Pharmacology and mechanism of action
Pyrimethamine is a diaminopyrimidine which is structurally related to trimethoprim. It is effective against erythrocytic stage of Plasmodium (P) falciparum and less so against P. vivax, P. ovale and P. malariae. Pyrimethamine also inhibits the sporogony in the mosquito, resulting in a decrease of transmission of the infection within the community
[1].
The mechanism of action of pyrimethamine is related to its inhibition of dihydrofolic reductase necessary for the folic acid synthesis in the parasite. Pyrimethamine acts slowly and is not recommended as monotherapy for acute malaria attacks. Resistance to pyrimethamine developed soon when the drug was used on a large scale as monoprophylaxis
[1]. In resistant strains, the enzyme dihydrofolic reductase binds to pyrimethamine several hundred times less than in sensitive strains
[2]. This high grade resistance is probably a onestep mutation and cannot be overcome by increasing the dose. However, when combined with long-acting sulphonomides (sulphadoxine), the effect of pyrimethamine is potentiated and the risk of developing resistant strains is far less.
Preparations
Pyrimethamine combined with sulphadoxine.
• Fansidar® (Roche). Tablets (pyrimethamine 25 mg plus sulphadoxine 500 mg). Solution for intramuscular injection (pyrimethamine 10 mg/ml and sulphadoxine 200 mg/ml).
Antiprotozoal veterinary drug
Pyrimethamine is a widely used broad-spectrum antiprotozoal veterinary medicine. In addition to this, pyrimethamine can also be applied to the breeding of the aquatic product and is also capable of enhancing the disease resistance capability of aquaculture fish. However, pyrimethamine has significant side effects with being used in excess amount being able to cause damage to the reproductive system of livestock and poultry, and is also difficult for recovery. Pyrimethamine has inhibitory effect on the primary exo-erythrocytic stage of Plasmodium falciparum and vivax malaria, and is a good preventive medicine. Although it has no significant effects on the malaria gametocyte but when the drug-containing blood enters into the mosquito body, it can affect the development of gametocytes inside mosquitoes, thus being able to interrupt the transmission. The mechanism of action is to inhibit the dihydrofolate reductase and affect the utilization of folate, causing reduction of the nucleic acid synthesis and inhibiting the malaria parasite reproduction. Meanwhile pyrimethamine and chloroquine, through lowering the level of oxidative stress and apoptosis, can exert protective effect on the placental pathology after the infection of malaria and can also reduce the proportion of infected red cells in the blood, therefore being able to achieving a excellent therapeutic effect on malaria. It also has enrichment effect in aquatic body. Upon going beyond a certain range, it can cause hemolytic anemia after being eaten and even has direct toxicity on the central nervous system.
This product is mainly used for the prevention of malaria and can also be used for the treatment of toxoplasmosis. It has inhibitory effects on the primary exo-erythrocytic stage of some kinds of Plasmodium falciparum and vivax malaria and is a good preventive medicine. Owing to its slow excretion rate, it has a long-lasting effect with the preventive effect of one-time medication being able to be maintained for more than 1 week. Although this product has no significant effects on the malaria gametocyte but when the drug-containing blood is inhaled into the mosquito body, it can affect the development of gametocytes inside mosquitoes, thus being able to interrupt the transmission. Because of its effect on inhibiting the exo-erythrocytic stage of Plasmodium, it is usually used in combination with primaquine for the prevention of recurrence.
This product appears as white crystalline powder; it is odorless and tasteless. It is insoluble in water, slightly soluble in ethanol, chloroform and acetone and soluble in dilute acid. It has a melting point of 232~235 ℃.
Figure 1 is a structural formula of pyrimethamine
Role and purpose
This product can inhibit dihydrofolate reductase, causing failure of the conversion from dihydrofolate into tetrahydrofolate, resulting in decreased synthesis of nucleic acid, so that the parasite propagation is inhibited. It is mainly used for the prevention of malaria, it can also be combined with primaquine to prevent relapse of malaria.
Pharmacokinetics
After oral administration, gastrointestinal absorption is complete but very slow with the blood concentration reaching peak after four hours. Plasma protein has a binding rate of 80%. It is mainly distributed in tissues such as liver, lung, kidney and other tissues as well as in milk. The half-life is about 90 hours with only 10% to 20% of the prototype drug being excreted through the urine at 5 to 7 days after administration. The effective concentration of the blood can be maintained for two weeks. Therefore, single-time medication can has its preventive effect be maintained for more than 1 week.
