The emergence of peripheral blood progenitor cells to support intensive chemotherapy for patients with breast cancer.

Increasing evidence supports the hypothesis that "dose" is critical to the clinical outcomes of cytotoxic chemotherapy for patients with breast cancer. Clinical trials continue to investigate whether higher doses of chemotherapy lead to proportionate improvements in the outcomes of patients. Delivery of dose-intensive chemotherapy has been facilitated by technological advancements in supportive care. Improved antiemetics have led to increased patient tolerance of the most acute symptoms of aggressive chemotherapeutic dosing. Chemotherapy-induced myelosuppression may be minimized in a lineage-specific manner by appropriate use of hematopoietic cytokines such as filgrastim (granulocyte colony-stimulating factor), sargramostim (granulocyte-macrophage colony-stimulating factor), and/or epoetin alfa (erythropoietin). However, cumulative myelotoxicity occurs with dose-intensive chemotherapy over multiple cycles despite adjunctive cytokine support. Additionally, no cytokine has yet been demonstrated to support platelet production to any clinically significant degree although several regulators of platelet production (such as thrombopoietin, IL-6, and IL-11) are in clinical trials. Many cytokines can induce the mobilization of hematopoietic progenitor and stem cells from the bone marrow into the circulating blood pool, where these cells may be harvested. Clinical use of these cytokine-mobilized peripheral blood progenitor cells (also known as PBPCs or, commonly, as blood stem cells) has documented the effectiveness of these cells to reconstitute multilineage blood production following very high-dose chemotherapy. The ease with which PBPCs can be collected and their reproducible clinical effectiveness to support patients through intensive treatment protocols have led to a virtual elimination of bone marrow as the source of cellular support for myeloablative chemotherapy in many transplant centers. Novel investigative approaches are also possible with PBPCs. In this review, the historical background of PBPCs is summarized, and the potential benefits (including economic advantages) of PBPCs to support dose-intensive chemotherapy for treating breast cancer are discussed. While dose intensification of breast cancer chemotherapy to the degree requiring PBPC support remains controversial and, in most centers, investigational, there is no doubt that PBPCs are an effective adjunct to the hematopoietic support of patients undergoing transplant-level cytotoxic treatments. Further study will undoubtedly lead to increased use of PBPCs in novel treatments for patients with breast cancer and other solid tumors.