Macrophages and cancer

The uncontrolled growth of metastases resistant to conventional therapeutic modalities is a major cause of death from cancer. Data from our laboratory and others indicate that metastases arise from the nonrandom spread of specialized malignant cells that preexist within a primary neoplasm. These metastases can be clonal in their origin, and different metastases can originate from different progenitor cells. In addition, metastatic cells can exhibit an increased rate of spontaneous mutation compared with benign nonmetastatic cells. These data provide an explanation for the clinical observation that multiple metastases can exhibit different sensitivities to the same therapeutic modalities. These findings suggest that the successful therapy of disseminated metastases will have to circumvent the problems of neoplastic heterogeneity and the development of resistance. Appropriately activated macrophages can fulfill these demanding criteria. Macrophages can be activated to become tumoricidal by interaction with phospholipid vesicles (liposomes) containing immunomodulators. Tumoricidal macrophages can recognize and destroy neoplastic cells in vitro and in vivo, leaving non-neoplastic cells uninjured. Although the exact mechanism(s) by which macrophages discriminate between tumorigenic and normal cells is unknown, it is independent of tumor cell characteristics such as immunogenicity, metastatic potential, and sensitivity to cytotoxic drugs. Moreover, macrophage destruction of tumor cells apparently is not associated with the development of tumor cell resistance. Macrophages are found in association with malignant tumors in a definable pattern, suggesting that the most direct way to achieve macrophage-mediated tumor regression is in situ macrophage activation. Intravenously administered liposomes are cleared from the circulation by phagocytic cells, including macrophages, so when liposomes containing immunomodulators are endocytosed, cytotoxic macrophages are generated in situ. The administration of such liposomes in certain protocols has been shown to bring about eradication of cancer metastases. Macrophage destruction of metastases in vivo is significant, provided that the total tumor burden at the tart of treatment is minimal. For this reason, we have been investigating various methods to achieve maximal cytoreduction in metastases by modalities such as chemotherapy or radiotherapy prior to macrophage-directed therapy. It is important to note that even the destruction of 99.9% of cells in a metastasis measuring 1 cm² would leave 10⁶ cells to proliferate and kill the host. The ability of tumoricidal macrophages to distinguish neoplastic from bystander nonneoplastic cells presents an attractive possibility for treatment of the few tumor cells which escape destruction by conventional treatments. Macrophage-directed therapy has been studied in several human protocols, yielding important biological information about the use of liposome-encapsulated macrophage activators in cancer patients. Currently, Phase II and Phase JB protocols are being conducted based on the results from Phase I trials.