Highlights
- Stringent control of myeloproliferation (WBC < 10 × 10⁹/L and Monocytes < 1 × 10⁹/L) is a powerful, independent predictor of overall survival (OS) in proliferative CMML.
- The persistence of leukocytosis or monocytosis after six cycles of treatment increases the hazard of death fivefold (HR = 5.38), regardless of bone marrow blast response.
- Classical monocytes (cMo) and immature granulocytes (iGRAN) identified by flow cytometry serve as superior biomarkers for quantifying disease burden and predicting outcomes.
- Combining cMo and iGRAN thresholds allows for a more refined risk stratification, distinguishing patients with a median OS of 35.1 months versus 15.3 months.
Background
Chronic Myelomonocytic Leukemia (CMML) is a complex hematologic malignancy characterized by overlapping features of myelodysplastic syndromes (MDS) and myeloproliferative neoplasms (MPN). Clinically, it is classified into dysplastic (MD-CMML) and proliferative (MP-CMML) subtypes, with the latter historically associated with a more aggressive disease course and poorer prognosis. The current therapeutic landscape largely relies on hypomethylating agents (HMAs) such as decitabine or cytoreductive therapies like hydroxyurea.
Traditionally, treatment response in CMML has been evaluated using International Working Group (IWG) criteria, which focus heavily on bone marrow blast percentage and hematologic improvement. However, in the proliferative subtype, the systemic burden of myeloproliferation—manifested as leukocytosis, monocytosis, and splenomegaly—often dictates clinical morbidity and mortality. Whether the mitigation of this proliferation provides a survival benefit independent of bone marrow response has remained a subject of intense debate. Furthermore, traditional complete blood counts (CBC) lack the granularity to distinguish between functional cells and the specific dysplastic clones driving the disease. This review synthesizes recent evidence from the DACOTA trial to address whether stringent cytoreduction should be a primary therapeutic goal in advanced proliferative CMML.
Key Content
The DACOTA Trial: Decitabine vs. Hydroxyurea
The evidence for stringent cytoreduction primarily stems from the landmark DACOTA trial (NCT02214407), which randomized 120 patients with advanced proliferative CMML to receive either decitabine (an HMA) or hydroxyurea. This study provided a unique platform to evaluate how different mechanisms of cytoreduction—epigenetic modulation versus ribonucleotide reductase inhibition—impacted survival through the lens of myeloproliferative control.
Patients were evaluated after 3 and 6 cycles using both traditional bone marrow aspiration and peripheral blood metrics. Interestingly, across both treatment arms, a significant proportion of patients (approximately 56-59%) failed to achieve stringent blood count normalization, maintaining monocytes > 1 × 10⁹/L or WBC > 10 × 10⁹/L. This persistence was not merely a failure of the drug but a reflection of the inherent aggressiveness of the underlying clone.
The Prognostic Power of Peripheral Blood Metrics
The most striking finding from the analysis of the DACOTA cohort was the correlation between blood count normalization and survival. At the 6-cycle landmark, patients who failed to achieve stringent cytoreduction (WBC ≤ 10 × 10⁹/L and Monocytes ≤ 1 × 10⁹/L) faced a hazard ratio (HR) for death of 5.38 (p = 0.0003).
Crucially, this survival disadvantage remained statistically significant after adjusting for:
1. Baseline CMML-specific Prognostic Scoring System (CPSS) scores.
2. The specific treatment arm (Decitabine vs. Hydroxyurea).
3. The persistence of bone marrow blast excess.
This suggests that in MP-CMML, the “peripheral burden” is not just a secondary symptom but an independent driver of mortality. This shifts the focus of clinical management from a purely blast-centric view to a more holistic approach that prioritizes the normalization of systemic white cell counts.
Novel Flow Cytometric Biomarkers: cMo and iGRAN
While standard CBC provides a count of total monocytes and granulocytes, flow cytometry allows for the identification of specific subsets. The DACOTA researchers focused on classical monocytes (cMo, defined as CD14++, CD16-) and immature granulocytes (iGRAN).
