What is the recommended management for patients with CALR‑mutated myeloproliferative neoplasms, including essential thrombocythemia and primary myelofibrosis?

Management of CALR-Mutated Myeloproliferative Neoplasms

Management of CALR-mutated MPNs follows the same risk-stratified approach as other Ph-negative MPNs, with treatment decisions based on thrombotic risk rather than mutation status, though emerging evidence suggests CALR-mutated patients may have distinct clinical characteristics and potentially inferior responses to standard therapies.

Risk Stratification Framework

The management algorithm differs between essential thrombocythemia (ET) and myelofibrosis (MF):

For CALR-Mutated Essential Thrombocythemia

Risk stratification uses the IPSET-Thrombosis model 1:

• Very Low-Risk (age ≤60 years, no JAK2 mutation, no prior thrombosis):

  • Observation without cytoreductive therapy
  • Monitor for thrombosis, acquired von Willebrand disease, or major bleeding

• Low-Risk (age ≤60 years, with JAK2 mutation OR CALR mutation, no prior thrombosis):

  • Aspirin 81-100 mg daily for vascular symptoms OR observation
  • Important caveat: Retrospective data suggest low-dose aspirin may not be beneficial in low-risk CALR-mutated ET, though current evidence is insufficient to withhold aspirin 1
  • Manage cardiovascular risk factors aggressively

• Initiate cytoreductive therapy when:

  • New thrombosis occurs
  • Symptomatic thrombocytosis develops
  • Progressive leukocytosis
  • Disease-related symptoms (pruritus, night sweats, fatigue)
  • Vasomotor symptoms unresponsive to aspirin

For CALR-Mutated Myelofibrosis

Risk stratification determines transplant eligibility and timing 2:

• Intermediate-2 or High-Risk Disease:

  • Ruxolitinib as first-line therapy for splenomegaly
  • Allogeneic stem cell transplantation for transplant-eligible patients

• Intermediate-1 Risk Disease:

  • Ruxolitinib for highly symptomatic splenomegaly
  • Allogeneic transplant if presenting with:
    • Refractory transfusion-dependent anemia
    • Peripheral blood blasts >2%
    • Adverse cytogenetics
    • High-risk mutations (ASXL1, EZH2, IDH1/IDH2, SRSF2)

Current Standard Therapies

Cytoreductive Options (When Indicated)

For both ET and MF requiring cytoreduction 2:

• Hydroxyurea (first-line)
• Recombinant interferon alpha (second-line for PV, applicable to ET)
• Ruxolitinib (JAK1/JAK2 inhibitor for MF)

Critical Clinical Considerations for CALR-Mutated Patients

CALR-mutated patients present with distinct disease characteristics 3:

• Typically younger at diagnosis
• More severe disease features at ruxolitinib initiation (higher blast counts, lower hemoglobin, worse marrow fibrosis)
• Longer median time from diagnosis to treatment (2.6 vs 0.7 years for JAK2-mutated)
• Inferior symptom responses to ruxolitinib (56.1% vs 66.7% in JAK2-mutated patients, p=0.04)
• Numerically lower spleen response rates at 6 months

Emerging CALR-Targeted Therapies

While not yet standard of care, novel immunotherapeutic approaches specifically targeting mutant CALR are in clinical development 4, 5, 6:

• Monoclonal antibodies (Fc-silent antagonists): Block mutant CALR-MPL signaling, showing rapid hematologic remissions with minimal toxicity in early trials
• Bispecific T-cell engagers: Demonstrate selective mutant-cell killing
• CAR-T cells: Show potent preclinical activity
• Antibody-drug conjugates: Selective targeting of mutant clones
• Peptide vaccines: Induce T-cell responses but have not achieved molecular responses to date

These represent a potential paradigm shift from symptom management to disease-modifying treatment, as they offer clonal specificity that current JAK inhibitors lack 5.

Practical Management Algorithm

1. Confirm CALR mutation subtype (Type 1 vs Type 2) and assess for co-mutations (ASXL1, SRSF2, EZH2, IDH1/2) 2, 7

2. Risk stratify using IPSET-Thrombosis for ET or DIPSS/MIPSS for MF

3. For low-risk ET:

   • Consider observation or aspirin (noting potential reduced benefit in CALR-mutated cases)
   • Avoid cytoreductive therapy unless symptomatic

4. For high-risk ET or symptomatic disease:

   • Initiate hydroxyurea
   • Consider interferon alpha in younger patients desiring disease modification

5. For MF:

   • Intermediate-2/high-risk: Ruxolitinib first-line; evaluate for transplant
   • Anticipate potentially lower symptom response rates compared to JAK2-mutated patients
   • Monitor for disease progression more closely given inferior ruxolitinib responses

6. Monitor for clinical trial availability of CALR-targeted immunotherapies, particularly in patients with inadequate responses to standard therapy

Common Pitfalls

• Do not withhold aspirin in low-risk CALR-mutated ET based solely on retrospective data; evidence remains insufficient 1
• Do not assume equivalent ruxolitinib efficacy across mutation types; CALR-mutated patients show inferior symptom responses 3
• Perform bone marrow biopsy before initiating cytoreductive therapy to rule out progression to myelofibrosis 1
• Use aspirin cautiously in patients with acquired von Willebrand disease or extreme thrombocytosis (>1,000 × 10⁹/L) due to bleeding risk 1