ALBERT

Description: Although it has been known for more than 60 years that the cause of sickle cell disease is polymerization of a hemoglobin mutant, hydroxyurea is the only drug approved for treatment by the US Food and Drug Administration. This drug, however, is only partially successful, and the discovery of additional drugs that inhibit fiber formation has been hampered by the lack of a sensitive and quantitative cellular assay. Here, we describe such a method in a 96-well plate format that is based on laser-induced polymerization in sickle trait cells and robust, automated image analysis to detect the precise time at which fibers distort (“sickle”) the cells. With this kinetic method, we show that small increases in cell volume to reduce the hemoglobin concentration can result in therapeutic increases in the delay time prior to fiber formation. We also show that, of the two drugs (AES103 and GBT440) in clinical trials that inhibit polymerization by increasing oxygen affinity, one of them (GBT440) also inhibits sickling in the absence of oxygen by two additional mechanisms.

Description: Background β-globin gene transfer into hematopoietic stem cells (HSCs) could reduce or eliminate sickle cell disease (SCD)-related manifestations. LentiGlobin for SCD gene therapy contains autologous CD34+ cells transduced with the BB305 lentiviral vector (LVV), encoding a human β-globin gene with the anti-sickling T87Q mutation (βA-T87Q). The safety and efficacy of LentiGlobin for SCD is being evaluated in the ongoing Phase 1/2 HGB-206 Study (NCT02140554). The initial 7 patients (Group A) were treated with LentiGlobin made from bone marrow harvested HSCs. The protocol was modified to improve HbAT87Q production by including pre-harvest red blood cell (RBC) transfusions, increasing the total busulfan exposure, and using a refined LentiGlobin manufacturing process (Group B, n=2). An additional modification was made for Group C patients where HSC collection by plerixafor mobilization followed by apheresis was instituted. Data from these Group C patients are discussed here. Results from patients in Groups A and B are reported separately. Methods Patients (≥ 18 years) with severe SCD (including those with recurrent vaso-occlusive crisis [VOC] and acute chest syndrome [ACS]) were screened for eligibility. Patients received 240 µg/kg of plerixafor 4-6 hours prior to HSC collection via apheresis. CD34+ cells were transduced with BB305 LVV. Patients underwent myeloablative busulfan conditioning and subsequent LentiGlobin drug product (DP) infusion. Patients were monitored for adverse events (AEs), engraftment, vector copy number (VCN), total hemoglobin (Hb) and HbAT87Q expression, hemolysis markers, and SCD clinical manifestations. Data are presented as median (min-max). Results: As of 7 March 2019, 19 Group C patients, aged 26 (18-36) years, had initiated mobilization/apheresis and 13 patients were treated with LentiGlobin for SCD gene therapy. Median DP VCN, % transduced cells, and CD34+ cell dose in the 13 treated patients were: 3.8 (2.8-5.6) copies/diploid genome (c/dg), 80 (71-88) %, and 4.5 (3.0-8.0) x 106 CD34+ cells/kg, respectively. The median follow-up was 9.0 (1.0-15.2) months. Twelve patients achieved neutrophil and platelet engraftments at a median of 19 (15-24) days and 28 (19-136) days, respectively. As of the data