eliminates leukemic cells in a high percentage of patients and permits 
engraftment (24). 
A number of studies in AML using autologous marrow engraftment 
after preparation with busulfan/cytoxan have shown promising results 
(24). In studies from Johns Hopkins Hospital, AML patients in second 
or subsequent complete remission who had marrow removed, treated 
with 4HC and then reinfused after treatment with busulfan/cytoxan 
had an estimated event-free survival at 2 years of 30%. By historical 
review, none of these patients would remain disease free with 
conventional chemotherapy for relapsed AML more than a median of 
6 months (33). 
2.3 Mechanism of Relapse 
Although ABMT may offer advantages over conventional chemotherapy, the 
major cause of treatment failure remains relapse or disease progression. 
When ABMT is undertaken for solid tumors, relapse is generally at the site 
of original disease, implying that supralethal chemo-radiotherapy has not 
eradicated the malignancy. The mechanism of relapse following ABMT for 
marrow cell derived malignancies such as AML is less clear. Two 
explanations have been offered as to why the incidence of relapse in these 
diseases is far higher after ABMT than it is after AlloBMT. The first is that 
the alloreactive T lymphocytes present in allogeneic marrow play a major role 
in the recognition and elimination of residual host malignant cells (26). Since 
these cells are absent from autologous marrow, relapse is more likely after 
ABMT. This explanation suggests that relapse after ABMT for hematological 
malignancy occurs for the same reason as relapse after ABMT for solid tumor 
- persistence of residual disease in the host . An alternative explanation is that 
even though cryopreserved autologous marrow is harvested in remission, it 
nonetheless contains residual malignant cells. In this case, relapse is due to 
residual disease not in the host, but in the rescuing marrow (33-35). It is 
likely that both factors contribute to relapse, although the relative 
contribution of each in any given disease is unknown. 
The mechanism of relapse following ABMT for marrow derived malignancy 
is an important issue to resolve. The fear that stored marrow contains 
residual malignant cells has led to intensive investigation of the value of 
marrow purging prior to storage and subsequent reinfusion. Purging may be 
undertaken with monoclonal antibodies (35), with cytotoxic agents active in 
vitro, or by physicochemical means (33,34). In animal and pre-clinical human 
studies all of these methods reduce contamination with malignant cells if these 
are deliberately added to the marrow, but none have been shown to reduce 
the risk of relapse in naturally occurring disease. Marrow for ABMT is 
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