Clinical Significance of Mixed Phenotype in Adult Patients with Philadelphia Chromosome-Positive Acute Leukemia–no Prognostic Impact in the Imatinib Era.

Mixed phenotype acute leukemia (MPAL) has historically been known as biphenotypic acute leukemia (BAL), and novel diagnostic criteria for this disease entity are described in the World Health Organization (WHO) classification 4th edition. As the most common recurrent genetic abnormality observed in MPAL is the bcr-abl fusion gene, Philadelphia chromosome-positive MPAL (Ph+MPAL) has been recognized as one distinctive disease entity. The prognosis of Ph+B-cell acute lymphoblastic leukemia (Ph+B-ALL) has been dramatically improved with the introduction of imatinib, and the goal of this study was to determine whether imatinib results in a survival benefit in the context of Ph+MPAL.

We retrospectively analyzed 42 consecutive adult patients who were diagnosed with Ph+AL between January 2001 and March 2012 at Gunma University Hospital and Saiseikai Maebashi Hospital in Gunma, Japan. Ph+AL was diagnosed based on detection of the bcr-abl fusion gene with the polymerase chain reaction method and the presence of more than 20% of blasts in the peripheral blood and/or bone marrow. Patients with a previous history of chronic myelogenous leukemia were excluded. The lineage of leukemia cells was defined according to the WHO classification 4^th edition. All patients received intensive chemotherapy and concurrent administration of imatinib. The c²-test was used for comparison of binary variables. The Mann-Whitney U test was used for comparison of continuous variables. Overall survival (OS) rate was estimated by the Kaplan-Meier method and were compared using the log-rank test. P < 0.05 was considered as statistically significant.

According to the WHO classification 4th edition, 13 (31%) patients were categorized as Ph+MPAL (positive for both myeloid and B-cell lineage), 27 (64%) patients were categorized as B-cell lineage acute lymphoblastic leukemia (Ph+B-ALL), and two (5%) patients were categorized as acute myeloid leukemia (Ph+AML). Patients with Ph+AML were excluded from this study, as the number of patients was relatively small. Of the 40 Ph+AL patients, 23 patients were men, and 17 were women, and the median age was 53 years (range, 16–75 years). Age, sex, white blood cell counts, lactate dehydrogenase levels, and the prevalence of additional cytogenetic abnormalities at diagnosis were not significantly different when comparing the groups, although patients with Ph+MPAL showed significantly higher frequency of major bcr-abl gene than those with Ph+B-ALL (69% and 19%, respectively; p < 0.01). Immunophenotypic analysis revealed that Ph+MPAL patients expressed CD10 and CD34 with significantly lower frequency than Ph+B-ALL patients. Notably, positivity of myeloid antigens (CD13 and 33) was similar between both groups. The complete response (CR) rates after the initial induction therapy were not significantly different when comparing Ph+MPAL and Ph+B-ALL (100% vs. 85%, respectively, p = 0.14). Likewise, the 5-year-OS rate was similar when comparing patients with Ph+MPAL and Ph+B-ALL (55% vs. 53%, respectively, p = 0.87). Of the 13 patients with Ph+MPAL, six patients received AML-type chemotherapy, and seven patients received ALL-type chemotherapy as the initial induction therapy. All patients achieved CR after the initial induction therapy, and there was no significant difference in 5-year OS according to the therapeutic strategy (AML-type vs. ALL-type), (50% vs. 63%, respectively, p = 0.71). Among 12 patients younger than 65 years old who were alive at >3 months after the diagnosis, eight patients underwent allogeneic hematopoietic stem cell transplantation (allo SCT). The 5-year OS was significantly better for patients who underwent allo-SCT than for those who received chemotherapy alone (58% vs. 20%, respectively, p = 0.05).

Among adult Ph+AL patients, mixed phenotype was more frequently observed than expected, and Ph+MPAL patients showed unique clinical features, including immunophenotype and the type of bcr-abl fusion gene. Although BAL has been considered as a negative prognostic factor, Ph+MPAL patients showed comparable prognosis to those with Ph+B-ALL who received imatinib-containing intensive chemotherapy. Therefore, the established theory that mixed phenotype is associated with poor outcomes should be revisited among these patients.

No relevant conflicts of interest to declare.