Prognostic significance of myeloperoxidase positivity of blast cells in acute myeloblastic leukemia without maturation (FAB: M1): an ECOG study.

The relationship between cell features by light microscopy and therapeutic outcome of 72 patients with acute myeloblastic leukemia (FAB M1) were investigated. The patients were divided into two groups according to myeloperoxidase-(MPO) positive percentage of blast cells, namely a low (less than 50%) and high (greater than 50%) MPO group. No remarkable morphological difference of blast cells between these two groups was observed. The 38 patients with low MPO showed a significantly lower complete response rate (CR) (52.6%) than the 34 patients with high MPO (85.3%) (p = 0.003) that included 10 CAE-positive patients and 2 ANAE-positive CAE-negative patients. The low MPO group included 5 CAE-positive patients with granulocyte dysplasia and 7 additional patients with alpha naphthyl acetate esterase (ANAE) positivity who were chloroacetate esterase (CAE) negative. Low positivity of blast cells may be due to a defective enzyme expression in the former but also may be a reflection of 'monocytic' aberrant expression in the latter. The low MPO group in M1 with this heterogeneous population is less likely to achieve CR with chemotherapy, while the high MPO group with a higher CR rate suggests a more pure entity within the myeloblastic leukemias referred to as M1. After further refinement by eliminating 24 cases that were 'esterase positive' (ANAE and/or CAE), the results were still the same, suggesting that the more immature blast populations were in the low MPO group.