Mgta-145, in Combination with Plerixafor in a Phase 1 Clinical Trial, Mobilizes Large Numbers of Human Hematopoietic Stem Cells and a Graft with Immunosuppressive Effects for Allogeneic Transplant

Abstract

Background . The majority of hematopoietic stem cell (HSC) transplants are performed using peripheral blood mobilized with granulocyte-colony stimulating factor (G-CSF) given over 5 days. The goal of a successful transplant is to reliably mobilize optimal numbers of HSCs necessary for rapid and consistent multilineage engraftment. Infusion of mobilized allogeneic grafts results in significant acute and chronic graft-versus-host disease (GvHD) in up to 80% of allogeneic transplant recipients. A reliable and rapid method to mobilize HSC-rich grafts with reduced GvHD potential would be clinically meaningful. In a Phase 1 study of normal volunteers, MGTA-145 (GroβT), a CXCR2 agonist, when combined with plerixafor, a CXCR4 inhibitor, robustly and rapidly mobilized sufficient HSCs for a safe transplant after only a single day of dosing and apheresis/collection. Here, we phenotypically and functionally profile these mobilized grafts obtained from human volunteers and show that MGTA-145 + plerixafor mobilizes grafts with >10-fold higher engraftment potential (as measured by SCID-repopulating units in NSG mice), a marked reduction in xenogeneic GvHD, and enhanced overall survival compared to G-CSF or plerixafor alone grafts. Results . In healthy donors, a peak of 40 CD34+ cells/μL were mobilized with MGTA-145 + plerixafor (n=12 donors). 11 of 12 (92%) of these donors mobilized >20 CD34+ cells/μL with single day dosing compared to only 8 of 14 (57%) achieving the same CD34+ cell target treated with plerixafor alone. Eight donors were mobilized with a single dose of MGTA-145 + plerixafor and apheresed on the same day. A median of 4x106 (1.5-7.0x106) CD34+ cells/kg were obtained (n=8 donors) from a median 20 (13-20) L collection. 35.8 (18.5-40.9)% of these cells were CD90+CD45RA-, a CD34+ subset enriched for HSCs, compared to only 6.9 (5.3-9.0)% with G-CSF (p<0.001, n=3 donors). Mechanistically, MGTA-145 bound to CXCR2 on neutrophils and led to a modest and transient increase in plasma concentrations of matrix metalloproteinase 9 (MMP- 9), a downstream target on neutrophils. To assess engraftment, we transplanted mobilized peripheral blood cells from healthy donors after a 5-day regimen of G-CSF or a single dose of plerixafor alone or MGTA-145 + plerixafor at limit dilution into sublethally irradiated primary and secondary NSG mouse recipients (n=3 cell doses, n=7-8 mice/group). Multilineage human engraftment was measured by flow cytometry 16 weeks post-transplant and SCID-repopulating cell (SRC) number was calculated (Figure 1A). MGTA-145 + plerixafor mobilized grafts (n=4 donors) led to a 23-fold increase in engraftment compared to G-CSF mobilized grafts (p<0.001, n=3 donors) and 11-fold higher engraftment compared to plerixafor mobilized grafts (p<0.001, n=3 donors). Immune cell subsets (B, T, and NK cells and cell subsets) mobilized by MGTA-145 + plerixafor were similar to those mobilized by plerixafor alone. While CD3+ T-cell numbers were comparable between MGTA-145 + plerixafor and plerixafor alone, MGTA-145 + plerixafor mobilized 0.2 (0.0-0.6) x108/kg CD8+ T-cells, constituting 1.8 (0.5-4.8)% of the graft, a number and proportion significantly lower than that mobilized by either G-CSF or plerixafor alone. To determine the effect of the mobilization regimen on xenogeneic GvHD, we developed a xenograft GvHD model in NSG mice where 6x106 PBMCs from various graft sources were infused into sublethally-irradiated animals (n=3-6 donors per graft source). Notably, MGTA-145 + plerixafor mobilized grafts resulted in significantly less GvHD than G-CSF (p<0.01) or plerixafor (p<0.001) grafts (Figure 1B). In vivo cellular subset depletion studies suggested that the GvHD protective effect in MGTA-145 + plerixafor grafts may be in part due to immunosuppressive monocytes which were not present, or present to a lesser degree, in grafts from donors mobilized with G-CSF or plerixafor. Conclusions . These data demonstrate that MGTA-145 + plerixafor is a rapid, reliable, and G-CSF free method to obtain high numbers of HSCs with durable engraftment potential and a graft with highly immunosuppressive properties. These data suggest that MGTA-145 + plerixafor is an effective single-day mobilization/collection regimen for both autologous and allogeneic stem cell transplantation resulting in enhanced engraftment and reduced GvHD in this xenograft model. Disclosures Goncalves: Magenta Therapeutics: Current Employment, Current equity holder in publicly-traded company, Patents & Royalties. Hyzy:Magenta Therapeutics: Current Employment, Current equity holder in publicly-traded company. Hammond:Magenta Therapeutics: Current Employment, Current equity holder in publicly-traded company. Falahee:Magenta Therapeutics: Current Employment, Current equity holder in publicly-traded company. Howell:Magenta Therapeutics: Current Employment, Current equity holder in publicly-traded company. Pinkas:Magenta Therapeutics: Current Employment, Current equity holder in publicly-traded company. Schmelmer:Magenta Therapeutics: Current Employment, Current equity holder in publicly-traded company. Hoggatt:Magenta Therapeutics: Consultancy, Current equity holder in publicly-traded company. Scadden:Magenta Therapeutics: Consultancy, Current equity holder in publicly-traded company, Membership on an entity's Board of Directors or advisory committees. Devine:Magenta Therapeutics: Consultancy. DiPersio:Magenta Therapeutics: Membership on an entity's Board of Directors or advisory committees. Savage:Magenta Therapeutics: Current Employment, Current equity holder in publicly-traded company. Davis:Magenta Therapeutics: Current Employment, Current equity holder in publicly-traded company.

Hematopoietic stem cell collection for sickle cell disease gene therapy

“…In a b-thalassemia mouse model, mobilization of HSCs with tGROb þ plerixafor facilitated their transduction by a systemically administered adenoviral vector encoding g-globin, providing preclinical proof of concept for "in vivo" gene therapy [94]. Administration of tGROb to healthy donors caused minimal changes in neutrophil levels or their activation, supporting its potential safety for individuals with SCD [91,92].…”

“…Multiple studies in animal models including nonhuman primates, and clinical studies of healthy individuals and patients with MM, have demonstrated that tGROb þ plerixafor mobilizes HSCs within minutes, has a longer peak of mobilization, and results in improved HSC engraftment compared to G-CSF-mobilized HSCs [85][86][87][88][89][90]. Compared to plerixafor alone, the addition of tGROb was safe for HSC mobilization in healthy volunteers and gave rise to greater numbers of repopulating HSCs with durable engraftment in patients with MM [91][92][93]. In a b-thalassemia mouse model, mobilization of HSCs with tGROb þ plerixafor facilitated their transduction by a systemically administered adenoviral vector encoding g-globin, providing preclinical proof of concept for "in vivo" gene therapy [94].…”