Mepacrine, also called quinacrine or by the trade name Atabrine, is a medication with several uses. It is related to chloroquine and mefloquine. Although formerly available from compounding pharmacies, as of August 2020 it is unavailable in the United States. Medical uses The main uses of mepacrine are as an antiprotozoal, antirheumatic, and an intrapleural sclerosing agent. Antiprotozoal use include targeting giardiasis, where mepacrine is indicated as a primary agent for patients with metronidazole-resistant giardiasis and patients who should not receive or cannot tolerate metronidazole. Giardiasis that is very resistant may even require a combination of mepacrine and metronidazole. Mepacrine is also used off-label for the treatment of systemic lupus erythematosus, indicated in the treatment of discoid and subcutaneous lupus erythematosus, particularly in patients unable to take chloroquine derivatives. As an intrapleural sclerosing agent, it is used as pneumothorax prophylaxis in patients at high risk of recurrence, e.g., cystic fibrosis patients. Mepacrine is not the drug of choice because side effects are common, including toxic psychosis, and may cause permanent damage. See mefloquine for more information. In addition to medical applications, mepacrine is an effective in vitro research tool for the epifluorescent visualization of cells, especially platelets. Mepacrine is a green fluorescent dye taken up by most cells. Platelets store mepacrine in dense granules. Mechanism Its mechanism of action against protozoa is uncertain, but it is thought to act against the protozoan's cell membrane. It is known to act as a histamine N-methyltransferase inhibitor. It also inhibits NF-κB and activates p53. History Antiprotozoal Mepacrine was initially approved in the 1930s as an antimalarial drug. It was used extensively during the second World War by Allied forces fighting in North Africa and the Far East to prevent malaria. This antiprotozoal is also approved for the treatment of giardiasis (an intestinal parasite), and has been researched as an inhibitor of phospholipase A2. Scientists at Bayer in Germany first synthesised mepacrine in 1931. The product was one of the first synthetic substitutes for quinine although later superseded by chloroquine. Anthelmintics In addition it has been used for treating tapeworm infections. Creutzfeldt–Jakob disease Mepacrine has been shown to bind to the prion protein and prevent the formation of prion aggregates in vitro, and full clinical trials of its use as a treatment for Creutzfeldt–Jakob disease are under way in the United Kingdom and the United States. Small trials in Japan have reported improvement in