Hepatic uptake and storage of warfarin. The relation with the target enzyme vitamin K 2,3-epoxide reductase

HH Thijssen and LG Baars

Department of Pharmacology, University of Limburg, Maastricht, The Netherlands.

The mechanisms of the reported dose-dependent warfarin pharmacokinetics were investigated using [14C]warfarin. When administered in microdoses (9 micrograms i.v.) to rats (male Wistars, 270-300 g), a steep distribution phase (T1/2 = 0.25 hr) was followed by a relatively slow beta-phase (T1/2 = 40 hr). The observed volume of distribution was 390 ml. This pharmacokinetic behavior contrasted highly with the one seen for higher (greater than 0.2 mg/kg) doses (unlabeled) warfarin; volume of distribution = 45 ml, T1/2 = 12.5 hr. If a "macrodose" (0.2 mg/kg) preceded (16 hr) the "microdose," "normal" pharmacokinetics were observed for the latter, suggesting a saturable "deep compartment." The administration of 4-hydroxycoumarins (i.e., acenocoumarol, phenprocoumon and warfarin) after the microdose of [14C]warfarin was in its beta-phase caused a rapid rise of plasma [14C]warfarin indicating [14C]warfarin to be displaced from the "deep compartment." The rate of appearance of [14C]warfarin was 0.3 hr-1 irrespective the 4- hydroxycoumarin used. The hepatic distribution of [14C]warfarin was investigated and the effect of a displacer thereupon. Fifty-three hours after the [14C]warfarin administration, the liver contained about 40% of the dose; 45% of it was bound to microsomes. The administration of acenocoumarol (0.2 mg/kg) at 48 hr, halved the liver content. [14C]warfarin was redistributed from microsomes (-65%) and from the 10,000 X g pellet (-65%) into the cytosol (+260%) and the plasma (+320%). Microsomal bound [14C]warfarin in vitro could not be washed out or be displaced unless dithiothreitol (50 mM) was included in the washing buffers.(ABSTRACT TRUNCATED AT 250 WORDS)

Volume 243, Issue 3, pp. 1082-1088, 12/01/1987
Copyright © 1987 by American Society for Pharmacology and Experimental Therapeutics