In a study,lasofoxifene caused a minor 16 % decrease in pro-thrombin time without changing warfarin pharmacokinetics.
Clinical evidence,mechanism, importance and management
In 12 healthy postmenopausal women, lasofoxifene 4mg on day one then 500 micrograms daily for 13 days had no effect on pharmacokinetics of R-or S-warfarin when a single 20mg dose of warfarin was given on day 8. Conversely, maximum prothrombin time was decreased by 16%, with a similar decrease in maximum INR from 1.9 to 1.6 (See reference number 1).
Because this slight decrease in warfarin effects has been seen with raloxifene, , authors suggested that it might be because oestrogenic compounds increase plasma concentrations of vitamin K-dependent clotting factors (See reference number 1).
The authors suggest that small decrease in warfarin effect with lasofoxifene may not be clinically relevant. Nevertheless,they say that until more data are available on longer-term concurrent use, it is recommended that prothrombin times should be monitored when starting or stopping lasofoxifene (See reference number 1). This seems a sensible precaution.
1. Quellet D,Bramson C, Carvajal-Gonzalez S, Roman D, Randinitis E, Remmers A, GardnerMJ. Effects of lasofoxifene on the pharmacokinetics and pharmacodynamics of single-dosewarfarin. Br J Clin Pharmacol (2006) 61, 741–5.