cut-off, engraftment was not yet evaluable in 1 patient at 1-month post-infusion. All patients stopped red blood cell (RBC) transfusions within about 3 months post-LentiGlobin gene therapy. Median total hemoglobin (Hb) and Hb fractions in patients at various time points are shown in Figure 1. Median HbS levels were at or below 50% in all patients with at least 6 months follow-up. The median total Hb at last visit in 8 patients with at least 6 months of follow-up, was 11.5 (10.2-15.0) g/dL, with a corresponding HbAT87Q median contribution of 5.3 (4.5-8.8) g/dL and a median HbS 5.7 (4.8-8.0) g/dL. Of these 8 patients, 6 had a history of VOCs or ACS. The median annualized VOC+ACS rate in these patients was 5.3 (3-14) pre-treatment and decreased to 0 (0-2) post-treatment. One Grade 2 VOC was observed 3.5 months post-treatment. No ACS or serious VOCs were observed in Group C patients' post- treatment. Lactate dehydrogenase, reticulocyte count, and total bilirubin at last visit post-LentiGlobin infusion were 225.0 (130.0-337.0) U/L, 150.0 (42.1-283.0) 109/L, 22.2 (3.42-39.3) µmol/L, respectively, trending towards normalization. The most common non-hematologic Grade ≥ 3 AEs were febrile neutropenia (n=10) and stomatitis (n=7) post-DP infusion. Serious AEs were reported in 6 patients post-LentiGlobin treatment, most common being nausea and vomiting. To date, there have been no DP-related AEs or graft failure, vector-mediated replication competent lentivirus detected, or clonal dominance reported. Longer follow-up and additional patient data will be presented. Summary The safety profile of LentiGlobin gene therapy for SCD remains consistent with single-agent busulfan conditioning and underlying disease. Patients in HGB-206 Group C experienced high-level, sustained expression of gene-therapy derived hemoglobin, with median HbS levels reduced to ~50% and median total Hb levels of 11.5 g/dL at 6 months. The cessation of clinical complications (no ACS or serious VOCs) and decreased hemolysis suggest a strong therapeutic effect after LentiGlobin gene therapy in patients with SCD. Disclosures Kanter: Peerview: Honoraria; NHLBI: Membership on an entity's Board of Directors or advisory committees; Rockpointe: Honoraria; SCDAA: Membership on an entity's Board of Directors or advisory committees; Novartis: Consultancy, Honoraria; Imara: Consultancy; Jeffries: Consultancy; Modus: Consultancy; Guidepoint Global: Consultancy; GLG: Consultancy; Cowen: Consultancy; bluebird bio, Inc: Consultancy; Medscape: Honoraria; Sangamo: Consultancy. Kwiatkowski:Terumo: Research Funding; Novartis: Research Funding; Apopharma: Research Funding; Imara: Consultancy; Celgene: Consultancy; bluebird bio, Inc.: Consultancy, Research Funding; Agios: Consultancy. Schmidt:German Cancer Research Center, Heidelberg, Germany: Employment; GeneWerk GmbH, Heidelberg, Gemrany: Equity Ownership. Miller:bluebird bio, Inc.: Employment, Equity Ownership. Pierciey:bluebird bio, Inc.: Employment, Equity Ownership. Huang:bluebird bio, Inc.: Employment, Equity Ownership. Ribeil:bluebird bio, Inc.: Employment, Equity Ownership. Thompson:Baxalta: Research Funding; Novartis: Consultancy, Research Funding; Celgene: Consultancy, Research Funding; bluebird bio, Inc.: Consultancy, Research Funding. Walters:AllCells, Inc: Consultancy; TruCode: Consultancy; Editas Medicine: Consultancy.

