Importance of timing in cyclophosphamide therapy of MOPC-315 tumor-bearing mice.

The timing of cyclophosphamide (CY) administration after tumor inoculation was found to be critical for successful ther apy of MOPC-315 tumor-bearing mice. Following inoculation with 3.5 x 106 viable tumor cells, a single i.p. injection of CY (15 mg/kg) into mice bearing 10to 25-mm (Days 8 to 14) tumors cured most mice, whereas injection into mice bearing nonpalpable (Day 4) tumors cured only a few of the mice. The time interval between tumor inoculation and CY administration rather than the tumor size was critical for successful therapy since mice bearing nonpalpable tumors 12 to 13 days postinoculation with 105 viable tumor cells were cured by CY therapy. Furthermore, CY therapy of mice bearing large (19-mm) tumors was not curative for mice that had been treated previously when their tumors were nonpalpable. A curative injection of CY into mice bearing large tumors resulted in an augmented ability of their spleen cells to mount a cytotoxic antitumor response upon in vitro immunization with mitomycin C-treated s'timulator tumor cells. This was not further augmented by depletion of glass-adherent cells and was accompanied by a decrease in the percentage of cells bearing surface MOPC-315 myeloma protein in the spleen. The level of antitumor cytotoxicity ex hibited by in w'fro-immunized spleen cells from CY-treated mice was equivalent to that exhibited by in wfro-immunized spleen cells from untreated tumor-bearing mice that prior to in vitro immunization were depleted of glass-adherent cells. Since depletion of glass-adherent cells from tumor bearer spleen cells prior to in vitro immunization was shown to result in greater augmentation of antitumor cytotoxicity than that ob tained by depletion of tumor cells (25), the present data suggest that in addition to the drug's tumoricidal activity, it also elim inates other suppressor elements in the spleen. Mice cured of tumors following CY therapy exhibited a high degree of antitumor immunity as judged in vivo by their ability to reject a large tumor challenge and in vitro by the ability of their spleen cells to mount a "secondary type" antitumor response upon in