Hypoxia Imaging Using PET and SPECT: The Effects of Anesthetic and Carrier Gas on [64Cu]-ATSM, [99mTc]-HL91 and [18F]-FMISO Tumor Hypoxia Accumulation

Abstract

BackgroundPreclinical imaging requires anaesthesia to reduce motion-related artefacts. For direct translational relevance, anaesthesia must not significantly alter experimental outcome. This study reports on the effects of both anaesthetic and carrier gas upon the uptake of [64Cu]-CuATSM, [99mTc]-HL91 and [18F]-FMISO in a preclinical model of tumor hypoxia.Methodology/Principal FindingsThe effect of carrier gas and anaesthetic was studied in 6 groups of CaNT-bearing CBA mice using [64Cu]-CuATSM, [99mTc]-HL91 or [18F]-FMISO. Mice were anaesthetised with isoflurane in air, isoflurane in pure oxygen, with ketamine/xylazine or hypnorm/hypnovel whilst breathing air, or in the awake state whilst breathing air or pure oxygen. PET or SPECT imaging was performed after which the mice were killed for organ/tumor tracer quantitation. Tumor hypoxia was confirmed. Arterial blood gas analysis was performed for the different anaesthetic regimes. The results demonstrate marked influences on tumor uptake of both carrier gas and anaesthetic, and show differences between [99mTc]-HL91, [18F]-FMISO and [64Cu]-CuATSM. [99mTc]-HL91 tumor uptake was only altered significantly by administration of 100% oxygen. The latter was not the case for [18F]-FMISO and [64Cu]-CuATSM. Tumor-to-muscle ratio (TMR) for both compounds was reduced significantly when either oxygen or anaesthetics (isoflurane in air, ketamine/xylazine or hypnorm/hypnovel) were introduced. For [18F]-FMISO no further decrease was measured when both isoflurane and oxygen were administered, [64Cu]-CuATSM did show an additional significant decrease in TMR. When using the same anaesthetic regimes, the extent of TMR reduction was less pronounced for [64Cu]-CuATSM than for [18F]-FMISO (40–60% versus 70% reduction as compared to awake animals breathing air).Conclusions/SignificanceThe use of anaesthesia can have profound effects on the experimental outcome. More importantly, all tested anaesthetics reduced tumor-hypoxia uptake. Anaesthesia cannot be avoided in preclinical studies but great care has to be taken in preclinical models of hypoxia as anaesthesia effects cannot be generalised across applications, nor disease states.

A Comparison of the Behavior of 64Cu-Acetate and 64Cu-ATSM In Vitro and In Vivo

“…At the time examined, the radiocopper is not localized in areas of strong hypoxia as delineated by immunohistochemistry. The decrease in tumor retention was more pronounced for 64 Cu-acetate than 64 Cu-ATSM (34). Muscle retention was unaffected by the inhaled gas.…”

64Cu-ATSM Reflects pO2 Levels in Human Head and Neck Cancer Xenografts but Not in Colorectal Cancer Xenografts: Comparison with 64CuCl2

“…Importantly, the different imaging time points and anesthetic procedures applied in the different studies complicate a direct comparison of results. It has previously been shown that both anesthetics and level of oxygen in anesthetic gas mixtures can influence tissue uptake of both 64 Cu-ATSM and copper (16,17). Therefore, to avoid fluctuations in uptake, the level of the anesthetic gas mixture was strictly kept at a constant level during all procedures from tracer injection until completion of pO 2 probe measurements.…”

“…Therefore, to avoid fluctuations in uptake, the level of the anesthetic gas mixture was strictly kept at a constant level during all procedures from tracer injection until completion of pO 2 probe measurements. However, the experimental setup has likely contributed to slower accumulation and clearance from the background than would be seen when the tracer is allowed to distribute in an unanesthetized animal (16,17).…”

“…64 Cu-ATSM has a high tumor-to-background ratio and has shown promising results in small clinical studies, in which the tumor-to-muscle ratio was able to predict treatment outcome (11)(12)(13). However, the robustness of 64 Cu-ATSM as a marker of hypoxia has been questioned, because preclinical studies have reported temporal changes in tumor uptake and cell-type-specific differences in hypoxia selectivity (14)(15)(16)(17)(18)(19)(20)(21)(22)(23)(24)(25)(26). Table 1 provides an overview of major studies using 64 Cu-ATSM in small-animal tumor xenograft models.…”

Positron emission tomography to assess hypoxia and perfusion in lung cancer

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