Cyclophosphamide-induced Changes in the Cellular Composition of a Methylcholanthrene-induced Tumor and Their Relation to Bone Marrow and Blood Leukocyte Levels¹

A single i.p. injection of cyclophosphamide (CY) (6 mg) induced regression of a methylcholanthrene-induced fibrosarcoma (MCA/76-9) frequently to a nonpalpable mass. The extent of regression was dependent on dose of drug and time of administration in relation to tumor size, but even under optimal conditions few (<2%) permanent regressions were recorded. Well-marked histological changes were observed after CY injection, involving an initial increase in macrophage numbers followed by an accumulation of mature and immature granulocytes. These cells largely replaced neoplastic cells by 7 to 10 days after CY injection. Changes observed histologically were verified and quantitated after enzymic disaggregation of tumors at various times before and after CY injection. A bioassay was used to assess the number of tumorigenic cells associated with the tumor mass after CY injection by injecting graded doses of cells from enzymically disaggregated tumors into normal recipients. Within 1 hr after CY injection, more than 10⁶ tumor cells were required for a take (compared with 10² cells from control tumors). However, by 3 days fewer cells (10⁴) were required, and by 10 days only 10² cells were needed. Bone marrow and peripheral blood leukocyte numbers decreased rapidly after CY injection, but absolute numbers returned to control values by 7 to 10 days. However, differential counts on blood smears indicated that lymphocyte counts required at least 21 days to return to control values, whereas the number of monocytes and granulocytes was elevated above those of control mice by Days 7 to 10. The possibility is discussed that the observed intratumor cellular changes might reflect a variety of different reactions unrelated to tumor regression or recurrence.