Gert Riethmüller, C. Klein, Oleg Schmidt-Kittler, P. Kufer
University of Munich, Munich, Germany

Antibody Therapy for Minimal Residual Cancer: The Target, the Effects and the Perspectives

In most cancer patients, metastases arise from latent cells already disseminated at a time when the primary lesion is small and curable. It is generally accepted that these residual cells after removal of the primary tumor are the equivalent of what is understood by the term minimal residual cancer. Due to their extreme rarity as well as latency the nature of these cells has so far remained largely unknown. Nevertheless, it is likewise generally agreed that these cells represent the most suitable target for immunological intervention such as active or passive immunization. The detection of disseminated cancer cells by immunocytochemical staining in bone marrow has enabled several prospective trials to study their prognostic impact on the development of clinical metastasis. Some of the more recent clinical trials have shown that the finding of these cells in patients with locally restricted cancer has a significant prognostic impact for overall survival and/or relapse-free survival (1,2). An attempt was therefore warranted to study the biology of the early disseminated cells in more detail. A first major objective was to get an insight into their genomic make-up as well as their gene expression profile, the intention being to define potential new targets for therapy specifically designed for minimal residual cancer. Because of the extreme rarity of the cells in bone marrow single immunofluorescent tumor cells were isolated by micromanipulation from bone marrow cell suspension. A novel technique was then applied to isolate and amplify by PCR global DNA or mRNA from single cells. Subsequent comparative genomic hybridisation revealed extensive genetic heterogeneity among cells isolated from individual patients displaying multiple distinct chromosomal aberration (3,4). Microarray analysis of RT-PCR amplified mRNA demonstrated distinct patterns of gene expression in various individual cells concordant with epithelial-mesenchymal transformation, cell migration, or DNA repair respectively. The finding of consistently elevated expression of defined genes led to the identification of a first candidate target molecule the protein of which could be detected by monoclonal antibodies on disseminated tumor cells.

In continuation of previous therapeutic trials in colorectal cancer patients with minimal residual disease, we studied the expression of EpCAM on individual metastatic cells. Except for rare instances of EpCAM expression by non-epithelial cells, the molecule proved to be a reliable surface marker for disseminated tumor cells in bone marrow. In breast cancer patients, presenting with more than 20 tumor cells per 106 bone marrow cells the depleting effect of an antibody infusion could be assessed by cell counts in bone marrow aspirates obtained before and after antibody administration. A significant depletion of metastatic cells was detected that was particularly pronounced on EpCAM positive cells (5). This demonstration of an in vivo effect of an EpCAM antibody together with the positive outcome of a large randomised clinical trial warranted the development of a fully human EpCAM antibody as well as novel bifunctional antibody fusion proteins with EpCAM specificity. Further studies are directed at the establishment of a multiplex RT-PCR assay for MAGE gene expression as a specific test of rare metastatic cancer cells in bone marrow. The feasibility of this assay as a surrogate for monitoring antibody therapy in minimal residual cancer is currently under study. From these studies it becomes evident, that a future, stronger concerted effort on minimal residual cancer will be more rewarding than the attempts to treat terminally ill patients.

References:

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Klein, CA, Seidl S, Petat-Dutter K, Offner S, Geigl JB, Schmidt-Kittler O, Wendler N, Passlick B, Huber RM, Schlimok G, Baeuerle PA, Riethmüller G. Combined transcriptome and genome analysis of single micrometastatic cells. Nature Biotechnology, in press

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