Side effects
1. Upon administration of high dose can cause acute poisoning symptoms such as fatigue, nausea, vomiting, abdominal pain, fever, cyanosis, jaundice, splenomegaly, etc. Upon this condition, drug administration should be promptly discontinued and the patients should be subject to gastric lavage, rehydration and symptomatic treatment. Because of the sweet taste of this product, it is more prone for children to be subject to mistakenly administration and poisoning, special attention should be paid.
2. Long-term administration can interfere with the in vivo utilization of folic acid, producing megaloblastic anemia. Therefore, the patients should be subject to regularly monitoring of blood. If the above issue happens, we should treat with leucovorin.
Precautions
1. Lactating women and patients of renal dysfunction should take with caution. Pregnant women in early phase should be disabled. Children less than 1 year of age should not use.
2. This product has slightly fragrance without bitter taste and should be protected from the reach of children whose mistaken administration can lead to poisoning, convulsions (Children under age 6 who takes 50~100mg can get poisoning and die). Barbiturates can confront its role in the central excitability.
3. This product, after single-time overdose of long-term continuous administration can cause bone marrow suppression and gastrointestinal function inhibition, resulting in megaloblastic anemia and leukopenia with timely withdrawal leading to self-healing. Giving formylation CF may alleviate the bone marrow function.
4. Adults, after single dose administration of 150~200mg have the risk of poisoning. It often occurs of nausea, vomiting, headache, dizziness and other symptoms within 1 to 5 hours with convulsions and coma occurring in severe cases. Children under 6 years of age can die upon administration at draught of 50~100mg and get poisoning and die, it should be given attention. First aid measures: apply gastric lavage, vomiting; drink a lot of sugar or 10% carrot juice. The patients can further subject to administration of the glucose infusion and diuretics. Patients of spasms and convulsions should be subject to infusion of thiopental.
5. Patients of renal damage, unconsciousness, 6-GPD deficiency and giant cell anemia should take with caution.
This information is edited by Xiongfeng Dai from Chemicalbook.
Medicine interactions
Being used in combination with dihydrofolate synthetase inhibitors (such as sulfadoxine or dapsone) can double block the folate metabolism of malaria parasite and enhance its performance, delay or prevent the emergence of drug-resistant strains of insects. It can’t be used in combination with other kinds of dihydrofolate reductase inhibitor because it can enhance the toxicity.
Dosage
Prophylaxis: start administration at 1 to 2 weeks before entering into the affected area. It is generally recommended to keep administration to until 6 to 8 weeks after leaving the affected areas once per week and 25 mg per time. Children take once per week with 0.9 mg/kg per time with the highest dose limited for adults.
For Plasmodium falciparum caused chloroquine-resistant strains: take 50 mg daily with 2 times. For children, take 0.3 mg/kg at 3 times per day with the course of three days.
Toxoplasmosis: Daily 50~100 mg, administrate at draught at a dose of 25 mg after 1 to 3 days with a course of 4 to 6 weeks. For children, take 1 mg/kg with 2 times. After 1-3 days, change to 0.5 mg/kg daily at 2 times with a course of 4 to 6 weeks.
References
1. Black RH, Canfield CJ, Clyde DF, Peters W, Wernsdorfer WH (1986). Chemotherapy of Malaria, 2nd edn, edited by L.J.Bruce-Chwatt. (Geneva: World Health Organization), pp. 77–80.
2. The biology of malaria parasites. Technical Report Series no 743 (1987). (Geneva: World Health Organization).
3. Friman G, Nyström-Rosander C, Jonsell G, Björkman A, Svendsrup B (1985). Agranulocytosis associated with malaria prophylaxis with Maloprim. BMJ, 286, 1244–1245.
4. Pyrimethamine. Therapeutic Drugs, edited by Sir Colin Dollery (1991), (London: Churchill Livingstone), pp. P314–P317.
Chemical Properties
White Solid
Uses
Dihydrofolate reductase inhibitor; generally used in combination with other antimicrobial agents. Antiprotozoal (Toxoplasma); antimalarial
Uses
This powerful inhibitor of dihydrofolate reductase is used for preventing and treating
malaria caused by plasmodia P. vivax, P. malariae, P. ovale, including P. falciparum.
Pyrimethamine, an antagonist of folic acid, exhibits antimicrobial action against
causative agents of malaria and simultaneously possesses sporontocide action. It is also
effective with respect to the causative agent of toxoplasmosis. It is used for preventing
malaria and treating toxoplasmosis. It can only be used for preventative measures; however,
because resistance develops quickly and because of the fact that the maximal effect is
achieved by using it in combination with sulfadoxine, a combined drug which is prescribed
under the name fansidar, which contains a pyrimethamine–sulfadoxine ratio of 1:20.
