Technetium-99m-labeled
human serum albumin (99mTc-HSA)
has been utilized as a blood pool imaging agent in the clinic for
several decades. However, 99mTc-HSA has a short circulation
time, which is a critical shortcoming for a blood pool imaging agent.
Herein, we developed a novel 99mTc-labeled HSA with a long
circulation time using click chemistry and a chelator, 2,2′-dipicolylamine
(DPA), (99mTc-DPA-HSA). Specifically, we examined the feasibility
of copper-free strain-promoted alkyne-azide cycloaddition (SPAAC)
for the incorporation of HSA to the [99mTc (CO)3(H2O)3]+ system by adopting a chelate-then-click
approach. In this strategy, a potent chelate system, azide-functionalized
DPA, was first complexed with [99mTc (CO)3(H2O)3]+, followed by the SPAAC click reaction
with azadibenzocyclooctyne-functionalized HSA (ADIBO–HSA) under
biocompatible conditions. Radiolabeling efficiency of azide-functionalized
DPA (99mTc-DPA) was >98%. Click conjugation efficiency
of 99mTc-DPA with ADIBO–HSA was between 76 and 99%
depending on the number of ADIBO moieties attached to HSA. In whole-body
in vivo single photon emission computed tomography images, the blood
pool uptakes of 99mTc-DPA-HSA were significantly enhanced
compared to those of 99mTc-HSA at 10 min, 2, and 6 h after
the injection (P < 0.001, 0.025, and 0.003, respectively).
Furthermore, the blood activities of 99mTc-DPA-HSA were
8 times higher at 30 min and 10 times higher at 3 h after the injection
compared to those of conventional 99mTc-HSA in ex vivo
biodistribution experiment. The results exhibit the potential of 99mTc-DPA-HSA as a blood pool imaging agent and further illustrate
the promise of the pre-labeling SPAAC approach for conjugation of
heat-sensitive biological targeting vectors with [99mTc
(CO)3(H2O)3]+.