Cu-BTSCs show significant promise as PET imaging agents. With a half-life of 12.7 h,
64Cu can be conveniently shipped between sites and does not require an on-site cyclotron [
27‐
29], while
62Cu, with a half-life of 9.7 min is produced by a portable generator [
61], allowing flexibility with regard to labeling and imaging strategies. The lipophilic Cu(II)-BTSC complexes freely diffuse into the cell, where they become reduced to Cu(I)-BTSC intermediates. If sufficient oxygen is available, they are rapidly re-oxidized by molecular oxygen and are able to leave the cell intact. If intracellular oxygen levels are insufficient to re-oxidize the complex, it dissociates, releasing the radiocopper payload into the cell, where it is sequestered, giving rise to a hypoxia-dependent PET signal [
62‐
64] (Fig.
1b). Cu-diacetyl-bis(N4-methylthiosemicarbazone) (Cu-ATSM) was the first BTSC demonstrated to exhibit hypoxia selectivity and is the most widely evaluated complex for both tumor [
65] and cardiac hypoxia imaging [
25]. It is highly cell permeable and clears rapidly from both normoxic tissues and blood [
32,
53] generating high contrast images within minutes (compared with several hours when using
18FMISO).
64Cu-ATSM has been shown to deposit
64Cu in both hypoxic and ischemic isolated perfused hearts [
50•,
51,
54], as well as in the in vivo regionally occluded canine myocardium [
31]. In the one small clinical trial that we are aware of, Takahashi et al. compared the cardiac retention of
62Cu-ATSM and
18FDG in seven patients with coronary artery disease [
56]. Of these, six had prior infarcts but were clinically stable, while the seventh had unstable angina.
18FDG PET imaging delineated regions of increased cardiac glucose metabolism in five patients, but
62Cu-ATSM retention was only observed in the patient with unstable angina [
56]. Taken together, these studies suggest that while Cu-ATSM can detect extreme degrees of hypoxia in tumors and experimental models of extreme cardiac ischemia [
62], it may not be sensitive enough to detect the subtle hypoxia thought to characterize chronic cardiac ischemic syndromes. By modifying their ligand backbone, however, the reduction potential of the Cu-BTSC complexes can be altered in order to deposit radiocopper in cells at different degrees of hypoxia, while properties important to optimizing imaging quality, such as lipophilicity or serum protein affinity, can be controlled by differently alkylating their terminal amino groups [
31,
66‐
68]. There is, therefore, a potential to create a large catalog of structurally related analogs of Cu-ATSM that may be better suited to imaging cardiac hypoxia. We have embarked upon a program of designing, screening, validating, and characterizing such analogs and have thus far identified two complexes in this family,
64Cu-CTS and
64Cu-ATS which may be an improvement on Cu-ATSM for cardiovascular applications [
50•,
51,
52].