The Antibody-Drug Conjugate (ADC) IMGN779 Is Highly Active in Vitro and in Vivo Against Acute Myeloid Leukemia (AML) with FLT3-ITD Mutations

IMGN779 is a CD33-targeted ADC utilizing DGN462, a novel DNA-alkylating agent consisting of an indolino-benzodiazepine dimer containing a mono-imine moiety. CD33 is expressed on the surface of about 90% of AML cases, with elevated levels of CD33 found in cases having molecular markers associated with poor prognosis, including mutations in FMS-like tyrosine kinase 3 (FLT3). The internal tandem duplication mutation (FLT3-ITD) is the most common FLT3 mutation, present in about 20-25% of AML cases. Patients with FLT3-ITD AML have a worse prognosis than those with wild-type (WT) FLT3, with an increased rate of relapse and a shorter duration of response to induction chemotherapy.

IMGN779 was found to demonstrate targeted activity against AML cell lines in vitro, with IC₅₀ values ranging from 2-3,000 pM. The MV4-11 cell line, which has a FLT3-ITD mutation, was the most sensitive to IMGN779 of the cell lines tested, with an IC₅₀ of 2 pM. We evaluated the in vivo activity of IMGN779 against MV4-11 xenografts in SCID mice; IMGN779 was highly active (T/C = 1 %) at a single dose of 0.6 mg/kg (conjugate dose, 10 µg/kg DGN462 dose), resulting in complete tumor regressions (CR) in 3/6 animals and partial regressions (PR) in 6/6 animals. A DGN462-ADC to a non-relevant target was inactive (T/C = 95%) at the same dose, demonstrating that the activity of IMGN779 was due to its CD33 targeting. IMGN779 has previously been shown to be highly active against AML xenograft models without FLT3-ITD mutations, at minimally efficacious doses of 0.6 mg/kg (10 µg/kg DGN462), demonstrating that the presence of FLT3-ITD does not confer resistance to IMGN779 treatment.

IMGN779 was also highly active in vitro against primary patient AML cells isolated from peripheral blood or bone marrow samples. Patient AML cells with FLT3-ITD were more sensitive to IMGN779 compared with FLT3 WT AML samples. IC₅₀ values in FLT3-ITD samples ranged from 10 to 300 pM. CD33 expression was generally greater on FLT3-ITD leukemic blast cells than on FLT3 WT blasts, which likely contributed to their increased sensitivity to IMGN779. In long term cultures, IMGN779 showed a dose dependent decrease in leukemic stem cell (LSC) colony formation using an AML patient sample with both FLT3-ITD and NPM1 mutations, which are an even worse prognostic marker than FLT3-ITD alone. In contrast, colony formation increased in normal bone marrow, indicating that normal hematopoietic stem cells (HSCs) were spared.

The differential expression of CD33 on LSC compared to HSCs makes CD33 an attractive target for treatment of AML, with the potential to eliminate LSCs and, thus, minimal residual disease in FLT3-ITD AML. The potent in vitro activity of IMGN779 against FLT3-ITD AML cell lines and primary patient FLT3-ITD AML progenitor cells and LSCs and its high level of CD33-targeted in vivo activity in a FLT3-ITD AML xenograft model support the advancement of IMGN779 as a potential treatment for AML, including FLT3-ITD AML.