Vincristine Sulfate Liposome Injection (VSLI, Marqibo®) in Combination with UK ALL-R3 Induction Chemotherapy for Children, Adolescents and Young Adults with Relapsed Acute Lymphoblastic Leukemia (ALL): A Therapeutic Advances in Childhood Leukemia and Lymphoma (TACL) Consortium Trial

Introduction: Vincristine intensification has the potential to improve outcomes in ALL, but severe neurotoxicity has prohibited escalation beyond standard capped doses. Vincristine sulfate liposome injection, VSLI (Marqibo®) is a liposomal formulation of aqueous vincristine that optimizes pharmacokinetics, prolongs circulating half-life, increases tissue penetration, and may be better tolerated than standard vincristine. VSLI received accelerated FDA approval for adults with relapsed/refractory (r/r) ALL, at a dose of 2.25 mg/m ²/dose without a dose cap. A phase I trial of VSLI in children and young adults with r/r ALL demonstrated safety, tolerability, and evidence for single-agent activity (Shah NN, et al. Pediatric Blood Cancer, 2016). Studies of VSLI with combination chemotherapy in children have not been conducted. With emerging data supporting improved outcomes for patients (pts) with r/r ALL who proceed to immunotherapy with low-burden disease, identifying safe and effective reinduction regimens to reduce disease burden remains a priority.

Methods: This multicenter pilot study conducted through the TACL consortium was designed to assess safety and feasibility of VSLI as replacement for standard vincristine with combination chemotherapy for individuals between the ages of 2-21 years with r/r ALL. Two dose levels of VSLI were utilized without a dose cap: Dose level 1 (DL1) 1.5 mg/m ²/dose; and Dose level 2 (DL2) 2.0 mg/m ²/dose. Treatment success was determined by both 1) the absence of a dose-limiting toxicity (DLT) (as defined by any > Grade 3 regimen related non-hematologic toxicity which precluded completion of the pre-specified treatment course or from proceeding to subsequent therapy within 7 weeks) and 2) completion of the prescribed treatment regimen. Adverse event grading was based on CTCAE v4.03. Bone marrow aspirate was used for standard morphologic assessment of response with central flow cytometric analysis for minimal residual disease (MRD) determination with a cut-off of <0.01% of mononuclear cells considered as MRD negative. This analysis reports the toxicities and feasibilities for Cohort A which constituted the 4-drug UK ALL-R3 induction regimen consisting of VSLI, dexamethasone, mitoxantrone, and asparaginase (pegaspargase or Erwinia).

Results: A total of 10 pts with r/r B-ALL, median age 13.4 (range 5.0-17.3 years) were treated on Cohort A. Pts were heavily pre-treated, with 3 having relapsed after prior allogeneic stem cell transplantation; 8 of 10 had an M3 marrow (>/= 25% blasts). The first 4 pts were treated at DL1. All 4 were evaluable for toxicity and response and met the criteria for treatment success, facilitating dose escalation to DL2. At DL2, 4 pts were treated, with 2 experiencing DLT, including one grade (Gr) 5 event (Table 1). Two additional pts were subsequently treated at DL1 and achieved treatment success, confirming safety and feasibility of DL1 with a VSLI dose of 1.5 mg/m ² in combination with UK ALL-R3 4-drug induction. Nine of 10 (90%) pts achieved a morphologic complete remission (CR), 5 of which were MRD negative. The median VSLI dose administered at DL1 was 2.3 mg (range, 1.8-3.2 mg) and at DL2 was 2.2 mg (range, 1.5-3.3 mg) demonstrating the feasibility of dosing beyond the standard vincristine dose-cap of 2 mg. Transient aspartate aminotransferase (AST) and alanine aminotransferase (ALT) elevations were seen in 8 pts, 6 of whom had a grade 1-2 toxicity. Gr 3 hepatic toxicities were seen in 3 pts: 1 each with ALT elevation and hyperbilirubinemia; ALT and AST elevation; gamma-glutamyl transferase elevation. Toxicities were generally consistent with the UK ALL-R3 regimen.

Conclusions: In children with r/r ALL, the combination of UK ALL-R3 with VSLI was highly effective with an acceptable safety profile. DL1 (1.5 mg/m ²/dose) was found to be the maximum tolerated dose in combination with this intensive re-induction regimen. The trial continues to enroll at DL1 in the expansion phase to obtain additional safety and response data with this 4-drug regimen. Additionally, 2 cohorts have been added to gain further experience with VSLI in combination with 3-drug induction (Cohort B: UK ALL-R3 without mitoxantrone) and with ALL maintenance chemotherapy (Cohort C: dexamethasone, methotrexate, mercaptopurine). VSLI has potential as a vincristine dose-intensification strategy in combination with reinduction chemotherapy for r/r ALL in pediatric patients.

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