducer. Praziquantel (PZQ) is an isoquinoline derivative that redistributes calcium in the parasite, and inhibits phosphoinositide metabolism. Pyrantel pamoate is the drug of choice for pinworm, and is also effective against some roundworms. It acts as a depolarizing neuromuscular blocker that activates nicotinic receptors and inhibits cholinesterase.
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