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Pharmacology

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Classification of antithrombotic drugs.
 
Antiplatelet drugs
  1. Aspirin
     
    Inhibiting platelet cyclooxygenase (COX)-1
  2. Thienopyridines
     
    The thienopyridines include ticlopidine and clopidogrel, drugs that target P2Y12, a key ADP receptor on platelets.
Mechanism of Action
 
By inhibiting phosphodiesterase
 
Dipyridamole blocks the breakdown of cAMP. Increased levels of cAMP reduce intracellular calcium and inhibit platelet activation. 
  1. GPIIb/IIIa Receptor Antagonists
     
    Abciximab, eptifibatide, and tirofiban

Extra Edge: (Ref. Hari. 18th ed., Pg- 992)

 

New Antiplatelet Agents:

  1. Direct-acting reversible P2Y12 antagonists
    1. Cangrelor
    2. Ticagrelor
  2. Orally active inhibitors of protease-activated receptor 1 (PAR-1), the major thrombin receptor on platelets.
    1. Vorapaxar
    2. Atopaxar

 

Site of action of antiplatelet drugs. 
 
Anticoagulants
  1. Heparin
     
    Activating antithrombin (previously known as antithrombin III)
Side Effects
  1. Bleeding                      
  2. Thrombocytopenia:                      
  3. Osteoporosis
  4. Elevated Levels of Transaminases: Without a concomitant increase in the level of bilirubin.

Extra Edge: (Ref. Hari. 18th ed., Pg- 967)

 

Heparin-Induced Thrombocytopenia (HIT)

 

HIT is not associated with bleeding and, in fact, markedly increases the risk of thrombosis. HIT results from antibody formation to a complex of the platelet-specific protein platelet factor 4 (PF4) and heparin. The antiheparin/PF4 antibody can activate platelets through the FcRIIa receptor and also activate monocytes and endothelial cells.

 

HIT can occur after exposure to low-molecular-weight heparin (LMWH) as well as unfractionated heparin (UFH), although it is about 10 times more common with the latter.

Drugs Associated with an Increased Risk of Generalized Osteoporosis in Adults
 
Glucocorticoids
Cyclosporine
Cytotoxic drugs
agonists
Anticonvulsants
Excessive alcohol
Aromatase inhibitors  
Excessive thyroxine
Aluminum
Gonadotropin-releasing hormone
Heparin
Lithium
  1. Low-Molecular-Weight Heparin
     
    Mechanism of action: Factor Xa inhibition, Thrombin inhibition
     
    Example: Enoxaparin, Tinzaparin, Dalteparin
  2. Pentasaccharide
     
    Mechanism of action: Factor Xa inhibition
     
    Example: Fondaparinux, Idraparinux, Rivaroxaban, Apixaban
  3. Direct thrombin inhibitors
     
    Mechanism of action: Thrombin inhibition
     
    Example: Lepirudin, Argatroban, Bivalirudin, Ximelagatran, dabigatran (oral drug).  
Newer factor Xa inhibitor:
  1. Rivaroxaban        
  2. Apixaban Both are given orally.
Extra Edge:
Dabigatran, an oral thrombin inhibitor, R
ivaroxaban & Apixaban both are oral Factor Xa inhibitor, are used for short-term thrombo prophylaxis after elective hip or knee replacement surgery.
 
Dabigatran is used as an alternative to warfarin for stroke prevention in patients with atrial fibrillation. (Harrison 18th, edition, page-992)
 
For most indications, warfarin is administered in doses that produce a target INR of 2.0 – 3.0 LQ 2012). An exception is patients with mechanical heart valves, where a target INR of 2.5 – 3.5 is recommended.
 
(Ref. Hari. 18th ed., Pg- 992)
 
New Antiplatelet Agents:
  1. Direct-acting reversible P2Y12 antagonists
    1. Cangrelor
    2. Ticagrelor
  2. Orally active inhibitors of protease-activated receptor 1 (PAR-1), the major thrombin receptor on platelets.
    1. Vorapaxar
    2. Atopaxar


Dabigatran 

  1. It is an oral anticoagulant from the class of the direct thrombin inhibitors.
  2. It is an alternative to warfarin as the preferred orally administered anticoagulant since it does not require frequent blood tests for international normalized ratio (INR) monitoring while offering similar results in terms of efficacy.
  3. There is no specific way to reverse the anticoagulant effect of dabigatran in the event of a major bleeding event, unlike warfarin. 
Laboratory findings in various platelet and coagulation disorders
Condition PT PTT BT Platelet count
Vitamin K deficiency or warfarin Prolonged Normal or mildly prolonged Unaffected Unaffected
DIC Prolonged Prolonged Prolonged Decreased
Von Willebrand disease Unaffected Prolonged Prolonged Decreased and/or Rejected
Hemophilia Unaffected Prolonged Unaffected Unaffected
Aspirin Unaffected Unaffected Prolonged Unaffected
Thrombocytopenia Unaffected Unaffected Prolonged Decreased
Liver failure, early Prolonged Unaffected Unaffected Unaffected
Liver failure, end-stage Prolonged Prolonged Prolonged Decreased
Uremia Unaffected Unaffected Prolonged Unaffected
Congenital afibrinogenemia Prolonged Prolonged Prolonged Unaffected
Factor V deficiency Prolonged Prolonged Unaffected Unaffected
Factor X deficiency as seen in amyloid purpura Prolonged Prolonged Unaffected Unaffected
Glanzmann's thrombasthenia Unaffected Unaffected Prolonged Unaffected
Bernard-Soulier syndrome Unaffected Unaffected Prolonged Decreased or unaffected
Factor XII deficiency (LQ 2012) Unaffected Prolonged Unaffected Unaffected




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