What is advanced systemic mastocytosis?

Systemic mastocytosis is a clonal mast cell disease in which abnormal mast cells accumulate in organs such as the bone marrow, liver, spleen, gastrointestinal tract and bones. Most patients have non-advanced disease, but a smaller group develop advanced systemic mastocytosis, where mast cell infiltration causes measurable organ damage.

Advanced systemic mastocytosis is defined by organ damage directly caused by mast cell infiltration. These are known as C findings. Examples include low blood counts, liver dysfunction with portal hypertension or ascites, enlarged spleen with hypersplenism, malabsorption with weight loss, and significant bone disease such as large osteolytic lesions or pathological fractures.

The key diagnostic concept: C findings

Advanced systemic mastocytosis is defined by organ damage directly caused by mast cell infiltration. These are known as C findings. Examples include low blood counts, liver dysfunction with portal hypertension or ascites, enlarged spleen with hypersplenism, malabsorption with weight loss, and significant bone disease such as large osteolytic lesions or pathological fractures.

KIT D816V: why molecular testing matters

Most adults with systemic mastocytosis carry the KIT D816V mutation, which drives abnormal mast cell growth and survival. Testing for KIT D816V helps confirm the diagnosis, assess disease burden and guide targeted treatment choices.

A standard next-generation sequencing panel may miss KIT D816V when the variant allele frequency is low. If clinical suspicion remains high, more sensitive techniques such as digital droplet PCR or allele-specific PCR may be needed, especially in patients with indolent or low-burden disease.

Current treatment options

Bezuclastinib: an emerging KIT D816V inhibitor

Bezuclastinib is an investigational, next-generation KIT D816V inhibitor being studied in systemic mastocytosis. Its key mechanistic distinction is potent activity against mutant KIT D816V while aiming to spare wild-type KIT and related kinases.

Unlike avapritinib, bezuclastinib has been described in clinical development as having minimal brain penetration. This is clinically important because it may reduce concerns about cognitive adverse effects and intracranial bleeding risk. However, all targeted therapies can still have side effects and require careful specialist monitoring.

Supportive care remains essential

Even when targeted treatment is used, patients with systemic mastocytosis usually need a personalised supportive care plan. This may include trigger avoidance, emergency medication, assessment of anaphylaxis risk, bone density monitoring, vitamin D optimisation, osteoporosis treatment when indicated, and review of gastrointestinal symptoms, nutrition and weight loss.

Practical action points for patients

• Ask whether your disease is non-advanced or advanced systemic mastocytosis.
• Confirm whether KIT D816V has been tested using a sufficiently sensitive method.
• Carry adrenaline auto-injectors if prescribed and ensure you know when and how to use them.
• Discuss bone density monitoring, calcium and vitamin D status, and osteoporosis prevention.
• Before avapritinib, platelet count and bleeding risk must be carefully reviewed by the treating specialist.
• Consider review in a centre with experience in mast cell disorders when the diagnosis or treatment plan is uncertain.

References and further reading

1. World Health Organization and international consensus classifications of systemic mastocytosis and advanced systemic mastocytosis.
2. Valent P, Akin C, Hartmann K, et al. Updated diagnostic criteria and classification of mast cell disorders.
3. DeAngelo DJ, Radia DH, George TI, et al. Avapritinib in advanced systemic mastocytosis: EXPLORER and PATHFINDER clinical trial data.
4. Gotlib J, Kluin-Nelemans HC, George TI, et al. Midostaurin in advanced systemic mastocytosis.
5. Cogent Biosciences. Bezuclastinib APEX study updates in advanced systemic mastocytosis.
6. American Academy of Allergy, Asthma and Immunology. Mastocytosis patient information and clinical resources.

What is FcRn?

FcRn stands for neonatal Fc receptor. Despite the word neonatal, FcRn is not only important in newborn babies. It remains active throughout life and is found in several tissues, including vascular endothelial cells, immune cells and epithelial surfaces. Its main role is to regulate the survival of IgG antibodies and albumin in the circulation.

