Prognostic relevance of CD20 in adult B-cell precursor acute lymphoblastic leukemia

We read with great interest the recent article by Maury et al. regarding the influence of CD20 expression on prognosis in adult B-cell precursor acute lymphoblastic leukemia (BCP-ALL).1 This report described their experience with 143 Philadelphia chromosome negative adult BCP-ALL patients treated with GRAALL-2003, a pediatric-like protocol with high dosage of L-asparaginase.2 Their cohort included 46 (32%) cases with at least 20% of cells expressing CD20. Univariate analysis of CD20 positivity showed no statistically significant effect on overall survival (OS), cumulative incidence of relapse (CIR), and event-free survival (EFS). However, the authors found that CD20 positivity was associated with higher CIR and shorter EFS among the subgroup of patients with high leukocyte count (>30×10/L); they also found CD20 expression correlated with higher CIR e in multivariate analysis of the cohort.

We conducted a similar study analyzing the impact of CD20 expression in BCP-ALL. We evaluated 119 patients diagnosed with de novo BCP-ALL between the ages of 18 and 60 (median 40 years) at our institute. There were 49 (41%) patients with at least 20% expression of CD20. All patients were initially treated with a modified pediatric protocol including L-asparaginase similar to the one used in the GRAALL-2003 study, as previously described.3,4 Philadelphia chromosome positive patients received tyro-sine kinase inhibitors in addition to this chemotherapy. In total, 32 patients received allogeneic stem cell transplants; these included 14 (29%) of the CD20-positive cases and 18 (26%) of the CD20-negative cases (P=0.83). CD20 positivity was associated with low platelet counts (P=0.004) but not with any other characteristics at diagnosis, including age (median 40.1 years vs. 40.0 years) and expression of CD19, CD13, and CD33.

In univariate analysis using the log-rank test, we found no effect of CD20 expression on OS, CIR, or EFS (P=0.18, P=0.40, and P=0.15, respectively). Leukocyte count greater than 30×10/L was associated with poorer OS (median 19.9 months vs. not reached, P<0.001), CIR (48% vs. 26% at two years, P=0.01) and EFS (median 10.6 months vs. not reached, P<0.001). Philadelphia chromosome positivity correlated with poorer OS (median 40.8 months vs. not reached, P=0.01), CIR (48% vs. 23% at two years, P=0.04), and EFS (median 19.3 months vs. not reached, P=0.004). Age over 40 had a significant adverse impact on OS (median 80.6 months vs. not reached, P=0.05) but not CIR (P=0.41) or EFS (P=0.10). Allogeneic stem cell transplants were associated with shorter OS (median 40.6 months vs. 83.0 months, P=0.02), but had no significant difference on cumulative incidence of relapse (P=0.46) or EFS (P=0.10).

Multivariate analysis with Cox’s proportional hazards regression was performed on CD20 as well as the four factors identified as significant above: high leukocyte count, Philadelphia chromosome positivity, older age, and allogeneic stem cell transplant. Only high leukocyte count was an independent predictor of an adverse outcome for OS (HR=2.9 [95% CI 1.4–5.8], P=0.003), CIR (HR=2.7 [95% CI 1.1–6.8], P=0.03) and EFS (HR=2.8 [95% CI 1.5–5.22], P=0.002). CD20 positivity did not significantly influence OS (P=0.11) or CIR (P=0.18), and had a trend towards a favorable EFS (HR=0.6 [95% CI 0.3–1.1], P=0.07; Table 1).

Based on the findings by Maury et al., we also performed subgroup analysis of CD20 expression in the low and high leukocyte count groups. Like them, we found no effect of CD20 expression on outcome in the low leukocyte count subgroup (OS: P=0.48, CIR: P=0.35, EFS: P=0.34). However, in contrast to their findings, we found no association between CD20 positivity and adverse outcome among the high leukocyte count subgroup (OS: median 40.2 months vs. 12.4 months, P=0.17, Figure 1A; CIR: 47% vs. 52% at two years, P=0.63, Figure 1B; EFS: median 33.1 months vs. 9.6 months, P=0.18, Figure 1C).

According to the analysis of Maury et al., we then excluded the 14 Philadelphia chromosome positive patients in this high leukocyte count subgroup. Among the 14 remaining patients, there were 5 positive for CD20. All achieved complete remission (CR) on first induction, 2 later relapsed, and one died of relapsed disease. Of the 9 CD20-negative patients, 8 achieved CR on first induction, 2 later relapsed, and 7 died: 3 were disease-related and 4 (in CR) were due to other causes. Four of 5 CD20-positive patients received at least 80% of the planned L-asparaginase dose, as well as 6 of the 8 CD20-negative patients who achieved CR (P=1.00). Three of the 9 CD20 negative patients had the t(4;11) karyotype. Contrary to the results of Maury et al., we found no correlation between CD20 and CIR (P=0.35) or EFS (P=0.16) among this subgroup; surprisingly, CD20-positive patients in this group appeared to have a longer OS (median not reached vs. 9.6 months, P=0.023).

Table 1.Multivariate analysis for overall survival, cumulative incidence of relapse, and event-free survival in 119 patients with adult BCP-ALL.

The article by Maury et al. was the second to report on the prognostic significance of CD20 in adult BCP-ALL. The first was by Thomas et al. who described their experience with 110 patients treated with the VAD/CVAD regimen and 143 patients treated with the hyper-CVAD protocol.5 Philadelphia chromosome positive patients were included. They found that CD20 positivity was an independent predictor of earlier relapse and shorter OS. The discrepancy between our results and those of Thomas et al. may be primarily due to differences in treatments, especially our intensive use of L-aspiraginase. In recent years, such non-myelotoxic drugs, as adopted from pediatric regimens, have been shown to lead to marked improvements in adult BCP-ALL treatment.2,4,6 Conflicting results surrounding CD20 expression have also been reported in pediatric BCP-ALL. Borowitz et al.7 found that increased CD20 intensity predicted a poorer EFS among a group of patients treated with POG protocols, while Jeha et al.8 reported that CD20 expression was not an adverse factor with St Jude protocols. Differences in chemotherapy regimens may account for much of the observed differences of CD20 significance.

Figure 1.Influence of CD20 expression on the following outcomes in high leukocyte count (>30x10⁹/L) adult BCP-ALL patients (includes the Philadelphia chromosome subset): (A) Overall survival, (B) Cumulative incidence of relapse, and (C) Event-free survival.

The study by Maury et al. and our study looked at similarly sized cohorts treated with similar protocols. Univariate analysis of both cohorts showed no statistical significance of CD20. However, differences arise in the multivariate and subgroup analyses, even when Philadelphia chromosome positive cases were excluded: Maury et al. found poorer outcome associated with CD20 expression while our results showed a trend towards a better outcome among CD20-positive patients. Much of this difference may be attributed to the limited size of both cohorts. Given the variance in results among these studies, the prognostic significance of CD20 expression in B-cell precursor acute lymphoblastic leukemia needs to be reassessed within each treatment regimen, using prospective studies with a larger sample size.

Acknowledgments

we thank the staff of the Department of Medical Hematology and Oncology at the University Health Network for their outstanding care of the patients in this study.