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The mucin MUC1 is greatly increased in breast cancer and is a potential target for immunotherapy. In mice, MUC1 conjugated to oxidized mannan (MUC1-mannan fusion protein [M-FP]) targets the mannose receptor and induces a high frequency of cytotoxic T lymphocytes and anti-tumor responses. On this basis, three phase I trials were performed in patients with adenocarcinoma to evaluate the tnxicity and the immunologic responses to mannan MUC1. Forty-one patients with metastatic or locally advanced carcinoma of the breast (trial 1), colon (trial 2), and various adenocarcinomas (trial 3) received increasing doses of M-FP (1 to 300 μg). The immunizations were given at weekly intervals (weeks 1 to 3) and repeated in weeks 7 to 9. Cyclophosphamide (to increase cellular immunity) was given on weeks 1 and 4. M-FP was given intramuscularly in trial 1 and intraperitoneally in trial 2. No toxic effects occurred, and delayed-type hypersensitivity responses were present only as a microscopic lymphocytic infiltration. Overall, approximately 60% of the patients had high-titer MUC1 immunoglobulin G1 antibody responses, with the intraperitoneal route yielding approximately 10-fold higher responses. Cellular responses (proliferation, cytotoxic T cells, or CD8 T cells secreting tumor necrosis factor-α αnd interferon-γ in response to MUC1 stimulation in vitro) were found in 28% of the patients, which was similar to that seen without cyclophosphamide. In most patients, disease progressed, but in five it remained stable. In addition, there were no objective responses. M-FP is not toxic and induces immune responses that were amplified by the intraperitoneal route of immunization. Cyclophosphamide was of no benefit.