Pharmacokinetics of vitamin K1 in low-birth-weight neonates

The pharmacokinetics of vitamin K1 was studied in 21 newborn infants. 11 neonates had received no parenteral loading dose prior to the study (group I), while 10 had been injected 5-10 mg vitamin K1 at birth (group II). At postnatal age 2-9 h, 1 mg of vitamin K1 was injected intravenously, and small samples of blood (less than or equal to 500 microliter) were collected at different times during 6 h. Serum vitamin K1 and its epoxide were assayed by high-performance liquid chromatography (HPLC). In both groups, the disappearance curve showed two exponential components: a fast distribution component during the 1st h and a slower elimination component during the next 5 h. In group I, the plasma half-life of the first component was between 18 and 52 min (median 23 min), and the half-life of the second was between 67 and 179 min (median 109 min). Both half-lives were significantly higher in group II. The volumes of distribution were suggestive of distribution into plasma during the first phase and roughly into the extracellular water for the second component. Epoxide was detected in most patients 15 min after vitamin K1 injection, and after 3 h its concentration was higher than the concentration of vitamin K1. These data suggest that the kinetics of vitamin K1 in neonates is not very different from that in adults. The newborn infant is able to oxidize vitamin K1, a phenomenon in keeping with the gamma carboxylation of glutamic acid.