Andexanet alfa (PRT4445, known by the brand name Andexxa®) is a recombinant modified human factor Xa (FXa) protein which was designed to mimic the activity of FXa (Lu G et al. 2013). Andexanet alfa competitively binds to FXa inhibitors preventing their interaction with native Xa (reviewed by Kaatz S et al. 2017). Compared to native FXa, the recombinant FXa variant (andexanet alfa) lacks: (a) the activation peptide of native FX, which was replaced with the hexapeptide RKRRKR linker enabling expression of the mature FXa, (b) a fragment of 34 residues in the membrane-binding GLA domain containing the 11 GLA residues and (c) the catalytic activity due to serine-to-alanine S419A substitution in the active site of FXa (Lu G et al. 2013). The modifications (b) and (c) result in a catalytically inactive FXa variant, which is unable to convert prothrombin to thrombin. The absence of the GLA domain prevents andexanet alfa from competing with native FXa for binding the plasma membrane and assembling a membrane-bound prothrombinase complex (Lu G et al. 2013; reviewed by Kaatz S et al. 2017). Clotting and chromogenic FXa activity assays showed that andexanet alfa is capable of reversing the anticoagulant effects of FXa inhibitors, including direct inhibitors known as 'xabans' such as rivaroxaban and apixaban, as well as indirect inhibitors like enoxaparin (Lu G et al. 2013, 2020; Siegal D et al. 2017; Kalathottukaren MT et al. 2018; Mehrotra S et al. 2022; reviewed by Abuan I et al. 2019; Carpenter E et al. 2019). In FXa enzyme activity assay, andexanet alfa binds to betrixaban, rivaroxaban and apixaban with nanomolar affinities which are comparable to those of native FXa (Lu G et al. 2013). Isothermal titration calorimetry (ITC) data support these findings by showing that andexanet alfa binds with high affinity both rivaroxaban and edoxaban with Kd values in the nanomolar range (Kalathottukaren MT et al. 2018). Andexanet alfa is believed to bind and block FXa inhibitors without exhibiting any anticoagulant activity.