Antiprotozoal

Antiprotozoal Drugs

• PM 716 course, Chapter 52, Antiprotozoal Drugs, by Richard M. Rocco, PhD
• PM 719 C12 Antimalarial course

Case Study (Patient)

• 9-year-old boy with intense chills, high fever (4 days), drenched in sweat, exhausted
• Diarrhea, nausea, abdominal pain
• Immigrated to the US from West Africa 3 weeks prior to illness

Case Study (Workup)

• Workup included tests to rule out bacterial and viral infections
• Thin-film blood smear for blood-born parasites confirmed malaria

Case Study (Treatment)

• Treat acute malaria with IV artesunate or IV quinine or quinidine
• If quinine or quinidine is used, monitor for potential cardiac toxicities

Plasmodium falciparum

• One of four infectious protozoal parasites
• Only P. falciparum causes life-threatening illness
• Four major stages in parasite development: ring, trophozoite, schizont, gametocyte

Malaria

• Anopheles mosquito inoculates sporozoites into human host
• Sporozoites infect liver cells (asexual stage) and mature into merozoites
• Merozoites release from liver, invade red blood cells (RBCs), feed on hemoglobin (Hb) and mature into trophozoites
• Trophozoites divide in RBCs into 16-32 new parasites
• RBC ruptures, releasing new parasites
• Anemia results, pain from sickle cell trapping in capillaries, oxygen starvation from anemia

Malaria (Epidemiology)

• 300-500 million cases worldwide per year
• 41% of the world's population lives in areas with endemic malaria
• Malaria kills 3 million humans per year
• P. vivax and P. falciparum account for 95% of cases, with P. falciparum being the more severe infection

Sickle Cell Trait

• Normal hemoglobin (Hb) contains two alpha and two beta globulin chains
• In sickle cell Hb, a single amino acid substitution occurs in one beta chain (trait) or both beta chains (disease)
• Glutamic acid is replaced with valine

Malaria (Sickle Cell)

• Sickle cell red blood cells (RBCs) are infected with the parasite similar to normal RBCs
• Decreased oxygen tension causes sickle shape in sickle cell RBCs
• Sickled cells removed from circulation in ~4 days (compared to 40 days for normal RBCs)
• Parasite dies before maturity

Malaria (Treatment: The Three "R" Problems)

• (1) Recrudescence: residual parasites remain, not destroyed by the drug, lie dormant in RBCs, and reemerge
• (2) Relapse: sporozoites lie dormant in the liver
• (3) Resistance: antimalarial drugs no longer work

Quinine

• Drug derived from the bark of the Cinchona tree
• Discovered by Native South Americans in the Andes of Peru
• Taken back to Europe by 16th-century Western explorers
• Called the "Cardinal's Cure"
• Isolated from Cinchona bark in 1820
• ADRs (Adverse Drug Reactions) include tinnitus, headache, nausea, dizziness, flushing, visual disturbances
• May cause hemolytic anemia in G6PD deficiency
• MOA: Parasite feeds on human hemoglobin to produce toxic heme which parasite sequesters into protective "cells"
• Quinine appears to block the parasite's ability to sequester toxic heme

Quinine & Quinidine

• Both derived from the bark of the Cinchona tree (Andes mountains)
• Quinidine is the dextrorotatory stereoisomer of quinine
• Schizonticide against all four species of human malarial parasites
• Exact mechanism is unknown
• ADRs: cinchonism (tinnitus, headache, etc.)
• Hemolysis (red cells) can occur
• Reduce dosage in renal insufficiency

Quinacrine (Atabrine)

• Yellow pills issued before WWII to South Pacific military personnel, not used today
• Required 3x daily oral dosing
• Causes major GI disturbances and turns skin yellow
• Non-compliance is a major issue due to ADRs
• Guadalcanal battle (1942-1943) up to 75% of hospitalizations due to malaria and not war wounds

B.B. Brodie (1909-1989)

• Goldwater Memorial Hospital, Roosevelt Island, NYC
• Considered one of the founders of modern pharmacology
• Determined quinacrine is effective at reduced dosing
• Lower doses prevent yellow skin and GI disturbances

Chloroquine

• Not effective against liver stage parasites
• Same MOA as quinine; blocks the parasite's ability to pack toxic heme into storage cells (hemzoins)
• Parasite develops a cell wall pump to block drug entry into the cell
• Drug of choice since 1940s, resistance not an issue
• Synthetic drug; high volume
• Drug causes heme buildup in RBCs (toxic to parasite)
• Side effects vary widely; cardiac changes (QRS widening, T-wave changes)