Flow cytometric analysis revealed that these biomarkers are highly sensitive indicators of the disease clone. After only 3 cycles of treatment, higher absolute counts of both cMo and iGRAN independently predicted poorer overall survival. There was no significant interaction with the treatment arm, meaning these biomarkers reflect the biology of the disease rather than a specific drug reaction.
By integrating these two flow-derived metrics, researchers identified a “low-risk” subgroup (cMo ≤ 0.94 × 10⁹/L and iGRAN ≤ 0.40 × 10⁹/L) representing 28% of the cohort. These patients enjoyed a median OS of 35.1 months, compared to only 15.3 months for those exceeding these thresholds. This nearly two-year difference in survival highlights the precision that flow cytometry brings to CMML prognosis.
Pathophysiological Implications
The significance of cMo and iGRAN reflects the underlying pathophysiology of CMML. The expansion of classical monocytes is a hallmark of the disease, often driven by mutations in the RAS pathway or TET2/SRSF2 co-mutations. Immature granulocytes represent the dysregulated myelopoiesis and the “left shift” characteristic of the MPN component. The persistence of these specific populations suggests that the therapeutic agent—whether decitabine or hydroxyurea—has failed to sufficiently suppress the dominant neoplastic clone, even if the blast count appears controlled.
Expert Commentary
The Shift Toward Proliferation-Targeted Therapy
The findings from Selimoglu-Buet et al. suggest a paradigm shift in how we treat proliferative CMML. For years, the oncology community has used the same response criteria for MDS and CMML. However, CMML is a distinct entity where the proliferative component is just as lethal as the dysplastic one. The fact that persistent leucocytosis carries a higher HR for death than bone marrow blasts in this cohort is a call to action for more aggressive cytoreductive strategies.
Utility of Flow Cytometry in Routine Practice
One of the most practical takeaways is the validation of flow-defined cMo and iGRAN. While many centers already use monocyte partitioning for the *diagnosis* of CMML, this evidence supports its use for *response monitoring*. Clinicians should consider incorporating these flow metrics into their post-treatment assessments to identify patients who may need an early switch in therapy or a more rapid move toward allogeneic hematopoietic stem cell transplantation (HSCT).
Limitations and Controversies
A limitation of this study is its focus on a specific subset of advanced, proliferative patients. It remains unclear if these stringent cytoreduction goals apply equally to lower-risk MD-CMML patients. Additionally, while the study shows that *persistent* counts are bad, it does not yet prove that *forcing* counts down with more toxic therapy will necessarily improve the biology of the disease. There is always the risk of inducing prolonged cytopenias (anemia and thrombocytopenia) in the quest for lower WBC counts.
Conclusion
In advanced proliferative CMML, the normalization of peripheral blood counts is more than a clinical convenience; it is a prerequisite for long-term survival. The DACOTA trial data clearly demonstrate that stringent cytoreduction—defined by WBC and monocyte thresholds—predicts OS independently of bone marrow response and baseline risk. The introduction of flow-defined biomarkers like cMo and iGRAN provides clinicians with a precise tool to measure treatment efficacy at a clonal level. Future clinical trials should consider using these peripheral metrics as co-primary endpoints to better reflect the survival outcomes of patients with this aggressive malignancy.
References
- Selimoglu-Buet D, et al. Does stringent cytoreduction improve survival in advanced proliferative chronic myelomonocytic leukemia?. Leukemia. 2024. PMID: 41803403.
- Itzykson R, et al. Decitabine versus hydroxyurea for advanced proliferative chronic myelomonocytic leukemia: results of the DACOTA trial. Lancet Haematol. 2023.
- Valent P, et al. Proposed diagnostic criteria for classical chronic myelomonocytic leukemia (CMML), CMML variants and pre-CMML conditions. Haematologica. 2019;104(10):1935-1949. PMID: 31221783.
- Solary E, et al. The role of monocytes in the pathogenesis of chronic myelomonocytic leukemia. Leukemia. 2023;37(1):1-10. PMID: 36418385.