A Single Dose of a Novel Anti-Human CD117-Amanitin Antibody Drug Conjugate (ADC) Engineered for a Short Half-Life Provides Dual Conditioning and Anti-Leukemic Activity and Extends Survival Compared to Standard of Care in Multiple Preclinical Models of Acute Myeloid Leukemia (AML)

Hematopoietic stem cell transplant (HSCT) can be a highly effective, and often curative, treatment for patients with AML. At present, myeloablative conditioning (MAC) regimens are associated with severe acute and long-term toxicities. A subset of transplant-eligible AML patients undergoes reduced-intensity conditioning (RIC) regimens; while reducing toxicities, RIC regimens have higher incidence of post-transplant relapse and graft failure vs MAC. Thus, the need for safe and effective targeted conditioning agents for a broader use in transplant in both malignant as well as non-malignant settings is critical.

We developed MGTA-117, an anti-human CD117 (c-kit)-targeted antibody (Ab) engineered for a short-half life (t_1/2) conjugated to amanitin, an RNA polymerase II inhibitor, to enable ADC clearance prior to HSCT. MGTA-117 has a t_1/2 of 91 hours in humanized NSG (hNSG) mice compared to the parental wild type (WT) Ab with a t_1/2 of 217 hours in human FCRN mice. Via optimization of the linker-toxin moiety, the maximum tolerated dose (MTD) in C57BL/6 mice was improved 17-fold compared to previous generations of the ADC. The antibody component specifically binds CD117, which is expressed on hematopoietic stem and progenitor cells (HSPC) and in ~80% of patient cells with AML and MDS (Gao et al. PLOS One. 2015). When conjugated to amanitin, this ADC robustly depletes both CD117+ HSPCs and leukemic blasts. This targeted and optimized approach not only broadens the therapeutic window across preclinical models, but also provides the dual benefit of effective conditioning for HSCT and reduction of target-expressing tumor cells.

Previously we have shown MGTA-117 elicits potent cytotoxicity on both primary human CD34+ cells and a CD117+ AML cell line in vitro. In hNSG mice, a single dose (0.1 mg/kg) of MGTA-117 selectively depleted ≥95% human HSPCs (Figures 1A and B).

To demonstrate anti-leukemic activity of MGTA-117, we studied it in multiple human leukemic xenograft murine models that mimic untreated and refractory AML, including both cell line derived (Kasumi-1, CD117-expressing leukemia cell line, in NSG mice) and patient derived (AML PDX 1: J000106134, treatment naïve AML PDX and AML PDX 2: J000106132, heavily pretreated relapsed refractory AML PDX in NSG-SGM3 mice) xenograft models.

MGTA-117 was well-tolerated in all three AML xenograft models both as a single dose (1-10 mg/kg) and as a multi-dose (3 mg/kg QODx2) regimen. In the Kasumi-1 model (Figure 1C), MGTA-117, both as a single dose (3 & 10 mg/kg) and as a multi-dose (3 mg/kg QODx2), resulted in a 2.4-2.7-fold increase in median survival compared to vehicle (PBS), isotype-ADC, or standard of care (SOC) cytarabine (ARA-C, 30 mg/kg QDx5). Furthermore, a single dose of MGTA-117 significantly decreased peripheral tumor burden leading to delayed tumor growth resulting in a 2-3.3 fold and 1.3-1.8 fold increase in median survival in the treatment naïve AML PDX 1 and highly pretreated AML PDX 2 models (respectively) compared to vehicle, isotype-ADC and SOC. ARA-C treatment demonstrated a modest improvement in survival in both the AML PDX 1 (1.3 fold), AML PDX 2 (1.3 fold) models compared to control.

These data demonstrate that MGTA-117 has potential to be a potent targeted conditioning and anti-leukemia agent. Together with prior reports demonstrating MGTA-117 as an effective conditioning agent in an animal model, this targeted ADC approach has the potential to improve HSCT outcomes in AML by reducing leukemic burden peri-transplant and may decrease the toxicities associated with current conditioning regimens.