Haploidentical Bone Marrow Transplant with Post-Transplant Cytoxan Plus Thiotepa Improves Donor Engraftment in Patients with Sickle Cell Anemia: Results of an International Multicenter Learning Collaborative

Introduction: Hematopoietic stem cell transplantation (HSCT) is the only curative treatment for sickle cell disease (SCD), though rarely used due to inadequate donor availability, associated regimen-related toxicities, and high mortality rate in adults. A reduced intensity, haploidentical bone marrow transplantation (Haplo-BMT) regimen with post-transplant cyclophosphamide (PTCy), reported by the Johns Hopkins Group (JHG) in adults with SCD, resulted in successful engraftment, minimal toxicity, abrogation of sickle cell related symptoms, but was associated with a high graft failure rate of 43% (Blood. 2012;120(22):4285-4291). A multi-institution learning collaborative was developed in 2013 with main objective of reducing the graft failure rate in individuals with SCD who undergo a haplo-BMT with PTCy from first-degree relatives.

Material and methods: The collaborative included 3 sites, similar eligibility criteria, monthly phone conferences for sharing of group learning experiences, and independent IRB approvals at each site, allowing for aggregate data sharing. A common conditioning regimen was used based on the JHG haplo-HSCT protocol - fludarabine 150 mg/m2, Cy 29 mg/kg, Thymoglobulin 4.5 mg/kg, and TBI 2Gy), and graft versus host disease (GVHD) prevention with PTCy 100 mg/kg, mycophenolate mofetil and sirolimus. However two additional modifications were systematically added which were believed to improve engraftment rates with two different strategies. In strategy 1, a Data Safety Monitoring Committee (DSMC) planned an interim analysis after enrollment of the first 5 participants using the JHG haplo-HSCT protocol, with modifications for > 20% mortality rate, graft rejection, or severe GVHD. In strategy 2, the participants in this group had preconditioning with 2 drugs for 3 months (azathioprine 3mg/kg/d, hydroxyurea 30 mg/kg/d) and hypertransfusion, plus thiotepa (10 mg/kg/day) on D-7. No DSMC was established. All donors were mobilized with filgrastim (5-10 μg/kg/d x5 days). Primary graft failure was defined as <5% myeloid chimerism and/or less than 5% donor cells by day +42 post-transplant; secondary graft rejection was defined as <5% donor myeloid chimerism >day +42 in patients with prior documentation of > 5% donor cells by day +42.

Results: All potential patients that sought haplo-BMT had a HLA haplodonor (n=34). Median total nucleated and CD34⁺ cell doses were 8.40 x 10⁸ /kg and 3.63 x 10⁶/kg respectively; median follow-up was 20.3 months (1.4-37.5). In strategy 1 (n=5), median age was 26.4yr (12-50), and 3 out of 5 had graft failure, (2 primary; 1 secondary), triggering the stopping rule. Subsequently, thiotepa alone (10 mg/kg/day; D-7) was added; for 7 patients transplanted in this cohort, median age was 18.1yr (7-26), 100% engrafted, median time to neutrophil engraftment was 25 days; 33.1 and 36.9 days for platelets >20 x 10⁹/L and >50 x 10⁹/L, respectively, with an EFS and OS of 100%, respectively. Main complications were 2 independent viral reactivation events (CMV and EBV), 2 patients had > grade 2 acute GVHD (gut) but no significant chronic GVHD developed in any participant. One patient with graft failure using the non-thiotepa approach, was re-transplanted >1 yr after initial haplo-BMT with the thiotepa alone containing regimen with successful engraftment. In strategy 2 (n=22), median age was 10yrs (3-18), 100% engrafted, median time to neutrophil engraftment was 17days; 36.5 and 38.5 days for platelets >20 x 10⁹/L and >50 x 10⁹/L, respectively. Mortality was 13.6% (3/22) mainly from infectious complications and macrophage activation syndrome; 2 patients had secondary graft failure (9.1%). The cumulative incidence of acute and chronic GVHD was 18.2%, respectively, all resolved; EFS and OS was 81.8% and 86.4% respectively. All patients were off immunosuppressive therapy > I year posttransplant.

Conclusion: We have provided preliminary evidence that haplo-BMT with PTCy plus thiotepa alone when compared to no thiotepa, improves donor engraftment without increasing morbidity or mortality. Preliminary evidence also indicates that there is no benefit for the 2 drug preconditioning regimen with azathioprine, hydroxyurea plus thiotepa when compared to addition of thiotepa alone. An NIH sponsored BMTCTN phase II trial, of haplo-BMT with PTCy plus thiotepa is underway to determine if this regimen maximizes donor engraftment, with reduced morbidity and mortality.