Hydroxyurea Use in Sickle Cell Disease
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Hemoglobinopathies Matthew M. Heeney, MD Associate Chief – Hematology Director – Sickle Cell Program Boston Children’s Hospital THE FAIRMONT COPLEY PLAZA HOTEL BOSTON, MA. SEPTEMBER 23-28, 2018 Faculty Disclosure Matthew M. Heeney, MD Personal financial interests in commercial entities that are relevant to my presentation(s) or other faculty roles: • Astra Zeneca Consultant, Clinical Trial funding • Pfizer Clinical Trial funding • Micelle Biopharma Consultant, Clinical Trial funding • Novartis Consultant Objective • Review the basic genetics of globins • Review – Qualitative disorders of hemoglobin • Sickle Cell disease • Other Hemoglobinopathies – Quantitative disorders of hemoglobin • Thalassemias 3 Hemoglobin • Four globular proteins (globins) – 2 α-like globins – 2 β-like globins • Four heme groups – One per globin chain – Reversibly bind O2 (CO2, NO) • Hb synthesis must be balanced and coordinated • All components are labile and toxic – globins, heme, iron 4 Globin Genes Nathan and Oski's Hematology of Infancy and Childhood, 7th Ed. 5 Globin Protein Synthesis Nathan and Oski's Hematology of Infancy and Childhood, 7th Ed. 6 Hemoglobin tetramers • Hemoglobins are distinguished by globin composition: % nl adult HbA α2β2 97-98% HbA2 α2δ2 2-3% HbF α2γ2 ≤2% S HbS: α2β 2 0% C HbC: α2β 2 0% HbH: β4 0% Hb Barts: γ4 0% 7 Global distribution of hemoglobin disorders • Distribution of thalassemia & sickle cell disease mirror worldwide distribution of malaria prior to 20th century. • Hypothesis (Haldane and others): heterozygous forms confer fitness - Thalassemia trait, sickle trait, G6PD etc.. protective against death from cerebral falciparum malaria. “Benefit” of trait WHO - Control of Hereditary diseases. 1996 outweighs homozygous risk. 8 Disorders of Hemoglobin Qualitative disorders – Structural variants or hemoglobinopathies – Production of abnormal globin chains Quantitative disorders – Thalassemias – Decreased (imbalanced) production of normal globin chains 9 Qualitative Disorders of Hemoglobin SICKLE CELL DISEASE 10 1910 First description of sickle cell anemia in a West Indian dental student with “peculiar elongated and sickle-shaped” red blood cells. James B. Herrick Presbyterian Hospital Chicago, Illinois Herrick JB. Peculiar elongated and sickle-shaped red blood corpuscles in a case of severe anemia. Archives of Internal Medicine 1910. 6(5): 517-211 11 1949 Established sickle cell anemia as a genetic disease in which affected individuals have a different forms of Linus Pauling hemoglobin in their California Institute of Technology blood Pasadena, CA Pauling L et al. Sickle Cell, A Molecular Disease. Science. 110 (2865): 543–548. 12 1956 Vernon Ingram and J.A. Hunt working at MIT, discovers the single amino acid change that causes hemoglobin to sickle Sickle cell anemia became the 1st genetic Vernon Ingram disorder whose molecular basis was Cavendish Laboratory, University of Cambridge known. Ingram VM. A Specific Chemical Difference between Globins of Normal and Sickle-cell Anemia Hemoglobins" Nature. 178 (4537): 792–794 13 Epidemiology Most common single gene disorder in African Americans 1/375 homozygous affected 1/12 are heterozygous carriers (~8%) Also affects other ethnicities: India Middle East Hispanic U.S. Prevalence: 80,000 – 120,000 U.S. Incidence: ~2000 live births annually 14 Sickle cell and malaria Distribution of endemic malaria Distribution of sickle cell allele Muntuwandi at en.wikipedia 15 Molecular Pathophysiology - Polymerization The sickle mutation is a single amino acid substitution at position 6 of β globin. Results in a hydrophobic region that is exposed in the deoxygenated state. Adapted from Rotter MA et al. Biophys J. 2005 Oct;89(4):2677-84.. 16 Bunn HF. NEJM. 1997 Sep 11;337(11):762-9 17 Delay time • Delay time: period during which Hb is deoxygenated, but not yet polymerized • If passage through the capillaries exceeds the delay time, Hb will aggregate and initiate sickling. 18 Cellular Pathophysiology Polymerization leads to: • Distortion of cell shape • Damage to RBC membrane • Abnormal permeability • Irreversible sickling Premature hemolysis = Anemia Impairment of RBC flow = Infarction 19 Hemolysis and Nitric Oxide depletion • ↓ NO • Endothelial dysfunction 20 Dichotomization of Pathophysiology? Adapted from Kato GJ et al. Blood Rev. 2007 Jan; 21(1): 37–47. 21 Manwani D and Frenette PS Blood. 