What is the mechanism of action of hydroxyurea in polycythemia vera, how does it reduce red‑cell mass, and why is macrocytosis a concerning side effect?

Mechanism of Action of Hydroxyurea in Polycythemia Vera

Hydroxyurea works in polycythemia vera by inhibiting ribonucleotide reductase, which immediately blocks DNA synthesis in rapidly dividing hematopoietic cells, thereby reducing the excessive production of red blood cells, white blood cells, and platelets characteristic of this myeloproliferative disorder. 1

How Hydroxyurea Reduces Red Cell Mass

The drug acts as a cytoreductive agent that specifically targets the clonal proliferation of blood cells in PV:

• Inhibits DNA synthesis without affecting RNA or protein synthesis, selectively suppressing the abnormal bone marrow proliferation 1
• Reduces hematopoietic progenitor cells: Studies demonstrate that hydroxyurea treatment significantly decreases CD34+ cells and erythroid colony formation in peripheral blood, directly reducing the pool of cells capable of producing excess red blood cells 2
• In PV patients, hydroxyurea reduces erythropoietin-independent erythroid colonies (EEC) from a mean of 74.4 to 8.0 colonies per 10⁵ cells, demonstrating its potent suppression of abnormal erythropoiesis 2

The drug achieves complete response (defined as hematocrit <45% without phlebotomy, platelet count <400 × 10⁹/L, WBC <10 × 10⁹/L, and no disease symptoms) in many patients, making it first-line cytoreductive therapy for high-risk PV 3.

Macrocytosis as a Concerning Side Effect

Macrocytosis becomes concerning because it represents a specific form of drug-induced dyserythropoiesis called "volumetric macrocytosis" that can complicate disease monitoring and may signal problematic bone marrow effects.

Why This Matters Clinically:

Volumetric macrocytosis is distinct from other forms:

• Unlike megaloblastic macrocytosis, these cells have increased mean corpuscular volume (MCV) but normal cell diameter 4
• The red cells are thicker, overhydrated, and contain increased hemoglobin 4
• This represents a drug-induced dyserythropoiesis that shares characteristics with neonatal red blood cells 4

Clinical concerns include:

1. Monitoring complications: The macrocytosis can mask or confuse assessment of disease control, making it harder to distinguish treatment effect from disease progression

2. Marker of bone marrow toxicity: Sustained hydroxyurea therapy causing volumetric macrocytosis may indicate cumulative bone marrow damage, which is particularly concerning given that:

   • Hydroxyurea can cause dose-limiting cytopenias (absolute neutrophil count <1.0 × 10⁹/L, platelet count <100 × 10⁹/L, or hemoglobin <10 g/dL) 3
   • There is concern about enhanced risk of acute leukemia when hydroxyurea is used long-term or followed by other myelosuppressive drugs 3

3. Treatment resistance indicator: The European LeukemiaNet defines resistance to hydroxyurea as requiring continued phlebotomy despite adequate dosing, or developing unacceptable hematologic toxicity at doses needed for disease control 3

Important Caveats:

• Use hydroxyurea with caution in young patients (<40 years) due to long-term leukemogenic concerns 3, 5
• Monitor complete blood counts closely to detect cytopenias early
• If macrocytosis develops with inadequate disease control or cytopenias, consider switching to interferon-α (non-leukemogenic alternative) or ruxolitinib (for hydroxyurea-resistant/intolerant patients) 3, 5
• The presence of hemoglobin <10 g/dL at the lowest effective hydroxyurea dose constitutes formal intolerance requiring therapy change 3