This blog series explores specific rare diseases and how CDMOs such as SK pharmteco can support the development of therapies for patients with high unmet needs. Previous entries in the series are available here.
This blog series explores specific rare diseases and how CDMOs such as SK pharmteco can support the development of therapies for patients with high unmet need. Previous entries in the series are available here.
What Is Mastocytosis? A Focused Overview for R&D and Clinical Development
Mastocytosis is a rare group of clonal mast-cell disorders. “Clonal” means that the abnormal mast cells arise from a shared disease-driving cellular origin, rather than from a temporary immune reaction. Mast cells are immune cells best known for their role in allergic responses, but in mastocytosis they accumulate abnormally in the skin, bone marrow, and other organs.
In adults, most clinically significant cases are classified as systemic mastocytosis (SM), meaning the disease also involves internal organs rather than only the skin. SM includes indolent SM (ISM), smoldering SM, and advanced SM (AdvSM). Advanced disease includes aggressive SM (ASM), SM with an associated hematologic neoplasm (SM-AHN), and mast cell leukemia (MCL). Updated WHO and International Consensus Classification (ICC) schemes from 2022–2024 have refined these categories and diagnostic thresholds, making classification more directly relevant to prognosis, trial eligibility, and treatment selection.1,2
The molecular center of gravity in adult systemic mastocytosis is KIT D816V. KIT is a receptor tyrosine kinase, a cell-surface signaling protein that helps regulate mast-cell growth and survival. In more than 80% of adult SM cases, a D816V substitution in exon 17 of KIT keeps the receptor switched on, even without its normal activating signal.1,3 The result is persistent mast-cell proliferation, survival, tissue infiltration, and mediator release.
For researchers, clinical scientists, and project managers, mastocytosis is therefore best understood as a rare, KIT D816V–driven myeloid neoplasm with unusually strong molecular biomarkers and a rapidly evolving targeted treatment landscape.
The Central Role of KIT D816V in Systemic Mastocytosis
Normal mast-cell development depends on KIT signaling. The D816V mutation alters the activation loop of the KIT kinase domain, stabilizing the receptor in an active conformation. This drives downstream signaling through pathways such as MAPK, PI3K/AKT, and JAK/STAT, which are involved in cell survival, proliferation, and inflammatory signaling. 1,3
Clinically, this biology helps explain the broad presentation of systemic mastocytosis. Mast-cell accumulation may occur in bone marrow, liver, spleen, and gastrointestinal tract. At the same time, activated mast cells release mediators that can produce flushing, anaphylaxis, abdominal pain, diarrhea, bone pain, fatigue, and other symptoms. Elevated serum tryptase, an enzyme released by mast cells, is commonly used as a biomarker of mast-cell burden and activation.
Sensitive molecular assays have made KIT D816V more than a diagnostic finding. PCR-based methods can detect the mutation in peripheral blood in most adults with systemic mastocytosis when sufficiently sensitive techniques are used. 3-4 This has pushed KIT D816V testing into routine diagnostic workflows and clinical trials, where quantitative allele burden can help track response to therapy. 2-4
From a clinical development perspective, KIT D816V is important for three reasons:
- First, it is a dominant driver mutation
- Second, it is pharmacologically targeted
- Third, its allele burden can be measured over time, creating a molecular bridge between mechanism of action, disease burden, and clinical response.1,5
Current Small-Molecule Therapies for Systemic Mastocytosis
The treatment landscape for systemic mastocytosis now spans symptom-directed therapy, cytoreductive therapy, and targeted inhibition of KIT D816V. This layered approach matters because mastocytosis is not only a proliferative disease; it is also a mediator-driven disease. Patients may need agents that reduce mast-cell burden, agents that control mediator-related symptoms, or both.
