Pharmacology 301.6Module 6

DRUGS & BLOOD Anticoagulants, anti-platelet &

fibrinolytics

Treatment of anemia

cancer

heart disease

other

lung

stroke injuries liver

Causes of death in Canada

1997: 661 deaths/100,000http://www.statcan.ca/english/Pgdb/health30b.htm

Heart disease and stroke = 1 of 3 deaths, due to clotting

Blood fluidity The endothelial lining is non-thrombogenic Balance between procoagulants (thromboxane,

thrombin, activated platelets, platelet factor 4) and anticoagulants (heparan sulfate, prostacyclin, nitric oxide, antithrombin)

1. heparin & derivatives – stimulate natural inhibitors of coagulant proteases (antithrombin)

2. coumarin anticoagulants – block multiple steps in the coagulation cascade

3. fibrinolytic agents – lyse pathological thrombi

4. antiplatelet agents – aspirin

The Hemostatic System

vasospasm platelets (5HT, TXA2)

platelet plug adhesion, activation, aggregation

Accidental injury vs. pathological injuryhypercholesterolemia, diabets,

hypertension

Coagulation cascade – platelet activation and coagulation

fibrin plug extrinsic, intrinsic (humoral)

Recanalization fibrinolysis

Platelet function

disruption of endothelium

platelet adhesion

platelet activation

platelet release

platelet aggregation

agonist binding

• thrombin

• serotonin

• ADP

• TXA2

Platelet adhesion and aggregation Platelet activation

Antiplatelet drugs

clotting

Increased cAMP

Prevents clotting

ADP

P2Y receptor

Lowers cAMP

Clopidogrelticlopidine

inhibitstimulates

TXA2 recep

Arachidonic acidAspirin

Thromboxane(from activated platelets)

GpIIb-IIIaReceptor for fibrinogen and platelet adhesion

Ca2+

Dipyridamole(prevents

breakdown by phosphodiesterase)

clotting

EptifibatideAbciximabTirofiban

Aspirin efficacy

Aspirin reduces clots by 15%, on average. 2% have a bleed, that is serious each year. Use in high risk clotters.

How does it work?Aspirin irreversibly inhibits platelet COX enzymePlatelets cannot synthesize new COX (no nucleus)No thromboxane (procoagulant, vasoconstrictor) synthesisLow dose aspirin (80-160 mg) does not inhibit endothelial COXProstacyclin (anticoagulant, vasodilator) formation not affected

Antiplatelet drugsTiclopidine (TICLID)- is a prodrug

Blocks platelet ADP receptor and prevents activation and aggregation Is often used in combination with aspirin (synergistic action), for angioplasty and stenting surgery To prevent secondary strokes and in unstable angina Severe neutropenia – 1% of patients

Clopidogrel (PLAVIX) Similar to ticlopidine and used same way Less incidence of neutropenia or thrombocytopenia Used in combination with aspirin

Factor IIa

Blood coagulation cascade

See the figure in textbook -Brenner’s

Activated partial thromboplastin time (aPTT) & prothrombin time

(PT) Blood clots in 4-8 min in a glass tube Chelation of ca2+ prevents clotting Recalcified plasma clots in 2-4 min Addition of negatively charged phospholipids and

kaolin (aluminium silicate) shortens clotting time to 26-33 sec – aPTT

Addition of ‘thromboplastin’ (a saline extract of brain – tissue factor and phospholipids) shortens clotting time to 12-14 sec – prothrombin time (PT)

Anticoagulants - Heparin Heparin is a glycoasminoglycan – alternating

glucuronic acid and N-acetyl-D-glucosamine residues – sulfate and acetyl groups.

Avg mol. wt - 12,000 daltons

Heparin is negatively charged Heparin HEPALEANHeparin HEPALEAN

Heparin – Source and function

Heparin - originally isolated from the liver Found in mast cells -storage of histamine

& proteases Rapidly destroyed by macrophages Normally not detected in the blood Heparan sulfate - similar to heparin but less

polymerized - contains fewer sulfate groups Found on the surface of endothelial cells and in

the extracellular matrix Interacts with circulating antithrombin to

provide a natural antithrombotic mechanism

Heparin & LMW Heparins difference in action

Heparin~ 45 saccaharide unitsMW ~ 13,500This reaction goes 1000 to 3000 times faster with heparin.