Description: Although sickle cell disease (SCD) has a variable clinical course, many patients develop end-organ complications that are associated with significant morbidity and early mortality. Myeloablative allogeneic HSCT (allo-HSCT) is curative but has been historically performed only in children younger than 16 years of age. Modest modifications in the conditioning regimen and supportive care have improved outcome such that the majority of children with a suitable HLA-matched sibling donor can expect a cure from this approach. However, adult patients have been excluded from myeloablative allo-HSCT because of anticipated excess toxicity resulting from accumulated disease burden. Efforts to use nonmyeloablative transplantation strategies in adults logically followed but were initially met with largely disappointing results. Recent results, however, indicate that nonmyeloablative allo-HSCT in adult patients with SCD allows for stable mixed hematopoietic chimerism with associated full-donor erythroid engraftment and normalization of blood counts, and persistence in some without continued immunosuppression suggests immunologic tolerance. The attainment of tolerance should allow extension of these potentially curative approaches to alternative donor sources. Efforts to build on these experiences should increase the use of allo-HSCT in patients with SCD while minimizing morbidity and mortality.

Description: Objective: Peripheral blood stem cell transplant (PBSCT) can now cure SCD in adults, but may result in a loss of future fertility. Little is documented regarding fertility preservation in women with SCD. The aim of this study was to perform fertility preservation for women with SCD scheduled for PBSCT. Design: Prospective cohort of women with SCD undergoing fertility preservation prior to PBSCT at a large research hospital. Materials and Methods: Patients underwent standard controlled ovarian hyperstimulation (COH) using an antagonist protocol cycle with leuprolide trigger under close multidisciplinary (including reproductive endocrinologists and hematologists) monitoring. All patients were continued on therapeutic or started on prophylactic anticoagulation prior to beginning COH, and maintained on hydroxyurea. Results: Nine reproductive aged women were screened; 1 declined participation, 1 was diagnosed with unrecognized premature ovarian insufficiency, 1 had her fertility preservation cycle canceled due to poor response to fertility medications and 2 patients are scheduled for upcoming cycles. The remaining four women (ages 20, 34, 24, 27) successfully underwent COH, transvaginal oocyte retrieval and cryopreservation of mature eggs (n= 8, 13, 15, 21 oocytes, respectively). The third patient underwent two cycles due to low mature oocyte yield from her initial cycle. Headaches were reported by patients 1 and 2 following gonadotropin injections, with a negative neurologic workup including MRI in patient 2. Patient 3 underwent an exchange transfusion on day 10 of stimulation, which did not adversely impact serum reproductive hormone levels. Patients 3 and 4 reported an acute exacerbation of their chronic pain during COH, which responded well to intravenous fluids, IV and oral pain medications. Therefore, despite severe SCD and co-morbid conditions, the side effects were manageable. There were no venous thrombotic events. Conclusion: These results provide support that fertility preservation can be safely performed in women with SCD under the care of a multidisciplinary team. The safest stimulation protocol (i.e. antagonist cycle with leuprolide trigger for final oocyte maturation) was successful in all patients despite multiple risk factors for failed leuprolide trigger. Fertility preservation is important not only before PBSCT to cure their underlying disease, but also because of high rates of premature ovarian insufficiency in the SCD population post-transplant. Support: Intramural NICHD and NHLBI, NIH. Disclosures No relevant conflicts of interest to declare.

Description: Abstract 4228 Human erythropoiesis is a dynamic complex multistep process that involves differentiation of early erythroid progenitors to enucleated red blood cells. Importantly, erythroid differentiation involves lineage-specific activation and restriction of gene expression. However, the mechanisms that play a role in erythropoiesis still remain incompletely understood. We previously demonstrated that erythropoietin-stimulated clone-1 (EP1), which is selectively expressed in normal human erythroid lineage cells shares 99.5% identity with the malignant fibrous histiocytoma amplified sequence 1(MFHAS1 or MASL1). MASL1 is an important oncogene that is highly expressed in malignant fibrous histiocytomas (MFH). MASL1 protein has several domains; ras, three leucine zipper, ATP/GTP-binding site and leucine-rich tandem repeat motif which are important structural or functional elements for interactions among proteins related to the cell cycle. However, the function of the MASL1 gene in erythropoiesis has not been studied. We hypothesized that MASL1 gene plays a role in the erythroid differentiation, and used a human liquid erythroid culture system to explore this concept. MASL1 mRNA and protein expression levels were significantly increased during the erythroid differentiation of CD34+ cells following EPO treatment. Conversely, small interfering RNA-mediated knock down of MASL1 in CD34+ cells resulted in a reduction of a double positive of transferrin receptor and glycophorin A (GPA) (14.1 ± 4.7%) when compared with mock (77.9 ± 4.4%) or control vector-transfected CD34+ cells (76.7 ± 8.8%) at day 14. May-Grunwald-Giemsa staining confirmed a phenotype of MASL1 knockdown CD34+ cells that most of cells were pro- or basophilic erythroblasts. Western blotting also showed a significant decrease in hemoglobin protein levels in MASL1 knockdown CD34+ cells. In addition, MASL1-knockdown in CD34+ cells demonstrated an interruption of Raf/MEK/ERK signaling pathway. Inhibition assay of SH3 domain of Son of Sevenless (SOS) which is an upstream adapter protein in EPO-induced erythroid differentiation, confirmed that a suppression of MASL1 reduced the phosphorylation of Raf/MEK/ERK kinases and delays erythroid differentiation of Epo-induced CD34+ cells. Taken together, our study provides novel insights into MASL1-regulated erythropoiesis through the Raf/MEK/ERK signaling pathway. Disclosures: No relevant conflicts of interest to declare.