Multikinase Inhibitors: Midostaurin as a First Targeted Therapy
Midostaurin is an oral multikinase inhibitor and was the first targeted agent approved for AdvSM. It inhibits KIT, including D816V, as well as FLT3 and several other kinases. 1
Clinical studies and real-world experience have shown that midostaurin can reduce mast-cell burden, improve organ function, and lower serum tryptase in some patients with AdvSM. However, responses may be incomplete, and adverse events such as gastrointestinal toxicity and cytopenias can require dose adjustment or treatment interruption.1,6
Even as more selective KIT inhibitors become more prominent, midostaurin remains scientifically and clinically relevant. It provides early proof that KIT inhibition could alter the biology of AdvSM. It also remains part of treatment practice in certain regions or clinical contexts because of access, reimbursement, clinician experience, and established treatment algorithms.
For translational researchers, midostaurin represents the first generation of KIT-directed therapy: broad kinase activity, clinically meaningful activity in AdvSM, and a useful benchmark for the next generation of more selective agents
Highly Selective KIT D816V Inhibition: Avapritinib
Avapritinib is a second-generation, highly selective KIT D816V inhibitor that has changed expectations for targeted therapy in SM. In advanced SM, the EXPLORER and PATHFINDER studies demonstrated deep clinical and molecular responses, including reductions in bone-marrow mast-cell burden, serum tryptase, KIT D816V allele fraction, and mast-cell–related organ damage.1, 5
Longer-term follow-up from PATHFINDER continues to support sustained activity and manageable safety over multiple years of treatment in AdvSM.5,7 Real-world comparative analyses also suggest that avapritinib may provide longer treatment duration and survival than historical or external control cohorts treated with midostaurin or cladribine in selected patients. 8,9 These findings have increased interest in avapritinib as an earlier treatment option for appropriate patients with advanced disease, while still requiring careful patient selection and safety monitoring.
Avapritinib has also expanded the treatment conversation in indolent systemic mastocytosis. Although ISM is generally not associated with the same organ-damage profile as AdvSM, many patients experience chronic and debilitating mediator-related symptoms. Data from PIONEER and subsequent follow-up studies indicate that avapritinib can reduce total symptom burden, decrease mast-cell burden, and improve quality of life in patients whose disease is driven by KIT D816V.3,10,11
The broader lesson for drug developers is clear: greater selectivity for KIT D816V can translate into more pronounced molecular responses, deeper symptom control, and a more direct relationship between target engagement and clinical outcome.
Other Targeted and Adjacent Therapeutic Strategies
Direct KIT inhibition is only one part of systemic mastocytosis management. Several other approaches remain important, especially because clinical presentation varies widely across subtypes.
Other KIT inhibitors and multikinase agents continue to be evaluated for specific patient groups, including those who do not tolerate or respond adequately to approved therapies (midostaurin or avapritinib).12 Cytoreductive therapies such as cladribine and interferon-α remain relevant in advanced disease, particularly when newer targeted agents are unavailable or clinically unsuitable. NCCN guidance stratifies these regimens by evidence level and clinical context.2,13
Mediator-directed therapies also remain central. H1 and H2 antihistamines, leukotriene receptor antagonists, cromolyn, and anti-IgE therapy such as omalizumab may help manage symptoms related to mast-cell activation.2,13 These treatments may not meaningfully reduce the malignant clone, but they can have a major effect on day-to-day disease burden.
For clinical trial designers and treatment-algorithm developers, mastocytosis therefore requires a dual lens: therapies must be evaluated not only for disease modification, but also for their ability to improve the symptoms that shape patient quality of life.
Gene and Cell-Based Therapy Concepts in Mastocytosis
Preclinical Gene Editing in Mastocytosis Models
No gene therapy is currently approved for mastocytosis, but the disease has several features that make it attractive for advanced therapeutic research: a defined driver mutation, measurable molecular burden, and clinically meaningful biomarkers.
Preclinical work has begun to explore gene-editing tools in mastocytosis models. One example is CRISPR/Cas9 engineering of the human mast-cell line HMC-1.2 to generate the HMC-1.3 subline. In this model, researchers corrected a V560G KIT mutation while leaving the D816V-KIT mutation intact. 14,15 This created a cleaner experimental system for studying the specific effects of D816V-KIT on signaling, cell survival, and drug sensitivity.