Low Mol. Wt. Heparin~ 15 saccaharide unitsMW ~ 4,500

circulates in the plasma - rapidly inhibits thrombin only in the presence of heparin

Antithrombin inhibits thrombin, Xa, IXa and to a lesser extent VIIa

Heparin – Toxicity - Hemorrhage

Hemorrhage – recent surgery, trauma, peptic ulcer disease, platelet dysfunction

Life-threatening bleeding can be reversed by protamine sulfate - 1 mg of protamine sulfate for every 100 U of heparin - slow iv infusion – 50 mg over 10 min)

Protamine sulfate interacts with platelets, fibrinogen, and other clotting factors - an anticoagulant effect – at higher doses

Anaphylactic reactions to protamine (a basic protein isolated from Salmon sperm)

Heparin-induced Thrombocytopenia

50% decrease in platelet count - <150,000/μl) Antibodies against complexes of heparin with

platelet factor 4 In 3-5% of patients 5 to 10 days after initiation of

heparin therapy Lower incidence with low mol wt heparin In 1/3 of pts is preceded by thrombosis Can be life-threatening Stop heparin immediately Alternative anticoagulants – lepirudin or

danaparoid

Low Molecular Weight Heparins Avg mol. wt 4,500 daltons - 15 monosaccharide units

Better absorbed - higher bioavailability

Longer biological half-life

More predictable dose-response - does not bind to plasma proteins, macrophages, or endothelial cells

Can be given s.c. without lab monitoring in an outpatient setting

Cleared unchanged by kidney (do not use in renal failure!) rather than by the reticuloendothelial system

Lower risks of thrombocytopenia and bleeding

Safety and use during pregnancy not evaluated

LMW heparins

Dalteparin (FRAGMIN) Enoxaparin (LOVENOX)

Uses:1. prevention of venous thromboembolism2. Treatment of venous thrombosis,

pulmonary embolism and unstable angina

3. prophylaxis following total knee arthroplasty

Other parenteral anticoagulantsDanaparoid (ORGARAN)

nonheparin glycosaminoglycans (84% heparan sulfate) Promotes inhibition of Xa by antithrombin Prophylaxis of deep vein thrombosis In patients with heparin-induced thrombocytopenia

Lepirudin (REFLUDAN) recombinant derivative of hirudin (a direct thrombin

inhibitor in leech) In patients with heparin-induced thrombocytopenia

Act

ion

of

Cou

mari

ns

Coumarins act here

Vitamin K

Coumarins are competitive inhibitors

Oral anticoagulants – 4-hydroxycoumarins

Gamma glutamic acid residues of clotting factors must be carboxylated for enzyme activity

Vit.K epoxide reductase

factors II, VII, IX, X, Prots C and S

Coumarins (warfarin)

• inhibits vitamin K reduction

• efficacy measured by INR (International Normalized Ratio), the patient’s PT divided by the PT in pooled plasma

• takes 4-5 days to become effective – active carboxylated factors in plasma need to be cleared

• small Vd, steep D-R curve, metabolized by CYP1A and CYP2C9 (interactions)

• Warfarin crosses placenta – is teratogenic – birth defects and abortion

• major indications: DVT, PE and atrial fibrillation

Warfarin COUMADINWarfarin COUMADIN

Warfarin – drug & other interactions

Any substance or condition is dangerous if it alters:

1. the uptake or metabolism of oral anticoagulant or vitamin K

2. the synthesis function or clearance of any factor or cell involved in hemostasis or fibrinolysis

3. the integrity of any epithelial surface

Warfarin - Clinical uses Prevent acute deep vein thrombosis or pulmonary embolism Prevent venous throboembolism in patients undergoing orthopedic or gynecological surgery Prevent systemic embolization in patients with myocardial infarction, prosthetic heart valves or chronic atrial fibrillation

