Long-term Survival after Hematopoietic Cell Transplant for Sickle Cell Disease Compared to the United States Population

Abstract

Hematopoietic cell transplant for sickle cell disease is curative but is associated with life threatening complications most of which occur within the first 2 years after transplantation. In the current era with interest in gene therapy and gene editing we felt it timely to report on sickle cell disease transplant recipients who were alive for at least 2-year after transplantation, not previously reported. Our objectives were to (1) report the conditional survival rates of patients who were alive for 2 or more years after transplantation (2) identify risk factors for death beyond 2 years after transplantation and (3) compare all-cause mortality risks to those of an age-, sex- and race-matched general population in the United States. By limiting to 2-year survivors, we exclude deaths that occur as a direct consequence of the transplantation procedure. De-identified records of 1149 patients were reviewed from a publicly available data source and 950 patients were eligible (https://picsure.biodatacatalyst.nhlbi.nih.gov). All analyses were performed in this secure cloud environment using the available statistical software package(s). The validity of the public database was confirmed by reproducing results from an earlier publication. Conditional survival estimates were obtained using the Kaplan-Meier method for the sub-cohort that had survived a given length (x) of time after transplantation. Cox regression models were built to identify risk factors associated with mortality beyond 2 years after transplantation. The standardized relative mortality risk (SMR) or the ratio of observed to expected number of deaths, was used to quantify all-cause mortality risk after transplantation and compared to age, race and sex-matched general population. Person-years at risk were calculated from an anchor date (i.e., 2-, 5- and 7-years) after transplantation until date of death or last date known alive. The expected number of deaths was calculated using age, race and sex-specific US mortality rates. The median follow up was 5 years (range 2-20) and 300 (32%) patients were observed for more than 7 years. Among those who lived for at least 7 years after transplantation the 12-year probability of survival was 97% (95% CI, 92%-99%). Compared to an age-, race- and sex-matched US population, the risk for late death after transplantation was higher as late as 7 years after transplantation (hazard ratio (HR) 3.2; P= .020) but the risk receded over time. Risk factors for late death included age at transplant and donor type. For every 10-year increment in patient age, an older patient was 1.75 times more likely to die than a younger patient (P= .0004). Compared to HLA-matched siblings the use of other donors was associated with higher risk for late death (HR 3.49; P= .003). Graft failure (beyond 2-years after transplantation) was 7% (95% CI, 5%-9%) and graft failure was higher after transplantation of grafts from donors who were not HLA-matched siblings (HR 2.59, P< .0001). Long-term survival after transplantation is excellent and support this treatment as a cure for sickle cell disease. The expected risk for death recedes over time but the risk for late death is not negligible.

Keywords: Late death; Long term survival; Sickle cell disease; hematopoietic cell transplant.