Antibody-Drug Conjugates Targeting CD45 Plus Janus Kinase (JAK) Inhibitors As Conditioning for Allogeneic Hematopoietic Stem Cell Transplantation

Abstract

<h3>Introduction</h3> Allogeneic hematopoietic stem cell transplantation (alloHSCT) is a potentially curative therapy for the treatment of hematologic diseases. AlloHSCT is associated with significant treatment-related morbidity and mortality from conditioning-related toxicities and graft-versus-host disease (GvHD), which limits its use to the most life-threatening indications, such as acute myeloid leukemia (AML). However, for conditions like AML that are primarily diagnosed in elderly patients, alloHSCT may be contraindicated due to poor patient functional status or medical comorbidities. Thus, an unmet clinical need exists for novel, highly-effective alloHSCT conditioning regimens with greater tolerability and minimal toxicity. <h3>Objectives</h3> Previously, antibody-drug conjugates (ADCs) composed of a CD45.2 antibody bound to the ribosome-inactivating protein saporin (CD45-SAP) were shown to allow high-level engraftment of syngeneic murine hematopoietic stem cells (HSCs). Here, we describe ADC-based conditioning regimens for murine alloHSCT which overcome immune barriers to engraftment imposed by minor histocompatibility antigen (miHA) and major histocompatibility complex (MHC) mismatches. <h3>Methods</h3> Mice were conditioned for HSCT with 3 mg/kg CD45-SAP 7 days prior to infusion of 10 × 10⁶ bone marrow cells. Haploidentical F1-to-parent (CB6F1→B6) and miHA-mismatched (BALB/c→DBA/2) HSCT models were used. For GvHD studies, a parent-to-F1 (B6→CB6F1) model was used in which recipients were infused with 25 × 10⁶ donor splenocytes after CD45-SAP treatment or sublethal irradiation. <h3>Results</h3> CD45-SAP combined with T cell depletion was sufficient for high-level engraftment in our alloHSCT models (Fig 1). The selective Janus kinase (JAK) inhibitor baricitinib, previously shown by our lab to prevent GvHD, also permitted HSC engraftment when mice received the drug during the peri-transplant period (Fig 2A-B). Graft stability in baricitinib-treated mice was optimized by either combining daily baricitinib treatment with pre-transplant T cell depletion, or by delivering baricitinib via continuous infusion (Fig 2C-D). Notably, CD45-SAP conditioned mice showed minimal morbidity or mortality from GvHD when infused with allogeneic splenocytes, whereas mice conditioned with sublethal irradiation developed acute GvHD. (Fig 3). <h3>Conclusion</h3> CD45-SAP combined with T cell depletion and/or pharmacologic immunomodulation with baricitinib effectively conditioned mice for engraftment of mismatched HSCs. Strikingly, mice conditioned with CD45-SAP did not develop GvHD even when infused with high doses of allogeneic splenocytes. These studies provide proof-of-principle evidence that targeted immunotherapeutics (ADCs plus JAK inhibitors) can prepare HSCT recipients for engraftment of allogeneic donor HSCs with minimal toxicities and without development of GvHD.