Blinatumomab in Pediatric Patients with Relapsed/Refractory B-Cell Precursor and Molecularly Resistant Acute Lymphoblastic Leukemia (R/R ALL): Updated Analysis of 110 Patients Treated in an Expanded Access Study (RIALTO)

Introduction:

Although survival rates in children and adolescents with acute lymphoblastic leukemia (ALL) have improved significantly, relapsed or refractory (R/R) ALL remains a leading cause of cancer-related deaths in pediatric patients. Blinatumomab is a bispecific T-cell engager (BiTE^®) immuno-oncology therapy that activates endogenous cytotoxic T cells to kill target B cells. We report the primary analysis results of RIALTO, an expanded access study, where pediatric patients with R/R ALL were treated with blinatumomab (NCT02187354).

Methods:

Enrolled in the study were children and adolescents >28 days and <18 years of age with R/R CD19+ ALL (defined as ≥2 relapses, relapse after allogeneic hematopoietic stem cell transplant [HSCT], or refractory to prior treatments) and ≥5% blasts or <5% blasts but with minimal residual disease (MRD) level ≥10⁻³. Blinatumomab was given as continuous infusion in a 6-week cycle (4 weeks of treatment and 2 weeks of treatment-free interval) for up to 5 cycles. Patients with <25% blasts were dosed at 15 µg/m²/day, whereas those with ≥25% blasts were dosed at 5 µg/m²/day (days 1-7 of cycle 1) followed by dose increase to 15 µg/m²/day. Any change in therapy (eg, HSCT) was off-protocol and per investigator preference. Primary endpoint was incidence of treatment-emergent (TE) and treatment-related (TR) adverse events (AEs). Secondary endpoints included incidence of morphological complete response (CR; <5% blasts) and MRD response (<10⁻⁴ blasts by PCR or flow cytometry) in the first 2 cycles, relapse-free survival (RFS), overall survival (OS), and HSCT rate after blinatumomab treatment. Data cutoff was September 27, 2018.

Results:

Of 110 patients enrolled (median age, 8.5 years [95% CI 0.4-17.0]), 60% were 7-17 years of age, 61% had <50% blasts at baseline, and 11% had <5% blasts (n=12; with MRD ≥10⁻³). Among 12 patients with <5% blasts and MRD-positive disease at baseline, 0 had prior relapse after HSCT and 2 had chromosome translocation mutations. Prior treatments included HSCT (41%) and blinatumomab (5%); 56% of patients had ≥2 relapses and 40% relapsed after HSCT (Table 1).

Of 98 patients with ≥5% blasts at baseline, 58 (59%) achieved CR (<5% blasts), 0 achieved partial remission (PR; ≥5 to <25% blasts), and 20 (20%) showed progressive disease (PD; ≥25% blasts) after the first 2 cycles. Of the 58 patients who reached CR, 39 (67%) achieved CR with full recovery of peripheral blood counts, 46 (47%) achieved an MRD response, and 36 (62%) proceeded to HSCT after achieving CR. The 2 patients with t(17;19) achieved CR with an MRD response. Of the 4 patients with germline trisomy 21 (Down syndrome), 3 achieved CR with an MRD response. Among the 12 patients with <5% blasts but with MRD ≥10⁻³ at baseline, 11 (92%) achieved CR and MRD response and 1 (8%) had disease progression (Table 2). Overall, the response rates were higher among patients with lower tumor burden at baseline.

Among 98 patients with ≥5% blasts at baseline, median OS was 13.1 months (95% CI, 9.8-21.3), with median follow-up time of 17.4 months. For patients reaching CR after the first 2 cycles, the median RFS was 8.5 months (95% CI, 3.4-NE), with a median follow-up time of 11.2 months; 38% of patients relapsed and 9% died.

Of 110 patients treated with blinatumomab, 99% experienced TEAEs, with 65% being grade ≥3, including neurologic events (6%), cytokine release syndrome (CRS, 2%), cytopenias (38%), elevated liver enzymes (13%), infections (18%), and neutropenia (14%). TRAEs were reported in 74% of patients; 36% were grade ≥3 and 26% were deemed serious. Grade ≥3 TRAEs included neurologic events (5%), CRS (2%), cytopenias (9%), elevated liver enzyme (4%), infections (5%), and neutropenia (6%). Due to TRAEs, 22% of patients interrupted treatment and 5% discontinued treatment. The 9 fatal AEs, unrelated to blinatumomab, occurred due to relapse and progressive nature of the disease (Table 3).

Conclusion:

Overall, the safety profile of blinatumomab in this expanded access study in pediatric patients with R/R ALL was tolerable and consistent with that in other blinatumomab clinical trials. Patients, including those with persistent MRD and genetic disorders at baseline, achieved high rates of CR and MRD responses with low rates of relapse and disease progression. These findings support blinatumomab as a suitable treatment option for pediatric patients with R/R ALL.

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