Gene Therapy with Reduced-Intensity Conditioning for Sickle Cell Disease

Abstract

Introduction

Autologous transplantation of gene-modified cells for treatment of sickle cell disease (SCD) involves myeloablative conditioning, with associated cytopenias, toxicities and long hospitalization. We report the first successful demonstration of gene therapy using reduced-intensity conditioning (RIC) for SCD, made possible with an enhanced lentiviral vector and melphalan dosing modified by pharmacokinetics

Objective

To evaluate gene therapy with RIC for SCD.

Methods

We report seven patients treated in a first-in-human Phase 1/2 study for SCD using RIC transplant (melphalan 140mg/m²) of autologous HSCs genetically modified with a lentiviral vector (GbG^M) encoding a modified γ-globin gene that expresses a potent anti-sickling fetal hemoglobin, HbF^G16D (ClinicalTrials.gov NCT02186418). Melphalan pharmacokinetics were performed to identify optimal area under the curve (AUC) to maximize engraftment of gene-modified cells.

Results

Seven patients received gene therapy as described in Figure 1. Minimum duration of follow-up is 2 years (range 2-7 years). No chemotherapy or product-related serious adverse events other than expected cytopenias were reported. The gene therapy product had relatively rich abundance of clones and engraftment was polyclonal with no evidence of clonal dominance on vector integration site analysis after gene transfer. All seven patients demonstrated sustained HbF^G16D expression with >80% reduction in severe vaso-occlusive events (VOE) (Figure 2A,B). Melphalan pharmacokinetics were performed after a single dose of 140 mg/m² and melphalan exposure is shown in Figure 2C. Patient 2 had reduced melphalan exposure due to renal hyperfiltration (estimated GFR = 200 mL/min/1.73 m²), which was associated with lower engraftment of transduced cells. Therefore, patients 6 and 7 received melphalan dosing adjusted for GFR, hematocrit and lean body mass, based on published melphalan PK modeling. As a result, six of seven patients had what appears to be adequate RIC melphalan exposure with melphalan AUC > 6.7 mg·h/L. All six demonstrated stable engraftment with a median vector copy number of 74% (range, 55-99%) compared to the infused product in peripheral blood at 6-12 months (Figure 2C). Median time to platelet engraftment was 20 days (35-36 days reported after busulfan) and to neutrophil engraftment 16 days (20-27 days reported after busulfan). Grade 4 thrombocytopenia was present a median of 5 days and grade 4 neutropenia a median of 8 days (Figure 3A,B). Median length of hospital stay was 24 days (range 17-32 days), shorter than 35 days (range 26-65 days) reported with busulfan.

Conclusion

Translation of sickle cell gene therapy to middle and low resource environments requires reduction in toxicity, cost and health care resource utilization. Our strategy of RIC and a modified vector produced in an academic environment is a first step towards this goal.