Which anticoagulant directly inhibits Factor Xa without requiring antithrombin III as a cofactor?

Explanation

## Direct Factor Xa Inhibitors vs. Heparin-Based Agents **Key Point:** Direct oral Factor Xa inhibitors (DOACs: rivaroxaban, apixaban, edoxaban) bind directly to the active site of Factor Xa and do NOT require antithrombin III (AT-III) as a cofactor. In contrast, heparin-based agents (UFH, LMWH, fondaparinux) all require AT-III as a mandatory cofactor. ## Comparison Table: Mechanism of Action | Agent | Class | Mechanism | AT-III Cofactor Required? | Route | | --- | --- | --- | --- | --- | | **UFH** | Heparin | Binds AT-III → inhibits Xa & IIa | **Yes** | IV/SC | | **Enoxaparin (LMWH)** | Heparin | Binds AT-III → inhibits Xa > IIa | **Yes** | SC | | **Fondaparinux** | Pentasaccharide | Binds AT-III → inhibits Xa only | **Yes** | SC | | **Rivaroxaban (DOAC)** | Direct Xa inhibitor | Direct active site inhibition of Xa | **No** | Oral | **High-Yield:** The distinction between AT-III-dependent and AT-III-independent mechanisms is frequently tested: - **Heparin-based agents** lose efficacy in AT-III deficiency (rare, but important in DIC or congenital deficiency) - **Direct Xa inhibitors** remain effective even if AT-III is depleted - This explains why DOACs have more predictable pharmacokinetics and do not require monitoring [cite:Harrison 21e Ch 139] **Mnemonic:** **DOXAC** = **D**irect **O**ral Xa inhibitors **A**re **C**ofactor-independent - Rivaroxaban, apixaban, edoxaban (all DOACs) - Contrast with heparin family: all require AT-III **Clinical Pearl:** Fondaparinux is a pentasaccharide that mimics the AT-III-binding domain of heparin but is highly selective for Factor Xa; it still requires AT-III as a cofactor, unlike true direct Xa inhibitors.