Evaluation of antidote effect of AT-N135Q-Pro394 in mice receiving a high dose of fondaparinux. In vivo antidote activity of AT-N135Q-Pro394 (∼ 20 μg/g) versus placebo (PBS) was estimated in mice pretreated with fondaparinux (0.5 mg/kg). Five minutes after intravenous administration of the antidote, blood was withdrawn by puncture in the vena cava, and plasma was obtained by a centrifugation. Anticoagulant activity of fondaparinux was evaluated in plasma of AT-N135Q-Pro394 or placebo-treated mice by measurement of anti-FXa activity as described for heparin derivative reversal properties of AT-N135Q-Pro 394 in plasma. Means ( ± SD) in each group were compared by unpaired t test and found significantly different with P < .05.