Pyridine-Containing 6-Hydrazinonicotinamide Derivatives as Potential Bifunctional Chelators for^99mTc-Labeling of Small Biomolecules

As a continuation of our interest in novel Tc-99m chelating systems, several pyridine-containing HYNIC (6-hydrazinonicotinamide) derivatives (L1-L5) have been synthesized and characterized by NMR (H-1 and C-13) and LC-MS. Tc-99m complexes of L1-L5 were prepared by the reaction of the HYNIC derivative with (99)mTCO(4)(-) in the presence of excess tricine and stannous chloride. Results from this study show that the attachment site of the linker is critical for the formation of macrocyclic Tc-99m complexes. For example, the pyridine-N in L3 is not able to bond to the Tc, because the lysine linker is attached to the 4-position. When the linker is at the 2-position, L1 forms the macrocyclic complex [Tc-99m(L1)(tricine)], but the radiochemical purity is relatively low. If the linker is attached to the 3-position of the pyridine ring, the HYNIC derivatives form macrocyclic complexes [Tc-99m(L)(tricine)] (L2, L4, and L5) in high yield (>95%). The HPLC data suggest that the macrocyclic complex [Tc-99m(L2)(tricine)] exists in solution as four isomers: two diastereomers and two conformational isomers. Diastereomers are due to a combination of the chirality of the lysine linker and of the Tc chelate. Replacing lysine with a pentamethylenediamine linker results in the macrocyclic complex [Tc-99m(L4)(tricine)] with two conformational isomers, which interconvert rapidly at room temperature. Changing the linker from pentamethylenediamine to hexamethylenediamine did not eliminate the minor isomer; but the percentage of the minor isomer was reduced from similar to10% for [Tc-99m(L4)(tricine)] to only 6% for [Tc-99m(L5)(tricine)]. The linker length is an important parameter to minimize the minor isomer. LC-MS data of complexes [Tc-19m(L)(tricine)] (L2, L4, and L5) are completely consistent with their proposed compositions. On the basis of these data, it is concluded that pyridine-containing HYNIC derivatives have the potential as bifunctional chelators for Tc-99m-labeling of small biomolecules if the linker is attached to the 3-position of the pyridine ring.