CD33 target validation and sustained depletion of AML blasts in long-term cultures by the bispecific T-cell–engaging antibody AMG 330

Abstract

Key Points• CD33 expression levels in AML correlate with specific disease characteristics.• Potent cytotoxicity against primary AML blasts is mediated by a CD33/CD3-bispecific antibody.Antibody-based immunotherapy represents a promising strategy to target and eliminate chemoresistant leukemic cells. Here, we evaluated the CD33/CD3-bispecific T cell engaging (BiTE) antibody (AMG 330) for its suitability as a therapeutic agent in acute myeloid leukemia (AML). We first assessed CD33 expression levels by flow cytometry and found expression in >99% of patient samples (n 5 621). CD33 was highest expressed in AMLs with NPM1 mutations (P < .001) and lower in AMLs with complex karyotypes and t(8;21) translocations (P < .001). Furthermore, leukemic stem cells within the CD34 1 / CD38 2 compartment displayed CD33 at higher levels than healthy donor stem cells (P 5 .047). In MS-5 feeder cell-based long-term cultures that supported the growth of primary AML blasts for up to 36 days, AMG 330 efficiently recruited and expanded residual CD3 1 /CD45RA 2 /CCR7 1 memory T cells within the patient sample. Even at low effector to target ratios, the recruited T cells lysed autologous blasts completely in the majority of samples and substantially in the remaining samples in a time-dependent manner. This study provides the first correlation of CD33 expression levels with AML genotype in a comprehensive analysis of adult patients. Targeting CD33 ex vivo using AMG 330 in primary AML samples led to T cell recruitment and expansion and remarkable antibody-mediated cytotoxicity, suggesting efficient therapeutic potential in vivo. (Blood. 2014;123(3):356-365)

Dual-targeting triplebody 33-16-123 (SPM-2) mediates effective redirected lysis of primary blasts from patients with a broad range of AML subtypes in combination with natural killer cells

“…A loose correlation between these two properties was observed for bulk AML cells (Figure 3), and similar correlations have been reported for the CD33-specific BiTE AMG330, 9 the CD123 antibody Talacotuzumab (CSL 362), 48,49 and the CD3 x CD123 DART agent MGD006. 56 While the target antigen density likely dictates the strength of binding of these agents to the target cells and the subsequent engagement of killer cells, it is unlikely that binding strength alone will translate 1:1 into an enhanced lytic activity.…”

“…A loose correlation between cytolytic and cytotoxic activity and target antigen density has been reported for the CD33-specific agents AMG330 9 and GO. 72 Therefore, we studied here, whether a similar correlation also existed for SPM-2 plus NK cells.…”

“…9 In this study surface densities of CD33 were determined for samples from 621 patients and CD33 was strongly expressed on the blasts from 99% of the patients. However, the surface expression level of CD33 was not the sole determinant of lytic activity, although the degree of lysis differed between strongly and weakly expressing cells.…”

“…Interestingly, monocytes and dendritic cells (DCs) showed strong expression of CD33, but no lysis of DCs by AMG 330 plus T cells was observed in RDL assays. 9 Data about lysis of monocytes and DCs by SPM-2 plus NK cells are not yet available.…”

“…Blasts from 85–90% of AML patients as well as normal myeloid progenitors and myelocytes express CD33. 8,9 Expression is restricted to normal and malignant hematopoietic cells, including AML-LSCs, 10–13 and therefore, CD33 is a promising target for immunotherapies of AML. 8,10,13 …”