Culture purging in leukemia: past, present, and future.

Over the past 5 years we have been evaluating the feasibility of using cultured marrow autografts to allow patients with chronic myeloid leukemia (CML) to receive intensive, potentially curative therapy. The rationale for this approach is based on two important findings. The first is that leukemic stem cells (operationally defined as Ph-positive long-term culture-initiating cells, or LTC-IC) are present in the marrow of many CML patients at relatively low levels by comparison to co-existing normal stem cells (i.e., Ph-negative LTC-IC). The second finding is that leukemic LTC-IC are selectively "purged" following their incubation in vitro for 10 days under LTC conditions. As a result, cultured CML marrow preparations show, on average, a 300-fold selection in favour of normal LTC-IC. However, there is considerable variation in the initial normal and leukemic LTC-IC content of marrow samples from different CML patients. Thus in only approximately one third of cases does the number of leukemic LTC-IC in the marrow decrease to undetectable levels within the first 10 days of culture with the number of co-existing normal LTC-IC remaining at or above 1/50th of the average value for normal marrow. We have now transplanted 22 such CML patients with 10 day cultured marrow autografts following their treatment with myeloablative therapy. Fifteen of these patients were in first chronic phase and 7 had more advanced disease. Hematologic recovery of neutrophils and platelets occurred in 16 in a time frame consistent with previous autografting experience in other hematologic malignancies and, in all of these, at least 75% of the dividing cells present in the marrow during this period of hematologic reconstitution were normal (i.e., Ph-negative). Hematologic recovery was inadequate in 5 patients, 3 of whom were rescued by re-establishing chronic phase disease following infusion of unmanipulated reserve autologous cells. There have been 5 deaths, all of which occurred early and only one of which was related to relapse of disease. Although minor populations of Ph-positive cells have eventually reappeared in all patients, administration of a-interferon post-autografting appears to be a useful approach to reducing this recurrent Ph-positive population. These results establish the feasibility of using cultured marrow autografts for the treatment of CML and suggest the potential of this approach as a starting point for the addition of other in vitro manipulations of normal and leukemic stem cells using growth factors and/or genetic vectors.