Antibody Drug Conjugates Targeted to CD45 or CD117 Enable Allogeneic Hematopoietic Stem Cell Transplantation in Animal Models
Authors: Palchaudhuri Rahul , Hyzy Sharon L. , Proctor Jennifer L , Adams Hillary L , Pearse Bradley R , Sarma Ganapathy , Aslanian Sharon , Gillard Geoff , Lamothe Tahirih L , Burenkova Olga , Brooks Melissa L. , Gabros Andrew D , McDonagh Charlotte F , Boitano Anthony E. , Cooke Michael P. 
Published: October 2019

Abstract
Introduction
Targeted antibody drug conjugates (ADCs) to mouse CD45 or mouse CD117 have recently been shown to effectively prepare immunocompetent mice for whole bone marrow transplants (Palchaudhuri et al. Nature Biotech 2016 34:738-745; and Czechowicz et al. Blood 2016 128:493). This new targeted approach to conditioning using ADCs has the potential to expand the utility of transplantation if it can be successfully translated to humans. The anti-CD45 or anti-CD117 antibodies used previously were coupled to saporin (SAP), a ribosome-inhibiting protein, which once internalized elicits cytotoxicity in a cell cycle-independent manner. Both anti-CD45-saporin (CD45-SAP) and anti-CD117-saporin (CD117-SAP) ADCs have been shown to effectively deplete bone marrow hematopoietic stem cells (HSCs) as single dosed agents, creating vacancies that enable efficient autologous HSC engraftment (>95% long-term donor chimerism).
Results
To further investigate and develop the utility of these tool ADCs in murine transplant models, we explored CD45-SAP (1.9 mg/kg, iv) and CD117-SAP (1 mg/kg, iv) in an allogeneic minor mismatch transplant model (Balb/c donor into DBA/2 recipients). The ADCs were used alone or in combination with an additional immune depleting agent, clone 30F11 (25 mg/kg, IP), a naked anti-CD45 antibody that mimics ATG by relying on effector function to enable potent peripheral B- and T -cell depletion. In addition to the lymphodepleting antibody, we included post-transplant Cytoxan (200 mg/kg, IP) to prevent GvHD. To compare the CD45-SAP and CD117-SAP to conventional conditioning methods, we investigated sub-lethal total body irradiation (TBI, 2Gy) or pre-transplant Cytoxan (200 mg/kg, IP) in combination with the immunosuppressants. Conditioned mice were transplanted with 2x107 whole bone marrow cells, and chimerism assessed over 12 weeks.
CD45-SAP or CD117-SAP in combination with immunosuppressants (30F11 and post-transplant Cytoxan) enabled >85% peripheral donor chimerism at 12 weeks post-transplantation. Multilineage reconstitution was observed in the T-, B- and myeloid cell compartments with >80%, >90% and >90% donor chimerism respectively in both CD45-SAP and CD117-SAP groups. In contrast, 2Gy TBI in combination with immunosuppressants (30F11 and post-transplant Cytoxan) resulted in only 5% donor engraftment. Multi-dosing with 30F11 (QDx3) plus 2Gy TBI and post-transplant Cytoxan increased the peripheral donor chimerism to 40%. Pre-transplant Cytoxan plus 30F11 (QDx3) and post-transplant Cytoxan yielded 20% donor chimerism. For all groups, stem cell chimerism in the bone marrow matched the peripheral chimerism.
Conclusion
These results indicate anti-CD45 or anti-CD117 ADCs may be used in combination with immunosuppression to enable highly efficient allogeneic transplants in a minor mismatch model (>85% donor chimerism). CD45-SAP and CD117-SAP were more effective at conditioning versus 2Gy TBI or pre-transplant Cytoxan. Future experiments will investigate anti-CD45 and anti-CD117 ADCs in additional allogeneic models.

Disclosures
Palchaudhuri: Magenta Therapeutics: Employment, Equity Ownership, Patents & Royalties; Harvard University: Patents & Royalties. Hyzy:Magenta Therapeutics: Employment, Equity Ownership. Proctor:Magenta Therapeutics: Employment, Equity Ownership. Adams:Magenta Therapeutics: Employment, Equity Ownership. Pearse:Magenta Therapeutics: Employment, Equity Ownership, Patents & Royalties. Sarma:Magenta Therapeutics: Employment, Equity Ownership. Aslanian:Magenta Therapeutics: Employment, Equity Ownership. Gillard:Magenta Therapeutics: Employment, Equity Ownership. Lamothe:Magenta Therapeutics: Employment, Equity Ownership. Burenkova:Magenta Therapeutics: Employment, Equity Ownership. Brooks:Magenta Therapeutics: Employment, Equity Ownership. Gabros:Magenta Therapeutics: Employment, Equity Ownership. McDonagh:Magenta Therapeutics: Employment, Equity Ownership, Patents & Royalties. Boitano:Magenta Therapeutics: Employment, Equity Ownership, Patents & Royalties. Cooke:Magenta Therapeutics: Employment, Equity Ownership, Patents & Royalties.