Sickle Cell Anemia

• Hemoglobin

HbA (adult) – α2β2

HbF (fetal) – α2γ2

• Sickle Cell Hemoglobin – a single E6V mutation in the β chain

HbA (adult) – α2β2s

Normal vs Sickled Erythrocytes

DeoxyHb Fibers in Sickle Erythrocyte

Inter-molecular Contacts of HbS fibers

Defects of Sickled Erythrocytes

• More rigid and adhesive – lodged in micro-vasculatures resulting in vascular occlusion

• Microinfarction – kidney, impaired its ability to concentrate urine and produce erythropoietin

• Altered ability to activate complement and defective granulocyte function - infections

• Splenic sequestration of sickled erythrocytes results in hemolytic anemia and splenomegaly

Treatments of Sickle Cell Disease

• Gene therapy• Prevention of infections - penicillin in children• Supportive managements of vaso-occlusive

crises - pain killers, chronic heparin therapy• Hydroxyurea increases HbF levels to 15-20%,

reducing frequency of vaso-occlusive crises• Prophylactic use, not for treatments of crises• Cytotoxic, side effects include GI effects (nausea,

vomiting, diarrhea), dermatologic effect (macular papular rash, pruritus) and risk of secondary neoplasm (leukemia) with prolonged use

• Hydroxyurea + Erythropoietin therapy?

REVIEW

Anemia

Anemia

• Caused by impaired rbc production

• Hypochromic anemia• Megaloblastic anemia• Aplastic anemia• Sickle Cell anemia

Hypochromic Anemia

• Microcytic rbc• Iron deficiency• Absorption:

– duodenum and jejunuum– transported in blood by transferrin– heme iron >>> non-heme iron– ferrous salts >>> ferric salts

• Cause:– dietary insufficiency; blood loss; interference of iron absorption

• Treatments:– oral therapy: ferrous sulphate administered under fasting– parenteral therapy: iron dextran injection (im or iv)

Megaloblastic Anemia

• Macrocytic rbc

• Vitamin B12 or folate deficiency

• Interrelationship of vitamin B12 and folate metabolism

– methyltetrahydrofolate donates its methyl group to vitamin B12

– active metabolite N5,10-methylene tetrahydrofolate supports the conversion of dTMP to dUMP necessary for DNA synthesis

Vitamin B12 deficiency

• Absorption– released from food and bound to Intrinsic Factor– absorbed through the mucosa of ileum– transferred by transcobalamin II in blood– uptake by liver or target cells

• Cause:– dietary insufficiency; deficiency of Intrinsic Factor (Addisonian

pernicious anemia); damage to ileal mucosa; deficiency of transcobalamin II (rare)

• Treatments:– oral therapy to supplement deficient diet– cyanocobalamin injection (im or sc) for absorption problems

Folate deficiency

• Absorption– Reduced and methylated– absorbed through the mucosa of duodenum and jejunum– transported in blood to liver or target cells– enterohepatic cycle of folate for reabsorption

• Cause:– dietary insufficiency; malnutrition and alcoholism; damage to

small intestine

• Treatments:– oral preparations– Folic acid injection for absorption problems

Aplastic Anemia

• Caused by disturbed stem cell kinetics

• Erythropoietin– growth factor to stimulate rbc production– produced primarily by the kidney– recombinant erythropoietin for treatment of anemia in anephric

patients; administered parenterally

• Myeloid Growth Factors– GM-CSF: granulocyte/macrophage colony-stimulating factor– G-CSF: granulocyte colony-stimulating factor– Recombinant forms for treatment of neutropenia

Sickle Cell Anemia

• Cause:– E6V mutation in the Hb β chain

• Treatments:– Hydroxyurea increase the expression of HbF (α2γ2)

REVIEW

Anti-thrombotic Drugs

Hemostasis and Thrombosis

• Blood Coagulation• Platelet aggregation

• Therapy of thrombosis– Anticoagulants– Anti-platelet drugs– Plasminogen activators

• Anticoagulants and anti-platelet drugs for the prevention of the formation of thrombi

• Plasminogen activators for lysis of existing thrombi

Heparin

• Mechanism of action:– negatively charged sugar subunits– binds to lysine residues of anti-thrombin III to activate it– neutralizes thrombin and other clotting factors

• Absorption:– highly charged– administered parenterally– crosses membranes poorly; drug of choice for pregnant women

• Complications:– hemorrhage– heparin-induced thrombocytopenia

Coumarins/Warfarin (Oral anticoagulants)

• Mechanism of action:– Vitamin K cycles between the KO and KH2 forms

– Vitamin KH2 is required for the conversion of Glu to Gla in some clotting factors

– Warfarin blocks the reduction of vitamin KO to vitamin KH2

• Absorption:– Given orally– 99% is albumin free in plasma, only the free form is active– crosses the placenta; cannot be used during pregnancy

• Complications:– hemorrhage– drug interactions

Anti-platelet drugs

• Aspirin

• Dipyridamole

• ADP receptor antagonists– Clopidogrel and Ticlopidine

• GPIIb-IIIa or fibrinogen receptor antagonists– Abciximab, Eptifibatide and Tirofiban