Warfarin - AntidoteVitamin K (oral or parenteral)

INR = (PTpt / PTref)ISI Target 2.0 to 3.0

Fibrinolytic process

t-PA has to bind here – localized ation

Streptokinase binds here – generalized action

Efficacy of thromobolytics

1.8% have serious bleeding;

0.7% have IC haemorrhage

Tissue plasminogen activator (t-PA) – (alteplase, ACTIVASE)

activates fibrin bound plasminogen (less systemic plasmin formation)

More expensive than streptokinase

Streptokinase (STREPTASE)

Binds plasminogen- coverts to plasmin Dissolve clots after myocardial infarction, deep vein thrombosis, massive pulmonary emboli Side effects: Bleeding, allergic reactions, hypotension, fever.

Summary

• we have lots of drugs that affect hemostasis

• they can inhibit platelet function, fibrin formation, or fibrinolysis.

• using combinations prevents more clots, but causes more bleeding.

• look at the risk/benefit ratio.

Anemia a reduction in the hemoglobin, hematocrit ( % of whole blood that is comprised of red blood cells) or red cell number Erythropoiesis - Pluripotent stem cells differentiate under the influence of growth factors (erythropoietin) to form erythrocytes controlled by a feedback system in the kidney - responds to changes in oxygen delivery - secretes erythropoietin (a glycoprotein) from peritubular interstitial cells - stimulates the marrow cells Feedback - disrupted by kidney disease, marrow damage or a deficiency in iron or an essential vitamin.

Anemia Iron deficiency is the most common cause

of anemia

Results in microcytic hypochromic anemia

Iron deficiency also affects iron-dependent enzymes such as cytochromes, catalase, peroxidase, xanthine oxidase and mitochondrial enzyme α-glycerophosphate oxidase

Iron deficiency has also been associated with learning problems in children

Iron in the body

mg/kg of body weightMale Female

Hemoglobin

31 28

Myoglobin and enzymes

6 5

Storage iron

13 4

Total 50 37

Ess

en

tial

iro

n

Treatment of Iron Deficiency

The ability of the patient to tolerate and absorb medicinal iron is important

Gastrointestinal tolerance to oral iron is limited Mainly absorbed only in the upper small

intestinal (delayed-release preparations ?)

Parenteral iron Iron dextran injection (INFED, DEXFERRUM)

Acute hypersensitivity, including anaphylactic reactions, can occur in from 0.2% to 3% of patients.

Iv is preferred – more reliable response Im route – more local side effects – skin

discoloration, long-term discomfort, concern about malignant change at injection site

Megaloblastic (macrocytic) anemias

Due to lack of folic acid or vitamin B12

Deficiency more common in older adults

Folate – food fortification – masks cobalamin deficiency (neurologic damage)

In pregnancy - prevention of folate deficiency and permanent neural tube defects in children minimized

Folate and Vitamin B12 Interaction

Tetrahydrofolate is necessary for DNA synthesis Cobalamin and folate are cofactors for tetrahydrofolate production Deficiency of either impairs cell division in the bone marrow while RNA and protein synthesis continues – enlarged erythrocytes Cobalamin deficiency – impairs synthesis of S-adenosylmethionine – necessary for proper nervous system functioning

Pernicious anemia

Lack of intrinsic factor – Vit. B12 not absorbed

Injury to parietal cells or autoantibodies Vitamin B12 - must be administered– is not

synthesized in body

Treating deficiencies Distinguishing B12 deficiency from folic acid

deficiency Folic acid will supply folate needed for DNA

synthesis Anemia corrected It DOES NOT correct the lack of

methionine and succinyl Co-A synthesis – this will cause neurological deficits

Folic acid therapy Rule out underlying cobalamin deficiencyFolinic acid (leucovorin calcium, citrovorum

factor) – 5-formyl derivative of tetrahydrofolic acid

To circumvent the inhibition of dihydrofolate reductase as a part of high-dose methotrexate therapy

To counteract the toxicity of folate antagonists such as pyrimethamine or trimethoprim

More expensive