Chronic Neutrophilic Leukemia (CNL) Expert Diagnosis & Targeted Therapy Access

Chronic Neutrophilic Leukemia (CNL) and CML (Chronic Myeloid Leukemia) are both conditions causing high white blood cell counts dominated by neutrophilic cells, but they are completely different diseases at the molecular level. CML is caused by the BCR-ABL1 fusion gene (Philadelphia chromosome) and is treated with TKIs targeting BCR-ABL1 (imatinib, dasatinib, etc.). CNL is caused by mutations in the CSF3R gene (G-CSF receptor) and BCR-ABL1 is negative. CNL also causes a mature neutrophilia (with minimal immature cells), while CML typically shows a full granulocytic left shift to myelocytes and promyelocytes on the blood film. CML is far more common; CNL is extremely rare. The distinction is critical because the treatments are completely different.

CSF3R (Colony-Stimulating Factor 3 Receptor) is the gene encoding the receptor for G-CSF — the hormone that normally drives neutrophil production. In CNL, somatic (acquired) mutations in CSF3R make this receptor permanently active without needing G-CSF to switch it on — driving autonomous, uncontrolled neutrophil production. The most common mutation, T618I, activates the JAK-STAT3/5 signalling pathway and identifies the majority (~90%) of CNL patients. CSF3R T618I testing is now a WHO 2022 diagnostic criterion for CNL, replacing the previous reliance on exclusion diagnosis alone. Critically, T618I immediately identifies sensitivity to ruxolitinib (a JAK1/2 inhibitor) — transforming diagnosis into a treatment selection tool.

CNL treatment depends on the CSF3R mutation type and disease risk. For patients with CSF3R T618I (membrane-proximal mutation, ~90% of cases), ruxolitinib (a JAK1/2 inhibitor) is the targeted medical therapy of choice, producing haematological responses (WBC normalisation, spleen reduction, symptom improvement) in approximately 60–70% of patients. For the rarer truncating CSF3R mutations, dasatinib (a SRC kinase inhibitor) is rationally preferred. Hydroxyurea is used for rapid cytoreduction and symptom control. Allogeneic stem cell transplantation (allo-SCT) is the only potentially curative option and should be considered for all eligible patients — it should not be deferred until medical therapy fails.

SETBP1 mutations — at specific hotspot positions in the SETBP1 gene — are found in approximately 20–30% of CNL cases and represent the most important adverse prognostic marker within CNL. Patients with SETBP1-mutated CNL have significantly shorter overall survival, higher risk of progressing to AML, and appear to have less durable responses to ruxolitinib than patients without SETBP1 mutation. SETBP1 co-mutation is the strongest argument for prioritising early allo-SCT in an eligible patient. All CNL patients should have comprehensive NGS mutation testing at diagnosis — not just CSF3R testing — specifically to identify SETBP1 and ASXL1 co-mutations.

Long-term disease control — and potentially prolonged disease-free survival — is achievable with allogeneic stem cell transplantation (allo-SCT) in eligible patients, making it the only approach with curative intent. Medical therapy with ruxolitinib or hydroxyurea controls the disease and improves symptoms but does not eliminate the CSF3R-mutant clone and does not prevent eventual disease progression or AML transformation. For patients who are not transplant-eligible due to age or comorbidities, medical therapy with ruxolitinib can provide meaningful disease control and quality-of-life improvement. The decision about transplant versus long-term medical therapy requires specialist haematology input at a centre experienced in rare MPNs and CNL.

Transformation to AML (≥20% blasts in blood or bone marrow) occurs in a significant proportion of CNL patients over time — more commonly and more rapidly than in other indolent MPNs, particularly in patients with SETBP1 or ASXL1 co-mutations. The transformed disease is classified as secondary AML and carries a poor prognosis with standard induction chemotherapy. AML arising from CNL is typically managed with intensive induction (CPX-351 or 7+3) in fit patients or HMA + venetoclax in less fit patients, followed by allo-SCT in remission where achievable.

Yes — CNL has a reported association with plasma cell neoplasms, particularly multiple myeloma and monoclonal gammopathy of undetermined significance (MGUS). In some cases, the CSF3R-mutant myeloid clone appears to be driven or stimulated by cytokines produced by the plasma cell population. This association is important clinically because: (1) all CNL patients should have evaluation for plasma cell dyscrasia at diagnosis (serum and urine protein electrophoresis, serum free light chains, bone marrow plasma cell assessment); and (2) management may require addressing both conditions. The distinction between reactive neutrophilia secondary to myeloma and true CNL with co-existing myeloma requires specialist haematopathology review.

CNL is genuinely one of the most diagnostically challenging haematological conditions for several reasons: it is extremely rare (many haematologists see none in their career); its defining feature (persistent mature neutrophilia) is also a common finding in many reactive conditions (infections, inflammatory diseases, smoking, medications); it requires specific exclusion of CML, other MPNs, and reactive causes; and until the discovery of CSF3R mutations, it was diagnosed by exclusion without a positive molecular marker. The WHO 2022 criteria now include CSF3R mutation as a positive diagnostic criterion, making the diagnosis more robust, but the test must be ordered specifically — many laboratories do not include it on standard myeloid mutation panels.

Yes. CancerFax supports patients with CNL — one of the rarest myeloid neoplasms — by facilitating CSF3R mutation testing (exons 14 and 17) and comprehensive co-mutation profiling (SETBP1, ASXL1, and others), coordinating second opinions with specialist haematologists experienced in rare MPNs and CNL, identifying ruxolitinib or dasatinib access based on confirmed CSF3R mutation type, assisting with transplant eligibility assessment and clinical trial access at specialist centres, and connecting patients with expert rare MPN programmes in India, China, and internationally. Please share your blood count, bone marrow report, and any available mutation test results via the CancerFax portal or contact our team to begin.