Targeting vectors
bound to a chelator represent a significant fraction
of radiopharmaceuticals used nowadays for diagnostic and therapeutic
purposes in nuclear medicine. The use of squaramides as coupling units
for chelator and targeting vector helps to circumvent the disadvantages
of several common coupling methods. This review gives an overview
of the use of squaric acid diesters (SADE) as linking agents. It focuses
on the conjugation of cyclic chelators, e.g., DOTA (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic
acid), as well as hybrid chelators like AAZTA5 (6-pentanoic
acid-6-amino-1,4-diazepine tetracetic acid) or DATA5m (6-pentanoic
acid-6-amino-1,4-diazapine-triacetate) to different targeting vectors,
e.g., prostate-specific membrane antigen inhibitors (KuE; PSMAi),
fibroblast activation protein inhibitors (FAPi), and monoclonal antibodies
(mAbs). An overview of the synthesis, radiolabeling, and in
vitro and in vivo behavior of the described
structures is given. The unique properties of SADE enable a fast and
simple conjugation of chelators to biomolecules, peptides, and small
molecules under mild conditions. Furthermore, SA-containing conjugates
could not only display similar in vitro characteristics
in terms of binding affinity when compared to reference compounds,
but may even induce beneficial effects on the pharmacokinetic properties
of these radiopharmaceuticals.