Molecular Assembly of Multifunctional 99mTc Radiopharmaceuticals Using Clickable Amino Acid Derivatives

Synthetic strategies that enable the efficient and selective combination of different biologically active entities hold great promise for the development of multifunctional hybrid conjugates useful for biochemical and medical applications. Starting from side-chain-functionalized N(alpha)-propargyl lysine derivatives, conjugates containing a Tc-99m-based imaging probe for SPECT and two different moieties (e.g., tumor-targeting vectors, pharmacological modifiers, affinity tags, or second imaging probes) can be assembled using the Cu-I-catalyzed alkyneazide cycloaddition in efficient one-pot protocols. This strategy was successfully applied to the preparation of a Tc-99m-labeled conjugate comprising a tumor-targeting peptide sequence (bombesin(7-14)) and a low-molecular-weight albumin binder, a pharmacological modifier that prolongs the blood circulation time of the conjugate. Evaluation of the conjugate in vitro and in vivo provided promising results for its use as an imaging agent for the visualization of tumors positive for the gastrin-releasing peptide receptor. The methodology presented herein provides an attractive synthetic tool for the preparation of multifunctional Tc-99m-based radiopharmaceuticals with significant potential for a multitude of applications.