A combination of pyrimethamine, sulfonamide, and quinine is the drug of choice for
acute attacks of malaria and its chloroquine-resistant forms.
Pyrimethamine in combination with sulfadiazine or trisulfapyrimidine is the drug of
choice for toxoplasmosis. A synonym of this combined drug is daraprim.
Uses
For the treatment of toxoplasmosis and acute malaria; For the prevention of malaria in areas non-resistant to pyrimethamine.
Pyrimethamine is a dihydrofolate reductase inhibitor, like the biguanides, and is structurally related to trimethoprim. It is seldom used alone. Pyrimethamine in fixed combinations with dapsone or sulfadoxine is used for treatment and prophylaxis of chloroquine-resistant falciparum malaria. The synergistic activities of pyrimethamine and sulfonamides are similar to those of trimethoprim/sulfonamide combinations. Resistant strains of Plasmodium falciparum have appeared world wide. Prophylaxis against falciparum malaria with pyrimethamine alone is therefore not recommended. Most strains of Plasmodium vivax have remained sensitive. Pyrimethamine is also used in combination with a sulfonamide for the treatment of Toxoplasmosis. It is slowly absorbed from the gastrointestinal tract with peak plasma levels 4–6 hours after dosing. Pyrimethamine is bound to plasma proteins and is extensively metabolized before excretion. Its elimination half-life is 3–5 days.
Indications
Pyrimethamine (Daraprim) is the best of a number of 2,4-
diaminopyrimidines that were synthesized as potential
antimalarial and antibacterial compounds. Trimethoprim
(Proloprim) is a closely related compound.
Pyrimethamine is well absorbed after oral administration,
with peak plasma levels occurring within 3 to 7
hours. An initial loading dose to saturate nonspecific
binding sites is not required, as it is with chloroquine.
However, the drug binds to tissues, and therefore, its
rate of renal excretion is slow. Pyrimethamine has a
half-life of about 4 days. Although the drug does undergo
some metabolic alterations, the metabolites
formed have not been totally identified.
Definition
ChEBI: An aminopyrimidine that is pyrimidine-2,4-diamine which is substituted at position 5 by a p-chlorophenyl group and at position 6 by an ethyl group. It is a folic acid antagonist used as an antimalarial or with a sulfonamide to treat toxoplasmo
is.
brand name
Daraprim (GlaxoSmithKline).
General Description
Odorless white crystalline powder. Tasteless. An antimalarial drug.
Air & Water Reactions
Pyrimethamine is a diaminopyrimidine which is structurally related to trimethoprim. It is effective against erythrocytic stage of Plasmodium (P) falciparum and less so against P. vivax, P. ovale and P. malariae. Pyrimethamine also inhibits the sporogony in the mosquito, resulting in a decrease of transmission of the infection within the community
[1].
The mechanism of action of pyrimethamine is related to its inhibition of dihydrofolic reductase necessary for the folic acid synthesis in the parasite. Pyrimethamine acts slowly and is not recommended as monotherapy for acute malaria attacks. Resistance to pyrimethamine developed soon when the drug was used on a large scale as monoprophylaxis
[1]. In resistant strains, the enzyme dihydrofolic reductase binds to pyrimethamine several hundred times less than in sensitive strains
[2]. This high grade resistance is probably a onestep mutation and cannot be overcome by increasing the dose. However, when combined with long-acting sulphonomides (sulphadoxine), the effect of pyrimethamine is potentiated and the risk of developing resistant strains is far less.
Reactivity Profile
Pyrimethamine together with sulphadoxine (Fansidar) is used in the treatment of P. falciparum malaria (cf. Sulphadoxine: Indications). Pyrimethamine is also valuable in the treatment of toxoplasmosis.
Fire Hazard
Pyrimethamine in combination with sulphadoxine (Fansidar) can cause severe cutaneous adverse reactions (cf. Sulphadoxine: Side effects). Agranulocytosis occurs quite frequently (1/2000) and fatalities have been reported when pyrimethamine is combined with dapsone
[3]. When given alone, life-threatening adverse reactions are very rare and the drug is generally well tolerated. Megaloblastic anaemia may, however, occur during long-term treatment with high doses (i.e. for toxoplasmosis) and can be prevented by folinic acid supplementation
[4].
Biological Activity
During long-term treatment with high doses, folinic acid supplement is usually given.