These engineered models are valuable because they help separate the contribution of KIT D816V from the effects of additional cooperating mutations. They also create platforms for testing combination therapies, resistance mechanisms, and synthetic-lethality strategies, approaches that target vulnerabilities created by the mutation rather than only inhibiting KIT itself.
Hypothetical Clinical Gene-Therapy Approaches
Currently, gene-therapy concepts in mastocytosis remain investigational and theoretical. Still, several approaches are worth watching because they mirror strategies being explored in other hematologic and stem-cell disorders.
- One potential strategy is ex vivo editing of hematopoietic stem and progenitor cells (HSPCs). These are the early blood-forming cells that give rise to mature blood and immune cells.
In a theoretical mastocytosis application, autologous HSPCs could be collected, edited outside the body using CRISPR or base-editing technologies to correct or inactivate KIT D816V, and reinfused after conditioning. This approach would face major challenges, including the need to target the malignant clone, preserve healthy hematopoiesis, and demonstrate acceptable off-target editing risk.
- A second concept is in vivo gene editing of KIT D816V–expressing cells.
In this scenario, editing machinery could be delivered directly into the body using a vector such as adeno-associated virus (AAV) or a lipid nanoparticle (LNP). The scientific challenge is substantial: delivery would need to reach the relevant mast-cell progenitors and spare healthy cells that depend on normal KIT signaling.
- A third and potentially more reversible approach is RNA targeting. Small interfering RNA or antisense oligonucleotides could theoretically reduce expression of mutant KIT transcripts or modulate key downstream pathways. Because RNA-targeted therapies do not permanently edit DNA, they may offer a different balance of durability, reversibility, dosing frequency, and regulatory considerations.
For now, these remain research directions rather than clinical options. However, the strong molecular architecture of systemic mastocytosis makes the disease a compelling model for future advanced-therapy medicinal products.
Biomarkers, Risk Scores and Companion Diagnostics in Mastocytosis
Modern mastocytosis development is biomarker-driven. This is one of the disease’s most important features for sponsors, clinical researchers, and CDMO teams supporting complex therapeutic programs.
KIT D816V Quantitation and Molecular Monitoring
Sensitive PCR-based assays can detect KIT D816V in peripheral blood and bone marrow in most adults with SM.3,4 Quantitative assessment of KIT D816V allele burden is increasingly used to support diagnosis, characterize baseline disease, and monitor response to targeted therapy.
In avapritinib studies, large reductions in KIT D816V allele burden often parallel reductions in serum tryptase, bone-marrow mast-cell burden, and clinical measures of disease activity.1,5,7 This makes allele-burden monitoring especially useful in studies where molecular response is being explored as a marker of depth and durability of response.
Extended next-generation sequencing (NGS) panels are also playing an important role. SM can coexist with other myeloid neoplasms, particularly in SM-AHN. Additional mutations in genes such as SRSF2, ASXL1, RUNX1, and NRAS can influence prognosis and treatment interpretation. These co-mutations are especially relevant when outcomes vary across advanced disease subtypes.
Mutation-Adjusted Risk Scores (MARS and MARS-R)
Risk stratification in advanced systemic mastocytosis increasingly integrates clinical and molecular information. The Mutation-Adjusted Risk Score, or MARS, combines clinical features with high-risk mutational status to estimate survival in AdvSM cohorts.16 Later refinements and real-world analyses have reinforced the prognostic value of both MARS and disease subtype.17
Updated approaches such as MARS-R incorporate newer data from patients treated with modern KIT inhibitors. These tools can help researchers stratify patients in clinical trials, interpret heterogeneous outcomes across AdvSM subtypes, and compare expected outcomes across treatment cohorts.
