Administration of BPX-501 Cells Following Aβ T and B-Cell-Depleted HLA Haploidentical HSCT (haplo-HSCT) in Children with Acute Leukemias

Background

Allogeneic hematopoietic stem cell transplantation (HSCT) is a well-established treatment for children with Acute Leukemia (AL). For patients lacking a compatible matched related or unrelated donor, HLA-haploidentical HSCT (haplo-HSCT) from a relative represents a viable alternative. Promising results were reported with a novel method of selective depletion of αβ T and B cells (Locatelli, Blood 2017). This approach is associated with limitations such as suboptimal adaptive immune reconstitution, increased risk of infection and disease relapse.

BPX-501 is an allogeneic product consisting of T cells modified to express the inducible caspase-9 (iC9) safety switch and truncated CD19 to allow monitoring and expansion of BPX-501 following transplant. The polyclonal nature of the BPX-501 provides broad virus and tumor-specific immunity, while the safety switch provides the unique ability to promptly and durably resolve graft-versus-host disease (GvHD) symptoms following the administration of rimiducid which induces dimerization and activation of iC9, inducing apoptosis of BPX-501.

Aims

To evaluate the safety and efficacy of BPX-501 administered after a αβ T and B-cell depleted haplo-HSCT in pediatric patients with AL in morphological complete remission (CR). The objective was to determine whether BPX-501 infusion can increase relapse-free survival (RFS) and overall survival (OS) through an enhanced graft-versus-leukemic (GvL) effect, while maintaining a low risk of GvHD.

Methods

This multicenter US (NCT03301168) and EU (NCT02065869), prospective trial utilizes αβ-T and B-cell-depleted haplo-HSCT followed by infusion of donor lymphocytes genetically modified with iC9 safety switch (BPX-501) in patients with malignant or non-malignant disorders. A subset of patients had acute high-risk leukemias (AML and ALL). BPX-501 was planned to be infused on day14±4 after the allograft. No post-transplant pharmacological GvHD prophylaxis was employed. Patients who develop GvHD resistant to conventional steroid therapy could receive ≥1 dose of rimiducid to activate iC9.

The efficacy-evaluable population (EEP) was defined as any patient with AL who received HSCT, BPX-501 infusion, and at least one follow-up assessment.

Results

At clinical cut-off (June 30, 2018), 100 patients (EU: 75, US: 25) with AL met the EEP definition. Median follow-up was 14.7 mos (1 - 40.6 mos). Key baseline characteristics are shown in Table 1.

The median time for neutrophil and platelet engraftment was 16 (15 - 17) and 12 (11 - 12) days, respectively. Four patients (4.1% [95% CI: 0 - 8%]) experienced primary graft failure. Of 96 evaluable patients, 21 patients developed Grade I-IV aGvHD (21.7% [95% CI: 13.5 - 29.8%]). Five patients developed Grade III-IV aGvHD (3.1% [95% CI: 0 - 6.5%]). Of 82 evaluable patients, 12 patients developed cGvHD (18.1% [95% CI: 8.2 - 22%]), with only three cases being moderate-severe.

Rimiducid was administered to 10 patients with steroid-resistant acute GvHD. Best overall clinical response of CR or PR post-rimiducid administration was seen in 8 patients (80%). Among responding patients, 7 patients (87.5%) had a CR.

Six patients died after transplantation (6.6% [95% CI: 1.4 - 11.7%]). Relapse Free Survival (RFS) was 82.2% (95% CI: 74.5 - 89.7%). Overall Survival (OS) was 90.1% (95% CI: 83.9 - 96.3%). Efficacy outcomes (TRM, RFS and OS) in AL subsets (AML and ALL) are shown in Table 2.

CD3⁺ and CD3⁺CD4⁺ T cells above 500 cells/ml were achieved by 180 and 270 days, respectively. IgA and IgM levels achieved normal values by 180 days. The percentage of circulating and median absolute BPX-501 cells at Day 100 were 9.96% ± 11.6% (0 - 54.9%) and 85.58 ± 165.57 cells/ul (0 - 1001 cells/ml), respectively.

Conclusion

The adoptive transfer of BPX-501 following αβ-T and B-cell depleted haplo-HSCT represents a novel and highly effective transplantation strategy for pediatric patients with AL. Compared to data from children receiving only αβ T and B-cell depleted haplo-HSCT or matched unrelated donor HSCT (https://bloodcell.transplant.hrsa.gov/research/transplant_data/us_tx_data/survival_data/survival.aspx), this novel approach resulted in a comparable risk of transplant-related mortality and a lower risk of recurrence. Rimiducid was also an effective treatment for patients who developed steroid-resistant GvHD.

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