Cardiac hypoxia imaging: second-generation analogues of 64Cu-ATSM.

UNLABELLED: Myocardial hypoxia is an attractive target for diagnostic and prognostic imaging, but current approaches are insufficiently sensitive for clinical use. The PET tracer copper(II)-diacetyl-bis(N4-methylthiosemicarbazone) ((64)Cu-ATSM) has promise, but its selectivity and sensitivity could be improved by structural modification. We have therefore evaluated a range of (64)Cu-ATSM analogs for imaging hypoxic myocardium. METHODS: Isolated rat hearts (n = 5/group) were perfused with normoxic buffer for 30 min and then hypoxic buffer for 45 min within a custom-built triple-γ-detector system to quantify radiotracer infusion, hypoxia-dependent cardiac uptake, and washout. A 1-MBq bolus of each candidate tracer (and (18)F-fluoromisonidazole for comparative purposes) was injected into the arterial line during normoxia, and during early and late hypoxia, and their hypoxia selectivity and pharmacokinetics were evaluated. The in vivo pharmacokinetics of promising candidates in healthy rats were then assessed by PET imaging and biodistribution. RESULTS: All tested analogs exhibited hypoxia sensitivity within 5 min. Complexes less lipophilic than (64)Cu-ATSM provided significant gains in hypoxic-to-normoxic contrast (14:1 for (64)Cu-2,3-butanedione bis(thiosemicarbazone) (ATS), 17:1 for (64)Cu-2,3-pentanedione bis(thiosemicarbazone) (CTS), 8:1 for (64)Cu-ATSM, P < 0.05). Hypoxic first-pass uptake was 78.2% ± 7.2% for (64)Cu-ATS and 70.7% ± 14.5% for (64)Cu-CTS, compared with 63.9% ± 11.7% for (64)Cu-ATSM. Cardiac retention of (18)F-fluoromisonidazole increased from 0.44% ± 0.17% during normoxia to 2.24% ± 0.08% during hypoxia. In vivo, normoxic cardiac retention of (64)Cu-CTS was significantly lower than that of (64)Cu-ATSM and (64)Cu-ATS (0.13% ± 0.02% vs. 0.25% ± 0.04% and 0.24% ± 0.03% injected dose, P < 0.05), with retention of all 3 tracers falling to less than 0.7% injected dose within 6 min. (64)Cu-CTS also exhibited lower uptake in liver and lung. CONCLUSION: (64)Cu-ATS and (64)Cu-CTS exhibit better cardiac hypoxia selectivity and imaging characteristics than the current lead hypoxia tracers, (64)Cu-ATSM and (18)F-fluoromisonidazole.

Demonstration of the retention of 64Cu-ATSM in cardiac myocytes using a novel incubation chamber for screening hypoxia-dependent radiotracers.

OBJECTIVE: We have designed a low-cost, reusable incubation system that allows cells to be cultured in either plated or suspension culture under complete atmospheric control for radiotracer characterization. We demonstrate its utility here in the first quantification of the hypoxia-dependent accumulation of Cu-diacetyl bis(N4-methylthiosemicarbazone) (Cu-ATSM) in adult rat ventricular myocytes (ARVMs). MATERIALS AND METHODS: ARVMs were allowed to adhere overnight in 9 cm culture plates (2×10 cells/dish) or were used in suspension culture, placed inside the chamber and equilibrated with either oxic (95 or 21% O2/5% CO2) or anoxic gas (95% N2/5% CO2). Cu-ATSM of 100 kBq was administered, and the cells were incubated for 30 or 60 min. Cells were then harvested, counted and fractionated to determine intracellular Cu biodistribution. RESULTS: After 1 h, the average cellular Cu retention in plated ARVMs under oxygenated conditions was 23.9 ± 2.5 mBq/cell (95% O2), increasing to 27.3 ± 5.1 under 21% O2 (P<0.05) and to 36.1 ± 3.1 under 0% O2 (P<0.05). When ARVMs were cultured in suspension, normoxic-hypoxic contrast was less marked but still significant [63.2 ± 14.1 vs. 53.4 ± 10.9% mBq/cell after 30 min (P<0.05)]. Sixty percent of tracer accumulated in the cytosol, and, although total cellular retention increased during hypoxia, there was no enrichment in any particular cellular compartment. CONCLUSION: This apparatus allows the conduction of radiotracer uptake studies in cells under complete atmospheric control, as evidenced by our first demonstration of the hypoxia-dependent uptake of Cu-ATSM in ventricular myocytes. It is ideally suited for screening, validating and characterizing novel hypoxia-selective radiotracers.