Interleukin 2 Receptor-α (CD25) Expression Is Associated with Shortened Overall Survival and Resistance to Induction Therapy with Plerixafor and Decitabine in Older Patients with Newly Diagnosed Acute Myeloid Leukemia (AML)

Introduction: Interleukin 2 receptor-α (CD25) expression on myeloid leukemic blasts may be a marker for chemotherapy-resistant leukemia stem cells (Saito et al., 2010) and has been associated with poor overall survival (OS) in AML patients (pts) <60 years treated with cytotoxic chemotherapy (Terwijn et al., 2009; Gonen et al., 2012; Cerny et al., 2013). The prognostic impact of CD25 expression in older pts remains unclear. We therefore retrospectively analyzed CD25 expression in baseline bone marrow (BM) of newly diagnosed AML pts >60 years enrolled in a Phase I clinical trial combining the CXCR4 antagonist, plerixafor and the DNA methyltransferase inhibitor, decitabine (Roboz et al., 2013).

Methods: BM aspirates were available for 69 newly diagnosed older AML pts treated with 1-4 cycles of 5 days of plerixafor combined with 10 days of decitabine. Pts with favorable risk cytogenetic or mutational profiles were excluded from the clinical trial. Multi-parameter flow cytometry was used to evaluate the expression of CD25 in blast and progenitor (CD34+) populations. Cells were gated on CD45^dim/SSC^lo characteristics. Pts were considered positive (CD25+) when greater than 10% of the gated population expressed CD25.

Results: Of 69 pts, 58 were evaluable for survival and 57 for response; one pt died prior to scheduled response assessment. Of 58 pts evaluated at baseline (pre-treatment BM), 20 (34.5%) were CD25+ vs. 38 (65.5%) CD25-negative (CD25-). CD25+ pts had significantly inferior median OS (152 days vs. 419 days, p=0.003) and were at higher risk of dying within 1 year of diagnosis, relative risk (RR) 1.58 (95% 1.04-2.41). Similarly, pts surviving less than 1 year had significantly higher percentages of CD34+ cells expressing CD25 than those who lived greater than 1 year (7.82% vs. 4.77%, p=0.028). CD25+ pts were less likely to respond to therapy, RR 1.90 (95% CI 1.23-2.93) and, in turn, pts who were resistant to therapy had higher baseline CD25 expression level than those who responded (7.82% vs. 4.87%, p=0.033).

Five CD25+ pts (25%) and 12 CD25- pts (32%) received an allogeneic transplant. Transplanted CD25+ pts had improved OS vs. CD25+ pts without transplant, (median OS not reached (NR) vs. 107 days), p=0.005. In contrast, there was no significant difference in survival between CD25- pts with and without allogeneic transplant, p=0.96. Also, there was no difference in median OS between CD25+ pts receiving transplant vs. CD25- pts (median OS NR vs. 419 days), p<0.40. There was no difference in survival between CD25+ pts with intermediate risk cytogenetics vs. CD25+ pts with unfavorable cytogenetics (OS 153 vs. 172, p=0.77). Lastly, CD25+ pts had significantly worse OS compared to CD25- pts with unfavorable cytogenetics, (median OS 152 vs. 333 days) respectively, p=0.05.

Compared to CD25- pts, CD25+ pts were older (median age 74.5 vs. 71.5, p=0.096), more likely to be male (75% vs. 47.3%, p=0.055) and had higher baseline WBC (19x1000/uL vs. 5x1000/uL, p=0.089) and pretreatment lactate dehydrogenase (LDH) (median 365 vs. 271, p=0.04). Analysis of diagnosis BM blast percentage yielded no difference between CD25+ and CD25- pts (62% and 46%, p=0.44). Sixteen (80%) and 4 (20%) of CD25+ patients had intermediate and unfavorable cytogenetics vs. 21 (55%) and 17 (44%) CD25- pts respectively, p=0.09. No significant difference between groups was noted when evaluating the mutational status of TET2, TP53, RUNX1, DNMT3A, NPM1, or FLT3.

Conclusions: Interleukin 2 Receptor-α expression on leukemic blasts is known to correlate with poor prognosis and OS in young pts with AML who have been treated with cytotoxic chemotherapy. We have demonstrated that >10% CD25 expression on CD34+ blasts is associated with poor OS and resistance to therapy in AML pts > 60years of age treated with the combination of plerixafor and decitabine. Pts with >10% CD25 expression on CD34+ cells were at increased risk of death within one year and increased risk of resistance to induction therapy. Thirty-four percent of the pts in this study were CD25+, consistent with previous reports (Terwijn et al., 2009). This study highlights the importance of CD25 expression on CD34+ leukemic cells in determining prognosis, OS, response to hypomethylating agent therapy and benefit of transplant in older pts with newly diagnosed AML. Further investigations into the aggressive nature of CD25+ AML, mechanisms of resistance and novel therapeutics are ongoing.