VI Autologous Bone Marrow Transplantation.pdf - Blog Science ...

AUTOLOGOUS STEM CELL HARVEST AND EVALUATION OF
TUMOR INVOLVEMENT
Our ability to detect small numbers of cancer cells in HSC harvest is limited.
Routine morphological stains are heavily relied upon. Often diseases with cytogenetic
markers can benefit from karyotypic analysis, but the cells have to be
able to undergo mitosis, and screening large numbers (over 100) of chromosome
preparation is very tedious. The polymerase chain reaction (PCR) offers the possibility
of extremely high sensitivity (>1 in 1 million), where specific markers
(e.g., translocations) are available to identify the tumor cells 4
. The technique is
particularly prone to technical variability, and there have been problems of standardization
in multicenter studies. Fluorescence in situ hybridization (FISH) offers
similar potential sensitivity, and these may become the methods of choice as
the technology improves. Tumor-directed monoclonal antibodies can pick up
one tumor cell in up to 100,000 normal cells in diseases such as neuroblastoma
and small cell lung cancer. Flow cytometry is of similar sensitivity and can detect
rare aneuploid cancer cells 4
.
PURGING TECHNIQUES
The aim of purging is to remove clonogenic tumor cells from the marrow or
PBSC preparation without adversely affecting the engraftment potential of the
normal stem cells. The most important method to decrease the risk of relapse
with AuBMT is to reduce the tumor burden in the patient prior to bone marrow
harvest (BMH) 2
. With this approach, the need to depend on ex vivo purging will
be less. Ex vivo purging can be achieved by negative selection in which the tumor
cells are eliminated, or by positive selection, in which the pluripotent hematopoietic
stem cells are selectively enriched. Animal models have shown that
injection of as few as 25 promyelocytic leukemia cells could establish a lethal tumor
in 50% of the recipients. Extrapolation to humans, indicated that this dose
would translate up to 3,500 leukemic cells are needed for causing leukemia 6
.
Based on this model, transplant recipients receiving 15-150 tumor cells in the
graft would have a 1-5% risk of disease relapse originating from these cells. Depending
upon the sensitivity of the method used to detect tumor involvement,
the number of cancer cells likely to be reinfused may be as high as lO^lO 7
(0.01-
0.1% infiltration of a harvest of 10 10
nucleated cells) in a remission marrow with
a correspondingly increased risk of contributing to relapse.
In our experimental model, we utilize cell lines alone and in combination
with irradiated and un-irradiated human bone marrow cells to simulate the ex
vivo purging conditions. Our investigations showed that a combination of 4-
Hydroperoxycyclophosphamide (4-HC) and VP-16 was very effective in eradicating
lymphoma and AML cells. Therefore, 4-HC and VP-16 combination was
chosen for clinical use 78
. Definitive proof of the clinical value of purging remains
to be established; however, preliminary results have suggested improved
outcome in patients receiving purged grafts 4
- 10
.
Disease relapse following HSC transplantation may be from residual tumor
in the recipient and/or from cancer cells that were reinfused with the graft. To
date it has not been possible to determine which source is predominant. Recurrences
in sites where reinfused stem cells transiently persist, e.g., the liver and
lung, suggest that reinfused cells may contribute to relapse. Studies are currently
146 SIXTH INTERNATIONAL SYMPOSIUM ON AUTOLOGOUS BONE MARROW TRANSPLANTATION