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

Abstract

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.

64Cu-CTS: A Promising Radiopharmaceutical for the Identification of Low-Grade Cardiac Hypoxia by PET

“…Hueting et al have recently suggested in this journal that the biodistribution and pharmacokinetics of 64 Cuacetate in tumor-bearing mice is similar to that of 64 Cu-ATSM when assessed after 2 or 16 h, and that 64 Cu-acetate exhibits hypoxia selectivity in cultured cancer cells, suggesting that 64 Cu-bis(thiosemicarbazone) complexes dissociate quickly in vivo and that the resultant biodistribution observed by PET is that of free copper, questioning the validity of 64 Cu-ATSM as a hypoxia tracer (34). We and other groups have observed no such nonspecific uptake of ionic 64 Cu 21 salts in normoxic or hypoxic myocytes in culture (15), CHO cells (35), or isolated buffer-perfused hearts (17,21). The wide variation in pharmacokinetics and early biodistributions that the different bis(thiosemicarbazones) have been shown to display, with some being hypoxia-selective (ATSM, ATS, CTS) and some not (pyruvaldehyde-bis(N4-methylthiosemicarbazone) [PTSM], glyoxal-bis(N4-methylthiosemicarbazone) [GTSM]), and with some crossing the blood-brain barrier (GTSM, PTSM, ATSM) and some not (ATS) (36,37), would also argue against a common nonselective mechanism of early tissue uptake.…”

“…1) exhibits rapid first-pass uptake and fast clearance from normoxic tissues (11) and blood (12), and rapid retention in hypoxic tissues to generate excellent images of tissue hypoxia within minutes. Experimentally, tissue 64 Cu accumulation from 64 Cu-ATSM has been demonstrated only in extreme models of acute hypoxia and ischemia in isolated perfused hearts, and in regionally occluded canine myocardium in vivo (13,14), appearing to have a hypoxia-selective threshold of 1 mm Hg or lower (11,15). Although this degree of hypoxia is compatible with the survival and radioresistance of cancer cells, for which it was designed, it is too severe for the long-term survival of cardiac myocytes (normal cardiac mitochondrial partial pressure of oxygen is 10-35 mm Hg) (16).…”

Pitfalls and Limitations of SPECT, PET, and Therapeutic Radiopharmaceuticals

“…61 Although these protocols were cumbersome, they could be automated and had the added advantage of greatly reducing the volume of eluate, which improved the efficiency of subsequent labeling reactions. The eluates of the newer 68 Ga generators meet the specifications of the European Pharmacopoeia; thus, they do not require purification and open the potential for truly kitbased procedures for preparation of 68 Ga-labeled radiopharmaceuticals.…”

“…The early commercial versions of the ionic 68 Ge/ 68 Ga generator had dual problems of metal contaminants in the eluate and 68 Ge breakthrough. 61 Accordingly, either cationic or anionic preconcentration protocols were developed to purify the eluate before the labeling reaction.…”

“…67 The hypoxiaimaging agent 64 Cu ATSM is also susceptible to radiolysis and requires stabilization with ascorbic acid. 68 The pH of the 18 F-FDG solution had a significant effect if the product was sterilized by autoclaving. Preparations at neutral pH showed substantial decomposition, whereas pH 5.5 was found to be optimal for stability.…”