VITAMIN K1

Биологические функции

Both vitamins K3 and K4 may produce hyperbilirubinemia and kernicterus in neonates as well as hemolysis in neonates and glucose-6-phosphate–deficient patients. In fact, the only advantage of vitamins K3 and K4 over vitamin K1 is that whereas absorption of vitamin K1 requires the presence of bile, absorption of vitamins K3 and K4 does not, because they are absorbed via a passive process directly from the intestine. This may be a slight advantage for patients with cholestasis or severe pancreatic dysfunction. Only vitamin K1, however, is appropriate therapy for bleeding associated with warfarin and superwarfarin anticoagulation. Vitamin K2 is not used therapeutically.

Механизм действия

Vitamin K antagonists, such as warfarin, produce their effect on blood coagulation by interfering with the cyclic interconversion of vitamin K and vitamin K 2,3-epoxide. Vitamin K is an essential cofactor necessary for the posttranslational carboxylation of the glutamic acid residues on the N-terminal portions of the specific clotting factors (II, VII, IX, and X) and anticoagulant proteins, such as protein C. This γ-glutamyl carboxylation results in a new amino acid, γ-carboxyglutamate, which through chelation of calcium ions causes the proteins to undergo a conformational change. This change in tertiary structure allows the four vitamin K–dependent clotting factors to become activated and bind to the negatively charged phospholipid membranes during clotting cascade activation.

Фармакокине?тика

The half-life of vitamin K1 is quite short—only 1.7 hours via the intravenous route and 3–5 hours via the oral route. When given orally, vitamin K1 is absorbed directly from the proximal small intestine in an energy-dependent and saturable process that requires the presence of bile salts. These kinetic features argue for administration in divided doses rather than larger, single daily doses. The typical starting point for adults with drug-induced hypoprothrombinemia is 2.5 to 10 mg of vitamin K1 orally, repeating in 12 to 48 hours if needed. In cases of ingestion of long-acting superwarfarin rodenticides (e.g., brodifacoum), therapy may be 125 mg/day for weeks or months. Practically speaking, because vitamin K1 is dispensed as 5-mg tablets, superwarfarin-poisoned patients may require 10 to 30 tablets every 6 hours.

Клиническое использование

Vitamin K1 (phytonadione, Mephyton) is the form of vitamin K most often used therapeutically. Vitamin K1 is safe for use in infants, pregnant women, and patients with glucose-6-phosphate deficiency.

Профиль безопасности

Moderately toxic by subcutaneous route. An experimental teratogen. When heated to decomposition it emits acrid smoke and irritating fumes.