4-Hydroxycoumarins

4-Hydroxycoumarins belong to a class of vitamin K antagonist (VKA) anticoagulant drug molecules derived from coumarin by adding a hydroxy group at the 4 position to obtain 4-hydroxycoumarin, then adding a large aromatic substituent at the 3-position (the ring-carbon between the hydroxyl and the carbonyl). The large 3-position substituent is required for anticoagulant activity.

The primary mechanism of the 4-hydroxycoumarin drugs is the inhibition of vitamin K epoxide reductase. These compounds are not direct antagonists (in the pharmaceutical sense) of vitamin K, but rather act to deplete reduced vitamin K in tissues. For this reason vitamin K antagonizes their effect, and this has led to the loose terminology of vitamin K antagonism.

Although 4-hydroxycoumarin itself is not an anticoagulant, it is an important fungal metabolite from the precursor coumarin, which is also not an anticoagulant, and its production leads to further fermentative production of the natural anticoagulant dicoumarol. This happens in the presence of naturally occurring formaldehyde, which allows attachment of a second 4-hydroxycoumarin molecule through the linking carbon of the formaldehyde, to the 3-position of the first 4-hydroxycoumarin molecule, giving the semi-dimer the motif of the drug class. Dicoumarol appears in spoiled sweet clover silages and is considered a natural chemical substance of combined plant and fungal origin; mycotoxin also shares these properties. The identification of Dicoumarol in 1940 is the precursor of the drug class known as 4-Hydroxycoumarin. See warfarin for this history.

The synthetic drugs in the 4-hydroxycoumarin class are all noted primarily for their use as anticoagulants, though they can have several additional effects. All affect the normal metabolism of vitamin K in the body by inhibiting the enzyme vitamin K epoxide reductase which recycles vitamin K to active form. As such, these compounds form the most important and widely used subset of vitamin K antagonist drugs, but other such drugs exist which do not have the 4-hydroxycoumarin structure. All the vitamin K antagonist agents diminish the amount of available vitamin K in the body, and thus inhibit the action of vitamin K-dependent enzymes that are critically involved in the production of active forms of certain clotting factors, and certain other metabolic processes involving the binding of calcium ion.

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