Conditioning for alloengraftment in FA

In this issue of Blood, Saha et al¹ describe the use of an anti-CD45 antibody-drug conjugate (ADC) with a pyrrolobenzodiazepine (PBD) payload to facilitate alloengraftment in 3 murine models of Fanconi anemia (FA). The authors demonstrate that antimurine CD45-PBD conditioning resulted in successful multilineage engraftment in major histocompatibility complex (MHC)-mismatched FANCA, FANCC, and FANCG mice, models that represent the vast majority of mutation subclasses in patients with FA. They show that at the selected intermediate dose with rapid pharmacokinetics (PK), anti-CD45-PBD achieved equivalent engraftment to total body irradiation (TBI) conditioning in both WT and FANCA mice transplanted from MHC-mismatched donors but with significantly reduced acute graft-versus-host disease (GVHD) and without serious toxicity. In a disease such as FA, which is characterized by chromosomal instability, standard transplant conditioning agents are very toxic. This work lays a foundation for further development of anti-CD45-ADC conditioning for patients with FA.

Hematopoietic stem cell (HSC) transplantation is the standard of care for the treatment of many hematological disorders. In HSC transplantation, engraftment of transplanted donor stem cells is limited by the availability of bone marrow (BM) niches normally occupied by host HSCs.² Studies have shown that depletion of host HSCs in a directed fashion could vacate BM niches and facilitate engraftment of donor HSCs.³ Conventional myeloablative regimens achieve optimal engraftment by nonspecifically destroying recipient HSCs as well as other BM components but cause significant acute and chronic toxicity and associated morbidity and mortality.⁴ In addition, tissue damage caused by myeloablative regimens contributes to the proinflammatory environment that promotes GVHD and hinders engraftment in allotransplantation.⁴ 

FA is an inherited DNA repair disorder characterized by BM failure, developmental abnormalities, myelodysplasia, leukemia, and solid tumor predisposition.⁵ FA allogeneic transplantation is particularly sensitive to conditioning regimen-related toxicity and GVHD. Although small molecule-targeting HSC-niche interaction may transiently open BM niche to facilitate donor HSC engraftment in FA mice,⁶ monoclonal antibodies can augment alloengraftment without chemotherapy or radiotherapy and should have less off-target and off-tumor toxicity. An approach targeting CD117, which is important for HSC proliferation, adhesion, and survival in the niche, by a blocking anti-mouse CD117 monoclonal antibody (mAb), ACK2, was shown to reduce host HSCs to a sufficient magnitude to allow donor BM engraftment in Fanca^−/− recipient mice.⁷ Recent studies using an ADC, anti-CD117-soporin (SAP), showed better donor engraftment than ACK2 alone.⁸ However, since CD117 is not expressed on lymphocytes, the anti-CD117 mAb-based approach leaves behind host lymphoid cells that may participate in donor graft rejection and host niche and cytokine competition with newly produced donor lymphocytes, and the broader expression of CD117 on other cell types is also a concern for potential toxicity.

CD45 is widely expressed in hematopoietic cells and participates in the regulation of lymphocyte activation and development. Because mature leukocytes and BM progenitor cells all express CD45, mAb-based approaches targeting CD45 for recipient HSC depletion have been explored and show promise for hematopoietic cell specific allotransplantation conditioning, by opening marrow niches and permitting donor stem cell engraftment. In a recent study, Fanca^−/− mice were conditioned with anti-CD45-SAP, which resulted in HSC depletion and donor alloengraftment comparable to cyclophosphamide treatment but with less toxicity.⁹ 

In a previous study, Saha et al¹⁰ reported the development of a PBD-based anti-CD45-ADC immunotoxin in mouse transplantation models and showed that its use for conditioning resulted in stable, multilineage donor engraftment. In the follow-up study, Saha et al now show that single-dose anti-CD45-PBD facilitated stable, multilineage chimerism in 3 distinct FA mouse models. The new data demonstrated that anti-CD45-PBD, in a dose-dependent manner, profoundly depleted host stem cell–enriched Lineage-Sca1⁺cKit⁺ cells within 48 hours without clinical toxicity. As shown in the figure, anti-CD45-PBD conditioning of Fanca knockout recipient mice resulted in significantly reduced GVHD compared with myeloablative TBI after MHC-disparate donor BM^+/− donor T-cell transplantation. The anti-CD45-PBD conditioning led to ∼50% donor chimerism 57 days posttransplantation, comparable to that by TBI conditioning, but the GVHD score and survival of the anti-CD45-PBD-conditioned recipients were significantly better than that of the TBI groups (from Figure 7 in Saha et al, which begins on page 2201). Despite potential ADC-mediated cell and organ toxicity that might be exaggerated in FA mice, anti-CD45-PBD-conditioned recipients survived long-term posttransplantation with no clinical GVHD signs. Interestingly, the low inflammatory and tissue injury responses observed in the CD45-ADC conditioning vs lethal TBI conditioning allowed for the infusion of donor T cells that may improve immune surveillance against infections and neoplasms. Additionally, the anti-CD45-PBD conditioning may also deplete host antigen-presenting cells that can play a key role in driving GVHD. Therefore, this anti-CD45-ADC conditioning regimen represents a significant improvement for allotransplantation, especially suitable to FA with minimal regimen-related toxicity, GVHD, and possibly secondary malignancies.

Importantly, the characterized PK/pharmacodynamics (PD) and liver toxicity profiles of anti-CD45-PBD provide a clear therapeutic window in the mouse models.^1,10 To move the findings to the clinic, it is essential to verify the efficacy and toxicity of similar regimens in human HSCs. To this end, the authors have generated preliminary data in rhesus macaques showing that antihuman CD45-ADC is at least as effective as lethal TBI in ablating monkey HSCs. It will be important to see if the antihuman CD45-ADC behaves similar to or better than the mouse anti-CD45-ADC in ablating human HSCs in vitro and in immunodeficient mouse xenograft, which would be needed for future US Food and Drug Administration Investigational New Drug consideration. It remains to be seen how the similar anti-CD45-SAP agent⁹ compares with the anti-CD45-PBD regimen. Careful selection of the anti-hCD45-ADC dose, definition of PK/PD, monitoring potential toxicity and risk for GVHD, and longer-term effects on FA-associated malignancies will all be crucial to successfully translate this elegant approach to clinical HSC transplantation practice.

Conflict-of-interest disclosure: The author declares no competing financial interests.