Pharmacodynamics of rFIXFc and rFIX in FIX-deficient mice. FIX-deficient mice were dosed with rFIXFc at 219 IU/kg body weight (5 per group, 6 groups, n = 30) or rFIX at 200 IU/kg body weight (4 or 5 per group, 6 groups, n = 28) on days 0, 4, and 8. Plasma samples were collected by cardiac puncture at 15 minutes and 96 hours after each dose, and clotting activity was measured with the use of a FIX activity assay. Plasma was also collected by tail bleeds at 8, 24, 48, and 72 hours after each dose. rFIXFc levels were measured in all of the samples with the use of an ELISA specific for rFIXFc. (A) Measured versus calculated activity. Clotting activity for rFIXFc was measured with the use of a FIX activity assay 15 minutes and 96 hours after 3 doses. The specific activity for this lot of rFIXFc was determined to be 43.8 (±5.4) IU/mg. On the basis of this activity (in IU/mg) and the measured protein levels, a calculated plasma clotting activity level was determined for time points at 15 minutes and for 8, 24, 48, 72, and 96 hours after each dose. (B) In FIX-deficient mice treated with up to 3 doses of rFIX at 200 IU/kg body weight, FIX levels were measured with FIX-specific ELISA. With the use of the measured specific activities of rFIXFc and rFIX, it was possible to compare calculated clotting activity for all samples analyzed by ELISA.