Intermittent Blood Flow in a Murine Tumor: Radiobiological Effects1

Little is known about how and why hypoxia arises in tumors, i.e., whether hypoxia is a chronic process resulting from diffusion limitations or occurs more acutely due to transient changes in blood perfusion. We have investigated the nature of hypoxia in the murine squamous carci noma SCC VII using a new fluorescence-activated cell-sorting technique which facilitates isolation of viable tumor cells as a function of their distance from the blood supply. The technique utilizes the DNA binding/ diffusion properties of the bisbenzamide fluorochrome Hoechst 33342. This compound has a very short distribution half-life from the blood after i.v. injection but remains bound within tumor cells even after disaggregation, redistributing with a half-life greater than 2 h. Cells can thus be sorted on the basis of their staining intensity (proximity to the blood supply), and varying the Hoechst 33342 administration protocol provides the basis for elucidating transient changes in blood flow that result in acute radiobiological hypoxia. Using this technique, we have demonstrated that acute hypoxia results from transient changes in blood perfusion in 500-mg SCC VII tumors. Independent confirmation of the intermittent blood flow has been obtained using histolÃ3gica!techniques.