Update on the Cure Sickle Cell Initiative

At the time this issue went to press, it was announced that the pharmaceutical sponsor bluebird bio, Inc., had temporarily suspended its phase I/II and phase III LentiGlobin™ gene therapy studies to investigate two cases of myeloid malignancy that have arisen in treated patients.

For many years, the only U.S. Food and Drug Administration–approved drug for sickle cell disease (SCD) was hydroxyurea, which leads to increases in fetal hemoglobin and subsequently retards HbS-induced sickling of red blood cells (RBCs). This effect, along with hydroxyurea-induced reduction of circulating leukocytes and other anti-inflammatory effects, are associated with decreased prevalence of vaso-occlusive crises, acute chest syndrome, stroke, and death. Other recently approved drugs that target the sequelae of HbS polymerization include L-glutamine, voxelotor, and crizanlizumab. These drugs target pathophysiologic processes downstream of the HbS point mutation and have varying and limited clinical impact. Thus, a curative option that would significantly impact the pathophysiology of HbS is desirable, and several approaches are currently being evaluated. This article aims to meet the practicing hematologist’s need for information about the current development of curative therapies, to help them inform their patients and answer their questions about the pros and cons of curative therapy for SCD.

A Brief History of the Development of Curative Therapies. Allogeneic hematopoietic stem cell (HSC) transplantation using sibling donors is currently the only standard curative option for SCD. However, a related HLA-matched stem cell donor is found in only less than 20 percent of patients. Graft failure and transplant-related mortality remain challenges, and moreover, because of the inherent risk of graft-versus-host disease (GVHD), allogeneic transplant recipients often require prolonged immunosuppression.

Gene therapy, which involves genetic modification of the patient’s autologous HSCs, could potentially serve as an attractive alternative to allogeneic transplantation. The concept of gene therapy, specifically the introduction of new genes to treat blood diseases, goes back to the 1970s. Since that time, modern molecular biology tools have developed the ability to clone human genes as complementary DNA (cDNA), and to generate therapeutic gene transfer vehicles (e.g., vectors derived first from bacteria and subsequently from viruses) that can introduce cloned genes into mammalian cells. Throughout the past 20 to 30 years, viral backbones have undergone many modifications to achieve efficient production, optimize transduction rates, and limit the chance of insertional mutagenesis. Lentiviral vectors are currently preferred over the previously used gamma retroviral vectors because of their increased ability to transduce non-dividing primitive cells such as HSCs. Moreover, ex vivo culture of HSC with lentiviral vectors is shorter, thus limiting loss of HSC stemness.

The clinical trials presently underway in the United States and Europe employ somewhat different therapeutic strategies to target the underlying pathophysiology of SCD. In one strategy, the newly introduced gene encodes a β globin gene with anti-sickling properties. This therapeutic β globin gene derivative, βT87Q-globin, interferes with HbS polymerization and thus reduces sickling. A phase I/II multicenter clinical trial involving more than 50 patients showed highly promising results in eliminating vaso-occlusive crises and transfusion dependence. This study was presented by Dr. Alexis Thompson at the 2020 ASH Annual Meeting (abstract #677).

Another strategy involves introduction of a gene encoding a short hairpin RNA that silences the BCL11A transcription factor, which is critical for switching off HbF production after birth. Therefore, HbF production in RBCs is reactivated. This strategy is attractive because high levels of erythrocyte HbF in patients with SCD are associated with reduced morbidity and mortality. Highly encouraging results of a phase I study involving six patients suggest that this also is a promising approach for HbF induction in patients with SCD.¹ 

The ability to alter or edit specific regions of the patient’s own genomic DNA using the CRISPR-Cas9 nuclease system is another attractive novel gene therapy approach currently being applied in gene therapy trials for thalassemia and SCD. Here, the nuclease and guide RNA are introduced into the patient’s HSC by either a nonviral or viral delivery vehicle. A phase I trial utilizing this editing technology to target the BCL11A enhancer region also shows promising efficacy and safety, albeit after limited follow-up thus far.² 

