A Phase 2 Study of Ruxolitinib with Chemotherapy in Children with Philadelphia Chromosome-like Acute Lymphoblastic Leukemia (INCB18424-269/AALL1521): Dose-Finding Results from the Part 1 Safety Phase

Background: Philadelphia chromosome-like acute lymphoblastic leukemia (Ph-like ALL) is a high-risk subtype occurring in 15-30% of older children and adolescents/young adults (AYAs) with B-ALL. Ph-like ALL is associated with high relapse rates and poor survival despite intensive multi-agent cytotoxic chemotherapy. Development of successful treatment strategies to decrease relapse and improve cure rates in patients with Ph-like ALL remains a major therapeutic gap.

Rearrangements of cytokine receptor-like factor 2 (CRLF2-R) with frequent concomitant JAK2 point mutations occur in 50% of Ph-like ALL cases and induce constitutive JAK/STAT and other kinase signaling. An additional 15-20% of Ph-like ALL harbors other JAK pathway alterations, such as JAK2 or EPOR rearrangements, that similarly activate JAK/STAT signaling. Ruxolitinib is a potent, selective JAK1/JAK2 inhibitor with demonstrated activity in preclinical Ph-like ALL models and clinical safety as monotherapy in children with relapsed/refractory cancers. We report the initial safety of ruxolitinib in combination with post-induction chemotherapy in children and AYAs with newly-diagnosed high-risk (HR) Ph-like ALL with CRLF2-R or other JAK pathway alterations treated on the non-randomized, 2-part phase 2 study INCB18424-269 (AALL1521; NCT02723994).

Methods: Patients aged 1-21 years at time of diagnosis with HR B-ALL and eligible Ph-like genetic lesions who had completed 4-drug induction chemotherapy as per the Children's Oncology Group (COG) AALL1131 study (NCT02883049) were eligible to participate. Patients were stratified into 4 cohorts by genetic alterations and end-induction flow cytometric minimal residual disease (MRD) status: cohort A = CRLF2-R JAK-mutant, MRD+; B = CRLF2-R JAK-wild-type, MRD+; C = other JAK pathway alterations, MRD+, D = any CRLF2-R or JAK pathway alteration, MRD-. Patients commenced treatment on the INCB18424-269/AALL1521 study at consolidation with ruxolitinib orally twice daily in combination with augmented Berlin-Frankfurt-Münster (aBFM) post-induction chemotherapy as per AALL1131. Five discontinuous dose levels (10-50 mg/m²/dose 14-days-on/14-days-off per cycle [DL-2 to DL2]) and one continuous DL1b (40 mg/m²/dose × 28 days per cycle) of ruxolitinib with aBFM chemotherapy were explored via a standard rolling 6 design. Dose-limiting toxicities (DLTs) were assessed through Day 29 of delayed intensification (DI) and defined as hematologic and non-hematologic toxicity with higher grade or more prolonged duration than observed in children with treated with identical chemotherapy (without ruxolitinib) on other COG HR B-ALL trials. Pharmacokinetic (PK) and pharmacodynamic (PD) analyses were conducted using serial blood samples obtained from patients during consolidation therapy.

Results: Forty patients (pts) were enrolled in Part 1 (cohort A, n=10; B, n=9; C, n=5; D, n=16). Four patients discontinued study treatment before the Day 29 DI timepoint, 3 of whom were replaced. Patients had a median age of 14 years, and 67.5% were male. Treatment-emergent adverse events occurred in all patients and included anemia (75%), platelet count decrease and/or thrombocytopenia (65%), febrile neutropenia (72.5%), and AST or ALT increase (57.5%). Thirty-three patients had Grade 3/4 events deemed possibly related to ruxolitinib without identified DLTs. Eleven patients in Part 1 (27.5%) discontinued study therapy for various reasons: CNS relapse (2 pts), end-consolidation MRD+ (4 pts), multi-system organ dysfunction (MSOD; 2 pts), elective MRD- stem cell transplantation (1 pt), psychosocial/compliance issues (2 pts). One patient died of septic shock and MSOD not attributed to ruxolitinib. Preliminary analysis of plasma drug levels at 4 hours post-dose was consistent with the known PK profile of ruxolitinib. PD studies demonstrated dose-dependent inhibition of target phosphoproteins and, importantly, sustained inhibition of phosphorylated STAT5 with continuous ruxolitinib dosing at DL1b.

Discussion: These findings demonstrate safety and tolerability of ruxolitinib in combination with intensive multi-agent chemotherapy in children and AYAs with newly-diagnosed HR CRLF2-R/JAK pathway-mutant Ph-like ALL and support continued investigation of treatment efficacy in Part 2 of this trial.