Rodolfo A. Medina, Erika Mariotti, Davor Pavlovic, Karen P. Shaw, Thomas R. Eykyn, Philip J. Blower, Richard Southworth

The subtle hypoxia underlying chronic cardiovascular disease is an attractive target for PET imaging, but the lead hypoxia imaging agents (64)CuATSM and (18)FMISO trapped only at extreme levels of hypoxia and hence are insufficiently sensitive for this purpose. We have therefore sought an analog of (64)CuATSM better suited to identify compromised but salvageable myocardium, validated using parallel biomarkers of cardiac energetics comparable to those observed in chronic cardiac ischemic syndromes. Rat hearts were perfused with aerobic buffer for 20 min, followed by a range of hypoxic buffers (using a computer-controlled gas mixer) for 45 min. Contractility was monitored by intraventricular balloon, energetics by (31)P NMR spectroscopy, lactate and creatine kinase release spectrophotometrically, and HIF1α by Western blotting. We identify a key hypoxia threshold at a 30% buffer O2 saturation which induces a stable and potentially survivable functional and energetic compromise: LV developed pressure was depressed by 20%, and cardiac phosphocreatine was depleted by 65.5 ± 14% (p<0.05 vs control), but ATP levels were maintained. Lactate release was elevated (0.21 ± 0.067 versus 0.056 ± 0.01 mmol/L/min, p<0.05), but not maximal (0.46 ± 0.117 mmol/L/min), indicating residual oxidative metabolic capacity. HIF1α was elevated, but not maximal. At this key threshold, (64)CuCTS selectively deposited significantly more (64)Cu than any other tracer we examined (61.8 ± 9.6% injected dose versus 29.4 ± 9.5% for (64)CuATSM p<0.05). The hypoxic threshold which induced survivable metabolic and functional compromise was 30% O2. At this threshold, only (64)CuCTS delivered a hypoxic:normoxic contrast of 3:1, and it therefore warrants in vivo evaluation for imaging chronic cardiac ischemic syndromes.