Comparison of Awake and Anesthetized Non-human Primate
Benzodiazepine BPND with [11C]Flumazenil and Equilibrium Analysis
Sandiego CM 1, Mulnix T 1, Jin X 1, Fowles K 1, Ford S 1, Gallezot JD1,
Lee D 2, Campbell DW 2, Abbott A 2, Gunn RN 3, Wells L 3, Rabiner EA3,
Castner SA 2, Williams GV 2, Cosgrove K2, Carson RE 1
1
Yale University, Department of Diagnostic Radiology, New Haven, CT, USA,
2
Yale University, Department of Psychiatry, New Haven, CT, USA,
3
GSK, London, UK
Objectives: Neuroreceptor imaging in the awake non-human primate (NHP) is ideal for
improving upon translational research approaches in humans. NHP-PET studies are typically
conducted under anesthesia, affecting the interpretability of receptor binding measures.
Specifically, anesthetic agents can enhance the affinity of GABA to GABAA receptors. Isoflurane
dose-dependently increased DVR of [11C]flumazenil, a GABAA benzodiazepine receptor
antagonist, compared with the awake condition in humans [1]. The goal of this study is to
compare [11C]flumazenil BPND in anesthetized and awake studies in the NHP. We hypothesized
that BPND would be higher in anesthetized vs. awake monkeys. To perform awake NHP studies
we used motion-tracking technology to permit NHP imaging without head restraint. Furthermore,
scan time was minimized to reduce stress by using bolus/infusion methods, allowing for
acquisition only during the equilibrium phase.
Methods: Monkeys were trained to sit in a custom chair, tilted back 45º while the Focus-220
scanner was tilted forward 45º. The Vicra system was used to track motion, with its tool
mounted onto the animal’s scalp via skin glue. [11C]Flumazenil was administered as a
bolus/infusion (Kbol=90 min). Animals were positioned in the scanner ~45 min post-injection, and
listmode and motion-tracking data were acquired for 25-35 min. The listmode data were binned
into sub-frames based on an intra-frame motion threshold of 2 mm and minimal sub-frame
duration of 3 sec [2]. Sub-frames were reconstructed with FBP and resliced to a reference
orientation using the motion data. A transmission image from an anesthetized study for the
same animal was resliced to the awake reference orientation and used to reconstruct subframes with attenuation and scatter correction [3]. Isoflurane anesthetized monkeys were
scanned for 90 min with the same bolus/infusion protocol. Unpaired anesthetized (n=20) and
awake (n=3) datasets were analyzed by equilibrium analysis. To assess the quality of
equilibrium, percent change per min (PCPM) in radioactivity concentration was calculated
across animals for ROIs. Mean BPND images were computed for anesthetized and awake
images in NHP template space, using the pons as a reference region where,
BPND=(CROI/CPONS)-1.
Results: Mean PCPM were generally positive for anesthetized and negative for awake
conditions, respectively: frontal(+0.130.30,-0.330.54), occipital(+0.230.13,-0.440.40)*,
temporal(+0.170.13,-0.270.68), insula(+0.160.15,-0.270.68)*, and pons(+0.260.45,-1.091.12), *p<0.001. Mean BPND values between awake and anesthetized conditions showed close
agreement when analyzed by regions, BPND(awake) =1.15BPND(anesthetized)+0.059, r2=0.92,
or by individual pixels, BPND(awake)=1.02BPND(anesthetized)+0.48, r2=0.74, in average BPND
images (Fig. 1).
Conclusions: In this initial dataset, there was excellent agreement of [11C]flumazenil BPND
values between awake and anesthetized conditions. These data suggest that Kbol should be
adjusted for awake NHP-PET to avoid potential bias. BPND was slightly higher for awake
monkeys, in contrast with the human study, perhaps due to differences in equilibrium or stressinduced changes. Results in a larger cohort with an adjusted Kbol and cortisol measurements will
be necessary to fully characterize anesthesia-induced changes in [11C]flumazenil binding.
References
[1] Gyulai FE et al., Anesthesiology, 2001;95(3):585-93.
[2] Jin X et al., IEEE MIC Conference Record, 2010.
[3] Sandiego CM et al., NRM abstract, 2010.