99mTc-DTPA-Amino Acids Conjugate as Specific SPECT Pharmaceuticals for Tumor Imaging

Tc-99m-Diethylene triamine pentaacetic acid-bis (amide) conjugates have been synthesized and evaluated as a potential radiopharmaceutical for tumor imaging. The compounds were synthesized by the condensation reaction of DTPA bis(anhydride) with different l-amino acids (methyl tryptophan, and 5-hydroxy tryptophan) and were characterized on the basis of IR, NMR, and Mass spectroscopy. Tc-99m-labeled compounds were found stable for about 24 h under physiological conditions with more than 95% radiolabeling yield. Blood kinetic studies of all these complexes showed a bi-exponential pattern as well as quick wash out from the blood circulation. The biological t(1/2)(F) and t(1/2)(S) were found to be 20 +/- 0.001 min for DTPA-(Me-Trp)(2) and 18 +/- 0.001 min for DTPA-(5HT)(2) and t(1/2) (slow) 5 h 45 min +/- 0.001, 5 h 30 +/- 0.001 min for DTPA-(Me-Trp)(2), and DTPA-(5HT)(2), respectively. Imaging and biodistribution studies were performed in mice bearing Ehrlich ascites tumor (EAT) tumors in right thigh. Radioconjugate derived from l-5-hydroxytryptophan exhibited remarkable localization at tumor site; whereas radiotracer derived from l-methyl tryptophan shows relatively less accumulation at the tumor site. Tumor-to-muscles ratios were 5.07 +/- 0.001, and 4.2 +/- 0.001 at 1 and 4 h for Tc-99m-DTPA-(Me trp)(2) and 4.97 +/- 0.001 and 5.8 +/- 0.001 at 1 and 4 h after postinjection for Tc-99m-DTPA-(5HT)(2), respectively. The preliminary results with these amino acid based ligands are encouraging to carrying out further in vivo experiments for targeted tumor imaging.