Warfarin -- September 19, 2021

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In the early 1920s, there was an outbreak of a previously unrecognized cattle disease in the northern United States and Canada. Cattle were haemorrhaging after minor procedures, and on some occasions spontaneously.[85] For example, 21 out of 22 cows died after dehorning, and 12 out of 25 bulls died after castration. All of these animals had bled to death.

In 1921, Frank Schofield, a Canadian veterinary pathologist, determined that the cattle were ingesting moldy silage made from sweet clover, and that this was functioning as a potent anticoagulant. Only spoiled hay made from sweet clover (grown in northern states of the US and in Canada since the turn of the century) produced the disease.

Schofield separated good clover stalks and damaged clover stalks from the same hay mow, and fed each to a different rabbit. The rabbit that had ingested the good stalks remained well, but the rabbit that had ingested the damaged stalks died from a haemorrhagic illness. A duplicate experiment with a different sample of clover hay produced the same result.

In 1929, North Dakota veterinarian Lee M. Roderick demonstrated that the condition was due to a lack of functioning prothrombin.

The identity of the anticoagulant substance in spoiled sweet clover remained a mystery until 1940. In 1933 Karl Paul Link and his lab of chemists working at the University of Wisconsin set out to isolate and characterize the haemorrhagic agent from the spoiled hay. 

It took five years before Link's student Harold A. Campbell recovered 6 mg of crystalline anticoagulant. Next, Link's student Mark A. Stahmann took over the project and initiated a large-scale extraction, isolating 1.8 g of recrystallized anticoagulant in about 4 months. This was enough material for Stahmann and Charles F. Huebner to check their results against Campbell's, and to thoroughly characterize the compound. Through degradation experiments they established that the anticoagulant was 3,3'-methylenebis-(4-hydroxycoumarin), which they later named dicoumarol. They confirmed their results by synthesizing dicoumarol and proving in 1940 that it was identical to the naturally occurring agent.

Dicoumarol was a product of the plant molecule coumarin (not to be confused with Coumadin, a later tradename for warfarin). 

Coumarin is now known to be present in many plants, and produces the notably sweet smell of freshly cut grass or hay and plants like sweet grass; in fact, the plant's high content of coumarin is responsible for the original common name of "sweet clover", which is named for its sweet smell, not its bitter taste. 

They are present notably in woodruff (Galium odoratum, Rubiaceae), and at lower levels in licorice, lavender, and various other species. The name coumarin comes from the French pronunciation of coumarou, the Indian name for the tree of the tonka bean, which noteably contains a high concentration of coumarin. However, coumarins themselves do not influence clotting or warfarin-like action, but must first be metabolized by various fungi into compounds such as 4-hydroxycoumarin, then further (in the presence of naturally occurring formaldehyde) into dicoumarol, in order to have any anticoagulant properties.

Over the next few years, numerous similar chemicals (specifically 4-hydroxycoumarins with a large aromatic substituent at the 3 position) were found to have the same anticoagulant properties. The first drug in the class to be widely commercialized was dicoumarol itself, patented in 1941 and later used as a pharmaceutical. Karl Link continued working on developing more potent coumarin-based anticoagulants for use as rodent poisons, resulting in warfarin in 1948. 

The name "warfarin" stems from the acronym WARF, for Wisconsin Alumni Research Foundation + the ending -arin indicating its link with coumarin. Warfarin was first registered for use as a rodenticide in the US in 1948, and was immediately popular. Although warfarin was developed by Link, the Wisconsin Alumni Research Foundation financially supported the research and was assigned the patent.

After an incident in 1951, in which an army inductee attempted suicide with multiple doses of warfarin in rodenticide but recovered fully after presenting to a Naval Hospital and being treated with vitamin K (by then known as a specific antidote), studies began in the use of warfarin as a therapeutic anticoagulant. It was found to be generally superior to dicoumarol, and in 1954 was approved for medical use in humans. An early recipient of warfarin was US President Dwight Eisenhower, who was prescribed the drug after having a heart attack in 1955.

The exact mechanism of action remained unknown until it was demonstrated, in 1978, that warfarin inhibits the enzyme epoxide reductase, and hence interferes with vitamin K metabolism.

It has been posited that Lavrenty Beria and I. V. Khrustalyov conspired to use warfarin to poison Soviet leader Joseph Stalin. Warfarin is tasteless and colourless, and produces symptoms similar to those that Stalin exhibited.