Acute myeloid leukemia (AML), the most common type of acute leukemia in adults which is also known as acute nonlymphocytic leukemia, is a cancer of the blood and bone marrow. In healthy individuals, the bone marrow makes blood stem cells (immature cells). Over time, these blood stem cells become mature blood cells — either myeloid or a lymphoid stem cells.
In acute myeloid leukemia the presence of acquired genetic alterations in hematopoietic progenitor cells found in the bone marrow alter the normal mechanisms of cell growth, proliferation and differentiation, affecting these cells that would normally turn into white blood cells. As a consequence of the disease, myeloid stem cells become a type of immature white blood cell called myeloblasts (or myeloid blasts). Blocking differentiation as well as unchecked proliferation, AML results in the accumulation of myeloblasts at the expense of normal hematopoietic precursors.
If untreated, AML spread to the blood and to other parts of the body including lymph nodes, liver, spleen, brain and spinal cord and testicles, and quickly becomes fatal.
The American Cancer Society estimates approximately 18,860 new cases of AML and 10,460 deaths from the disease in the United States in 2014. Most patients will be adults. The disease is slightly more common among men than among women. The lifetime risk of getting AML for the average man is about 1 in 227; for the average woman the risk is about 1 in 278.
CD33 and acute myeloid leukemia
CD33, a 67-kDa glycoprotein, is expressed on the surface of about 90% myeloid leukemia cells as well as on normal myeloid and monocytic precursors. In AML, elevated levels of CD33 are found in cases having molecular markers associated with poor prognosis, including mutations in FMS-like tyrosine kinase 3 (FLT3). This makes CD33 an attractive target for monoclonal antibody-based therapy of acute myeloid leukemia.
Gemtuzumab ozogamicin (Mylotarg®; Wyeth/Pfizer), an anti-CD33 antibody conjugated to the antitumor antibiotic calicheamicin, improved survival in a subset of AML patients when combined with standard chemotherapy. Safety concerns led to US marketing withdrawal of this drug in 2010. However, as result of the demonstrated activity of gemtuzumab ozogamicin, CD33 remained a viable therapeutic target for AML.
Research has shown that activating mutations in the FMS-like tyrosine kinase 3 (FLT3) gene are recognized one of the most frequently encountered, and clinically challenging, class of mutations in acute myeloid leukemia. The internal tandem duplication mutation (FLT3-ITD), in turn, is the most common FLT3 mutation.
This mutation is present in about 20-25% of cases of de novo acute myeloid leukemia. Patients with FLT3-ITD AML have a particularly poor prognosis – notable worse – compared to patients with wild-type (WT) FLT3. with an increased rate of relapse and a shorter duration of response to induction chemotherapy. These patients often present with a more aggressive disease, have a significantly increase rate of relapse after remission and a shorter duration of response to induction chemotherapy.
In preclinical trials, IMGN779, a novel, CD33-targeting antibody-drug conjugate or ADC utilizing DGN462, a novel DNA-alkylating agent consisting of an indolino-benzodiazepine dimer containing a mono-imine moiety, being developed by ImmunoGen, Inc., Waltham, MA, showed to be a potential treatment for acute myeloid leukemia. Initial data confirming IMGN779’s potential were presented in June 2014 at the 19th Congress of the European Hematology Association (EHA) meeting in Milan, Italy (abstract# P802).
Now, in updated preclinical research, results presented at the 56th annual meeting and exposition of the American Society of Hematology, being held in San Francisco, December 6 – 9, 2014, 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.
Kathleen Whiteman, MS, and a team of researchers at ImmunoGen, and Paul Noordhuis, PhD, and his colleagues from the Department of Hematology, VU University Medical Center, Amsterdam, The Netherlands, evaluated the in vivo activity of IMGN779 against MV4-11 xenografts in SCID mice – which are severely deficient in B and T lymphocytes. At a single dose of 0.6 mg/kg (conjugate dose, 10 µg/kg DGN462 dose), IMGN779 was highly active (T/C = 1 %). The results was complete tumor regressions (CR) in 3/6 animals and partial regressions (PR) in 6/6 animals. 
The researchers also looked at DGN462-ADC to a non-relevant target and found that this was inactive (T/C = 95%) at the same dose, suggesting that the activity of IMGN779 was realy 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.
Peripheral blood or bone marrow samples
IMGN779 was also highly active in vitro against primary patient acute myeloid leukemia cells isolated from peripheral blood or bone marrow samples. Patient acute myeloid leukemia cells with FLT3-ITD were more sensitive to IMGN779 compared with FLT3 WT (“Wild Type”) acute myeloid leukemia 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 (“Wild Type”) blasts. This may have, most likely, contributed to their increased sensitivity to IMGN779.
The researchers also noted that in long term cultures, IMGN779 showed a dose dependent decrease in leukemic stem cell or LSC colony formation using an acute myeloid leukemia 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 or HSCs were spared.
The differential expression of CD33 on leukemic stem cell compared to hematopoietic stem cells makes CD33 an attractive target for treatment of acute myeloid leukemia, with the potential to eliminate leukemic stem cell and, thus, minimal residual disease in FLT3-ITD acute myeloid leukemia. The potent in vitro activity of IMGN779 against FLT3-ITD acute myeloid leukemia cell lines and primary patient FLT3-ITD acute myeloid leukemia progenitor cells and leukemic stem cell and its high level of CD33-targeted in vivo activity in a FLT3-ITD acute myeloid leukemia xenograft model support the advancement of IMGN779 as a potential treatment for acute myeloid leukemia, including FLT3-ITD acute myeloid leukemia.