|Systematic (IUPAC) name|
|Biological half-life||96 hours|
|ATC code||P01BD01 (WHO)
QP51AX51 (WHO) (combinations)
|PDB ligand ID||CP6 (PDBe, RCSB PDB)|
|Molar mass||248.71 g/mol|
Pyrimethamine (trade name Daraprim) is a medication used for protozoal infections. It is commonly used as an antimalarial drug (for both treatment and prevention of malaria), and to treat Toxoplasma gondii infections, particularly when combined with the sulfonamide antibiotic sulfadiazine when treating HIV-positive individuals. It works by inhibiting folic acid metabolism via dihydrofolate reductase.
It is on the World Health Organization's List of Essential Medicines, the most important medications needed in a basic health system.
- Sulfonamides inhibit dihydropteroate synthetase, an enzyme that participates in folic acid synthesis from para-aminobenzoic acid. Hence, sulfonamides work synergistically with pyrimethamine by blocking a different enzyme needed for folic acid synthesis.
- Folinic acid (leucovorin) is a folic acid derivative converted to tetrahydrofolate, the primary active form of folic acid, in vivo without relying on dihydrofolate reductase. Folinic acid reduces side effects related to folate deficiency in the patient.
It is primarily active against Plasmodium falciparum, but also against Plasmodium vivax. Due to the emergence of pyrimethamine-resistant strains of P. falciparum, pyrimethamine alone is seldom used now. In combination with a long-acting sulfonamide such as sulfadiazine, it is widely used, such as in Fansidar though resistance to this combination is increasing. It has been used in the treatment of actinomycosis and isosporiasis, and for the treatment and prevention of Pneumocystis jirovecii pneumonia.
In 2011, researchers discovered that pyrimethamine can increase β-hexosaminidase activity, thus potentially slowing down the progression of late-onset Tay–Sachs disease. It is being evaluated in clinical trials as a treatment for amyotrophic lateral sclerosis.
Mechanism of resistance
Resistance to pyrimethamine is widespread. Mutations in the malarial gene for dihydrofolate reductase may reduce its effectiveness. These mutations decrease the binding affinity between pyrimethamine and dihydrofolate reductase via loss of hydrogen bonds and steric interactions.
Pyrimethamine can cause a rash, and if higher doses are used (such as for toxoplasmosis), it can cause gastrointestinal symptoms such as nausea, vomiting, abdominal cramps, dry mouth, weight loss, and diarrhea. Central nervous system effects include headache, ataxia, and rarely seizures and haematologic side effects such as leucopenia and anaemia.
Pyrimethamine is contraindicated in patients with:
- Folate-deficiency anaemia
- Pregnancy, especially during the first trimester, due to the possible detrimental effects an antifolate such as pyrimethamine might have on organogenesis
It is a white, colourless, crystalline powder; it is practically insoluble in water and slightly soluble in ethanol, chloroform, and acetone. It is unstable in the presence of air and light. It is chemically a diaminopyrimidine derivative.
Mechanism of action
Pyrimethamine interferes with tetrahydrofolic acid synthesis from folic acid by inhibiting the enzyme dihydrofolate reductase (DHFR). Tetrahydrofolic acid is needed for DNA and RNA synthesis in many species, including protozoa. It has also been found to reduce the expression of SOD1, a key protein involved in amyotrophic lateral sclerosis.
The Nobel Prize-winning American scientist Gertrude Elion developed the drug at Burroughs-Wellcome (now part of GlaxoSmithKline) to combat malaria. Pyrimethamine has been available since 1953, and is not subject to any unexpired patent. However, in the United States, the market for this product is sufficiently small that no generic manufacturer has emerged. In 2010, GlaxoSmithKline sold the marketing rights for Daraprim to CorePharma. Impax Laboratories sought to buy CorePharma in 2014, and completed the acquisition, including Daraprim, in March 2015. In August 2015, the rights were bought by Turing Pharmaceuticals. Turing subsequently became known for a price hike controversy when it raised the price of a dose of the drug in the U.S. market from US$13.5 to US$750, a 5,500% increase.
Availability and price
In the United States, as of 2015, with Turing Pharmaceuticals' acquisition of the US marketing rights for Daraprim tablets, Daraprim has become a single-source and specialty pharmacy item, and the price of Daraprim has been increased. The cost of a monthly course for a person on 75 mg dose rose to about $75,000/month, or $750 per tablet. Outpatients can no longer obtain Daraprim from their community pharmacy, but only through a single dispensing pharmacy, Walgreens Specialty Pharmacy, and institutions can no longer order from their general wholesaler, but have to set up an account with the Daraprim Direct program. Presentations from Retrophin, a company formerly headed by Martin Shkreli, CEO of Turing, from which Turing acquired the rights to Daraprim, suggest that a closed distribution system could prevent generic competitors from legally obtaining the drugs for the bioequivalence studies required for FDA approval of a generic drug.
Martin Shkreli, CEO of Turing, defended the price hike by saying, "If there was a company that was selling an Aston Martin at the price of a bicycle, and we buy that company and we ask to charge Toyota prices, I don't think that that should be a crime." As a result of the backlash, Shkreli hired a crisis public relations firm to help explain his fund's move. Turing Pharmaceuticals announced on November 24, 2015 "that it would not reduce the list price of that drug after all" but they will offer various patient assistance programs. However, New York Times journalist Andrew Pollack noted that these programs "are standard for companies selling extremely high-priced drugs. They enable the patients to get the drug while pushing most of the costs onto insurance companies and taxpayers."
The price increase has been fiercely criticised by physician groups such as HIV Medicine Associates and Infectious Diseases Society of America.
In India, over a dozen pharmaceutical companies manufacture and sell pyrimethamine tablets and, multiple combinations of generic pyrimethamine are available for a price ranging from US$0.04–$0.10 each (3–7 rupees).
In Canada, the drug was reportedly discontinued in 2013 but hospitals may make the drug in-house when it is needed. As of December 2015[update], Daraprim imported into Canada directly from GSK UK is available from an online pharmacy for US$2.20 per tablet.
On October 22, 2015, Imprimis Pharmaceuticals announced it has made available compounded and customizable formulations of pyrimethamine and leucovorin in oral capsules starting as low as $99.00 for a 100 count bottle in the United States.
- Etoprine (and metoprine) is closely related structure but has an extra chlorine atom and is instead described as antineoplastic.
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