IgG is the most abundant antibody class in the blood. It plays an essential role in protection against infection, long-term immune memory and the response to vaccination. However, in some autoimmune diseases, the immune system produces IgG autoantibodies. These are antibodies that bind to the body’s own proteins. If these IgG autoantibodies are pathogenic, they may activate inflammatory pathways, interfere with normal receptor function, recruit complement, promote tissue injury or disturb normal cell signalling.

How FcRn protects IgG from breakdown

Cells constantly take up small amounts of fluid and proteins from the bloodstream. Many internalised proteins are sent to lysosomes, where they are broken down. IgG is different because FcRn binds to IgG inside acidic endosomes and diverts it away from lysosomal degradation. The IgG is then recycled back to the cell surface and released into the bloodstream.

This recycling process gives IgG a long biological half-life compared with many other circulating proteins. In healthy immunity, this is useful because protective antibodies remain available for longer. In antibody-mediated autoimmunity, the same protective recycling pathway may also prolong the survival of harmful IgG autoantibodies.

What is FcRn blockade?

FcRn blockade means using a medicine that prevents FcRn from rescuing IgG. When IgG can no longer bind effectively to FcRn, more IgG is directed towards lysosomal degradation. This leads to a reduction in total circulating IgG, including pathogenic IgG autoantibodies.

This approach is different from many traditional immunosuppressive treatments. FcRn blockade does not aim to shut down broad immune-cell activity. It does not directly deplete B cells, T cells or plasma cells. Instead, it reduces the circulating level of IgG antibodies that have already been produced. This is why the effect can be relatively rapid and reversible.

Why IgG autoantibodies matter in autoimmune disease

Autoimmune diseases are not all the same. Some are mainly driven by T-cell inflammation, some by cytokines, some by immune-complex formation, and some by disease-causing autoantibodies. FcRn blockade is most relevant where IgG autoantibodies have a clear pathogenic role.

In IgG-mediated autoimmune disease, autoantibodies may bind to cell-surface receptors, structural proteins, platelets, red blood cells, skin adhesion proteins or other self-antigens. The clinical effect depends on the target. In some conditions the antibody blocks normal receptor function. In others it activates complement, marks cells for clearance, or causes inflammation at a tissue surface.

The important principle is that lowering pathogenic IgG may reduce the autoimmune signal while leaving other immune mechanisms more intact than with broad immunosuppression. Immunoglobulin A, immunoglobulin M and many cellular immune functions are not directly targeted by FcRn blockade, although clinical monitoring remains essential because IgG is an important part of infection defence.

Examples of FcRn-targeting medicines

Several FcRn-targeting treatments have been developed or are under clinical investigation. These include efgartigimod alfa, rozanolixizumab, nipocalimab and batoclimab. They differ in molecular structure, route of administration, dosing schedule, regulatory status and clinical development programme.

Efgartigimod alfa is an engineered human IgG1 Fc fragment designed to bind FcRn and reduce IgG recycling. Rozanolixizumab is a humanised monoclonal antibody directed against FcRn. Nipocalimab and batoclimab are FcRn-blocking monoclonal antibodies being studied across several IgG-mediated autoimmune conditions. Availability in the UK and Europe depends on the licensed indication, local commissioning, specialist assessment and individual patient suitability.

How quickly does FcRn blockade work?

Because FcRn blockade increases the breakdown of circulating IgG, it may reduce IgG levels more quickly than treatments that depend on slowly altering antibody production. The timing of clinical improvement varies between conditions and between patients. A fall in antibody level does not always translate immediately into symptom improvement because tissue inflammation, complement activation, organ damage or downstream immune pathways may take longer to settle.

The effect is also reversible. When treatment is stopped, FcRn function gradually resumes and IgG levels may recover over time. This reversibility can be clinically useful, but it also means that repeated or maintenance treatment may be required in some chronic relapsing conditions.

Is FcRn blockade immunosuppression?

FcRn blockade is best understood as targeted immunomodulation rather than broad immune suppression. It lowers IgG by increasing IgG catabolism. It does not directly suppress bone marrow function, does not directly deplete lymphocytes and does not act like high-dose corticosteroids or conventional cytotoxic immunosuppressants.

However, IgG is important for immune protection. Patients being considered for FcRn blockade require specialist assessment of infection history, vaccination status, current medicines, immunoglobulin levels, pregnancy status where relevant, co-existing medical conditions and the need for monitoring during treatment.

