The evolving role of bone marrow transplantation in the treatment of chronic myelogenous leukemia.

Bone marrow transplantation is the only treatment that can result in long-term disease-free survival and possible cure in a significant number of patients with CML. Several prognostic features influence relapse and survival following allogeneic BMT for CML. The most important factor is treatment of patients during chronic phase. The timing of BMT in chronic phase CML remains controversial, because the Seattle findings that BMT done within a shorter interval from diagnosis to transplant was associated with improved survival has not been confirmed by the IBMTR. No factor can predict in the individual patient the timing of transformation, even in patients with low-risk chronic phase CML, but we believe that allogeneic BMT should be offered as soon as possible for newly diagnosed patients who have histocompatible siblings. More widespread application of BMT in CML is possible because of effective methods for preventing GVHD, the major cause of morbidity after allogeneic BMT. However, in vitro techniques for the depletion of donor marrow T cells have resulted in higher graft failure and relapse rates. More precise understanding of the immune mechanisms involved may permit more selective depletion techniques which not only abrogate GVHD but also permit sustained engraftment and preserve GVL effect. This may extend application of BMT for patients with mismatched related or histocompatible unrelated donors. It is of interest that cytogenetic relapse after BMT is not invariably followed by hematologic relapse. It is likely that the use of polymerase chain reaction techniques which detect the bcr-abl rearrangement at a very low level will identify the persistence of the malignant clone after allogeneic BMT in even more patients. At present, the significance of such findings is unclear, but further study of the kinetics of disappearance of the CML clone post-BMT may increase our understanding of the immune mechanisms involved in suppression of the malignant clone and determine whether in fact CML can be cured using BMT approaches.