Antiplatelets & Anticoagulants
A. Min Kang, MD
Medical Toxicology Fellow
Banner – University Medical
Center Phoenix
Goals
• Review hemostasis and lab testing.
• Review common antiplatelet and anticoagulant
agents.
• Discuss treatment options in overdose.
Overview
• Primary Hemostasis
▫ Platelets
• Secondary Hemostasis
▫ Coagulation Cascades
PRIMARY Hemostasis
Primary Hemostasis (Platelets)
GpIIb/IIIa –
vWF/fibrinogen –
GpIIb/IIIa
Activation: ADP receptors
Collagen – vWF - GpIb
Aspirin
• Irreversibly inhibits COX-1  inhibits TXA2
formation
Aspirin Overdose
• Platelet transfusion
• ddAVP may provide some short-term effect
Question
A 65 y/o M with CAD and recent cardiac stents
presents after reportedly overdosing on his clopidogrel
tablets.
What is the mechanism of action of clopidogrel?
a) Inhibit platelet adherence by blocking GpIb.
b) Inhibit platelet activation/aggregation by blocking
ADP.
c) Inhibit platelet aggregation by blocking GpIIb/IIIa.
d) Inhibit platelet aggregation by blocking COX.
ADP-R
P2Y12 Inhibitors
• Purine analogs – reversible antagonists
▫ Ticagrelor
▫ Cangrelor
P2Y12 Inhibitors
• Thienopyridines – irreversible antagonists
▫ All are prodrugs
▫ All associated with TTP, neutropenia, aplastic
anemia
Clopidogrel
• Prodrug
• Pharmacokinetics:
▫ Absorption: rapid, >50% oral
 May be dose-limited
▫ Metabolism:
 85% hydrolyzed to inactive clopidogrel-carboxylic acid
 Cytochromes (?2C19): 2-oxo-clopidogrel (intermediate)
 Hydrolyzed or converted to active thiol metabolite
▫ Elimination:
 t1/2 of thiol metabolite = 30 mins
P2Y12 Inhibitor Overdose
• Uncommon
▫ Platelet transfusion does not appear to work
▫ ddAVP partially reverses bleeding time in rats,
improves platelet function assays in humans
Glycoprotein IIb-IIIa Antagonists
• Blocks platelet GpIIb/IIIa receptor:
▫ Blocks binding of fibrinogen, vWF on activated
platelets
▫ Inhibits platelet aggregation
• Abciximab – chimeric (human-mouse) Fab
• Eptifibatide
• Tirofiban
SE pygmy rattlesnake
(Sistrurus miliarius
barbouri)
SECONDARY Hemostasis
Classical Coagulation
Extrinsic
Pathway
Common
Pathway
Fibrin
Clot
Intrinsic
Pathway
Question
A 35 y/o M with no PMH presents 2 hours after
ingesting a handful of a relative’s warfarin tablets
in a suicide attempt. He is asymptomatic and
coagulation labs (PT/INR & aPTT) are within
normal limits.
What is an appropriate next step?
a) Clear the patient for psychiatry.
b) Treat with Vitamin K now.
c) Treat with FFP now.
d) Admit and monitor labs for 24 hours.
Warfarin History
• In 1920s, livestock in Canadian
prairies and US plains started
dying from hemorrhage
• Moldy hay from sweet clover
(“sweet clover disease”)
• Coumarin became oxidized to
dicoumarol
• Work funded by Wisconsin Alumni
Research Foundation (WARF)
Warfarin History
• In 1948, Karl Link
promoted warfarin as a
rodenticide
• President Dwight
Eisenhower was given
warfarin in 1955
Warfarin
• Absorption:
▫ Complete GI absorption
▫ Peak concentration in ~90 mins
• Distribution:
▫ 99% protein bound
• MOA: Vitamin K antagonist
• PT is most sensitive since Factor 7 has shortest
half-life
Vitamin K Cycle
Vitamin K Cycle
Active
Inactiv
e
Vitamin K Cycle
Classical Coagulation
Extrinsic
Pathway
Common
Pathway
Factor 7
Tissue Factor
Factor 10
Factor 5
Intrinsic
Pathway
Factor 2
(Prothrombin)
Fibrin
Clot
PT vs. INR
• PT:
▫ Plasma + Ca2+ + [TF + phospholipids]
(thromboplastin)
▫ Commercial thromboplastins vary  leads to
different sensitivities to VKA
• INR:
▫ WHO proposed INR system in 1983 to standardize
testing
▫ INR = [PT / mean normal PT]ISI
▫ ISI indicates overall sensitivity of the reagent to
reductions in factors II, VII, and X
Classical Coagulation
Extrinsic
Pathway
Factor 7
Tissue Factor
PT/INR
Intrinsic
Pathway
Common
Pathway
Factor 10
Factor 5
PT/INR
Factor 2
(Prothrombin)
PT/INR
Fibrin
Clot
PT/INR
Question
A 49 y/o M presents to the ED with nausea, vomiting, and bloody
stools. He is tachycardic but has normal blood pressure. He
reports ingesting d-CON mouse poison on multiple occasions
over the past several weeks. Pertinent labs are listed:
Hgb = 9.9
Hct = 27.5
Platelet = 239
PT and PTT unmeasurable
Which of the following can correct the coagulopathy?