Aspirin

• Mechanism of action:– In activated platelets, arachidonic acid is released and

metabolized by cyclooxygenase to the potent platelet agonists PGH2 and TXA2

– Aspirin acetylates cyclooxygenase, rendering it inactive

• Complications:– Gastrointestinal bleeding– It also inhibits cyclooxygenase on endothelial cells to block the

formation of PGI2, a natural platelet inhibitor

• Recommended Uses:– Low doses

Dipyridamole

• Mechanism of action:– An increase in cAMP in platelets inhibits platelet function by

sequestering calcium into its platelet storage sites– Dipyridamole inhibits cAMP phosphodiesterases and increases

platelet cAMP by preventing its breakdown

• Complications:– Non-specific and not effective

• Recommended Uses:– In combination with warfarin to prevent thromboembolism in

patients with artificial heart valves

ADP Receptor Antagonists(Clopidogel and Ticlopidine)

• Mechanism of action:– Blocks the P2Y12, an ADP receptor on platelets

• Complications:– Nausea, dyspepsia, diarrhea, hemorrhage, leukopenia, anemia

• Recommended Uses:– In combination with low dose aspirin– In aspirin-intolerant and aspirin-resistant patients

GPIIb-IIIa Antagonists(Abciximab, Eptifibatide, Tirofiban)

• Mechanism of action:– Blocks fibrinogen binding to GPIIb-IIIa thereby inhibiting platelet

aggregation

• Complications:– Oral drugs not active

• Recommended Uses:– Administered parenterally

Plasminogen Activator(t-PA, Urokinase, Streptokinase)

• Fibrinolysis:– plasminogen is converted to plasmin which degrades fibrin clots

• Tissue-type plasminogen activator (t-PA):– serine protease synthesized by endothelial cells– fibrin specific

• Urokinase:– zymogen synthesized by kidney cells– fibrin specific

• Streptokinase:– produced by β-hemolytic streptococci– complexed with plasminogen to change its conformation– NOT fibrin specific, degrades both fibrinogen and fibrin

REVIEW

Anti-atherosclerotic Drugs

Atherosclerosis

• LDL in blood penetrates into the subendothelium and becomes oxidized.

• Oxidized LDL induces transmigration of monocytes and macrophages which ingest oxidized LDL to form foam cells and fatty streaks.

• SMC proliferation and deposition of extracellular matrix materials results in atherosclerotic plaque formation.

• Affects large and medium size arteries, major cause of heart attack and stroke

• Elevated LDL and TG levels are associated with increased risk

• HDL levels are inversely related to risk

Cholesterol and Triglyceride Metabolism

• Exogenous pathway:– Chylomicrons (CM) are degraded by lipoprotein lipase– Uptake of TG by adipose tissue and muscle– Transport of cholesterol in CM remnants to liver

• Endogenous pathway:– Liver synthesize and secrete VLDL– VLDL degraded by lipoprotein lipase to form IDL and LDL– Uptake of IDL and LDL by LDL receptor-mediated endocytosis– Transport of TG and cholesterol from liver to target cells

Cholesterol and Triglyceride Metabolism

• Reverse transport of cholesterol:– As cells die, cholesterol is released and trapped in HDL– Cholesterol in HDL is esterified by LCAT and transferred to

VDLD, which eventually is metabolized to IDL and LDL

• De novo cholesterol synthesis:– Liver is the major site– HMG-CoA is the rate limiting enzyme

• Enterohepatic Circulation:– Bile salt is synthesized from cholesterol in liver– Released to intestine and recycled

Drug Therapy

• Bile salt sequestrants (colestipol, cholestyramine)– Anion exchange resins bind negatively charged bile acid– Increased cholesterol conversion to bile acid– Increased cholesterol synthesis and LDL receptor in liver– Increased LDL uptake by liver and decreased serum LDL and

cholesterol levels– Increased HDL/LDL ratio

• Niacin (nicotinic acid)– Inhibits a hormone-sensitive lipase involved in lipolysis in

adipose tissue– Decreased free fatty acid available to the liver for TG synthesis – Decreased production and release of VLDL by liver– Decreased serum levels of VLDL, LDL, and TG– Decreased HDL clearance– Increased HDL/LDL ratio

Drug Therapy

• Lovastatin (statins)– HMG-CoA reductase inhibitors – Cells express more LDL receptor– Decreased cholesterol and VLDL production and release by liver– Decreased HDL clearance– Increased HDL/LDL ratio

• Fibrates (gemfibrozil)– Stimulate lipoprotein lipase– Increased VLDL clearance – Decreased serum TG and LDL– Decreased HDL clearance– Increased HDL/LDL ratio

Drug Therapy

• Ezetimibe– Blocks cholesterol uptake by jejumal enterocytes – Reduced cholesterol incorporation into chylomicrons and

delivery to liver– Increased expression of LDL receptor in hepatocytes– Decreased serum LDL levels– Increased HDL/LDL ratio

Ezetimibe• Action: inhibits dietary cholesterol uptake by jejunal

enterocytes by binding to a key mediator of cholesterol absorption – Neimann-Pick C1-Like1 (NPC1L1).

• Results: 1) reduction of cholesterol incorporation into chylomicrons and delivery to hepatocytes; 2) increased synthesis of cholesterol and LDL receptors in hepatocytes; 3) decreased serum LDL and cholesterol levels.

• Advantages: clinically safe; effective; used as monotherapy in statin-intolerant patients; also used in combination with statins in statin-tolerant patients for further reduction of serum LDL and cholesterol.

• Disadvantages: no effect on TG absorption; a new class of anti-atherosclerotic drug – long term effect not known.