Effect of flunarizine on micro-regional distributions of intravascular HbO2 saturations in RIF-1 and KHT sarcomas.

Flunarizine, a calcium channel-blocker, preferentially active on peripheral blood vessels and red blood cells, has previously been shown to increase not only tumor blood flow, but also both chemosensitivity and radiosensitivity. The mechanisms behind these effects are not clear at present, and different tumor lines have been demonstrated to respond quite differently to the same flunarizine treatment. To clarify the underlying mechanisms at the micro-regional level, the present investigation examined the effects of flunarizine on spatially-resolved distributions of intravascular oxyhemoglobin (HbO2) saturations. Tumors were implanted s.c. in the flanks of C3H mice and grown to a volume of 500-1000 mm3. Flunarizine was injected i.p. at 5 mg/kg; animals were anesthetized, and following a 20 min delay, tumors were quick-frozen. HbO2 saturations were determined cryospectrophotometrically at systematically selected sites across each tumor cross-section. Three primary questions were addressed: (a) Does flunarizine have a direct effect on tumor oxygen delivery as measured by intravascular HbO2 saturations? (b) Are changes in oxygen delivery uniformly distributed throughout the tumor volume? (c) Are HbO2 distributions for tumor lines of dissimilar radiobiological hypoxic fraction affected similarly by flunarizine? For KHT tumors, flunarizine substantially increased the % vessels greater than or equal to 10% saturated throughout the tumor volume, with a somewhat larger increase for the interior vessels than for the peripheral vessels. In contrast, for the RIF-1 tumors, HbO2 saturations following flunarizine remained near control levels. These results indicate that fundamental differences in vascular geometry, hemodynamics, or blood rheology may exist between the KHT and RIF-1 tumor lines, and furthermore, that the response of one tumor line to a given blood-flow modifier may be completely unrelated to that of another.