a) Platelet transfusion.
b) ddAVP.
c) Vitamin K.
d) Cryoprecipitate.
Anticoagulant Rodenticides
• 1st Generation:
▫ Warfarin
▫ Chlorophacinone
▫ Diphacinone
• 2nd Generation:
▫
▫
▫
▫
Brodifacoum
Difenacoum
Bromadiolone
Difethialone
EPA & Rodenticides
• In 2008, EPA tightened safety standards on
household rodenticide products
▫
▫
▫
▫
Bait stations
Block or paste poison must be inside the station
< 1 pound of poison
Cannot sell second-generation agents to
consumers
• Production of many d-CON products ceased
12/31/2014
• Distribution to retailers ended 03/31/2015
VKA Reversal
• Vitamin K1 (phytonadione) PO or IV
• FFP
▫ Time to cross-match and thaw
▫ Large volume
▫ Infection risk; TRALI (1 in 5,000)
• Prothrombin Complex Concentrates:
▫ No cross-match; Small volume; short infusion time
▫ 3-Factor Prothrombin Complex Concentrate (Bebulin; Profilnine):
 Factors 2, 9, 10
▫ 4-Factor Prothrombin Complex Concentrate (Kcentra):
 Adds Factor 7
▫ Most PCCs have small amount of heparin and AT
 Contraindicated in HIT
• rh7a
• ACCP recommends 4-factor PCC over FFP (Grade 2C)
4-Factor PCC (Kcentra)
• FDA approved for VKA-induced major bleeding
or need for urgent surgery/invasive procedure
• ~500 or ~1,000 unit vials
• Contains:
▫ Factors 2, 7, 9, 10
▫ Protein C, S
▫ Heparin, AT3
VKA Reversal
• Factor Eight Inhibitor Bypassing Activity
(FEIBA):
▫ AKA Anti-Inhibitor Coagulant Complex
▫ Indications: Prophylaxis and bleeding in
Hemophilia A/B with inhibitors
▫ Pooled plasma
 Factors 2, 9, 10 (non-activated)
 Factor 7 (activated)
 Factor 8
Question
A 50 y/o M presented to the ED after reportedly
injecting himself subcutaneously with 1,500 mg (17
mg/kg) of enoxaparin 3 hours prior.
Which of the following agents may be helpful if the
patient begins to have major bleeding?
a) Vitamin K.
b) Platelet transfusion.
c) Cryoprecipitate.
d) Protamine.
Heparins
• Bind antithrombin-3
• Heparin:
▫ Inhibits Factor 10a (depends on AT3)
▫ Inhibits Factor 2a
• LMWH: Inhibits Factor 10a > Factor 2a
▫ Enoxaparin, dalteparin, tinzaparin
Coagulation Tests
• aPTT:
▫ Plasma + Ca2+ + [phospholipids] (partial
thromboplastin)
▫ Tests Intrinsic Pathway
• Thrombin Time:
▫ Platelet-poor plasma + thrombin
Anti-Factor 10a Assay
• Sample should be drawn at peak plasma
concentration (~4 hrs post-dose for LMWH)
• Test Components:
▫ Synthetic Factor 10a substrate with chromophore
▫ Patient’s Factor 10a can cleave the chromophore
(if not inhibited by an anticoagulant)
▫ Less color change means more anticoagulant
Anti-Factor 10a Activity
Bates & Weitz (2005). Circulation 112: e53-60.