Companion Diagnostics and Integrative Development Workflows
Because systemic mastocytosis has a strong molecular driver, future trials are likely to require validated KIT D816V assays at defined time points. Many programs may also incorporate extended myeloid mutation panels to identify high-risk co-mutations and apply mutation-adjusted risk models.
Molecular endpoints, such as major reductions in KIT D816V allele burden, are likely to remain important secondary or exploratory endpoints alongside clinical, pathologic, and patient-reported outcomes .1,5,7,9
For drug developers, this creates a development environment where therapeutic strategy, diagnostic strategy, and biomarker strategy must be planned together from the beginning.
How These Developments Look from Different Professional Perspectives
Mastocytosis is scientifically rich, clinically complex, and operationally demanding to study. For researchers and potential CDMO customers, including scientists, project managers, procurement leads, and executives, the same landscape looks different depending on the decisions each group must make.
Perspective of Translational and Clinical Researchers
Researchers working in mastocytosis now have a well-defined driver mutation, a measurable molecular biomarker, and therapies that can directly target disease biology. KIT D816V is not only a diagnostic marker; it is a treatment target and a response marker.1,3,8,10,11
This creates opportunities to connect mechanisms, biomarker movement, and clinical benefits in a highly structured way. Small-molecule programs can explore selectivity, resistance, combination therapy, and molecular-response depth. Advanced-therapy programs can use engineered cell models to examine clonal evolution and test whether genetic or RNA-targeted interventions could meaningfully alter disease biology.14,15
For researchers, mastocytosis offers a rare-disease setting where the path from molecular mechanism to clinical endpoint is unusually visible.
Perspective of Project and Program Managers
For project and program managers, mastocytosis drug development requires careful planning because the patient population is small and clinically segmented. Trials may need to distinguish between ISM, smoldering SM, ASM, SM-AHN, and MCL. Each subgroup may have different endpoints, risk profiles, treatment histories, and regulatory expectations.
Biomarker logistics are also central. Protocols may require serial blood and bone-marrow sampling, KIT D816V allele-burden testing, serum tryptase monitoring, central pathology review, and NGS-based mutation profiling. These needs affect site selection, patient burden, sample handling, data management, and vendor coordination.
Real-world comparative data may also influence expectations for new therapies entering the field.8,9,15 New entrants will need to show not only target activity, but also a meaningful clinical rationale in a landscape where selective KIT inhibition has already raised the bar.
Perspective of Executives and Strategic Decision-Makers
At a strategic level, mastocytosis illustrates several broader trends in rare disease drug development.
Rare diseases with clear genomic drivers can support highly targeted therapeutic strategies, but those strategies often require substantial diagnostic and biomarker infrastructure. Additionally, long-term follow-up is increasingly important for demonstrating durability, informing health-technology assessments, and supporting post-approval development.5,7,10,11,15 Finally, early gene- and cell-based research may create future waves of innovation even when current clinical development remains concentrated on small molecules.
For organizations evaluating rare-disease programs, mastocytosis shows how scientific clarity can create opportunity, but also how operational complexity, biomarker execution, and long-term evidence generation shape the path to impact.
SK pharmteco and the Rare Disease Advancement Initiative
Mastocytosis underscores the scientific and operational challenges that often define rare-disease development: small patient populations, complex biology, specialized analytics, and the need to move promising programs from laboratory insight toward clinical and commercial readiness.
SK pharmteco recognizes the issues faced by patients, researchers, and therapy developers working in mastocytosis and other rare diseases. The Rare Disease Advancement Initiative was created to support programs that are ready to move beyond the lab, with thoughtful commercial flexibility designed to help drive meaningful and scalable impact.
Contact SK pharmteco to learn more.
References
- J. Gotlib, et al., Avapritinib for Advanced Systemic Mastocytosis, Blood, 140(15), 1667 (2022).
doi.org/10.1182/blood.2021014612
- NCCN: Guidelines® Insights: Systemic Mastocytosis. Version 3.2024, J. Natl. Compr. Canc. Netw., 22(2D), e240030 (2024).
