Mast cell activation syndrome (MCAS) targeted therapy

Stepwise pharmacotherapy for mast cell activation syndrome (MCAS), including H1/H2 antihistamines, mast cell stabilizers, antileukotrienes, and biologic options (omalizumab, avapritinib).

Evidence tier: Expert consensus. Reflects expert-consensus criteria, not a prospectively validated instrument. Apply clinical judgment and local policy.

Agent (class)	Role in MCAS	Dose / notes
H1 antihistamines, cetirizine, loratadine, fexofenadine, hydroxyzine (non-sedating preferred)	First-line: control urticaria, flushing, pruritus, rhinorrhea. Double or triple the standard dose in refractory MCAS (off-label, guideline endorsed). Sedating agents (hydroxyzine, diphenhydramine) for breakthrough or nighttime use.	Cetirizine 10-20 mg QD or BID; loratadine 10-20 mg QD; fexofenadine 180-360 mg QD. Non-sedating preferred for maintenance. Hydroxyzine 10-25 mg QHS for nocturnal symptoms.
H2 antihistamines, famotidine, ranitidine (note ranitidine withdrawn)	First-line adjunct: add H2 blockade to H1 for GI symptoms (nausea, reflux, cramping, diarrhea) and refractory urticaria. H1+H2 combo superior to either alone for dermatologic and GI manifestations.	Famotidine 20-40 mg BID. Always combine with H1, H2 monotherapy is insufficient for MCAS. Avoid ranitidine (withdrawn in 2020).
Mast cell stabilizers, cromolyn sodium (oral), ketotifen	Adjunct for GI MCAS (oral cromolyn) and systemic symptoms (ketotifen). Cromolyn sodium prevents mast cell degranulation in the GI lumen, useful for abdominal pain, diarrhea, nausea after meals.	Cromolyn sodium oral: 100-200 mg QID 30 min before meals. Ketotifen: 1-2 mg BID (not available in US, available Canada/Europe). Onset is slow (weeks); do not judge response before 4-6 weeks.
Leukotriene modifier, montelukast, zafirlukast	Adjunct for flushing, abdominal cramping, and exercise-induced symptoms. Mast cells release leukotrienes (LTC4, LTD4) during degranulation, CysLT blockade addresses this mediator pathway.	Montelukast 10 mg QHS. Zafirlukast 20 mg BID. Neuropsychiatric adverse events with montelukast (boxed warning), monitor mood/behavior, especially in adults with prior psychiatric history.
Aspirin, low to high dose for MCAS with prostaglandin-dominant flushing	Adjunct for patients with prostaglandin-dominant MCAS (predominant flushing, elevated urine 11β-PGF2α). Blocks COX-mediated prostaglandin synthesis.	Start 81 mg QD to test tolerance, but the effective antiprostaglandin range for flushing is substantially higher (commonly titrated to 325-650 mg BID under supervision), 81 mg daily alone will often read as a treatment failure. Caution: aspirin can paradoxically TRIGGER MCAS in patients with NSAID-sensitive phenotype, test with low dose in clinic first. Not appropriate for all MCAS.
Omalizumab (anti-IgE)	Second-/third-line biologic for refractory MCAS and mastocytosis-associated MCAS. IgE cross-linking drives mast cell activation, omalizumab dramatically reduces serum IgE and mast cell surface IgE receptor density, reducing activation threshold.	150 or 300 mg SC every 4 weeks. Note that CSU dosing is FIXED, it is NOT adjusted for body weight or serum IgE (unlike the allergic-asthma regimen, whose weight/IgE table does not apply here). 300 mg q4w is the dose used in most MCAS and mastocytosis series. Off-label for MCAS; multiple case series and retrospective cohorts support significant symptom reduction. FDA-approved for CSU, use this as the label anchor if needed for coverage.
Avapritinib (KIT D816V inhibitor)	Targeted therapy for systemic mastocytosis with KIT D816V. FDA-approved for advanced SM (200 mg) AND for indolent SM (25 mg, 2023) where symptoms are inadequately controlled by symptom-directed therapy.	25 mg PO QD (PIONEER trial dose for indolent SM with symptoms). FDA-approved for advanced SM at 200 mg QD; 25 mg for non-advanced SM (Indolent SM) FDA-approved 2023 (Ayvakit). Inhibits KIT D816V, the driver mutation in >90% of systemic mastocytosis. Not indicated in non-clonal MCAS, it targets KIT D816V and has not been studied in KIT D816V-negative MCAS, which is not the same as being shown ineffective.
Midostaurin (multi-kinase KIT inhibitor)	Advanced systemic mastocytosis (aggressive SM, mast cell leukemia, SM-AHN). Not used for indolent SM or pure MCAS. FDA-approved for advanced SM.	100 mg PO BID with food. Significant drug interactions (CYP3A4). Use limited to hematology/oncology setting for advanced SM. Not a MCAS biologic, included to distinguish from avapritinib.
Systemic corticosteroids, prednisone	For acute severe MCAS flares unresponsive to antihistamines. NOT for maintenance, long-term steroids in MCAS are not disease-modifying and cause significant harm.	Prednisone 0.5-1 mg/kg × 3-5 days for acute severe flare. Short burst only. Daily corticosteroid use is not appropriate for MCAS maintenance; escalate to omalizumab or avapritinib if flares are frequent.

Notes

MCAS diagnosis requires all three: (1) typical multisystem episodic symptoms of mast cell mediator release, (2) response to mast-cell-targeted therapy, AND (3) an event-related rise in SERUM TRYPTASE above the patient’s own baseline, greater than (1.2 x baseline) + 2 ng/mL. Urinary mediator metabolites (N-methylhistamine, 11-beta-PGF2-alpha, LTE4) are SUPPORTIVE only and do not satisfy criterion 3. Chromogranin A is NOT a valid MCAS marker, it is confounded by proton-pump inhibitors, renal impairment, cardiac disease, and neuroendocrine tumours. Never diagnose MCAS on symptoms alone.
Serum tryptase at baseline and during a flare is the key biomarker, but a persistently raised baseline (>11.4 ng/mL) does NOT by itself justify a bone marrow biopsy. Evaluate first for hereditary alpha-tryptasemia (TPSAB1 copy number), which affects roughly 5% of people and is the commonest cause of a tryptase in the 11-20 range, and send a high-sensitivity peripheral-blood KIT D816V. Triage to marrow biopsy on REMA score >=2, a positive blood KIT D816V, or hypotensive/Hymenoptera anaphylaxis without urticaria, not on the tryptase value alone.
Triggered MCAS (identifiable triggers) responds better to trigger avoidance + antihistamines; idiopathic MCAS often requires escalation to omalizumab or avapritinib.
Omalizumab is the most evidence-supported biologic for MCAS, multiple retrospective series show dramatic reduction in anaphylaxis episodes, urticaria, and mediator symptoms. Consider early escalation if H1+H2+cromolyn is insufficient.
Avapritinib at 25 mg QD is now FDA-approved for non-advanced systemic mastocytosis (indolent SM) with symptoms, the PIONEER trial showed significant reduction in mast cell burden and symptom scores.

Evidence & citations

Clinician decision support. Verify against the cited source. Not a substitute for clinical judgment. 100% on-device; no patient data is stored or transmitted.