Amodiaquine

• Similar to chloroquine
• Used in combination drugs (amodiaquine + artesunate, amodiaquine + sulfadoxine + pyrimethamine)

Artemisinin (ATN)

• Qinghaosu (pronounced Ching-how-sue), also known as Artemisia annua plant (sweet wormwood)
• Used in China for over 2000 years for fever reduction
• Plant related to family that produces absinthe, tarragon, sagebrush
• Zhou Hou Bei Ji Feng (Handbook of Prescriptions for Emergency Treatments) by Ge Hong (340AD) advocated the use of this herb for fevers and chills.
• Li Shizen's Compendium of Materia Medica (Ben Cao Gang Mu, 1546) advocated its use to treat malaria
• 50% of the world's population at malaria risk
• 250 million have malaria
• 1 million die yearly (primarily children in Africa)
• Mortality rate for serious infection is 20-50%
• All current drugs have shown resistance
• 1972: Artemisinin extracted from the Qinghaosu plant
• 1979: X-ray crystallography determined the structure
• One of the few naturally occurring endoperoxide compounds
• Semisynthetics produced to improve solubility
• Artemether is an example used in clinical trials.
• LD₅₀ in mice 4228 mg/kg
• In vitro studies show effectiveness against the malarial parasite
• In 1979, treatment with ATN for Malaria in 2099 patients exhibited a 100% success rate (Chinese Med J. 92:811, 1979)
• 1979-Vietnam malaria epidemic saw a 97% reduction in deaths due to ATN
• Very rapid symptom reversal (down to 0 within 8 hours).
• Clears within 2 days
• Destroys all chloroquine resistant strains and all plasmodial forms
• High cure rates with 5–7 day dosage regimens

ATN (Drug details)

• Drug derivatives (semisynthetics) are synthesized from ATN extracted from the plant
• Total synthesis is difficult
• Cultivated plants yield 2% dry weight of ATN
• Wild plants yield ~ 0.3%
• Plant takes 8 months to grow
• Chloroquine (synthetic introduced in the 1940s), costing 20 cents/dose, and ATN, costing 90 cents/dose
• WHO has ordered 100 million doses of ATN
• Global Fund will spend $450 million on this drug over the next five years
• Chloroquine-resistant strains of Plasmodium falciparum have developed worldwide
• ATN is the only drug able to treat large numbers of the population living in parasite resistant locations
• ATN has become the world's most effective new drug for malaria in the last 100 years
• Herbal product since discovery of quinine in 1530.
• Drug taken up into parasite-infected RBCs 100x more than non-infected cells
• Kills parasite at nanomolar concentrations
• Needs micromolar concentrations to harm mammalian cells
• Intercalates with parasite proteins, causing free radical damage to the RBC, resulting in parasite death

Riamet

• Combination of ATN derivative (artemether) and lumefantrine (secondary antimalarial)
• Widely used in Europe for travelers
• Required for preventing ATN resistance
• It took almost 400 years to discover a new antimalarial. Every drug since quinine has become resistant .

Amebiasis Treatment

• Infection with Entamoeba histolytica causes intestinal infections (colitis, dysentery), liver abscesses
• Metronidazole and tinidazole are drugs of choice to treat amebiasis, giardiasis, and trichomonas infection

Metronidazole & Tinidazole

• Nitro groups reduced by protozoans, forming toxic compounds for the organism
• ADRs include metallic taste, dry mouth
• Metronidazole has a disulfiram-like effect; avoid alcohol ingestion

Mefloquine

• Often used to treat chloroquine-resistant strains of P. falciparum
• Chemically related to quinine
• Mechanism of action is unknown
• Effective when used for prophylaxis
• FDA black box warning about potential neurological and psychiatric toxicities

Primaquine

• Drug of choice for eradicating dormant liver forms of P. vivax and P. ovale
• Active in the hepatic stage of all parasite forms
• Can be used for chemo prophylaxis
• Poor activity against red blood cell stages
• ADRs are common

Melarone

• Combination drug for treating malaria
• Atovaquone (250 mg) + proguanil (100 mg)
• PO daily dosing can be used for prophylaxis

Folate Inhibitors

• Mostly used in combination regimens
• Pyrimethamine (related to trimethoprim)
• Proguanil (biguanide derivative)
• Fansidar (sulfadoxine + pyrimethamine)
• Selective inhibitors of plasmodial dihydrofolate reductase (a key enzyme in folate synthesis)

Antibiotics

• Lumefantrine (available only in fixed combinations with artemether in Coartem and Riamet)
• See notes on artemisinin derivatives