Page 124 - 2019_04-Haematologica-web

E. Angelova et al.
The frequency of CD123 expression was higher in B- ALL than in T-ALL (164/183, 89.6% versus 13/30, 43.3%; P<0.0001), and within B-ALL the frequency of CD123 expression was higher in Ph+ versus Ph– B-ALL patients (96.6% versus 86.3%; P=0.033). (Figure 1A) The intensity of CD123 expression across groups of patients showed similar patterns. Namely, the median CD123 RFI was higher in B-ALL compared to T-ALL (14.45 versus 2.35; P<0.001), and within B-ALL it tended to be higher in Ph+ compared with Ph– patients, although the difference did not attain statistical significance (20.85 versus 11.8; P=0.08) (Figure 1B). We analyzed our findings using var- ious CD123 MFI-based cutoffs of 20%, 30% 50%, as well as RFI ≥10. All conclusions remained valid at cutoff levels of 20% and 30% as well as RFI ≥10 but not at the 50% cutoff level (data not shown).
In the T-ALL group (40% thymic; 33% early non-ETP; 20% ETP; 7% mature), the median CD123+ blast propor- tion was 17.95% (range, 0.5-93%) with a median RFI of 2.35 (range, 0-101). We identified a correlation (P=0.001) between T-ALL immunophenotypic subgroups and CD123 expression status; namely, 12/13 (92.3%) patients with CD123+ T-ALL had either ETP or early non-ETP immunophenotype.
Correlation between CD123 expression and clinical parameters
CD123 expression status did not correlate with mini- mal residual disease status at the end of induction. The median follow-up duration for the entire group was 23 months (range, 1-79 months) and was comparable for patients with CD123+ and CD123– ALL. There was no difference in Performance Status between CD123+ and CD123– ALL patients in any of the clinical subgroups (data not shown). At last follow up, 69 (32.4%) patients were dead. There was no difference in overall survival between patients with CD123+ and CD123– ALL within any of the subgroups. However, for T-ALL patients, the relapse-free survival rate was more favorable in those with CD123– disease compared with those with CD123+ disease (P=0.03) (Online Supplementary Figure S1). Similar results were obtained when patients under 18 years of
AB
age were excluded. Notably, the impact of CD123 expression on relapse-free survival correlated with the intensity of CD123 expression in adult T-ALL using RFI ≥10 vs. <10 as a cutoff (P=0.015) (Online Supplementary Figure S2). For patients with B-ALL, CD123 expression showed no correlation with relapse-free survival (Online Supplementary Figure S3).
We performed multivariate analysis using Cox regres- sion modeling (Wald backward stepwise method) to evaluate the impact of CD123 expression status on relapse-free survival alongside other prognostic variables that included: minimal residual disease status at com- plete remission, central nervous system involvement, and age (patients <18 years excluded). Analyses were conducted in each of the clinical subgroups separately. In the Ph+ B-ALL group only minimal residual disease status at complete remission was independently associated with relapse-free survival (P=0.019), and in the Ph– B-ALL group only central nervous system involvement was independently associated with relapse-free survival (P=0.005). In the T-ALL group, none of the variables was associated with leukemia-free survival.
In vitro anti-leukemia activity of IMGN632 in acute lymphoblastic leukemia
IMGN632 is a conjugate of the CD123-binding antibody G4723A with a novel DNA-alkylating IGN payload, DGN549. The numbers of binding sites for the antibody component of IMGN632 were quantified on leukemic blasts and on lymphocytes from 21 patients with newly diagnosed or relapsed/refractory B-ALL (Online Supplementary Figure S4). As described above, the blast pop- ulation was identified based on the CD45dim/SSClow/med/CD19+/CD34+ profile, while the lym- phocyte population was identified based on CD45bright/SSClow characteristics. Of the 21 samples ana- lyzed, 20 expressed the CD123 antigen, with a median number of ABC of 1,085 on leukemic blasts and 57 on lym- phocytes. There was no appreciable difference in the num- ber of ABC between blasts from newly diagnosed and relapsed/ refractory patients.
The cytotoxicity of IMGN632 was assessed in a panel of
Figure 1. Expression of CD123 on leukemic blasts in B and T acute lymphoblastic leukemia/lymphoma by multiparameter flow cytometry immunophenotyping.
(A) Percentage of leukemic blasts with CD123 expression based on mean fluorescence intensity relative to background in each of the groups of patients. CD123 expression was significantly different between B-acute lymphoblastic leukemia/lymphoma (ALL) and T-ALL (P<0.0001; Mann-Whitney U test). In addition, CD123 expression was significantly different between Philadelphia chromosome (Ph)-positive and Ph-negative patients (P=0.033; Mann-Whitney U test). (Lines: median, 25th-75th percentiles). (B) Relative fluorescence intensity (RFI) on leukemic blasts compared to non-leukemic gated events (lines: mean ± standard deviation). Ph: Philadelphia chromosome and/or BCR/ABL1 fusion status.
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haematologica | 2019; 104(4)