The synchronizing effect of the combination chemotherapy of vincristine (VCR) and cyclop-hosphamide (CPM) on C-1300 murine neuroblastoma was investigated in vivo. A synchronizing effect of intraperitoneally administered (i.p.) VCR was observed by flowcytometry using Fried's formula for analysis and autoradiography. VCR (0.1 mg/kg) i.p. induced a G_2+M phase accumulation after 6 hours and an S-phase accumulation 9 to 18 hours later. The combined treatment with VCR (0.1 mg/kg) followed by CPM (100 mg/kg) after 12 hours weekly caused remarkable inhibition of the tumor growth during 4 weeks. The improvement of the animal survival rates were not to good as the tumor growth inhibitory effect. This may be because of the timing of injection of CPM, which may induce an S-phase accumulation of normal bone marrow cells and increased animal toxicity. To use a synchroniation method in vivo, it is important to know the cell cycles of both tumor and normal cells.
