Longevity of pimonidazole adducts in spontaneous canine tumors as an estimate of hypoxic cell lifetime.

The longevity of pimonidazole adducts in tumors was quantified as an estimate of the lifetime of hypoxic cells. Pimonidazole was given before irradiation to 12 dogs bearing spontaneous tumors, and tumors were biopsied 24, 48 and 72 h later. Pimonidazole antigen was quantified in the biopsies using ELISA and immunohistochemistry. Pimonidazole antigen was detectable in the initial biopsy in all dogs. In 5 dogs the amount of detectable antigen decreased to less than 50% of the initial amount, in 5 other dogs the amount of detectable antigen decreased to an amount between 50 and 100% of the initial amount, and in 2 dogs the amount of antigen appeared to increase relative to the initial amount. Tumors with high initial adduct concentration were characterized by greater decreases in adduct concentration than tumors with low initial adduct concentration. Immunohistochemically, labeled cells were present in 11 of 12 tumors. The geographic area in tumor biopsies labeled immunohistochemically with pimonidazole adducts (labeled area fraction) tended to decrease over time in 6 dogs, remain stable in 4 dogs and seemingly increase in 1 dog. There was no relationship in individual tumors between the relative change in antigen concentration and the relative change in labeled area fraction. Hypoxic cells which bind pimonidazole may persist for days during fractionated radiation therapy, and the potential exists for them to exert a negative effect on the host.