An Anti-CD300f Antibody Drug Conjugate Depletes Hematopoietic Stem Cells and Primary Acute Myeloid Leukemia (AML): Facilitating a Targeted Conditioning Regimen for Allogeneic Stem Cell Transplantation in AML

Abstract

Allogeneic Hematopoietic Stem Cell Transplantation (allo-HSCT) significantly reduces the rate of relapse in Acute Myeloid Leukemia (AML) but comes at the cost of significant Treatment Related Mortality (TRM). Despite the reduction in relapse overall, it remains common, especially in high risk groups. There is increasing evidence that many cases of relapse are driven by residual AML that can be detected with minimal residual disease techniques. A large proportion of the morbidity that prevents most patients accessing allo-HSCT is due to nonspecific toxic conditioning agents which are required to remove recipient Hematopoietic Stem and Progenitor Cells (HSPC) allowing for successful engraftment. We propose that a targeted conditioning agent against AML and HSPC can facilitate a reduction in relapse and TRM in allo-HSCT. CD300f is expressed evenly across HSPC subtypes. CD300f has equivalent transcription and protein expression as CD33 on AML. We have developed an anti-CD300f antibody which efficiently internalises into target cells. Harnessing this ability to internalise, we have generated a highly potent anti-CD300f Antibody Drug Conjugate (ADC) with a pyrrolobenzodiazepine (PBD) warhead which selectively depletes AML cell lines in vitro. Colony Forming Units were inhibited when cultured in the presence of the ADC. The anti-CD300f ADC has a rapid onset of action with >99% cytotoxicity of the AML cell line HL-60 occurring within 24 hours. The ADC synergises with fludarabine, making it a natural combination to use in a minimal toxicity conditioning regimen. The growth of subcutaneous tumours in mice engrafted with the AML cell line U937 was significantly reduced by a single dose of our ADC (at 150 μg/kg or 300 μg/kg). In a bone marrow engraftment model with HL-60, our ADC prolongs the survival at a single dose of 300 μg/kg (Fig 1A). In a humanised mouse model, 85% of all CD34+ cells and 90% of the primitive CD34+ CD38- CD90+ population were depleted with a single injection of our ADC at 300 μg/kg (Fig1 B). There was significant reduction in primary AML cells with a single injection of our ADC at 300 μg/kg (Fig 1C). This proof of principle work establishes an anti-CD300f ADC as an attractive potential therapeutic to develop which will reduce the toxicity of allo-HSCT and the relapse rate afterwards. We have demonstrated that CD300f is expressed across HSPC and AML. Our antibody to CD300f is efficiently internalised and capable of delivering a PBD warhead. Our ADC synergised with fludarabine and has a rapid onset of action. We have demonstrated that our ADC prolongs survival in mouse cell line models as well as depletes both HSPC and primary AML in vivo. A targeted conditioning agent has the potential to reduce relapse risk and toxicity of allo-HSCT in AML.