2013:122(24):3892-3898 22 Vaso-occlusion Intra-cellular dehydration Endothelial Activation Hemolysis Inflammation Reperfusion Injury Nitric Oxide consumption 23 23 Sickle cell genotypes Genotype Characteristics Hb SS Anemia : Hb 7 - 9 g/dL “Sickle cell anemia” Smear: Irreversibly sickled cells Electrophoresis: Hb S, Hb F Hb SC Anemia: Hb 9 - 11 g/dL Smear: Sickle and target cells Electrophoresis: Hb S and Hb C Hb S/β0 thalassemia Anemia: Hb 7 - 9 g/dL Smear: Microcytosis with sickle and target cells Electrophoresis: Hb S, Hb F Hb S/β+ thalassemia Anemia: Hg > 10 g/dL Smear: Microcytosis with sickle and target cells Electrophoresis: Hb S, Hb F, and small %Hb A 24 Primary pathological processes in SCD Vaso-occlusion Chronic Hemolysis • Pain episodes / “crises” • Cholelithiasis • Acute chest syndrome • Folate deficiency • Avascular necrosis • Cardiomegaly • Splenic sequestration • High-output heart failure (mostly in children) • Liver disease from iron overload (with repeated • Renal insufficiency transfusions) • Proliferative retinopathy • Non-hemorrhagic stroke (HbSC > HbSS) • Pulmonary hypertension • Excess fetal loss • Leg ulcers • Priapism 25 Infection in SCD • Functional asplenia Sepsis from encapsulated organisms AA SS • Osteomyelitis Salmonella and Staphylococcus aureus • Aplastic crisis (Parvovirus B19) • Hepatitis B and C (from transfusion) 26 Acute chest syndrome (ACS) • Clinical syndrome of – Fever, – Respiratory symptoms (e.g. hypoxia, tachypnea) – New pulmonary infiltrate 27 Acute chest syndrome (ACS) • 2nd most common cause of hospitalization. • Most common cause of death in sickle cell anemia. • Increased mortality with poorly controlled asthma • Prevent atelectasis – Control chest pain, incentive spirometry • O2 and trial of bronchodilators • Often associated with atypical organisms (e.g. chlamydia, mycoplasma). – Empiric broad-spectrum antibiotics + macrolide • Simple or exchange transfusion may be life-saving 28 Stroke Ohene-Frempong et al Blood. 1998; 91(1):288-94. 29 Stroke Ohene-Frempong et al Blood. 1998; 91(1):288-94. 30 Stroke Risk Assessment Transcranial Doppler ultrasound (TCD) is a reproducible, non-invasive technique to predict 1° stroke risk in children < 170 cm/sec “Normal” 170 – 199 cm/sec “Conditional” > 200 cm/sec “Abnormal” 31 Stroke Risk Assessment R.R. of stroke with Abnormal TCD = 44 (95% CI = 5.5 - 346) Adams et al. Ann Neurol 1997; 42:699 -704 32 Transfusion therapy for stroke prevention Effect of chronic Effect of chronic transfusion on 1° transfusion on 2° stroke rate in an stroke probability. asymptomatic patients with elevated TCD. (“STOP” Trial) Effect on stroke rate Incident rate of stroke of discontinuing in SS children living in chronic transfusion in California (CA), asymptomatic before and after patients with publication of the elevated TCD from STOP Trial STOP Trial. (“STOP2” Trial) Adapted from: Platt OS. Hematology 2006.. 2006:54-57 (ASH Education Book 2006) 33 Acute Transfusion therapy • Goal – maximize O2-carrying capacity • Post –transfusion Hb should not exceed 10-11 g/dL Platt OS. Hematology 2006.. 2006:54-57 • Transfuse phenotypically matched blood for minor antigens C, E and Kell • Allo-sensitization should be reassessed 1-3 months after episodic transfusions 34 Chronic transfusion therapy Benefits: • Protection from CVA, ACS Risks • Iatrogenic Fe overload • Allo-sensitization • Unknown Infectious Risk 35 Perioperative management of SCD • Preoperative period – Admit to hospital 12 to 24 hours before surgery for hydration. – Treat obstructive lung disease with bronchodilators. – Simple transfusion to a Hb 10-11g/dL except for minor operations. • Intraoperative period – Maintain oxygen therapy with pulse oximetry. – Maintain hydration. – Prevent hypothermia. – Monitor blood loss and replace blood when necessary. • Postoperative period – Pulse oximetry, IV fluids, incentive spirometry. – Monitor for development of acute chest syndrome. 36 Role of fetal hemoglobin in sickle cell disease • Sickle cell phenotype only occurs after ~ 6months of age • Hemoglobin F levels are inversely correlated with disease severity / mortality • Biochemical evidence that gamma globin disrupts sickle globin polymerization 37 Hydroxyurea • The only FDA approved agent for sickle cell disease. • Old drug – New indication. • first synthesized in 1869 • anti-neoplastic since the 1960’s • mechanism of action unclear. • ribonucleotide reductase inhibitor. • Mechanism • ↑ Hemoglobin F • ↓ HbS polymerization and hemolysis • ↑ Hemoglobin • ↓ White Blood Cells (“Side-Effect”?) 38 Hydroxyurea – Clinical Trials MultiCenter Study of Hydroxyurea (MSH) (1992-1995) • Adults • ↑ Hemoglobin F; ↓ Hemolysis & Anemia; ↓ white blood cells • ↓ pain crises by 40%; ↓ acute chest syndrome by 50%; ↓ transfusions by 50% Charache S et al. N Engl J Med 1995;332:1317-1322. MSH Follow-Up (1996-2001) • 40% reduction in mortality after 9 years of follow-up Steinberg MH et al JAMA. 2003;289(13):1645-1651 MSH Follow-Up (1996-2010) • after 17.5 years of follow-up Steinberg MH et al. Am J Hematol 2010;85: 403–408. 39 MSH Follow-up – Cumulative Mortality Steinberg MH et al. Am J Hematol 2010;85: 403–408.