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Anticoagulant Pharmacology - Complete Guide

The Clotting Cascade (Foundation)

• Extrinsic pathway: triggered by tissue factor (TF) released after vascular damage → activates Factor VII → activates Factor X
• Intrinsic pathway: Factor XII activated by contact with collagen → XIIa → XIa → IXa → Factor X
• Common pathway: Factor Xa + Factor Va → converts prothrombin (II) → thrombin (IIa) → converts fibrinogen → fibrin clot

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PART 1: PARENTERAL ANTICOAGULANTS

1. Unfractionated Heparin (UFH)

• UFH inhibits both thrombin AND Factor Xa (requires the full heparin chain)
• A unique pentasaccharide sequence in heparin allows binding to AT-III

• Given IV (immediate effect) or subcutaneously (onset 1-2 hours)
• Half-life: ~1.5 hours
• Binds many plasma proteins → unpredictable pharmacokinetics
• Metabolized by monocyte/macrophage system; renally excreted
• Monitoring: aPTT (target 1.5-2.0x control); or anti-Xa levels

• Acute DVT/PE treatment
• ACS (NSTEMI, STEMI) - reduces death/MI by up to 60% with aspirin
• Post-op VTE prophylaxis (hip replacement, etc.)
• Anticoagulant of choice in pregnancy (does not cross placenta - too large)
• Bridging therapy during procedures

• Bleeding (most important) - reversible with protamine sulfate (1 mg per 100 units heparin)
• Heparin-Induced Thrombocytopenia (HIT) - immune-mediated platelet activation causing paradoxical thrombosis; requires stopping heparin immediately
• Osteoporosis (with long-term use)
• Hypersensitivity

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2. Low-Molecular-Weight Heparins (LMWHs)

• Half-life: 3-12 hours (longer than UFH)
• Anti-Xa peak: ~4 hours after SC injection
• Eliminated renally - dose reduce in renal impairment
• Monitor anti-Xa levels in obese, pregnant, renally impaired patients

• Treatment DVT/PE: 1 mg/kg SC twice daily (or 1.5 mg/kg once daily)
• ACS: 1 mg/kg SC bid (reduce 50% if creatinine >2 mg/dL or GFR <30)
• Prophylaxis: 40 mg SC daily

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3. Fondaparinux (Arixtra)

• Completely synthetic - no risk of HIT
• Given SC once daily
• Dose: 2.5 mg SC daily
• Eliminated renally (contraindicated if GFR <30)
• No reversal agent available (andexanet alfa may work but off-label)
• Not recommended for PCI/ACS without additional antithrombin anticoagulation (increased thrombosis risk during PCI)
• Good option in non-invasive ACS management (less bleeding than LMWH)

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4. Direct Thrombin Inhibitors (DTIs) - Parenteral

Bivalirudin (Angiomax)

• Mechanism: Directly inhibits thrombin (IIa) - both free thrombin AND clot-bound thrombin (heparin cannot reach clot-bound thrombin)
• Does NOT require AT-III
• IV infusion, monitored with aPTT (target 1.5-2.5x control if >4 hours)
• No HIT - drug of choice when HIT is present or suspected
• Dose: 0.75 mg/kg IV bolus, then 1.75 mg/kg/h during PCI
• Adjust for renal impairment (GFR <30 or dialysis)
• Associated with increased stent thrombosis vs UFH in some studies - use with caution unless high bleeding risk

Argatroban

• Direct thrombin inhibitor given IV
• Hepatically metabolized - preferred in renal failure
• Used for HIT treatment and HIT patients needing PCI

Lepirudin/Desirudin

• Recombinant hirudin derivatives (hirudin from leeches is the original direct thrombin inhibitor)
• Renally eliminated

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PART 2: ORAL ANTICOAGULANTS

5. Warfarin (Coumadin) - Vitamin K Antagonist

• Without active vitamin K, these factors are synthesized but non-functional
• Effect is delayed by 2-3 days (existing factor levels must deplete first)
• Factor VII (shortest half-life ~6 hours) falls first - PT/INR rises early
• Factor II (longest half-life ~72 hours) falls last - full anticoagulation takes several days

