Mechanisms of cardiovascular hypoxia detection by ⁶⁴Cu-ATSM

Abstract

⁶⁴Cu-Diacetyl-bis(N⁴-methylthiosemicarbazone) (⁶⁴Cu-ATSM) is a hypoxia-selective positron emission tomography (PET) tracer and has potential imaging applications in cardiology. To fully exploit and develop ⁶⁴Cu-ATSM, it is essential to understand its mode of action. It has been suggested that intracellular reductants, such as thiols, reduce the uncharged Cu(II)-ATSM complex to Cu(I)-ATSM- which dissociates to deposit its radiocopper in hypoxic tissue. It is also possible that this process may be accelerated by intracellular acidosis, which is a hallmark of ischaemic tissue. The sensitivity of Cu-ATSM to intracellular thiol status or pH would impact upon its hypoxia specificity and potentially restrict its diagnostic utility.
An isolated perfused rat heart model and cultured bovine aortic endothelial cells (BAEC) were therefore utilised to investigate the importance of intracellular thiol status and intracellular acidosis on the hypoxia selectivity and pharmacokinetics of ^64Cu-ATSM.
During these studies the hypoxia selective retention of ^{64Cu from 64Cu-ATSM was confirmed in isolated perfused rat hearts. It was also demonstrated that intracellular thiol concentration has no effect on 64Cu retention from 64Cu-ATSM in normoxic or hypoxic hearts. Conversely, hypoxic BAEC incubated with high concentrations of thiols displayed significantly higher 64Cu retention from 64Cu-ATSM, while thiol depletion had no effect. It was therefore concluded that while thiols may be necessary for the initial reduction of Cu-ATSM, physiological changes in thiol concentration do not affect Cu-ATSM pharmacokinetics, or underlie its hypoxia selectivity.}
It was also demonstrated that the hypoxic myocardium is not acidotic, and that acidosis cannot be a prime determinant of Cu-ATSM retention during hypoxia. Subsequent experiments suggest that acidosis may suppress ^{64Cu retention from 64Cu-ATSM during normoxia and hypoxia; but that this may be secondary to the oxygen-sparing effect that acidosis has on the hypoxic heart, by impairing cardiac contractility. These findings provide further insight into the mechanisms of Cu ATSM hypoxia selectivity, and support its use as a hypoxia-specific imaging agent.}

Date of Award	2014
Original language	English
Awarding Institution	King's College London
Supervisor	Richard Southworth (Supervisor) & Richard Siow (Supervisor)