Temperature dependence of binding of a fluorescent hypoxia probe

The rate of binding of a fluorescent probe for hypoxia, AF-2, is dependent upon cell temperature as well as oxygenation. Aerobic cells bind AF-2 about 5 times less rapidly at 37 degrees C than at 45 degrees C. Below 42 degrees C, the rate of AF-2 binding by aerobic cells increases by a factor of about two for each 10 degrees C, but above 42 degrees C, the rate increases 10-fold for each additional 10 degrees C. However, for cells incubated under nitrogen, these biphasic kinetics were not observed, and the rate of metabolism and binding increased only two-fold per 10 degrees C from 28 degrees to 45 degrees C. These differences in binding kinetics cannot be explained by a decrease in the rate of auto-oxidation of the nitro anion radical, but might involve differences in the available reducing equivalents in cells heated under air or nitrogen. The possibility of using fluorescent probes to measure temperatures of individual tumor cells during hyperthermic treatment is discussed.