Stem cell purging ex vivo.

Over the last several years there has been significant progress in developing several techniques designed to detect contaminating tumor cells in bone marrow (BM) and peripheral blood progenitor cells (PBPC). These techniques include immunohistochemistry, in situ hybridization (ISH), flow cytometry, clonogenic tumor assays, and polymerase chain reaction (PCR) based assays. These assays have detection capabilities ranging from 1 tumor cell in 100 normal cells (ISH) to 1 tumor cell in a million tumor cells (RT-PCR). These techniques have confirmed that BM and PBPC are frequently contaminated with tumor cells, with most studies suggesting higher tumor contamination in the BM as compared to the PBPC. Comparison of immunocytochemistry with the clonogenic assay has demonstrated good but not perfect correlation of these two techniques. Gene marking studies have confirmed that these tumor cells are viable and capable of contributing to disease relapse. Many of the assays are at least semiquantitative and are effective at monitoring for persistent residual tumor cell contamination after ex vivo processing of autografts including purging strategies and/or positive selection techniques. Preliminary data are conflicting, although some data suggest that patients receiving autografts with residual tumor cell contamination do worse than patients receiving autografts free of detectable tumor cells. Many purging techniques are capable of reducing tumor contamination by 3 to 5 logs; positive selection techniques are probably slightly less effective, reducing tumor contamination by 3 to 4 logs. The clinical benefit of purging remains to be demonstrated in randomized clinical trials.