Synthesis of Specific SPECT-Radiopharmaceutical for Tumor Imaging Based on Methionine: 99mTc-DTPA-bis(methionine)

Methionine-diethylenetriaminepentaaceticacid-methionine [DTPA-bis(Met)] was synthesized by covalently conjugating two molecules of methionine (Met) to DTPA and was labeled with Tc-99m in high radiochemical purity and specific activity (166-296 MBq/mu mol). Kinetic analysis showed Km of 12.95 +/- 3.8 nM and a maximal transport rate velocity (V-max) of 80.35 +/- 0.42 pmol mu g protein(-1) min(-1) of Tc-99m-DTPA-bis(Met) in U-87MG cells. DTPA-bis(Met) had dissociation constants (K-d) of 0.067 and 0.077 nM in U-87MG and BMG, respectively. S-35-methionine efflux was trans-stimulated by Tc-99m-labeled DTPA conjugate demonstrating concentrative transport. The blood kinetic studies showed fast clearance with t(1/2) (F) = 36 +/- 0.5 min and t(1/2) (S) = 5 h 55 min +/- 0.85 min. U-87MG and BMG tumors saturated at similar to 2000 +/- 280 mnol/kg of Tc-99m-DTPA-bis(Met). Initial rate of transport of Tc-99m-DTPA-bis(Met) in U-87MG tumor was found to be 4.68 x 10(-4) mu mol/kg/min. The tumor (BMG cell line, malignant glioma) grafted in athymic mice were readily identifiable in the gamma images. Semiquantitative analysis from region of interest (ROI) placed over areas counting average counts per pixel with maximum radiotracer uptake on the tumor was found to be 11.05 +/- 3.99 and compared ROI with muscle (0.55 +/- 0.13). The tumor-to-contralateral muscle tissue ratio of Tc-99m-DTPA-bis(Met) was found to be 23 +/- 3.3. Biodistribution revealed significant tumor uptake and good contrast in the U-87MG, BMG, and EAT tumor-bearing mice. In clinical trials, the sensitivity, specificity, and positive predictive values were found to be 87.8%, 92.8%, and 96.6%, respectively. Tc-99m-DTPA-bis(Met) showed excellent tumor targeting and has promising utility as a SPECT-radiopharmaceutical for imaging methionine-dependent human tumors and to quantify the ratio of MET+/HCY-.