Mechanism of action
The combination of pyrimethamine with a long-acting sulfonamide, sulfadoxine, which
blocks dihydrofolate synthesis by blocking incorporation of PABA into the dihydrofolate, is called
Fansidar, which produces sequential blockage of tetrahydrofolate synthesis similar to that reported
for treatment of bacterial infections. Plasmodium enzymes catalyzing folic acid
synthesis differ from those enzymes found in other organisms. A single bifunctional protein present
in Plasmodium sp. catalyzes the phosphorylation of 6-hydroxymethyl-7,8-hydropterin
(a pyrophosphokinase) and the incorporation of PABA into dihydropteroic acid. A second
bifunctional enzyme catalyzes the reduction of dihydropteroic acid and thymidylic acid synthesis.
As a result, the drug combination (Fansidar) appears to have improved drug-mediated disruption of
folic acid in Plasmodium sp.. This combination has been used with quinine for the
treatment and prevention of chloroquine-resistant malaria (Plasmodium falciparum, Plasmodium
ovale, Plasmodium vivax, and Plasmodium malaria). The combination therapy (Fansidar) has the
added advantage of being inexpensive, which is essential for successful therapy in developing
countries. When used on its own, pyrimethamine is a blood schizonticide without effects on the
tissue stage of the disease.
Clinical Use
Pyrimethamine has been recommended for prophylactic
use against all susceptible strains of plasmodia;
however, it should not be used as the sole therapeutic
agent for treating acute malarial attacks. As mentioned
previously, sulfonamides should always be coadministered
with pyrimethamine (or trimethoprim), since the
combined antimalarial activity of the two drugs is significantly
greater than when either drug is used alone.
Also, resistance develops more slowly when they are
used in combination. Sulfonamides exert little or no effect
on the blood stages of P. vivax, and resistance to the
dihydrofolate reductase inhibitors is widespread.
In addition to its antimalarial effects, pyrimethamine
is indicated (in combination with a sulfonamide) for the
treatment of toxoplasmosis.The dosage required is 10 to
20 times higher than that employed in malarial infections.
Side effects
Relatively few side effects are associated with the
usual antimalarial dosages. However, signs of toxicity are
evident at higher dosages, particularly those used in the
management of toxoplasmosis. Many of these reactions
reflect the interference of pyrimethamine with host folic
acid metabolism, especially that occurring in rapidly dividing
cells. Toxic symptoms include anorexia, vomiting,
anemia, leukopenia, thrombocytopenia, and atrophic
glossitis. CNS stimulation, including convulsions, may follow
an acute overdose.The side effects associated with the
pyrimethamine–sulfadoxine combination include those
associated with the sulfonamide and pyrimethamine
alone. In addition, there is evidence of a greater incidence
of allergic reactions, particularly toxic epidermal necrolysis
and Stevens-Johnson syndrome, with the combination.
This carries an estimated mortality of 1:11,000 to 1:25,000
when used as a chemoprophylactic.
Safety Profile
Poison by ingestion,
subcutaneous, and intraperitoneal routes.
Experimental teratogenic and reproductive
effects. Questionable carcinogen. Human
mutation data reported. When heated to
decomposition it emits very toxic fumes of
Cland NOx. Used as an antimalarial drug
for humans and to treat toxoplasmosis in
hogs.
Synthesis
Pyrimethamine, 2,4-diamino-5-(4-chlorophenyl)-6-ethylpyrimidine
(33.1.60), is synthesized from 4-chlorobenzycyanide, which upon condensation with
methyl ester of propionic acid in the presence of sodium methoxide gives the |?-ketonitrile
(33.1.58). Reacting this with ethyl orthoformate gives a methoxymethylene derivative
(33.1.59), which upon heterocyclization in pyrimidine using guanidine as the binucleophile forms the desired pyrimethamine (33.1.60).
Veterinary Drugs and Treatments
In veterinary medicine, pyrimethamine is used to treat Hepatozoon
americanum infections, and toxoplasmosis in small animals (often
in combination with sulfonamides). In horses, it is used to treat
equine protozoal myeloencephalitis, sometimes called equine toxoplasmosis.
In humans, pyrimethamine is used for the treatment of toxoplasmosis
and as a prophylactic agent for malaria.
Drug interactions
Potentially hazardous interactions with other drugs
Increased antifolate effect with sulphonamides,
trimethoprim, methotrexate and pemetrexed.
Antiepileptics: anticonvulsant effect of fosphenytoin
and phenytoin antagonised, also increased antifolate
effect.
Antimalarials: avoid concomitant use with
artemether/lumefantrine; increased antifolate effect
with proguanil.
Metabolism
Pyrimethamine is metabolised in the liver and slowly
excreted via the kidney, with up to 30% recovered in
the urine as parent compound over a period of several
weeks. Several metabolites have also been detected in
the urine, although data are lacking on the nature of
these metabolites, their route, rate of formation and
elimination, and any pharmacological activity, particularly
after prolonged daily dosing.
