The development of monoclonal antibodies (MAbs) to cytokeratins, which are integral components of the epithelial cytoskeleton, has made possible immunocytochemical detection of epithelial tumor cells. Importantly, this technique allows the detection of epithelial tumor cells that have metastasized from primary adenocarcinomas to secondary sites such as the bone marrow.

The aim of the study was not only to detect micrometastatic cells in bone marrow, but also to assess the expression of nuclear proliferation markers (Ki-67 and p120) and the erbB2 oncogene (also known as ERBB2) in these cells and, thus, hopefully improve prognostic precision

Bone marrow aspirates were obtained from both sides of the upper iliac crest of 532 patients having definitive diagnoses of either breast or gastrointestinal cancer. The presence of micrometastatic epithelial tumor cells in bone marrow was assayed using the MAb cytokeratin 2 (CK2) to cytokeratin component 18 (CK18), in combination with the alkaline phosphatase-anti-alkaline phosphatase immuno-staining technique. After primary screening of all marrow samples with MAb CK2, representative subgroups of CK18⁺ samples were selected for co-labeling with MAbs either to ErbB (n = 16), ErbB2 (n = 121), Ki-67 (n = 33), or p120 (n = 36) protein. An alternative labeling protocol based on the combination of immunogold and immunoenzymatic techniques was utilized to confirm the results derived from immuno-enzymatic double staining.

In total, single CK18-positive tumor cells were detected in 180 (33.8%) of 532 bone marrow aspirates, with few differences among patients with breast or gastrointestinal cancer in TNM stage M0 (i.e., no distant metastasis). In patients with overt metastasis (stage M1), however, the incidence of metastatic cells in marrow increased to 73.7% in breast cancer, 52.5% in gastric cancer, and 39.0% in colon cancer. Whereas expression of Ki-67 or p120 on micrometastatic cells was observed only in 11 (15.9%) of 69 cancer patients analyzed, ErbB2⁺/CK18⁺ cells were found in 48 (67.6%) of 71 breast cancer patients and 14 (28.0%) of 50 patients with gastrointestinal cancer (P = .0001). The incidence of ErbB2⁺/CK18⁺ cells was positively correlated with the clinical stage of tumor progression.

The high incidence of ErbB2 expression on micrometastatic breast cancer cells in the bone marrow suggests that these cells might have been positively selected during early stages of metastasis. The majority of these cells appear to be in a dormant state of cell growth.

Although support from clinical follow-up is still needed, this study demonstrates that, beyond the mere presence of micrometastatic cells in bone marrow, useful prognostic information can be obtained by analysis of additional cell growth markers. [J Natl Cancer Inst 85: 1419–1424, 1993]