• Oral bioavailability nearly 100%
• Highly protein-bound (albumin)
• Metabolized by CYP2C9 (primarily) and 2C19, 3A4
• Half-life: ~36-42 hours
• Monitoring: PT/INR - target INR typically 2.0-3.0 (or 2.5-3.5 for mechanical heart valves)

• Drugs that increase INR (potentiate warfarin): amiodarone, fluconazole, metronidazole, trimethoprim, NSAIDs, fluoroquinolones
• Drugs that decrease INR (inhibit warfarin): rifampin, carbamazepine, barbiturates, St. John's Wort
• Foods high in Vitamin K (leafy greens) decrease INR

• Minor bleeding / high INR: hold warfarin, give oral Vitamin K
• Major bleeding: IV Vitamin K + 4-Factor Prothrombin Complex Concentrate (PCC) or Fresh Frozen Plasma (FFP)
• Reversal with PCC is faster than FFP

• Bleeding (most common)
• Warfarin-induced skin necrosis - occurs early in therapy when Protein C drops; treat by stopping warfarin, giving Vitamin K, switch to heparin/DOAC
• Teratogenicity - contraindicated in pregnancy (crosses placenta; causes warfarin embryopathy)
• Purple toe syndrome (rare - cholesterol microembolism)

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6. Direct Oral Anticoagulants (DOACs)

Direct Factor Xa Inhibitors ("-xabans")

Direct Thrombin Inhibitor (Oral)

• Mechanism: Oral prodrug (dabigatran etexilate) converted to dabigatran; directly inhibits thrombin (IIa) - both free and clot-bound
• Dose: 150 mg BID (AF); 110 mg BID in elderly/high bleeding risk
• Half-life: 12-17 hours
• 80% renally excreted - avoid if GFR <30 mL/min (or <15 for some indications)
• Reversal agent: Idarucizumab (Praxbind) - humanized antibody fragment that binds dabigatran with 350x higher affinity than thrombin
• Must be taken with food (capsules must not be crushed - enteric coated)
• Requires functional kidneys - stop if GFR declines

• No routine monitoring required
• Fixed dosing (weight-independent in most cases)
• Fewer drug interactions
• No dietary restrictions (no vitamin K concern)
• Faster onset and offset
• Specific reversal agents available

• More expensive
• Reduce dose / avoid in severe renal impairment
• Less data in mechanical heart valves (warfarin still preferred)
• Adherence important due to short half-life (missing doses = rapid loss of effect)

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PART 3: COMPARISON TABLE - All Anticoagulants

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PART 4: INDICATIONS BY CONDITION

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PART 5: REVERSAL AGENTS SUMMARY

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PART 6: MONITORING TESTS

• aPTT (activated Partial Thromboplastin Time): monitors the intrinsic + common pathway; used for UFH and bivalirudin
• PT/INR (Prothrombin Time / International Normalized Ratio): monitors extrinsic + common pathway; used for warfarin
• Anti-Xa levels: monitors LMWH and fondaparinux when needed (renal impairment, obesity, pregnancy)
• ACT (Activated Clotting Time): used to monitor UFH during PCI/cardiac surgery
• Thrombin time (TT): very sensitive for dabigatran; prolonged even at low levels
• Ecarin Clotting Time (ECT): specific for direct thrombin inhibitors

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KEY CLINICAL PEARLS

1. HIT: Any drop in platelets >50% after 5-10 days of heparin = suspect HIT. Stop heparin immediately. Do NOT give platelets (paradoxically worsens thrombosis). Switch to argatroban or bivalirudin.
2. Warfarin + DOAC bridging: When starting warfarin, always overlap with heparin/LMWH for ≥5 days AND until INR is therapeutic for 2 days - to prevent the transient Protein C drop hypercoagulable state.
3. Pregnancy: Only heparin/LMWH are safe (do not cross placenta). Warfarin is teratogenic (weeks 6-12). DOACs - insufficient data, avoid.
4. Renal dosing: LMWHs, fondaparinux, dabigatran, edoxaban all accumulate in renal failure. UFH, warfarin, argatroban, and apixaban (least renally cleared DOAC) are safer.
5. DOACs in AF: Major trials - RE-LY (dabigatran), ROCKET-AF (rivaroxaban), ARISTOTLE (apixaban) - all showed non-inferiority or superiority to warfarin with less intracranial bleeding.

