31P NMR spectroscopic studies of the effects of cyclophosphamide on perfused RIF‐1 tumor cells

To determine whether direct cellular effects of chemotherapy are responsible for 31P NMR spectral changes observed in treated tumors in vivo, RIF-1 fibrosarcoma cells were examined in vitro before, during, and after treatment with 4-hydroperoxycyclophosphamide (4-HC), an activated form of cyclophosphamide. When RIF-1 cells were treated with 4-HC in a metabolically stable but nonproliferating state, the 31P NMR spectra were identical with those of untreated cells for up to 70 h. When actively proliferating RIF-1 cells were treated with 4-HC, the intensities of the nucleotide triphosphate resonances, which increased linearly during control cell growth, remained constant for 50 h or longer. These studies demonstrate that the bioenergetic improvement observed following treatment of RIF-1 tumors in vivo [S.-J. Li, J.P. Wehrle, S.S. Rajan, R.G. Steen, J.D. Glickson, and J. Hilton, Cancer Res. 48, 4736 (1988)] does not result from direct effects of cyclophosphamide metabolites on RIF-1 cell metabolism, but rather from indirect effects of treatment on tumor or host physiology.