Validating Activated CSF3R Signaling as a Therapeutic Target in CNL and Atypical CML

Prospective Evaluation of Ruxolitinib Efficacy for CNL/aCML Patients with Mutation of Colony Stimulating Factor 3 Receptor (CSF3R)

NCT02092324

Oregon Health and Sciences University Knight Cancer Institute

9 sites in the Unites States

~30 patients

This is an open-label, phase II, investigator-initiated study of ruxolitinib in patients 18 years or older with World Health Organization (WHO)-defined chronic neutrophilic leukemia (CNL) or atypical chronic myeloid leukemia (aCML). Disease-specific eligibility includes the following: 1) platelet count > 25,000/mm³; 2) discontinuation of alternative therapies such as hydroxyurea, or biologics such as interferon-α; and 3) adequate hepatic and renal function defined as ALT/SGPT and direct bilirubin < 4X ULN and creatinine clearance > 15cc/min, respectively. Subjects with CNL or aCML who have already been taking ruxolitinib as part of their standard of care may enroll as long as they meet the following criteria: 1) have pre-treatment cells or DNA in storage to allow sequencing of CSF3R or other relevant genes, 2) allow the co-investigators access to their medical records pertaining to treatment and to data relevant to study endpoints, and 3) meet other trial eligibility criteria.

 Treatment with ruxolitinib is initiated at a total daily dose between 5 mg and 40 mg (range from 5 mg qd to 20 mg bid). The starting dose is based both on existing guidelines used for treating patients with intermediate to high-risk myelofibrosis and on whether a patient is taking, concomitantly, drugs categorized as moderate or potent CYP3A4 inhibitors. Duration of treatment is 96 weeks; for patients with therapeutic benefit, the drug may be continued either on a commercial basis or via enrollment in an extension study sponsored by the manufacturer (Incyte). The primary objective is to determine hematologic response rate. Secondary objectives include evaluation of safety/tolerability, correlation of response with type of CSF3R mutation, degree of mutant allele burden reduction, and assessment of changes in the total symptom score as measured by a modified Myeloproliferative Neoplasm Symptom Assessment Form (MPN-SAF).

No standard of care exists for the rare entities CNL and aCML, and the estimated median overall survival for these disorders is in the range of 20 to 30 months. In 2013, gain-of-function mutations in CSF3R (the gene that encodes the G-CSF receptor) were identified in the majority of patients with CNL and in some patients with aCML (Maxson JE et al. N Engl J Med. 2013;368:1781-1790). CSF3R mutations cluster into two distinct regions of the receptor, with the majority occurring just extracellular of the transmembrane domain (membrane proximal mutations) and a small number resulting in truncation of the cytoplasmic tail (truncation mutations). The most common membrane proximal mutations (e.g., T618I and T615A) result in ligand-independent activation of CSF3R that initiates down-stream signaling through JAK2, and cells harboring membrane proximal CSF3R mutations are sensitive to the JAK1/JAK2 inhibitor ruxolitinib, with IC50s of ~100-200 nM. In mice transplanted with CSF3R T618I-expressing hematopoietic cells, a fatal myeloproliferative disorder characterized by overproduction of granulocytes and granulocytic infiltration of the spleen and liver developed. In this animal model, treatment with ruxolitinib lowered the white blood count and reduced spleen size with a concomitant gain in body weight (Fleischman AG et al. Blood. 2013;122:3628-3631). In addition, an index patient with CSF3R T618I-driven CNL treated with ruxolitinib exhibited an excellent clinical response with near normalization of white blood cell/ neutrophil counts and normalization of the platelet count (Maxson JE et al. N Engl J Med. 2013;368:1781-1790). Truncation mutations are less common, but data from primary bone marrow colony assays suggest that cells harboring truncation mutations of CSF3R may be similarly sensitive to ruxolitinib, though these cells are also sensitive to inhibitors of SRC-family kinases, such as dasatinib.

The preclinical findings cited above and anecdotal patient data underscore the need to investigate the therapeutic potential of JAK inhibition in CSF3R-mutated CNL and aCML. In addition to establishing the hematologic response rate and effects on other clinical endpoints, such as splenomegaly, this trial will evaluate whether there is potential for modification of the natural history of these poor-prognosis neoplasms. The trial will also correlate biomarkers of response, such as type of CSF3R mutation, quality of response, and whether meaningful effects on mutant allele burden are achievable, similar to the molecular remissions obtained by tyrosine kinase inhibitors in BCR-ABL1-positive CML. Patients who meet diagnostic criteria for CNL and aCML without molecular abnormalities of CSF3R may alternatively carry mutations in proteins downstream of this signaling axis or in parallel pathways. The collection of DNA for exome sequencing will permit identification of alternative or cooperating oncogenic driver mutations that contribute to the pathogenesis of CNL and aCML and may guide use of therapeutic agents such as ruxolitinib or of other signaling pathway inhibitors that are in development.