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Importance of Timing in Cyclophosphamide Therapy of MOPC-315 Tumor-bearing Mice¹

Department of Microbiology and Immunology, University of Illinois at the Medical Center, Chicago, Illinois 60612

The timing of cyclophosphamide (CY) administration after tumor inoculation was found to be critical for successful therapy 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 10- to 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 stimulator 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 exhibited by in vitro-immunized spleen cells from CY-treated mice was equivalent to that exhibited by in vitro-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 obtained by depletion of tumor cells (25), the present data suggest that in addition to the drug's tumoricidal activity, it also eliminates 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 vitro immunization.

¹ Supported in part by NIH Grant ROI CA-26480.

² In partial fulfillment of the requirements for a Ph.D. in Microbiology and Immunology in the Graduate College of the University of Illinois at the Medical Center.

³ To whom requests for reprints should be addressed, at P. O. Box 6998, Chicago, Ill. 60680.

Received 9/13/79. Accepted 3/19/80.

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