Current Results of Lentiglobin Gene Therapy in Patients with Severe Sickle Cell Disease Treated Under a Refined Protocol in the Phase 1 Hgb-206 Study

Background

β-globin gene transfer has the potential for substantial clinical benefit in patients with sickle cell disease (SCD). LentiGlobin Drug Product (DP) contains autologous CD34+ hematopoietic stem cells (HSCs) transduced with the BB305 lentiviral vector (LVV), encoding β-globin with an anti-sickling substitution (T87Q). The safety and efficacy of LentiGlobin gene therapy is being evaluated in the ongoing Phase 1 HGB-206 study (NCT02140554). Results in the initial 7 patients treated with LentiGlobin DP from steady state bone marrow harvested (BMH) HSCs using original DP manufacturing process (Group A) demonstrated stable HbA^T87Q production in all patients, but at levels below the anticipated target. The protocol was thus amended to include pre-harvest RBC transfusions, optimize myeloablation by targeting higher busulfan levels, and use a refined DP manufacturing process (Group B); additionally, HSC collection by plerixafor mobilization/apheresis was instituted (Group C). Data from patients in Group C, treated under the modified protocol with DPs manufactured from plerixafor-mobilized HSCs using the refined process, are reported here. Results in patients in Groups A and B are reported separately.

Methods

Patients with severe SCD (history of recurrent vaso-occlusive crisis, acute chest syndrome, stroke, or tricuspid regurgitant jet velocity of >2.5 m/s) were enrolled. Patients in Group C received ≥2 months of transfusions to reach Hb of 10 - 12 g/dL and <30% HbS before HSC collection. Patients received 240 μg/kg of plerixafor 4 - 6 hours before HSCs were collected by apheresis and CD34+ cells were transduced with the BB305 LVV at a central facility. Following myeloablative conditioning with busulfan, the DP was infused, and patients were monitored for adverse events (AEs), engraftment, peripheral blood (PB) vector copy number (VCN), HbA^T87Q expression, and HbS levels. Summary statistics are presented as median (min - max).

Results

As of 15 May 2018, 11 Group C patients (age 25 [18 - 35] years) had undergone mobilization/apheresis, 9 patients had DP manufactured (median 1 cycle of mobilization [1 - 3]) and 6 patients had been treated. Cell dose, DP VCN and % transduced cells in the 6 treated patients were: 7.1 (3 - 8) x 10⁶ CD34+ cells/kg, 4.0 (2.8 - 5.6) copies/diploid genome (c/dg) and 81 (78 - 88) % transduced cells. The median follow-up was 3.0 (1.2 - 6.0) months. Patients achieved neutrophil engraftment at a median of 19 (18 - 20) days. Platelet engraftment was achieved at a median of 28 (12 - 64) days in 4 patients; platelet engraftment was pending in 2 patients.

Two of 11 patients experienced 4 grade ≥3 AEs associated with plerixafor mobilization/HSC collection: 1 had vaso-occlusive pain and hypomagnesaemia, and the other had vaso-occlusive pain and non-cardiac chest pain. The toxicity profile from DP infusion to last follow-up in the 6 treated patients was consistent with myeloablative conditioning. Febrile neutropenia (n=5) and stomatitis (n=4) were the most common non-hematologic grade ≥3 AEs. Serious AEs were reported in 3 patients post-DP infusion: splenic hematoma, non-cardiac chest pain and mucosal inflammation. To date, there have been no DP-related AEs, graft failure, vector-mediated replication competent lentivirus, or clonal dominance.

In the 6 treated patients, PB VCN at last visit ranged from 1.4 - 2.9 c/dg. In the 3 patients with 3 months follow-up, total Hb levels were 11.7 g/dL, 9.8 g/dL and 9.2 g/dL, and HbA^T87Q levels were 4.7 g/dL, 3.2 g/dL and 3.5 g/dL. One additional patient with 6 months follow-up was off transfusions and had total Hb of 14.2 g/dL, of which 62% (8.8 g/dL) was vector-derived HbA^T87Q and 36% (5.1 g/dL) was HbS. All 4 patients had HbA^T87Q (median 39%) levels higher than or equal to HbS (median 31%) at the 3-month visit.

Summary

HGB-206 protocol changes and refined DP manufacturing have improved the LentiGlobin DP characteristics resulting in significantly improved outcomes. In addition, the HbA^T87Q expression is comparable to, or exceeds, HbS levels as early as 3 months post DP infusion. These data support the feasibility of plerixafor-mediated CD34+ cell collection in patients with severe SCD and the efficacy of gene therapy. The safety profile of LentiGlobin gene therapy remains consistent with single-agent busulfan conditioning. Additional data and longer follow-up will determine the clinical effect of increased HbA^T87Q/HbS ratios.