Tumor model studies of 131I-tetracycline and other compounds

Iodine-131-tetracycline (131I-TET) was prepared by allowing tetracycline hydrochloride to react with radioiodide in acidic methanol (labeling efficiency greater than 85%). This preparation was found to be stable at--4 degrees C for at least 72 hr. Some minimal in vivo breakdown did occur. The 131I-TET, 67Ga, and several 99mTc compounds were studied in a rat hepatoma model. The incorporation of the radiopharmaceuticals into the tumor occurred rapidly, with peak levels at 0.5 and 24 hr after injection for 131I-TET and 67Ga, respectively. The clearnace of the radiopharmaceutical from nonviable tumor was slower than for viable tumor, and by 72 hr after injection the greatest concentration of radioactivity appeared in the nonviable fraction. All normal tissues showed faster clearance than did tumor tissue, regardless of viability. Decreasing the quantity of 131I-TET injected increased the percent of uptake in the nonviable tumor tissue but had no effect on the viable tumor uptake. Of the 99mTc compounds studied, the phosphates developed the highest tumor-to-background ratios. Unfortunately these ratios were not as high as those achieved for 67Ga or 131I-TET.