- P. Nöldeke, et al., Avapritinib Reduces Symptoms and Mast Cell Burden in Systemic Mastocytosis, Allergy Asthma Clin Immunol. 21(40) (2025).
doi.org/10.1186/s13223-025-00986-z
- M. Jara-Acevedo, et al., Detection of the KIT D816V Mutation in Peripheral Blood of Systemic Mastocytosis Patients, Mod. Pathol., 28 (8), 1138 (2015). doi.org/10.1038/modpathol.2015.72
- A. Reiter, et al., Avapritinib in Patients with Advanced Systemic Mastocytosis (AdvSM): Efficacy and Safety Analysis from the Phase 2 PATHFINDER Study with 3-Year Follow-Up (EHA 2024 oral presentation; published abstract/poster).
- M. Heiblig, et al., Prognostic Characterization of Advanced Mastocytosis Patients Treated with Midostaurin According to Diagnosis and Mutation-Adjusted Risk Score: A Nation-Wide Ceremast Study of 170 Patients, Blood, 142 (Supplement 1), 1837 (2023).
doi.org/10.1182/blood-2023-185890
- A. Reiter, et al., MPN-274 Avapritinib in Patients with Advanced Systemic Mastocytosis (AdvSM): Efficacy and Safety Analysis From the Phase 2 PATHFINDER Study With 3-Year Follow-Up, Clin. Lymphoma Myeloma Leuk., 24 (Supplement 1) S425 (2024).
doi.org/10.1016/S2152-2650(24)01421-6
- A. Reiter, et al., Avapritinib Versus Midostaurin or Cladribine in Advanced Systemic Mastocytosis: A Retrospective Real-World External Control Study, Leukemia Res., 157, 107919 (2025).
doi.org/10.1016/j.leukres.2025.107919
- S. Jacobs, Study Suggests Avapritinib as First-Line Therapy in Advanced Systemic Mastocytosis, Rare Disease Advisor (2025).
- H.O. Elberink, et al., Long-Term Avapritinib Therapy Sustains Efficacy and Tolerability in Indolent Systemic Mastocytosis Longer-Term Analysis of the PIONEER Study (EAACI 2025 Congress abstracts/posters).
- P. Pyatilova, et al., Avapritinib Improves Disease Control and Quality of Life in Patients with Indolent Systemic Mastocytosis: First Results of the Real-World Evidence Study AVATAR, J. Allergy Clin. Immunol., 155(2), Supplement AB428 (2024).
doi.org/10.1016/j.jaci.2024.12.975
- A. Reiter, et al., KIT-Targeting Drugs in Advanced Systemic Mastocytosis: New Developments in Diagnosis, Prognostication and Treatment, Blood, 135 (16), 1365 (2020).
doi.org/10.1182/blood.2019000932
- NCCN: Clinical Practice Guidelines in Oncology: Systemic Mastocytosis. Version 1.2025 (2025).
- G. Bandara, et al., CRISPR/Cas9-Engineering of HMC-1.2 Cells Renders a Human Systemic Mastocytosis Model with a Single D816V-KIT Variant, Front. Immunol., 14, 1078 (2023).
doi.org/10.3389/fimmu.2023.1078958
- G.H. Falduto, et al., Genome Engineering of HMC-1.2 Cells Reveals the Impact of Additional KIT Variants on Transcriptional Programs and Drug Sensitivity, J. Immunol., 210 (Supplement 1) 67.02 (2023).
doi.org/10.4049/jimmunol.210.Supp.67.02 2023
- M. Jawhar, et al., Mutation-Adjusted Risk Score (MARS) for Advanced Systemic Mastocytosis. J. Clin. Oncol., 37 (31), 2846 (2019).
doi.org/10.1200/JCO.19.00640
- R. Lai, MARS Prognostic Score and Advanced SM Subtype are Highly Predictive of Survival, Rare Disease Advisor (2024).