In vitro and in vivo evaluation of a technetium-99m-labeled 2-nitroimidazole (BMS181321) as a marker of tumor hypoxia.

UNLABELLED: The 99mTc-labeled 2-nitroimidazole derivative BMS181321, previously studied in experimental models of myocardial and cerebral ischemia, has been evaluated in single-cell and tumor models.
METHODS: Accumulation of BMS181321 was studied in aerobic and hypoxic (<10 PPM O2) suspension cultures of Chinese hamster ovary (CHO) cells at 37 degrees C and the oxygen dependency and stability of the accumulated radioactivity determined. Biodistribution studies of the tracer after intravenous injection in C3H mice bearing three different murine solid tumors were performed noninvasively using a gamma camera, as well as invasively by determining blood and tissue levels of radioactivity from 10 min to 24 hr after injection.
RESULTS: Accumulation of BMS181321 in aerobic cells in vitro equilibrated within 5 min at a approximately 10-fold level over the external medium. Hypoxic cells showed a linear increase in radioactivity up to 4 hr for cell densities < or = 1 x 10(6)/ml. At higher cell densities (2-4 x 10(6)/ml) there was substantial depletion of radioactivity from the growth medium and increased alteration in the chemical state of the tracer that remained. Low O2 levels (approximately 40 ppm) inhibited the maximal accumulation rate by 50%. Approximately 30% of radioactivity accumulated under hypoxic conditions remained cell-associated after 24 hr. Following intravenous injection, the tracer rapidly distributed throughout the mouse and was predominately cleared through the hepatobiliary system. Blood levels of radioactivity cleared quickly and plateaued at approximately 4% of the total dose from 2-24 hr. Absolute uptake in the tumors was highest 10 min after injection, and the tumor-to-muscle activity ratios increased and plateaued from 4-8 hr at values of 3.5-4.0. Two drugs which affect blood flow and increase hypoxic cell fraction in these tumors, hydralizine and nitro-L-arginine, significantly increased levels of BMS181321 radioactivity over control levels with minimal effects on normal tissue retention.
CONCLUSION: These results suggest BMS181321 or an analog of it will be a useful agent to investigate the status of hypoxia in solid tumors experimentally and potentially in the clinic.