Hydroxyurea

Ribonucleotide reductase inhibitor and cytotoxic antimetabolite · also known as hydroxycarbamide, Hydrea, Droxia

Cytotoxic antimetabolite documented in two uncontrolled MCAS series, with extensive concurrent treatment and major monitoring boundaries.

How it relates to mast cells: MCAS patients
Types of studies cited: Case series
Approved in the US for: resistant chronic myeloid leukemia, locally advanced squamous cell head and neck cancers with chemoradiation, sickle cell anemia with recurrent moderate to severe painful crises
Catalog context: Emerging / refractory evidence
Clinical practice: Documented specialist use a refractory case series from one specialist practice record set · Treating-author report an earlier single-practice report from a different specialist · Treating-author report

What this relationship means: Studied in people diagnosed with mast cell activation syndrome. Current evidence consists of case reports, uncontrolled series, or retrospective review rather than randomised comparisons — so it establishes that people have taken it, not that it worked.

Mechanism of action

Hydroxyurea inhibits ribonucleotide reductase, limiting the deoxyribonucleotide supply required for DNA synthesis and repair. That established cytotoxic mechanism underlies its oncology and sickle-cell regulatory uses. The MCAS reports propose additional effects on mediator-related symptoms, but they did not directly measure mast-cell signalling, mediator release, or a selective mast-cell target.

The larger report reviewed one MCAS-focused practice and identified 26 hydroxyurea-exposed patients among 310 records. Twenty remained exposed long enough for the symptom-score analysis, while six stopped earlier after adverse events. Participants had extensive prior and concurrent treatment, the later assessment was partly retrospective, and medication changes were not standardized. The paper identifies which of its authors held the practice whose records were reviewed, which is why this carries a specialist-use tag rather than inferring the role from authorship.

An earlier five-person case series from a different specialist practice gives individual clinical histories. Those two first-party reports support separate documented-specialist-use signals. They remain uncontrolled descriptions from refractory practice populations, not randomized comparisons and not evidence that a cytotoxic exposure should be treated as a routine MCAS step.

Sources

Efficacy and toxicity of hydroxyurea in mast cell activation syndrome patients refractory to standard medical therapy: retrospective case series Peer-reviewed study 2022 PMID 35982335 · link checked 21 Aug 2026
Utility of hydroxyurea in mast cell activation syndrome Peer-reviewed study 2013 PMID 24192267 · link checked 21 Aug 2026
HYDREA current US prescribing information Drug label · link checked 21 Aug 2026
DROXIA current US prescribing information Drug label · link checked 21 Aug 2026

What this evidence cannot establish

Both MCAS reports are uncontrolled and used diagnostic approaches that should not be silently equated with consensus-1 MCAS. The larger retrospective practice review selected 26 people from 310 patients, retained 20 for the symptom analysis, used an average of six concomitant medications, and collected some later ratings retrospectively; changes in other care and recall or selection effects cannot be separated from hydroxyurea exposure. The earlier five-person report has the same case-series attribution limit. Hydroxyurea is a cytotoxic drug rather than a selective mast-cell stabilizer. Current labels carry severe myelosuppression, malignancy, embryo-fetal toxicity, fertility, interaction, handling, and laboratory-monitoring boundaries, and no US label includes MCAS.

What this page does not say

No dosing, and no claim that this works. The entry describes a published mechanism, the cited study types, and what those studies cannot establish — nothing further. Why the site is built this way.

Recently verified. Verified 21 Aug 2026