Description: Retroviral insertion site analysis was used to track the contribution of retrovirally transduced primitive progenitors to hematopoiesis after autologous transplantation in the rhesus macaque model. CD34-enriched mobilized peripheral blood cells were transduced with retroviral marking vectors containing the neo gene and were reinfused after total body irradiation. High-level gene transfer efficiency allowed insertion site analysis of individual myeloid and erythroid colony-forming units (CFU) and of highly purified B- and T-lymphoid populations in 2 animals. At multiple time points up to 1 year after transplantation, retroviral insertion sites were identified by performing inverse polymerase chain reaction and sequencing vector-containing CFU or more than 99% pure T- and B-cell populations. Forty-eight unique insertion sequences were detected in the first animal and also in the second animal, and multiple clones contributed to hematopoiesis at 2 or more time points. Multipotential clones contributing to myeloid and lymphoid lineages were identified. These results support the concept that hematopoiesis in large animals is polyclonal and that individual multipotential stem or progenitor cells can contribute to hematopoiesis for prolonged periods. Gene transfer to long-lived, multipotent clones is shown and is encouraging for human gene therapy applications.

Description: Hematopoietic cells can be highly enriched for repopulating ability based upon efflux of the fluorescent Hoechst 33342 dye by sorting for side population (SP) cells, a phenotype attributed to expression of ABCG2, a member of the ABC transporter superfamily. Intriguingly, murine studies suggest that forced ABCG2 expression prevents hematopoietic differentiation. We sought to determine the effects of forced expression of the ABCG2 gene in hematopoietic stem cells in the nonhuman primate model, a model with proven relevance to human hematopoiesis. We cloned the full-length rhesus ABCG2 (rh-ABCG2) cDNA using a series of primers spanning the entire sequence designed using the published human sequence. Sequence homology was greater than 96%. The rh-ABCG2 gene was then introduced into an MFGS based retroviral vector pseudotyped with the RD114 envelope. Mobilized human peripheral blood CD34-positive cells were transduced with either rh-ABCG2 or human GP91-phox vector with no other payload. All transductions were initiated with 4 x10e5 cells using X-VIVO10/1%HSA/4mM/L L-glutamine supplemented with 100ng/ml_FLT3L, 100ng/ml SCF, 100ng/ml TPO and polybrene (5 ug/ml). RD114 vector was concentrated by ultracentrifugation (83,000g 90minutes 4°C). Gene transfer rates to CFU of greater than 80% were achieved using both vectors with similar gene transfer rate estimated by flow cytometry. ABCG2-transduced human peripheral blood progenitor cells (PBPCs) acquired the SP phenotype, but showed significantly reduced growth compared to control (Day 8: cell counts 7.67+/− 2.54 vs. 17.83+/−6.64 x10e5 for ABCG2 and GP91-phox transduced cells, respectively p=0.0024, n=5). We then examined the engraftment of ABCG2-expressing stem and progenitor cells in the rhesus macaque autologous transplant model. GCSF/SCF mobilized PBPCs were collected from 2 animals and the CD34+ cells were divided and transduced with either vector and infused after lethal irradiation. In vivo marking levels post transplant measured in mononuclear cells and granulocytes from peripheral blood and bone marrow ranged initially from 0.5–4% by Realtime PCR, declined equally over time, and were similar between transduced fractions, with no discrepancy between bone marrow and peripheral blood marking. Furthermore, peripheral blood T cells, B cells and granulocytes expressed ABCG2 at levels predicted by vector copy number long term, and the differential of such cells within the SP gate matched that of the non-SP fraction demonstrating no block to differentiation in the large animal. In vitro studies showed selective protection against mitoxantrone among ABCG2-transduced rhesus PBPCs. Our results confirm the existence of rhesus-ABCG2, support its importance in conferring the SP phenotype, suggest no detrimental effect of its overexpression upon hematopoiesis, and imply a potential role for its overexpression as an in vivo selection strategy for gene therapy applications.

Description: Key Points Results based on our allogeneic model suggest genetic strategies aimed at correcting at least 20% of HSCs are necessary to reverse SCD. A minority of donor myeloid cells is adequate because of the vast differences in RBC survival between donor and recipient.