Myocardial hypoxia is a major factor in the pathology of cardiac ischemia and myocardial infarction. Hypoxia also occurs in microvascular disease and cardiac hypertrophy, and is thought to be a prime determinant of the progression to heart failure, as well as the driving force for compensatory angiogenesis. The non-invasive delineation and quantification of hypoxia in cardiac tissue therefore has the potential to be an invaluable experimental, diagnostic and prognostic biomarker for applications in cardiology. However, at this time there are no validated methodologies sufficiently sensitive or reliable for clinical use. PET imaging provides real-time spatial information on the biodistribution of injected radiolabeled tracer molecules. Its inherent high sensitivity allows quantitative imaging of these tracers, even when injected at subpharmacological (≥pM) concentrations, allowing the non-invasive investigation of biological systems without perturbing them. PET is therefore an attractive approach for the delineation and quantification of cardiac hypoxia and ischemia. In this review we discuss the key concepts which must be considered when imaging hypoxia in the heart. We summarize the PET tracers which are currently available, and we look forward to the next generation of hypoxia-specific PET imaging agents currently being developed. We describe their potential advantages and shortcomings compared to existing imaging approaches, and what is needed in terms of validation and characterization before these agents can be exploited clinically.

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 5/group) were perfused with normoxic buffer for 30 min and then hypoxic buffer for 45 min within a custom-built triple-g-detector system to quantify radiotracer infusion, hypoxiadependent 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% 6 7.2% for 64 Cu-ATS and 70.7% 6 14.5% for 64 Cu-CTS, compared with 63.9% 6 11.7% for 64 Cu-ATSM. Cardiac retention of 18 F-fluoromisonidazole increased from 0.44% 6 0.17% during normoxia to 2.24% 6 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% 6 0.02% vs. 0.25% 6 0.04% and 0.24% 6 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.

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.