99mTc-Tetraethylenepentamine-Folate--a new 99mTc-based folate derivative for the detection of folate receptor positive tumors: synthesis and biological evaluation.

A new radiopharmaceutical, 99mTc-Tetraethylenepentamine(TEPA)-Folate has been synthesized introducing TEPA to the gamma-carboxyl group of folic acid. This binds with 99mTc high efficiency at ambient temperature. The resulting 99mTc-N5-Folate is stable under physiological conditions at least for 24 h after radiocomplexation. TEPA is a known open chain pentamine (N5) chelator, its four-nitrogen act as the binding site for 99mTc. The folate membrane receptor binding of the 99mTc-TEPA-Folate by established human tumor cell lines (KB, U-87MG and MDA-MB-468) showed Kd in microM range in normal DMEM (10% serum, 10 microM folic acid). The blood kinetic studies showed more than 70% clearance within five minutes from the circulation. The KB cell line tumors in mice were readily identifiable in the gamma images and revealed major accumulation of radiotracer in liver, kidneys and intestines. High tumor uptake was shown in the tumor bearing nude mice; tumorto-blood ratios reached 2.68 +/- 0.52 and 5.5 +/- 1.47 at 1 and 4 h after post injection respectively. Surviving fractions as obtained in clonogenic assay were 1.02 +/- 0.07 and 1.03 +/- 0.05 in U-87MG and MDA-MB-468 cell lines respectively. The 99mTc-N5-Folate conjugate have promising utility as a receptor specific radiopharmaceutical for imaging neoplastic tissues known to over express folate-binding protein.