Mechanism of Action of Hydroxyurea
Hydroxyurea works primarily by inhibiting ribonucleotide reductase, an enzyme essential for DNA synthesis, through conversion to a free radical nitroxide that quenches the tyrosyl radical at the enzyme's active site. 1
Primary Mechanism: Ribonucleotide Reductase Inhibition
Hydroxyurea is absorbed orally and converted to a free radical nitroxide in vivo. This active metabolite diffuses into cells where it specifically targets the M2 protein subunit of ribonucleotide reductase by quenching the tyrosyl free radical at the active site, thereby inactivating the enzyme 1, 2. This inhibition:
- Blocks DNA synthesis selectively without interfering with RNA or protein synthesis
- Causes cell death specifically in S-phase of the cell cycle
- Synchronizes surviving cells by holding them in G1 phase
- Inhibits DNA repair processes in cells damaged by radiation or chemotherapy 2
The molecular mechanism involves hydroxyurea reacting with the radical site of the RNR protein, particularly at Tyr-176, which is essential for free radical formation. Hydroxyurea is converted to its oxidized form (compound III), while the tyrosyl radical is converted to a normal tyrosine residue, effectively shutting down the enzyme 3.
Secondary Mechanism: Nitric Oxide Pathway Activation
Beyond its cytotoxic effects, hydroxyurea has a distinct mechanism relevant to sickle cell disease treatment:
- Directly interacts with soluble guanylyl cyclase (sGC) through a free-radical nitroxide pathway, producing iron-nitrosyl derivatives 4
- Generates nitric oxide (NO) in erythroid cells, increasing intracellular cGMP levels 4, 5
- This NO/cGMP pathway activation leads to increased fetal hemoglobin (HbF) production 2, 5
- The elevated HbF reduces vaso-occlusive crises in sickle cell disease 2
Hydroxyurea can undergo redox reactions with heme-based enzymes like catalase and hemoglobin to produce NO, with specific hydrogen bonding interactions from distal residues (His75, Asn148, Gln168) stabilizing the reaction intermediates 6.
Additional Molecular Effects
Recent evidence suggests hydroxyurea also:
- Modulates microRNA (miRNA) expression, affecting hematopoiesis and globin switching 7
- Selectively reduces episomal DNA levels, potentially decreasing drug resistance from duplicated genes 2
Clinical Implications
The dual mechanism explains hydroxyurea's effectiveness in both:
- Malignancies (through cytotoxic DNA synthesis inhibition)
- Sickle cell disease (through NO-mediated HbF induction and reduced vaso-occlusive events)
The drug's ability to sensitize cells to radiation and create synergy with agents like bleomycin, cytarabine, and etoposide stems from its disruption of DNA repair mechanisms and destabilization of the iron center at the ribonucleotide reductase active site 2.