Pentostatin Plus Cyclophosphamide Decrease Graft Rejection in Nonmyeloablative Haploidentical Hematopoietic Cell Transplantation for Sickle Cell Disease

Sickle cell disease (SCD) deleteriously affects all organs, causing considerable morbidity and early mortality. Hematopoietic cell transplantation (HCT) is a potentially curative therapy that can reverse the SCD phenotype, stabilize, and possibly improve, abnormal organ function. Nonmyeloablative conditioning has been employed safely in adults with severe SCD. Whereas fewer than 10% of patients have an available HLA-matched sibling donor, haploidentical (haplo) donors greatly expand HCT availability. While early efforts at haplo HCT were plagued with high rates of acute graft rejection (50%); pentostatin, a purine antimetabolite, with the alkylating agent cyclophosphamide, prevents murine allograft rejection. Here we report preliminary data from a nonmyeloablative haplo peripheral blood HCT study employing pentostatin plus oral cyclophosphamide preconditioning.

This study analyzes data from adults who received haplo HCT for SCD at the National Institutes of Health (NIH) from January 2017 through December 2020. Inclusion criteria were: a history of stroke or cerebral vasculopathy, elevated tricuspid regurgitant velocity (TRV, ≥2.5 m/s) or pulmonary hypertension (PH), sickle hepatopathy, or recurrent vaso-occlusive crises (VOC) or acute chest syndrome while on hydroxyurea. The HCT conditioning included pentostatin, oral cyclophosphamide, alemtuzumab, and 400cGy total body irradiation (Figure 1).Graft-vs-host disease (GVHD) prophylaxis included post-transplant cyclophosphamide and sirolimus.

The cohort included 19 patients with a median age of 31 years. There were 12 men (63%); 95% had HbSS. Haplo donors included siblings (53%), parents (37%), and children (10%). Common transplant indications were elevated TRV/PH (34%), stroke/cerebral vasculopathy (23%), and recurrent VOC (20%). 63% of patients had two transplant indications, and 21% had 3 transplant indications.

The median CD34+ stem cell dose was 13.6 x10e6 CD34+ cells/kg. 95% of patients had primary engraftment; one experienced acute graft rejection (5%). Median neutrophil recovery occurred by 22 days, platelet recovery by 25 days, and red blood cell recovery by 30.5 days.

At 1-year post-HCT, overall survival was 95% and event-free survival 89%. At 3.3 years of median follow-up (range 1.6-5.2 years), overall survival is 84%, and event-free survival is 68%. Three patient deaths (15.8%) occurred; one due to intracranial hemorrhage at 2 months post-HCT and two after protracted courses of treatment-refractory Evans syndrome at 1.5 and 1.7 years post-HCT. In addition to the single patient with acute graft rejection, two patients had secondary graft failure and return of SCD at 2.6 and 2.9 years post-HCT. At 24-months follow-up, 8 (61.5% of evaluable) patients had full donor chimerism (donor myeloid and lymphoid chimerism ≥ 95%); nine (56% of evaluable) patients discontinued immunosuppression by the last follow-up. Figure 2 demonstrates that although donor myeloid chimerism (DMC) overall remained stable over time in most engrafted patients, four patients have experienced falling DMC.

Mean hemoglobin (9.1 ± 1.2 to 11.7 ± 2.5 g/dL, p=0.004), direct bilirubin (0.5 ± 0.2 to 0.3 ± 0.2 mg/dL, p=0.02), absolute reticulocyte count (330.8 ± 207 to 156.3 ± 109.2 K/mcL, p=0.004), lactate dehydrogenase (495.1 ± 264.1 to 303.9 ± 108 U/L, p=0.03), and TRV (2.6 ± 0.3 to 2.3 ± 0.5 m/s, p=0.01) improved within one year after HCT.

Though frank viral disease occurred in three patients (EBV post-transplant lymphoproliferative disorder requiring rituximab, chemotherapy, and viral-specific t-cell therapy), viral reactivation was a common complication, occurring in 12 patients (63%; EBV n=4, CMV n=7, HSV n=1) and was controlled with weaning immunosuppression, antiviral therapy, or both. One (5%) patient developed grade II acute GVHD, no one has experienced chronic GVHD.

This study demonstrates that nonmyeloablative haplo HCT with pentostatin plus cyclophosphamide preconditioning for SCD decreases acute graft rejection with minimal GVHD. Notable complications include viral disease, lethal immune phenomena, and falling DMC. Further exploration is required to examine the causal role of pentostatin plus cyclophosphamide versus other factors, such as the mixed chimerism state, in these complications. We have amended the protocol to increase myelosuppression with the goal of decreasing autoimmune complications and late graft failure.

No relevant conflicts of interest to declare.

pentostatin-- nonmyeloablative pre-conditioning