New 99mTc(CO)3Mannosylated Dextran Bearing S-Derivatized Cysteine Chelator for Sentinel Lymph Node Detection

The aim of the present study is to synthesize new mannosylated dextran derivative that can be labeled with Tc-99m for potential use in sentinel lymph node detection (SLND). The compound was designed to have a dextran with molecular weight of 10 kDa as a backbone, mannose for binding to mannose receptors of the lymph node and S-derivatized cysteine as a suitable chelator for labeling with [99mTc(H2O)3(CO)3]+precursor. Reaction of allyl bromide with dextran (MW 11800) yielded the intermediate allyl-dextran (1) with about 40% coupling. Addition of cysteine to allyl-dextran resulted in the S-derivatized cysteine, compound DC15 (2). The final product DCM20 (3) was obtained in good yield after in situ hydrolysis and activation of cyanomethyl tetraacetyl-1-thio-d-mannopyranoside and coupling to DC15. All derivatives were purified by ultrafiltration and characterized by NMR. DC15 and DCM20 were quantitatively labeled with 99mTc (>95% radiochemical purity) using the fac-[99mTc(OH2)3(CO)3]+precursor and ligand concentration of 1.5 × 10–6M at neutral pH. Both 99mTc-labeled compounds 99mTc(CO)3–DC15 (6) and 99mTc(CO)3–DCM20 (7) remained stable after 6 h incubation at 37 °C in the presence of excess histidine or cysteine, as well as even after 20-fold dilution and incubation for 24 h at room temperature. The characterization of the compounds 6and 7was performed by comparing their HPLC radiochromatograms with those of their rhenium surrogates Re(CO)3–DC15 (4) and Re(CO)3–DCM20 (5) respectively that were prepared using the precursor [NEt4]2fac-[ReBr3(CO)3] and characterized by IR and NMR spectroscopy. When injected subcutaneously from the foot pad of mice, 99mTc-labeled mannosylated dextran (7) showed accumulation in the popliteal lymph node (SLN in this model) higher than that of non-mannosylated analogue (6) and the 99mTc-phytate serving as standard. Compound 7also exhibited lower radioactivity levels at the injection site compared to 99mTc-phytate. The SPECT/CT studies in mice confirmed that 7accumulated in the popliteal lymph node allowing its clear visualization. The present findings demonstrate that compound 7(99mTc(CO)3-DCM20) is promising and merits further evaluation as a radiopharmaceutical for sentinel lymph node detection.