Targeting Bcl-2 and CD38 As Part of Personalized HSCT Conditioning Regimen in Chemorefractory Pediatric Leukemia

Introduction

The outcome hematopoietic stem cell transplantation (HSCT) in a cohort of children with chemorefractory leukemia is poor. The incidence of relapse exceeds 50% and survival varies from 10 to 40%. Additional attempts at remission induction with various combinations of chemotherapy are unlikely to improve the outcome and will contribute to toxicity. We hypothesized that personalized targeted therapy combined with high-dose chemotherapy may improve the outcome of allogeneic hematopoietic stem cell transplantation in a cohort of pediatric patients with refractory leukemia. Bcl-2 and CD38 were chosen as potential targets due to frequent expression in pediatric acute leukemias, availability of marketed targeted therapies, venetoclax and daratumumab, and expected non-overlapping toxicity profile of these agents and the conditioning regimen.

Materials and methods

A total of 16 pts with chemorefractory disease (T-ALL - 2, AML - 8, JMML - 6, 12 male, 4 female, median age 5,7 years), underwent HSCT between November 2017 and June 2018, median follow-up - 3 months (1,6-7). All pts were transplanted from haploidentical donors, had active disease (AD) at the moment of SCT, for 12 (75%) pts it was the first allogenic HSCT, for 4 pts it was the second HSCT. Median bone marrow leukemia burden before cytoreduction was 22% (3-75). Bcl-2 expression on the tumor cells was detected in all pts (100%) with the median expression of 69% (0,7-100), CD38 expression was detected in 10 pts (AML=7, ALL=2, JMML-1) with the median expression of 96% (71-100). Ten pts received treosulfan-based conditioning, 3 - busulfan-based and 3 -TBI-based. GVHD prophylaxis included tocilizumab at 8 mg/kg on day -1, post-transplant bortezomib and abatacept at 10 mg/kg on day -1, +7, +14, +28. Three pts received thymoglobulin 5mg\kg. According to the expression of Bcl-2 and CD38 on tumor cells, 9 patients (56%) received Daratumumab (anti-CD38 monoclonal antibody) on day -6, 15 patients (94%) received venetoclax at 300 mg/m²/day on days -7 to -2.

TCRαβ+/CD19+ depletion of PBSC with CliniMACS technology was implemented in all cases. The median dose of CD34+ cells in transplant was 11 x10⁶/kg (range 7-18), α/β T cells - 40x10³/kg (range 11- 139).

Modified (CD45RA-depleted) donor lymphocyte infusions (DLI) were administered to 15 pts, 9 pts received modified DLI on day 0.

Result

Primary engraftment was achieved in 13 (81%) of 16 pts. The median time to ANC and platelets recovery was 14 days (11-22). Engraftment was 100% (10 of 10) among patients with acute leukemia and 50% (3 of 6) among patients with JMML. Three patients with JMML had early disease progression. There were no significant toxic effects after HSCT and no cases of transplant-related mortality. The median NK- cells count by the day +30 was 0,185 x 10⁶/ml (range 0,019- 0,472), the median levels of αβ T cells and gd T cells were 0,045 x 10⁶ /ml (range 0 - 0,364) and 0,07 x 10⁶ /ml (range 0 - 0,349, respectively. Acute GVHD grade 1-2 was developed in 2 pts (15%), none of them required systemic immunosuppressive therapy. There were no cases of chronic GVHD. One (7,6%) patient with AML relapsed on day +61. Three pts (1 with AML and 2 with JMML) died from disease progression, 1 patient with JMML died from complications after the second HSCT. At the moment of reporting 12 pts (9 of 10 with acute leukemia and 3 of 6 with JMML) are alive, in complete remission with a median follow up of 3 months (1,5-7m).

Conclusion

We suggest that addition of venetoclax and datatumumab to the backbone of myeloablative haploidentical HSCT with αβ T cell depletion is not associated with increased toxicity and may lead to improved early outcomes in a cohort of pediatric patients with chemorefractory acute leukemia. This approach can be further tested in a prospective trial with the goal to increase the anti-leukemic efficacy of HSCT.