Hydroxychloroquine

4-aminoquinoline antimalarial and lysosomal immunomodulator · also known as HCQ, Plaquenil

Mixed four-person mast-cell-disease report and primary human mast-cell experiments, with separate randomised evidence in chronic urticaria.

How it relates to mast cells: MCAS patients
Types of studies cited: Case series , Human mast cells in vitro
Approved in the US for: uncomplicated malaria and prophylaxis where chloroquine resistance is not reported, rheumatoid arthritis in adults, systemic lupus erythematosus in adults, chronic discoid lupus erythematosus in adults
Randomised trials in: antihistamine-refractory chronic spontaneous urticaria
Catalog context: Emerging / refractory evidence

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

Hydroxychloroquine is a lysosomotropic 4-aminoquinoline whose US label covers specified malaria, rheumatoid-arthritis, and lupus indications rather than MCAS. Its anti-inflammatory and immunomodulatory mechanisms in rheumatic disease are not fully defined. The mast-cell paper instead examined whether accumulation in acidic cellular compartments could alter granule and lysosomal biology.

That 2018 report combined a retrospective four-person clinical description with experiments using primary human mast cells grown from blood progenitor cells. The laboratory work recorded changes in lysosomal and granule homeostasis, enzymatically active tryptase, selected inflammatory-mediator expression, and long-term cell survival. The clinical portion mixed two cutaneous-mastocytosis and two MCAS diagnoses, and every participant also had inflammatory rheumatism. It therefore documents exposure and a direct laboratory mechanism without supplying a controlled MCAS comparison.

Two small randomised studies separately evaluated hydroxychloroquine in chronic spontaneous or idiopathic urticaria. They provide adjacent-condition context, not an MCAS result or a measured human mast-cell effect. A registered single-group study in cutaneous and indolent systemic mastocytosis has an estimated sample and completion date, but ClinicalTrials.gov now marks its status unknown and carries no posted results.

Sources

Hydroxychloroquine as a novel therapeutic approach in mast cell activation diseases Peer-reviewed study 2018 PMID 30004016 · link checked 24 Aug 2026
Hydroxychloroquine in the treatment of anti-histamine refractory chronic spontaneous urticaria, randomized single-blinded placebo-controlled trial and an open label comparison study Peer-reviewed study 2017 PMID 28884989 · link checked 24 Aug 2026
Impact of hydroxychloroquine therapy on chronic urticaria: chronic autoimmune urticaria study and evaluation Peer-reviewed study 2004 PMID 15086698 · link checked 24 Aug 2026
PLAQUENIL current US prescribing information Drug label · link checked 24 Aug 2026
ClinicalTrials.gov record NCT05084872 — hydroxychloroquine in cutaneous or indolent systemic mastocytosis Trial registry NCT05084872 · link checked 24 Aug 2026

What this evidence cannot establish

The direct clinical report contained four people: two with cutaneous mastocytosis and two with MCAS, all of whom also had unclassified inflammatory rheumatism that supplied a separate clinical reason for exposure. It was retrospective, uncontrolled, used subjective symptom estimates, mixed diagnoses, and concurrent care, and cannot isolate an MCAS outcome. The authors disclosed a patent application concerning antimalarials in mast-cell activation diseases. The primary-human-mast-cell experiments used prolonged exposure outside the body and measured lysosomal, granule, mediator-expression, and survival endpoints; those findings do not predict a clinical outcome. The randomised studies were in chronic spontaneous or idiopathic urticaria, not MCAS, and one enrolled 60 people while the earlier study enrolled 21. The mastocytosis trial record remains stale with unknown status, estimated enrolment, and no posted results. The current label makes retinal toxicity, cardiomyopathy and ventricular arrhythmias, serious skin reactions, hematologic and neuromuscular toxicity, neuropsychiatric reactions, hypoglycemia, renal toxicity, and drug interactions central boundaries.

Registered trials

Phase 2/3 · Status unknown NCT05084872

cutaneous or indolent systemic mastocytosis with skin involvement · 30 planned — no final figure reported

Read from ClinicalTrials.gov on 24 Aug 2026

“Status unknown” is the registry's own flag for a record whose sponsor stopped updating it past the expected completion date. Where the enrolment figure is a plan rather than a count, nobody has reported what actually happened. Secondary trackers tend to keep showing the last confident-sounding status instead.

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 24 Aug 2026