Experiences of allogeneic hematopoietic cell transplantation following non-myeloablative conditioning regimen in severely comorbid patients with myelofibrosis: case series with a patient presenting with extensive extramedullary hematopoiesis.

We have performed allogeneic hematopoietic cell transplantation (allo-HCT) using a reduced intensity conditioning regimen for curative management of advanced myelofibrosis (MF). However, allo-HCT is rarely considered for elderly or patients with severe comorbidities due to high transplantation-related mortality. In those patients, an alemtuzumab-based non-myeloablative (NMA) conditioning regimen followed by stem cell transplantation could be a possible treatment that has been tried in sickle cell anemia showing stable mixed chimerism and improvement of the disease. However, it is uncertain whether this regimen can provide durable donor-dominant chimerism also in patients with MF. We planned a two-stage allo-HCT in four patients - initially aimed at mixed chimerism with NMA conditioning and then reinforced with additional stem cell infusion if graft failure occurred. In one case with extensive extramedullary hematopoiesis, causing blindness and paraplegia, we achieved stable complete donor-chimerism and complete molecular response with disappearance of bone marrow fibrosis after allo-HCT. Although this NMA regimen failed to achieve durable donor-chimerism, additional stem cell infusion showed a possible role for stable long-term chimerism with good clinical outcomes. Although it leaves room for further improvement, allo-HCT using an NMA conditioning regimen may be worth consideration for advanced MF patients with severe comorbidity, otherwise no appropriate treatment option is available.