Coumarin resistance in a Negro woman

High clearance of (S)-warfarin in a warfarin-resistant subject

A 30 year old black male required a 60 mg daily dose of warfarin to elicit a therapeutic anticoagulant response (normal warfarin dose 2.5-10 mg day-1; maximum 15 mg day-1). Hereditary warfarin resistance was suspected after compliance, diet, concurrent medication and any gastrointestinal disorder were eliminated as contributory causes. The disposition of vitamin K and vitamin K epoxide was examined in the propositus, his two sisters and 13 control black male subjects. Each subject was given an i.v. bolus dose (5 mg) of vitamin K prior to and after 2 weeks of warfarin therapy (5 mg day-1). The oral clearances of (S)- and (R)-warfarin were also measured in each subject during the last day of warfarin therapy. The mean (+/- s.d.) systemic clearance of vitamin K was similar in all subjects before (114 +/- 35 ml min-1) and after (112 +/- 40 ml min-1) warfarin therapy. The mean (+/- s.d.) AUC value for vitamin K epoxide was increased by warfarin treatment (6.5 +/- 5.4 micrograms ml-1 min before and 139 +/- 78 micrograms ml-1 min after) in all subjects. In the propositus, the oral clearance of (S)-warfarin (14.5 ml min-1) and the clearance ratio for (S)/(R)warfarin (2.6) differed by more than 7 standard deviations from the control group (4.3 +/- 1.1 ml min-1 and 1.2 +/- 0.2, respectively). In one sister of the propositus, the stereoselective disposition of warfarin was comparable with that of her brother ((S)-warfarin clearance = 16.2 ml min-1; and (S)/(R)-warfarin clearance ratio = 2.7).

Familial warfarin resistance

Warfarin, a coumarin anticoagulant, acts by interfering with the hepatic synthesis of the vitamin-K-dependent clotting factors. It is used clinically in the treatment or prophylaxis of venous thrombosis and embolisms. Resistance to the effects of the coumarin and indanedione anticoagulants has been reported in rats and man, but its incidence has been defined as extremely rare. Resistance has been described as relative, acquired, or hereditary. The first well-documented case was also the first report to identify a genetic basis for this resistance. Since that time, there has been only one other study that strengthened the evidence for a hereditary transmission, and only a few other reports have suggested hereditary influence as a reason for coumarin resistance. In this report, a patient who presented with a familial-type warfarin resistance is described. A discussion of previous reports and possible mechanisms for nonfamilial warfarin resistance is also included.