Safety considerations and monitoring

The safety profile depends on the specific medicine, the dose, the route of administration and the patient’s underlying condition. Reported adverse effects vary between products but may include headache, injection-site reactions, upper respiratory tract infections, nausea, diarrhoea, fever, rash or infusion-related symptoms. Because FcRn also participates in albumin homeostasis, some FcRn-targeting approaches may require attention to serum albumin, although the extent of albumin change differs between molecules.

Before treatment, clinicians may check full blood count, liver and kidney function, immunoglobulin levels, infection risk, vaccination history and disease-specific autoantibody markers. During treatment, monitoring may include symptom scores, clinical examination, infection surveillance, IgG levels where appropriate, treatment tolerance and review of concomitant medicines.

Patients should inform their clinician if they develop recurrent infections, persistent fever, new unexplained symptoms, severe headache, allergic-type symptoms, pregnancy, planned surgery, or if they are due to receive vaccines. Live vaccines and timing of vaccination should be discussed with the treating specialist.

How FcRn blockade differs from other antibody-lowering treatments

Plasma exchange can remove antibodies rapidly from the circulation, but it is a procedure-based treatment and may require vascular access. Intravenous immunoglobulin can modulate immune function through several mechanisms, including Fc receptor effects, complement modulation and anti-inflammatory signalling. B-cell targeted treatments reduce the formation of new antibody-producing immune responses but may take longer to affect established antibody levels and may have broader immune effects.

FcRn blockade occupies a different position. It is pharmacological rather than procedure-based, reduces IgG by accelerating its natural breakdown, and can be repeated according to the licensed product schedule and specialist treatment plan. It may be considered when the aim is to lower pathogenic IgG without using broad immune suppression, although it is not suitable for every patient or every autoimmune condition.

Who may benefit from specialist assessment?

A specialist immunology or relevant organ-specific specialist assessment may be appropriate for patients with suspected antibody-mediated autoimmune disease, especially where standard treatments have not provided adequate disease control, where corticosteroid exposure is problematic, or where there is a documented pathogenic IgG autoantibody associated with active disease.

Assessment should include confirmation of diagnosis, review of previous investigations, antibody testing, disease activity measurement, treatment history, infection history and discussion of the balance between potential benefit and risk. FcRn blockade should not be started purely because an autoantibody is present; the antibody must be interpreted in the full clinical context.

Summary

FcRn is a natural recycling receptor that protects IgG antibodies from degradation. This mechanism is useful for normal immune protection but may also prolong the survival of pathogenic IgG autoantibodies in antibody-mediated autoimmune disease. FcRn blockade interrupts this recycling pathway, increases IgG breakdown and can reduce circulating pathogenic IgG without directly suppressing the whole immune system.

This is an important development in modern immunology because it offers a targeted way to address IgG-driven disease mechanisms. Its role depends on the specific condition, the antibody involved, the licensed indication, treatment availability and the patient’s overall clinical situation.

References

1. Gjølberg TT, Andersen JT, Sandlie I. Targeting the neonatal Fc receptor in autoimmune diseases: pipeline and progress. BioDrugs. 2025.
2. Zhu L, et al. FcRn inhibitors: transformative advances and significant potential in autoimmune diseases. Front Immunol. 2025.
3. Yang CW, et al. Evaluating the clinical impact of FcRn inhibition in IgG-mediated autoimmune disease. Autoimmun Rev. 2025.
4. Seth NP, et al. Nipocalimab, an immunoselective FcRn blocker that lowers IgG, including pathogenic autoantibodies. Clin Pharmacol Ther. 2025.
5. Nilforoushzadeh MA, et al. FcRn inhibitors in immune thrombocytopenia: a comprehensive review of therapeutic advances and clinical outcomes. Transfus Apher Sci. 2025.
6. Yasuda M, et al. Opposing effects of efgartigimod and rozanolixizumab on serum albumin levels. Naunyn Schmiedebergs Arch Pharmacol. 2026.
7. European Medicines Agency. Vyvgart: efgartigimod alfa. European public assessment report.
8. European Medicines Agency. Rystiggo: rozanolixizumab. European public assessment report.