Lack of Pharmacokinetic Interaction between Aspirin and Warfarin

An open-label, randomized, two-phase crossover study was conducted on 36 healthy male volunteers to identify the effects of coadministration of aspirin (acetylsalicylic acid; ASA) and crystalline warfarin sodium (Coumadin((R))) on the elimination and disposition kinetics of ASA, salicylic acid (SA) and R- and S-warfarin enantiomers. Twenty-four subjects were administered single doses of 325 mg of ASA alone and in combination with 10 mg of crystalline warfarin sodium with a 1-week washout between ASA doses. ASA and SA pharmacokinetic parameters were determined after each dose. Twelve subjects were administered single doses of 10 mg of crystalline warfarin sodium alone and in combination with 325 mg of ASA with a 4-week washout between warfarin doses. R- and S-warfarin enantiomer pharmacokinetic parameters were determined after each dose. Pharmacokinetic parameters were compared using analysis of variance and 90% confidence intervals. ASA and SA AUCs (the area under the plasma concentration versus time curve from time zero to time infinity) respectively were 3.28 plus minus 0.80 and 66.99 plus minus 11.73 &mgr;g h ml(minus sign1) (ASA alone), and 3.22 plus minus 0.61 and 69.48 plus minus 15.79 &mgr;g h ml(minus sign1) (ASA with warfarin). R-warfarin and S-warfarin AUCs respectively were 33.9 plus minus 9.3 and 23.9 plus minus 16.0 &mgr;g h ml(minus sign1) (warfarin alone) and 33.6 plus minus 10.2 and 22.6 plus minus 14.7 &mgr;g h ml(minus sign1) (warfarin with ASA). The only pharmacokinetic parameter which was statistically significantly different when the combination was administered was the S-warfarin elimination rate constant (p < 0.05), but the difference (9.2% increase in the presence of ASA) was small and no significant difference was found in S-warfarin clearance. It is concluded that there is no pharmacokinetic interaction when a single dose of ASA 325 mg is coadministered with a single dose of crystalline warfarin sodium 10 mg.