• Compare to a
standard specific
for the
anticoagulant
• Results reported as
drug concentration
(units/mL)
Classical Coagulation
Extrinsic
Pathway
Factor 7
Tissue Factor
PT/INR
Intrinsic
Pathway
Common
Pathway
Factors 9, 11, 12
Factor 8
aPTT
Factor 10
Factor 5
PT/INR
aPTT
Anti-Factor 10a
Factor 2
(Prothrombin)
PT/INR
aPTT
Fibrin
Clot
PT/INR
aPTT
TT
Protamine
• Derived from fish sperm.
• Fully reverses heparin:
▫ 1 mg protamine for each 100 units heparin
• Neutralizes anti-2a activity of LMWH:
▫ Will reverse aPTT and TT
▫ Only 60% of anti-Factor 10a activity is reversed
• Increased risk for anaphylaxis:
▫ Previous use of protamine-sulfate insulin
▫ Vasectomy
▫ Fish sensitivity
Question
A 36 y/o M presents after a reported ingestion of a
handful of rivaroxaban ~1.5 hr prior to arrival. (He
does not take it chronically.)
Which lab test would most likely help determine if
he is telling the truth?
a)
b)
c)
d)
PT
aPTT
ECT
TT
Direct Factor 10a Inhibitors
• Inhibit bound and free Factor Xa
• Does not depend on AT3
Classical Coagulation:
Factor 10a Inhibitor
Extrinsic
Pathway
Common
Pathway
Factor 7
Tissue Factor
X
Factor 10
Factor 5
Intrinsic
Pathway
Factors 12, 11, 9
Factor 8
Factor 2
(Prothrombin)
Fibrin
Clot
Indications
• Nonvalvular atrial fibrillation
• Deep vein thrombosis (DVT) & pulmonary
embolism (PE) treatment
• DVT prophylaxis after hip/knee replacement
(rivaroxaban, apixaban)
Rivaroxaban and PT
Arachchillage et al (2014). Thromb Haemost 112: 421-3.
Apixaban and PT/aPTT
PT
aPTT
Dale et al (2014). J Thromb Haemost 12: 1810-5.
Testing for 10a Inhibitors
Medication PT
aPTT
Anti-Factor 10a
Assay
rivaroxaban
Less sensitive than PT
Increased (need
standard for
calibration)
apixaban
edoxaban
Sensitivity depends
on thromboplastin
reagent
ECT & TT are not affected.
Minimal effect
Less sensitive than PT
Question
A 65 y/o M taking rivaroxaban for atrial fibrillation
presents with a lower GI bleed.
Which of the following can reverse the
coagulopathy of rivaroxaban?
a)
b)
c)
d)
Vitamin K.
Platelet transfusion.
Cryoprecipitate.
4-factor PCC.
Factor Xa Inhibitor Overdose
•
•
•
•
•
•
•
Activated charcoal
HD not useful
4-factor PCC (Eerenberg et al, 2011)
FEIBA
rhF7a
3-factor PCC
Andexanet alfa
▫ Inactive Factor 10a mimic
▫ Andexanet Video
Question
A 35 y/o F overdosed on her bottle of dabigatran.
She is brought to the hospital with active GI
bleeding and is noted to be hypotensive and
tachycardic. Which of the following would have the
highest potential of reversing this coagulopathy?
a) Vitamin K
b) Platelets
c) FFP
d) FEIBA
Thrombin & Direct Thrombin Inhibitors
(DTIs)
• Thrombin
▫ Converts fibrinogen  fibrin
▫ Activates platelets
Dabigatran
(Pradaxa)
• Pharmacokinetics:
▫ Absorption: 3-7% PO bioavailability
▫ Distribution: 35% protein binding
▫ Metabolism:
 Prodrug quickly & completely hydrolyzed by serum
esterases to active dabigatran
 20% conjugated to active glucuronides
▫ Elimination:
 80% renal
 t1/2 = 12-17 hrs; prolonged with renal insufficiency
• Testing:
▫ TT most sensitive, followed by ECT and aPTT
ECT (Ecarin Clotting Time)
• Ecarin is metalloprotease from saw-scaled viper
(Echis carinatus) venom
• Only affected by DTIs
▫ Not affected by heparin, warfarin, LA
Favaloro et al (2011). Pathology 43: 682-92.
