Sickle cell anemia (SCA) is a globally prevalent, monogenic, life-threatening blood disorder with a complex pathology that remains obscure. A deepened understanding of the malady in the recent years has led to pharmaceutical advancements that target pathophysiology and ultimately ameliorate associated multivariate clinical manifestations. Abnormal cell to cell interactions, endothelial adhesion, induced oxidative stress, intracellular erythrocyte dehydration and concentration levels of fetal hemoglobin are a few factors know to play a key role in microvascular obstruction in individuals with Hemoglobin S. All of which have recently been manipulated in vitro to pharmaceutically alter clinical complications, disease progression and mortality patterns.
The purpose of this paper is to provide a condensed overview of recent discoveries made within SCA pathology and subsequent pathologic based therapeutic agents. A general overview of the molecular basis of the malady, global prevalence and the efforts toward advancing stem cell transplantation technology will also be discussed.
Sickle cell disease (SCD) denotes a group of pleiotropic hemoglobinopathies that mechanically affect the body’s vasculature. Of the various forms, Sickle Cell Anemia or homozygous Hemoglobin S (HbS) is the most common. It is characterized by the production of abnormally shaped red blood cells, which can obstruct and damage circulation; initiating a cascade of atypical cell interactions. Decreased organ perfusion to the kidneys, spleen, nervous, cardiovascular, respiratory and urinary systems become imminent in the absence of appropriate pathophysiology specific therapy (Lanzkron, Carroll, & Haywood, 2010). Sickle cell crisis is the most prevalent clinical complication associated with SCA and usually precedes multivariate symptoms correlating with decreased organ perfusion and functioning.
There are 100 million carriers of heterozygous Hemoglobin AS or sickle cell trait (SCT) worldwide. Global prevalence suggests a long standing selective pressure against falciparum malaria and consecutive carrier migration from malaria endemic regions(Gong et al., 2012). SCT carriers demonstrate minimal, if any clinical complications throughout one’s lifetime.
Nevertheless, 300,000 babies are born to these carriers each year with a 25% chance of their children inheriting SCD. The benefits of SCT genotyping and fetal screening has directly impacted mortality and morbidity patterns in resource accessible regions. Limited access to frontline therapies and proper medical follow in malaria endemic and developing regions are great attributors to disease prevalence. Moreover, a deepened understanding of the complex pathology of SCA in the past decade has encouraged researchers to shy away from panoptic therapies and gear toward development of pathology modifying interventions that ultimately decline global incidences on a greater scale.
Inheritance of the sickle cell gene causes a non-conservative missense mutation at the beta globin chain(Manwani & Frenette, 2019). The exchange of hydrophobic valine with hydrophilic glutamic acid on the 6th position of beta globin induces hemoglobin S to sickle in anoxic and acidic microenvironments; also referred to as the deoxy-sickle hemoglobin phase. Mechanical obstruction of the micro-vessels by recurring red cell sickling has been tipically estimated as the sole reason for decreased organ perfusion and tissue infarction; yet recent scientific progress proves otherwise. Further demystification of the mechanistic complexity of HbS has led to pharmaceutical advancements aimed to target abnormal endothelial interactions, inflammation and red cell dehydration in individuals with the life compromising disease.
Manipulation of vasoactive factors in vitro has demonstrated encouraging results for new oral therapeutic agents anticipated to impact individuals in developing regions. Frontline SCD therapies such as hydroxyurea and blood transfusions vary in clinical efficacy from patient to patient and is not readily accessible in impoverished regions. Stem cell transplantation is the only curable option for SCD yet advancements in donor transplantation and remain limited. Although a thorough investigation of the complex, multivariate pathophysiology of SCD does not fall within the scope of this review, emphasis will be placed on disease targeting agents that are being studied to date that perpetuate a less hindering clinical outcome in those suffering from SCA.
