Decitabine-Induced Early Platelet Response, a Predictor of Favorable Outcome during Hypomethylating Treatment of MDS, Is Associated with In Vivo Megakaryocytic Differentiation

Introduction

Despite broad use of hypomethylating agents (HMAs) in MDS/AML treatment, the number of established outcome predictors is still very limited (Stomper and Lübbert, Sem Hematol 2019). One of them, the early, isolated and sometimes massive increase of platelets, is recurrently observed in patients with MDS treated with HMAs. It is not observed with non-HMA cytidine analogs such as low-dose cytarabine. In HMA-treated MDS patients, an early platelet increase is a predictor of overall and leukemia-free survival (van den Bosch et al., Leuk Res 2004; van der Helm et al., Br J Haematol 2011). HMAs induce cellular differentiation in vitro, by gene reactivation in the malignant cells. However, evidence for HMA-induced in vivo differentiation is still very limited. We hypothesized that the megakaryocytic cell lineage in MDS is a target for HMA-induced cellular maturation in vivo.

Methods

We systematically analyzed the bone marrow morphology of 34 higher-risk MDS patients (median age: 71.5 years, range 51-79) before and after 1 cycle of treatment with the HMA decitabine (DAC). All patients had been treated at a single center within 3 prospective clinical trials (Wijermans et al., Ann Hematol 2005; Lübbert et al., J Clin Oncol 2011). One treatment cycle consisted of 45 mg/m² DAC per day (15 mg/m² intravenously over 4 hours every 8 hours) for 3 consecutive days, repeated 6 weeks later. The early platelet response was evaluated after 1 cycle of DAC treatment. Based on the criteria of the International Working Group, an absolute increase in platelet count of 30x10⁹/l or more compared to the pre-treatment count was defined as a platelet response. The histological analysis of the bone marrow specimens was performed by an experienced hematopathologist blinded to the treatment timepoints. Up to 200 megakaryocytes (MK) per sample were quantified at a magnification of 400 x using chloroacetate esterase staining.

Results

Thirteen of 34 patients (38%) showed a platelet response already after 1 cycle of DAC treatment, 21 (62%) did not. The median pre-treatment platelet count did not differ in patients with or without an early platelet response (median of 34x10⁹/l in both groups, range 7-169 and 8-265, respectively). After 1 cycle of DAC treatment, patients with a platelet response had a median platelet count of 117x10⁹/l (range 78-281), patients without this response had a median platelet count of 32 x10⁹/l (range 4-155).

Overall survival (OS) was measured from the time of early platelet response assessment after completion of 1 treatment cycle, i.e. after 6 weeks. The presence of a platelet response after 1 DAC cycle was associated with a longer OS compared to the absence of this early platelet response: median of 26.6 versus 14.0 months (p=0.04).

Both pre- and post-treatment bone marrow biopsies of patients with an early platelet response showed higher numbers of MK, as well as significant differences in MK morphology compared to biopsies from patients without an early platelet response. Regarding MK numbers, increased MK density in specimens of patients with an early platelet response was observed both before (mean MK number per high power field 6.2 vs. 2.6, p=0.02) and after the application of DAC (mean MK number 10.4 vs. 3.1, p=0.01). Regarding MK maturation stage, more pre-treatment juvenile MK (on average 32.4% vs. 20.5% of all MK, p=0.03) and MK with typical myeloproliferative stigmata (present in 5/13 vs. 2/21 biopsies) were observed in patients with an early platelet response, compared to patients without this response. Regarding the induction of megakaryocytic maturation during this early treatment phase, more post-treatment "naked", mature MK nuclei indicative of active platelet shedding (on average 9.5% vs. 3.8% of all MK, p=0.01), were noted in patients with an early platelet response than in patients without an early platelet response.

Conclusions

This is, to the best of our knowledge, the first systematic hematopathological analysis of changes in quantitative and morphological MK features in bone marrow specimens of MDS patients during HMA treatment. DAC, which has in vitro differentiation-inducing effects on megakaryoblastic cells, induced maturation also of dysplastic MK in vivo in higher-risk MDS patients with an early platelet response to this HMA. The predictive value of an early platelet increase, a very easy-to-determine parameter, during this type of treatment is confirmed.