BCR/ABL+ CML Stem Cells (CD34+/CD38-) Express High Levels of CD33 and Are Responsive to a CD33-Targeting Drug: a New Potential Concept for Eradication of CML Stem Cells.
Authors: Herrmann Harald , Cerny-Reiterer Sabine , Gleixner Karoline V. , Blatt Katharina , Herndlhofer Susanne , Agis Hermine , Rabitsch Werner , Streubel Berthold , Mitterbauer-Hohendanner Gerlinde , Sperr Wolfgang R. , Sillaber Christian , Valent Peter 
Published: October 2019

Abstract
Abstract 3382
Siglec-3 (CD33) is an established therapeutic target in acute myeloid leukemia (AML). We and others have shown that CD33 is expressed on immature CD34+/CD38- stem cells in AML. We here report that leukemic stem cells obtained from patients with chronic myeloid leukemia (CML) display high levels of CD33, and that CD33 may serve as a potential target in CML. As assessed by multi-color flow cytometry, CD34+/CD38-/CD123+ CML progenitor cells in chronic phase (CP) were found to express significantly higher levels of CD33 compared to normal CD34+/CD38- bone marrow stem cells (figure). By contrast, similar levels of the cell surface antigen MDR1 (CD243) were detected when comparing normal and CML progenitors. In chronic phase (CP) CML, CD33 was found to be expressed homogeneously on most or all CD34+/CD38- stem cells. In patients with accelerated (AP) or myeloid blast phase (BP), CML stem cells also co-expressed CD33, but the levels of CD33 varied from donor to donor, and in one patient, most CML stem cells appeared to be CD33-negative cells. In two patients with CML, CD34+/CD38- cells were highly enriched by cell sorting (purity >98%) and found to contain CD33 mRNA in qPCR analysis. The presence of BCR/ABL and thus the leukemic origin of these cells was confirmed by FISH analysis and PCR. We then examined the effects of the CD33-targeted drug gemtuzumab/ozogamicin (GO) on growth of primary CML cells. As assessed by 3H-thymidine uptake, GO produced growth inhibition in leukemic cells in all patients tested (CP, n=13; AP, n=3). The effects of GO on leukemic cell growth were dose-dependent and occurred at relatively low concentrations, with IC50 values ranging between 1 and 100 ng/ml. GO effects were also seen in precursor-enriched Lin-negative CML cells (n=3). As assessed by Annexin-V staining, GO was found to induce apoptosis in CD34+/CD38- CML progenitor cells. Next we investigated drug combination effects. In these experiments, GO was found to synergize with nilotinib and bosutinib in producing growth inhibition in primary CML cells. In conclusion, CD33 is expressed abundantly on immature CD34+/CD38- progenitor cells in CML. Whether GO can be employed to eradicate residual leukemic stem cells in CML patients alone or in combination with BCR/ABL kinase inhibitors remains at present unknown.
Figure: Expression of CD33 on CD34+/CD38- cells in normal bone marrow (n=7) and patients with CML in chronic phase (CP, n=16) or advanced phase (AP/BP, n=11) of the disease. Figure:. Expression of CD33 on CD34+/CD38- cells in normal bone marrow (n=7) and patients with CML in chronic phase (CP, n=16) or advanced phase (AP/BP, n=11) of the disease.

Disclosures:
Valent: Novartis: Research Funding; Bristol-Myers Squibb: Research Funding.