99mTc-Labeling of Hydrazones of a Hydrazinonicotinamide Conjugated Cyclic Peptide

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Abstract
Eight HYNICtide hydrazones (three with aliphatic substituents and five with aromatic groups) were studied for their potential use as the final intermediate for preparation of RP444, a new radiopharmaceutical under development for imaging thrombosis. The goal of this study is to screen various hydrazones through stability testing and radiolabeling and find those which are able to remain stable without significant degradation in the manufacturing process and at the same time are reactive to produce enough free hydrazine in situ for successful 99mTc-labeling. In an initial screening study, only hydrazones 6 and 8, which contain aliphatic substitients, gave satisfactory (≥90%) yields of RP444 using 50 °C and 30 min of heating. However, their solution instability excludes them from being used as commercial reagents. Hydrazones 1 and 4 gave ≥90% yields when the reaction mixtures were heated at 80 °C for 30 min. Both hydrazone 1 and hydrazone 4 can be used as the final intermediate for preparation of RP444. The combination of 40 mg of tricine, 1−10 mg of TPPTS, 20−40 μg of hydrazone 1 or 4 for 50 mCi of [99mTc]pertechnetate, 20−50 μg of stannous chloride, pH 4.5 ± 0.5, and heating at 80 °C for 30 min gives the best yield for RP444. It is surprising that hydrazones 1 and 4 have both the solution stablity with respect to decomposition and to reaction with aldehydes and ketones and yet are able to hydrolyze in situ to produce enough free HYNICtide for the 99mTc-labeling.