Treosulfan-based conditioning before hematopoietic SCT: more than a BU look-alike.

Allogeneic hematopoietic SCT is a curative treatment for a variety of hematological malignancies and genetic diseases. There is a continuous search for novel conditioning regimens that will reduce SCT-related toxicity while retaining maximal antimalignancy effect. Treosulfan (L-threitol-1,4-bis-methanesulfonate; dihydroxybusulfan) was initially used in the treatment of certain solid tumors. Preclinical studies showed that it has a myeloablative effect on committed and non-committed stem cells. It has potent immunosuppressive characteristics, more prominent than its related chemotherapy agent BU, which makes it an attractive candidate for the use in conditioning regimens before allo-SCT. It is also associated with a favorable toxicity profile with little extramedullary toxicity. The combination of fludarabine and treosulfan was explored in several studies in patients not eligible for standard myeloablative conditioning, and data are rapidly emerging. This regimen is associated with consistent engraftment. A limited non-relapse mortality (NRM) rate in the range of 9-28% was observed. This rate is promising considering the patients selected and results from low rates of organ toxicity as well as acute and chronic GVHD. The regimen was also associated with low relapse rates of 5-30% depending on disease status at SCT. Together with low NRM rate, this resulted in favorable survival in the range of 40-80%. Promising results were seen in myelodysplastic syndrome (survival 36-70%) and leukemia in remission (60-70%). Randomized prospective studies will be needed to better define the role of treosulfan-based regimens in SCT.