Treatment with Anti-CD19 BiTE^® Blinatumomab in Adult Patients with Relapsed/Refractory B-Precursor Acute Lymphoblastic Leukemia (r/r ALL) Post-Allogeneic Hematopoietic Stem Cell Transplantation

Introduction: Current therapies for patients with r/r ALL who have had prior allogeneic hematopoietic stem cell transplantation (alloHSCT) have very poor outcomes. Improvements in the therapeutic options available for adult r/r ALL are required. Blinatumomab is a bispecific T-cell engager (BiTE^®) antibody construct that redirects cytotoxic T cells to lyse CD19-positive B cells. The aim of the present analysis was to characterize a subset of patients with r/r ALL and prior alloHSCT before treatment with blinatumomab from a large confirmatory open-label, single-arm, multicenter phase 2 study (Topp MS et al Lancet Oncol 2015;16(1):57-66).

Methods: Eligible patients (≥18 years) had Philadelphia chromosome-negative r/r ALL with 1 of the following negative prognostic factors: primary refractory, first relapse within 12 months of first remission, relapse within 12 months of alloHSCT, or second or greater salvage. Patients with active acute or chronic graft-versus-host disease (GvHD) were excluded. Patients were required to stop all immunosuppressive GvHD therapy within 2 weeks before starting blinatumomab. A total of 189 patients were enrolled and received blinatumomab by continuous intravenous infusion (4 weeks on/2 weeks off) for up to 5 cycles. The primary endpoint was complete remission or complete remission with partial hematologic recovery (CR/CRh) within the first 2 cycles. Secondary endpoints included overall survival (OS), relapse-free survival (RFS), and adverse events (AEs).

Results: 64 (34%) patients had received alloHSCT before study enrollment; 10 patients had 2 prior alloHSCTs. Donor types primarily included 29 (45%) matched sibling and 31 (48%) unrelated, with 34 (59%) patients receiving myeloablative conditioning regimens (donor chimerism data were unavailable). Among those with prior alloHSCT, median age (range) was 32 (19-74) years. At baseline, 23 (36%) patients had 1 prior relapse, 24 (38%) had 2 prior relapses, and 17 (27%) had ≥3 prior relapses; 28 (44%) patients had relapsed post-alloHSCT. Of the 55 patients who had received previous salvage therapy, 38 (69%) had received salvage therapy after last alloHSCT and prior to blinatumomab. Median time (range) between the last alloHSCT and subsequent relapse was 6 (1-33) months. Median time from last prior alloHSCT to first dose of blinatumomab was 10 (3-40) months. Nineteen (30%) patients had a history of GvHD, and 42 (66%) had ≥50% bone marrow blasts at start of treatment as assessed by a central laboratory. Patients received blinatumomab for a median of 2 (1-5) cycles.

Efficacy data are presented in Table 1. Overall, 45% (29/64; 95% confidence interval [CI], 33-58) of patients achieved CR/CRh within the first 2 cycles, with similar rates of remission also observed in the alloHSCT-naïve (42%; 52/125) group. With a median follow-up of 8.8 months, median (95% CI) OS was 8.4 (4.2-9.4) months for the 64 patients with prior alloHSCT treated with blinatumomab. Of the 29 responders (CR, n=18 and CRh, n=11), 22 (76%) had a minimal residual disease (MRD) response and 19 (66%) achieved a complete MRD response. Median RFS (95% CI) was 6.1 (5.0-7.7) months. 9/29 (31%) responders subsequently underwent another alloHSCT.

In total, 56 (88%) patients had grade ≥3 treatment-emergent AEs, with the most frequent including neutropenia (22%), febrile neutropenia (20%), anemia (17%), and thrombocytopenia (14%). Six patients reported treatment-emergent GvHD (two grade ≥ 3) during blinatumomab treatment, 3 of whom had GvHD in skin. Eight patients had fatal treatment-emergent AEs, which included 1 due to gastrointestinal hemorrhage, 1 due to respiratory failure, and 6 due to infection/infestation; 1 of these (candida infection) was considered to be possibly related to treatment by the investigator. Of the subjects who had treatment-emergent fatal AEs, none were in remission at the time of death.

Summary: In this heavily pretreated group of patients with r/r ALL and prior alloHSCT, single-agent blinatumomab was able to induce a CR/CRh rate of 45%, with an AE profile consistent with that previously reported. Post-alloHSCT patients who had relapsed performed equally as well as those without prior alloHSCT. To prolong remission in this poor outcome patient group, the addition of other immunotherapies to the treatment regimen may be considered for future investigations.

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