The folate receptor is overexpressed in a wide variety of human tumors. Con
jugates of folate have been shown to be selectively taken up by tumor cells
via the folate receptor. In this study, a novel radiopharmaceutical, Tc-99
m-6-hydrazinonicotinamido-hydrazido (HYNIC)-folate, was synthesized and eva
luated for its efficacy as a targeted agent for the imaging of tumors that
overexpress the folate receptor. Methods: HYNIC-folate was synthesized and
radiolabeled with Tc-99m using tricine and trisodium triphenylphosphine-3,3
',3 "-trisulfonate as coligands. The receptor binding properties of Tc-99m-
HYNIC-folate were studied in cultured tumor cells that overexpress the fola
te receptor. The tumor-localizing properties of 99mTc-HYNIC-folate were the
n evaluated in C57BL/6 mice bearing subcutaneously implanted folate recepto
r-positive syngeneic tumors. Tissue distribution was determined at two diff
erent time points, and gamma camera images were collected on two animals. R
esults: The folate receptor-mediated uptake of Tc-99m-HYNIC-folate by cultu
red tumor cells was approximately 300 times higher than the nonspecific bin
ding determined in the presence of 1 mmol/L free folic acid. Excellent tumo
r selectivity was also shown in the animal model; tumor-to-blood ratios rea
ched 55 +/- 19 and 81 +/- 6 at 4 and 24 h after injection, respectively. Tu
mor uptake of the radiotracer was blocked by the co-injection of 100 mu g f
ree folate. Tumors were clearly identifiable on the gamma camera images, wi
th the kidneys and the bladder as the only normal organs showing high level
s of the radiotracer. Conclusion: Tc-99m-HYNIC-folate is a promising, novel
receptor-specific radiopharmaceutical with potential applications in the i
maging of human tumors.