The Basics of Gene Therapy As a Medical Procedure. HSC transplantation of either allogeneic HSC or gene-modified autologous HSC is conducted according to a carefully planned multistep protocol. The pretransplant regimen includes multiple measures such as HSC mobilization and procurement, RBC exchange transfusion, and marrow ablation. These steps pose significant challenges due to the inflammatory aspects of SCD. Procurement of adequate numbers of HSC is critical for the in vitro gene modification process but also likely impacts HSC engraftment efficiency. HSC procurement for transplantation by bone marrow harvest has yielded inconsistent numbers of viable HSC and can trigger SCD-related morbidities such as acute chest syndrome in some patients. This practice has been mostly replaced by HSC mobilization to the peripheral blood and collection by apheresis. Granulocyte colony-stimulating factor has been widely used as an effective mobilizing agent in patients with malignancy undergoing transplantation. However, this drug is contraindicated in SCD, as it can trigger severe adverse events. The CXCR4 antagonist plerixafor is a more recently developed mobilizing agent that is well tolerated and effective in healthy allogeneic stem cell donors and has been used successfully in the current gene therapy trials for SCD. However, the degree of mobilization in patients with SCD is often suboptimal, as are the yields of purification by apheresis. As a result, patients may require multiple mobilization-apheresis interventions. Most gene therapy protocols have implemented RBC exchange transfusions as a pretransplant preparative regimen to prevent the occurrence of SCD-related morbidities associated with the HSC mobilization/apheresis procedure. Prior to infusion of the gene-modified HSC, myeloablative cytotoxic therapy is administered to “make room” for the autologous transplant. Thus, analogous to autologous stem cell transplantation for malignant disorders, patients undergoing gene therapy require in-hospital monitoring of infections and blood transfusion support during the marrow recovery period. The long-term risks of genotoxic conditioning regimens include infertility and occurrence of secondary malignancy. Other potential long-term risks relate to engraftment durability as well as the potential for off-target effects of either vectors and/or editing components. Multiple working groups and committees of the National Institutes of Health (NIH) Cure Sickle Cell Initiative (www.curesickle.org) have been established to address these pre- and post-transplant issues (Table).

Although patients and clinicians may choose gene therapy to stop the relentless recurrence of pain episodes, many patients will arrive to gene therapy with significant prior damage to the brain, heart, lungs, or kidneys. Such underlying damage may worsen during the physiological stresses of the bone marrow conditioning regimen and recovery periods, and we do not know at this time what damage, if any, will be reversible once predominantly normalized red cells are circulating. Thus, gene therapy, even when successful, may not be the “cure” the patient expects. Additionally, most gene therapy schemes do not result in the production of red cells with predominantly hemoglobin A. For the most part, they result in production of cells containing both an anti-sickling Hb (e.g., AT87Q, HbF) as well as HbS. We do not yet know exactly how normally these cells will behave in the circulation. It is unclear to what extent progressive organ damage might occur after gene therapy and whether such progression will be related to specific measurable results of gene therapy such as percent HbF.

In 2017, NIH Director Francis Collins formed the Cure Sickle Cell Initiative to accelerate all aspects of the progress of curative therapies to the bedside. The initiative encompasses a programmatic effort to support all aspects of development of curative therapies. Another important role of the initiative is to support growth in the understanding of gene therapy and its broader implications for patients, family members, caregivers, and the public. Thus, the initiative is engaging members of those groups, soliciting their input, and using a variety of means to better understand lay, patient, and family knowledge and attitudes, with the aim of having these individuals involved in the whole process of program building and clinical trial design. Among the questions being asked are, “What should the definition of a ‘cure’ be?” Additionally, the initiative is supporting economic analyses of the many costs of SCD to the health care system, to the affected individual, and to society, with and without curative therapy. Mental health implications of gene therapy clinical trial participation will also be examined.

The Cure Sickle Cell Initiative is also supporting the development of resources for clinical trial enrollment, such as universal standardized methods of assessment of disease severity and donor intake forms to determine donor eligibility for different clinical trials (see previous article in the November-December 2018 issue of The Hematologist).

There is a need to develop alternative safer, nongenotoxic (e.g., antibody mediated) approaches given the long-term sequelae of the current myeloablative conditioning regimens. Preparative regimens that target the bone marrow microenvironment either pharmacologically or via blood transfusion might lead to more efficient HSC mobilization and favor durable engraftment with a high degree of chimerism. In vivo gene therapy — the direct delivery to the patient of nucleases packaged in viral or nonviral vehicles — could be conducted on a large scale, as it obviates the need for an autologous transplant. As an alternative to gene therapy, pharmacologic HbF reactivation by small molecule regulators targeting BCL11A or other genes may have comparable impact on SCD morbidity and mortality. Although such treatment would entail long-term medication use, it is more likely to be widely implemented.

The authors wish to thank Victoria H. Coleman-Cowger, PhD, and LaTonya Kittles, MS, for helpful review and edits of the manuscript.