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🩸 How Anticoagulants Work — Simple Explanation

First, understand the problem they solve:

Imagine your blood is like a river. When a pipe (blood vessel) gets damaged, your body sends clotting workers to patch the hole. That's good. But sometimes these workers go overboard and block the whole pipe — causing a dangerous clot (DVT, stroke, heart attack). Anticoagulants are like supervisors that slow down the workers.

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🔵 GROUP 1: PARENTERAL ANTICOAGULANTS (given by injection)

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🔹 A. Indirect Thrombin Inhibitors (Heparins)

Heparin's job: Wake up the sleeping guard (AT-III) and make it work 1000x faster.

UFH (Unfractionated Heparin)

• Big molecule, activates AT-III
• AT-III then kills both Thrombin AND Factor Xa
• Given IV or SC
• Think of it as: "Heparin hands AT-III a weapon and says — go kill both thrombin and Xa!"

LMWH (Low Molecular Weight Heparins)

• Smaller piece of heparin
• Also activates AT-III, BUT the molecule is too short to grab thrombin
• So AT-III only kills Factor Xa (not thrombin)
• Think of it as: "LMWH is a smaller key that only opens one lock (Factor Xa)"
• More predictable, given SC, no routine blood monitoring needed

Fondaparinux & Danaparoid

• Synthetic - not real heparin but mimics the key part
• Fondaparinux = just the 5-sugar sequence that binds AT-III
• Result: Only Factor Xa is inhibited
• Zero risk of HIT (no real heparin involved)

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🔹 B. Direct Thrombin Inhibitors

Imagine thrombin as the villain boss. Instead of waking up a guard, these drugs walk straight up to the villain and block its active site (the mouth it uses to convert fibrinogen → fibrin).

• No AT-III needed
• Works even on clot-bound thrombin (heparin can't reach this!)
• Key use: patients with HIT (allergic to heparin) - these drugs have NO heparin, so no allergy
• Given IV only

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🟢 GROUP 2: ORAL ANTICOAGULANTS

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🔹 C. Vitamin K Antagonists (Warfarin, Acenocoumarol, etc.)

Warfarin blocks VKORC1 — so Vitamin K cannot be recycled — so new clotting factors come out "dead" and non-functional.

• Works SLOWLY (2-3 days) because existing factors must naturally die off first
• Monitored with INR (target 2-3)
• Reversed with Vitamin K or Prothrombin Complex Concentrate (PCC)
• CANNOT be used in pregnancy (crosses placenta, causes birth defects)

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🔹 D. Direct Factor Xa Inhibitors ("-xabans")

Imagine Factor Xa is a key step in an assembly line. These drugs directly plug Factor Xa's active site — like sticking chewing gum in the machine.

• No AT-III needed (unlike heparins)
• Oral pills, fixed dose, no monitoring
• Stop Factor Xa → prothrombin cannot become thrombin → no fibrin → no clot
• Reversed with Andexanet alfa (a fake Factor Xa that acts as a "sponge" to absorb the drug)

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🔹 E. Oral Direct Thrombin Inhibitor

This is the oral version of the "punch thrombin directly" approach.

• Swallowed as a prodrug → gets converted to active dabigatran in the body
• Directly sits in thrombin's active site and blocks it
• No AT-III needed
• 80% excreted by kidneys - avoid in severe renal failure
• Reversed with Idarucizumab (an antibody that grabs dabigatran like a magnet)

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🟡 GROUP 3: IN VITRO ANTICOAGULANTS (used outside the body only)

The calcium story: Clotting factors need calcium ions as a "glue" between steps. These agents simply steal the calcium so the cascade cannot proceed. They only work in a tube/bag — you can't use them in a patient because removing calcium from blood in the body is dangerous.

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🧠 Super Simple Summary Table

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