Mgta-145 / Plerixafor-Mediated HSC Mobilization and Intravenous HDAd5/35++ Vector Injection into Mice Allows for Efficient In Vivo HSC Transduction and Stable Gene Marking in Peripheral Blood Cells of CD46-Transgenic and Thalassemia Mice

Background.In vivo hematopoietic stem cell (HSC) gene therapy represents a simpler approach to treating hemoglobinopathies without the need for myelosuppressive conditioning and autologous HSC transplantation. We developed a helper dependent adenovirus (HDAd5/35++)-based platform that enables efficient in vivo transduction of mobilized HSCs via CD46. Transduced HSCs can be positively selected by low-dose O6BG/BCNU-treatment to achieve ~90% marking rates in peripheral blood. Initial proof-of-concept in murine models as well as in rhesus macaques demonstrates high level of g-globin expression after gene addition by a Sleeping Beauty transposase. While the current mobilization regimen-4 days of G-CSF injection followed by an injection of AMD3100/plerixafor on day 5-mobilizes HSCs from the bone marrow to the periphery, several issues exist. Despite widespread use as a mobilization agent in oncology, G-CSF is contra-indicated in patients with sickle cell disease. Additionally, G-CSF results in unselective bone marrow cell mobilization, which leads to leukocytosis and elevated numbers of cytokine-producing cells in the periphery that come into contact with HDAd particles, leading to high cytokine levels. Mobilized (committed) bone marrow cells in the periphery also sequester HDAd thus reducing the effective dose for primitive HSCs. Further, the five-day treatment regimen and high costs associated with G-CSF + plerixafor justify the development of an alternative mobilization regimen. A single-day, G-CSF-free mobilization regimen that mobilizes a high proportion of HSCs may therefore be preferred for in vivo gene therapy. Results. Here we tested HSC mobilization by truncated MGTA-145, a CXCR2 agonist, and plerixafor in the context of in vivo HSC transduction. CD46-transgenic animals were mobilized with GCSF + plerixafor (5 days) or with MGTA-145 + plerixafor (same-day treatment) and then injected one hour later with an integrating HDAd5/35++ mgmt/GFP vector. MGTA-145 + plerixafor resulted in robust mobilization of HSCs, less leukocytosis and no significant elevation of cytokines, as observed with G-CSF + plerixafor. With both mobilization regimens, after in vivo selection with O6BG/BCNU, >90% of PBMCs expressed GFP and marking rates were stable long-term. Mice were sacrificed 12 weeks after in vivo transduction and bone marrow lineage-negative cells were harvested for transplantation into secondary recipients. Stable transgene expression (>90% at week 16 after tra