Ex vivo and in vivo evaluation of the norepinephrine transporter ligand [11C]MRB for brown adipose tissue imaging

Abstract

Introduction: It has been suggested that brown adipose tissue (BAT) in humans may play a role in energy balance and obesity. We conducted ex vivo and in vivo evaluation using [(11)C]MRB, a highly selective NET (norepinephrine transporter) ligand for BAT imaging at room temperature, which is not achievable with [(18)F]FDG.

Methods: PET images of male Sprague-Dawley rats with [(18)F]FDG and [(11)C]MRB were compared. Relative [(18)F]FDG or [(11)C]MRB retention at 20, 40 and 60 min post-injection was quantified on awake rats after exposing to cold (4°C for 4h) or remaining at room temperature. Rats pretreated with unlabeled MRB or nisoxetine 30 min before [(11)C]MRB injection were also assessed. The [(11)C]MRB metabolite profile in BAT was evaluated.

Results: PET imaging demonstrated intense [(11)C]MRB uptake (SUV of 2.9 to 3.3) in the interscapular BAT of both room temperature and cold-exposed rats and this uptake was significantly diminished by pretreatment with unlabeled MRB; in contrast, [(18)F]FDG in BAT was only detected in rats treated with cold. Ex vivo results were concordant with the imaging findings; i.e. the uptake of [(11)C]MRB in BAT was 3 times higher than that of [(18)F]FDG at room temperature (P=0.009), and the significant cold-stimulated uptake in BAT with [(18)F]FDG (10-fold, P=0.001) was not observed with [(11)C]MRB (P=0.082). HPLC analysis revealed 94%-99% of total radioactivity in BAT represented unchanged [(11)C]MRB.

Conclusions: Our study demonstrates that BAT could be specifically labeled with [(11)C]MRB at room temperature and under cold conditions, supporting a NET-PET strategy for imaging BAT in humans under basal conditions.

Fig. 1

PET images (50-70 min post-injection) on isoflurane-anesthetized rats from left to right: A-[¹⁸F]FDG room temperature control, B-[¹⁸F]FDG cold-exposed rat, C-[¹¹C]MRB room temperature control, D-[¹¹C]MRB cold-exposed rat, and E-[¹¹C]MRB following blockade by unlabeled MRB. BAT is denoted by arrows. The amounts of injected activity from left to right were 10.73, 10.878, 29.23, 31.45 and 40.875 MBq.

Fig. 2

Time course assessment at 20 to 60 min after tracer administration of either [¹⁸F]FDG or [¹¹C]MRB in both non-stimulated (basal state, room temperature) and activated BAT (after cold exposure, 4°C) from ex vivo studies. Data are shown as mean %ID/g of tissue ± SE.

Fig. 3

Comparison of the uptake of [¹⁸F]FDG vs. [¹¹C]MRB at the basal state. Note that the uptake of [¹⁸F]FDG was lower than that of [¹¹C]MRB in BAT (P = 0.0088). Data are shown as mean %ID/g of tissue ± SE. *P <0.05.

Fig. 4

Significant blocking effect on the [¹¹C]MRB uptake with unlabeled MRB or nisoxetine (2mg/kg, intravenous injection 30 min prior tracer) was detected in NET-specific retention regions, e.g. BAT and heart. Data are shown as mean %ID/g of tissue ± SE. *P <0.05.