Increasing fetal hemoglobin concentration
Increased levels of fetal hemoglobin (HbF) has long been understood to improve SCA related clinical complications. Having 2 alpha and 2 gamma chain tetramers physiologically inhibits HbF from entering the deoxy-sickle hemoglobin phase. Contrary to adult hemoglobin, fetal hemoglobin decreases the likelihood of HbS polymerization. During gestation, fetal hemoglobin predominates until its regression to less than 1% at nearly 6 to 12 months after birth. This phenomenon accounts for why most SCD infants are asymptomatic in the first few months. Moreover, baseline fetal hemoglobin concentrations fluctuate amongst haplotypes of the sickle cell gene. Senegal and Saudi-Indian haplotypes manifest higher HbF levels in adulthood and thus demonstrate a milder clinical expression contrary to the other beta S-globin haplotypes. Pharmacologically inducing fetal hemoglobin levels strongly correlates with a reduction in episodes of pain crises, hemolysis and longer survival.
For over 30 years, Hydroxyurea (HU) remains the therapeutic standard for modifying SCD pathogenesis and increasing HbF. HU is mechanistically known to inhibit the formation of deoxyribonucleotides, thereby trapping cells in the synthesis stage of the cell cycle and stimulating erythropoiesis. Consequentially, the intercalation of fetal and adult hemoglobin (HbA) hinders the polymerization of HbS and subsequent red cell sickling. HU related studies have also proved that clinical efficacy is not only a result of HbF production but also a demotion of abnormal blood cell interactions, endothelial adherence, blood viscosity and premature intravascular hemolysis. The Multi-Center Study of Hydroxyurea (MSH) was a prominent placebo controlled randomized trial for adults in 1999 that revealed a 44% reduction in pain crisis in the placebo group. Furthermore, fewer episodes of acute chest syndrome, splenic sequestration, transfusion induced hospitalizations and priapism were also reported. In 2011, HU was considered by the Food and Drug Administration to be an appropriate therapy for children over 2 years old. The BABY HUG study was the first placebo-controlled randomized trial of HU in adolescents that even included infants with mild clinical complications. Of note, the placebo group demonstrated a 5 times higher dactylitis occurrence and 3 times higher incidence of acute chest syndrome.
Hydroxyurea for children is arguably underused in malarial-endemic regions where the greatest sickle cell burden prevails(Diallo & Tchernia, 2002). The NOHARM, placebo controlled multicenter study was one of the first of its kind to test the efficacy of HU on children in malaria-endemic Uganda. HU recipients had demonstrated benefits similar to trials performed in non-malarious regions; including a less frequent SCA-related clinical outcome, a notable decrease in reticulocyte formation, splenic sequestration and the need for blood transfusion compared to the placebo group. Despite the encouraging results, the demand for optimal HU dosing and proper regimen monitoring remains a challenge. With the maximum dosage being 35mg/kg in order to avoid cytopenia and toxicity, HbF induction via hydroxyurea is considered lower than the magnitude expected to completely prevent SCA related clinical complications. In fact, studies have proven that HbF induction via Hydroxyurea varies amongst individuals with reports of baseline regression status post 2 years of treatment. Moreover, rates of non-adherence to HU secondary to an unvaried treatment outcome is also a contributor of the delayed decline in disease prevalence. Thus, the need for a robust HbF inducing agent is imminent in effectively treating mortality rates in heath-impoverished regions.
Pharmacological advancements in HbF biochemistry resulted in the genetic manipulation of HbF gene silencer, DNMT1. 5-aza-2’-deoxycytidine (decitabine) functions to bind and deplete DNMT1 and therefore upregulate fetal hemoglobin in erythrocytes. The first human study on the efficacy of decitabine in HbF induction was performed in 2017 and demonstrated an encouraging outcome for larger trials forthcoming. The randomized trial administered decitabine with the oral multidrug resistant transporter, tetrahydrouridine (THU) in order to expand bioavailability and counteract pharmacologic limitations. The randomized, phase I/II study revealed an increase in fetal hemoglobin levels to 80% of total erythrocytes in the treated group. A decline in levels of cellular adhesion and reticulocyte production were also noted. Although Decitabine has potential toxicities as does HU, this HbF inducer demonstrates a greater efficacy even in patients non-responsive to Hydroxyurea. While the demand for more research remains, oral interventions that genetically manipulate the HbF gene may allow less follow-up need in regions that may not have access.