Dabigatran Lab Tests
PT
aPTT
Anti-10a TT
Sensitivity Prolonged but
No effect
depends on non-linear and can
assay
underestimate
ECT
Most sensitive Sensitive but no
standard for
calibration
- HEMOCLOT Thrombin Inhibitor
- Modified TT assay
- Not affected by low fibrinogen levels (sample is mixed with
normal plasma)
- Standards for hirudin, argatroban, dabigatran
Dabigatran Overdose
• In vitro: Activated charcoal binds
• Consider HD (~2/3 removed) but rebound
documented
• FEIBA
▫ Reversed some coagulation tests in vitro
▫ Decreased bleeding time in rat tail model
• 4-factor PCC
▫ Did not reverse coagulation tests in vivo (Eerenberg et
al 2011)
▫ Decreased bleeding in rabbit model
• No evidence for desmopressin, tranexamic acid,
aminocaproic acid
Dabigatran Overdose: PER977
• Idarucizumab
• PER977:
▫ Binds heparin, LMWH, direct factor 10a
inhibitors, dabigatran (DTI)
Summary: Classical Coagulation
Extrinsic
Pathway
Factor 7
Tissue Factor
PT/INR
Intrinsic
Pathway
Common
Pathway
Factors 9, 11, 12
Factor 8
aPTT
Factor 10
Factor 5
PT/INR
aPTT
Anti-Factor 10a
Factor 2
(Prothrombin)
PT/INR
aPTT
ECT
Fibrinogen
&
Fibrin Clot
PT/INR
aPTT
ECT
TT
Summary: Testing
Class
Prolonged
Normal
Overdose
Treatment
VKA
PT/INR, aPTT
ECT, TT
Vitamin K, PCC-4,
FFP
Heparins
aPTT; Anti-Factor
Xa; TT
PT may change with
extreme UFH doses
ECT
Protamine
(does not fully
reverse anti-10a
activity of LMWH)
Direct Factor Xa
Inhibitors
PT; Anti-Factor Xa
ECT; TT
aPTT may change but
less sensitive
Charcoal
PCC-4
Andexanet
Direct Thrombin
Inhibitors
TT > ECT > aPTT
PT may change
depending on assay
Charcoal
HD
FEIBA
Idarucizumab
Anti-Factor
Xa
If you are in Arizona and have an out-ofstate cell phone, dial (602) 253-3334 to
reach the poison center.
References
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Ageno et al (2012). Oral anticoagulant therapy: Antithrombotic Therapy and Prevention of Thrombosis, 9th ed:
American College of Chest Physicians Evidence-Based Clinical Practice Guidelines. Chest 141: e44S-88S.
Ansell et al (2014). Use of PER977 to reverse the anticoagulant effect of edoxaban. N Engl J Med 371: 2141-2.
Arachchillage et al (2014). Comparative sensitivity of commonly used thromboplastins to ex vivo therapeutic
rivaroxaban levels. Thromb haemost 112: 421-3.
Cuisset et al (2010). Clopidogrel resistance: what's new? Arch Cardiovasc Dis 103: 349-53.
Eerenberg et al (2011). Reversal of rivaroxaban and dabigatran by prothrombin complex concentrate: a
randomized, placebo-controlled, crossover study in healthy subjects. Circulation 124: 1573-9.
Garcia et al (2012). Parenteral anticoagulants: Antithrombotic Therapy and Prevention of Thrombosis, 9th ed:
American College of Chest Physicians Evidence-Based Clinical Practice Guidelines. Chest 141: e24S-43S
Gehrie & Laposata (2012). Test of the month: The chromogenic antifactor Xa assay. Am J Hematol 87: 194-6.
Godier et al (2015). Inefficacy of platelet transfusion to reverse ticagrelor. N Engl J Med 372: 196-7.
Hankey & Eikelboom (2011). Dabigatran etexilate: a new oral thrombin inhibitor. Circulation 123: 1436-50.
Holbrook et al (2012). Evidence-based management of anticoagulant therapy: Antithrombotic Therapy and
Prevention of Thrombosis, 9th ed: American College of Chest Physicians Evidence-Based Clinical Practice
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Lindhoff-Last et al (2010). Assays for measuring rivaroxaban: their suitability and limitations. Ther Drug Monit
32: 673-9.
Morishima & Kamisato (2015). Laboratory measurements of the oral direct factor Xa inhibitor edoxaban:
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Pollack et al (2015). Idarucizumab for Dabigatran Reversal. N Engl J Med.
Taylor et al (2013). Is platelet transfusion efficient to restore platelet reactivity in patients who are responders to
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