Quantification of cardiac sympathetic nerve density with N-11C-guanyl-meta-octopamine and tracer kinetic analysis

Abstract

Most cardiac sympathetic nerve radiotracers are substrates of the norepinephrine transporter (NET). Existing tracers such as (123)I-metaiodobenzylguanidine ((123)I-MIBG) and (11)C-(-)-meta-hydroxyephedrine ((11)C-HED) are flow-limited tracers because of their rapid NET transport rates. This prevents successful application of kinetic analysis techniques and causes semiquantitative measures of tracer retention to be insensitive to mild-to-moderate nerve losses. N-(11)C-guanyl-(-)-meta-octopamine ((11)C-GMO) has a much slower NET transport rate and is trapped in storage vesicles. The goal of this study was to determine whether analyses of (11)C-GMO kinetics could provide robust and sensitive measures of regional cardiac sympathetic nerve densities.

Methods: PET studies were performed in a rhesus macaque monkey under control conditions or after intravenous infusion of the NET inhibitor desipramine (DMI). Five desipramine dose levels were used to establish a range of available cardiac NET levels. Compartmental modeling of (11)C-GMO kinetics yielded estimates of the rate constants K1 (mL/min/g), k2 (min(-1)), and k3 (min(-1)). These values were used to calculate a net uptake rate constant K(i) (mL/min/g) = (K1k3)/(k2 + k3). In addition, Patlak graphical analyses of (11)C-GMO kinetics yielded Patlak slopes K(p) (mL/min/g), which represent alternative measurements of the net uptake rate constant K(i). (11)C-GMO kinetics in isolated rat hearts were also measured for comparison with other tracers.

Results: In isolated rat hearts, the neuronal uptake rate of (11)C-GMO was 8 times slower than (11)C-HED and 12 times slower than (11)C-MIBG. (11)C-GMO also had a long neuronal retention time (>200 h). Compartmental modeling of (11)C-GMO kinetics in the monkey heart proved stable under all conditions. Calculated net uptake rate constants K(i) tracked desipramine-induced reductions of available NET in a dose-dependent manner, with a half maximal inhibitory concentration (IC50) of 0.087 ± 0.012 mg of desipramine per kilogram. Patlak analysis provided highly linear Patlak plots, and the Patlak slopes Kp also declined in a dose-dependent manner (IC50 = 0.068 ± 0.010 mg of desipramine per kilogram).

Conclusion: Compartmental modeling and Patlak analysis of (11)C-GMO kinetics each provided quantitative parameters that accurately tracked changes in cardiac NET levels. These results strongly suggest that PET studies with (11)C-GMO can provide robust and sensitive quantitative measures of regional cardiac sympathetic nerve densities in human hearts.

Keywords: hydroxyephedrine; metaiodobenzylguanidine; norepinephrine transporter; positron emission tomography.

FIGURE 1.

Comprehensive compartmental model of a sympathetic nerve radiotracer with optimal kinetic properties (A). Arrow thicknesses are drawn in approximate proportion to the magnitude of the rate constants. If the NET transport rate of the tracer (k₃) is less than the efflux rate from interstitium back into plasma (k₂), this prevents tissue uptake of tracer from being ‘flow-limited’. Furthermore, if the vesicular storage rate (k₅) is very rapid relative to efflux from neuronal axoplasm (k₄), and the tracer is effectively trapped in vesicles (k₆ = 0 or small), the two neuronal compartments kinetically behave as a single trapping compartment. This allows the use of the simplified compartmental model (B) for quantitative analysis of kinetic data from PET studies.

FIGURE 2.

Structure of N-¹¹C-guanyl-(−)-meta-octopamine (¹¹C-GMO). The carbon-11 label is incorporated into the guanidine group (*).

FIGURE 3.

Kinetics of ¹¹C-MIBG, ¹¹C-HED and ¹¹C-GMO in isolated working rat hearts (A). In each case, an 10 min constant infusion of tracer was performed to measure neuronal uptake rates (K_up; mL/min/g wet), then the heart switched to normal heart perfusate to study efflux rates from neuronal spaces. For comparison, the isolated rat heart kinetics of ¹¹C-GMO are shown along with those of ¹⁸F-FDG (B). Note the different y-axis scales used in panels A and B.

FIGURE 4.

Reverse-phase HPLC/radiodetection analysis of radiometabolite formation in plasma (A). The fraction of activity associated with intact ¹¹C-GMO was determined for each sample, and the % intact vs. time data fit to a mathematical function to characterize the metabolic breakdown of ¹¹C-GMO (B).

FIGURE 5.

Representative transaxial microPET images of cardiac ¹¹C-GMO retention. Shown are summed images (final four dynamic frames) for a control study (far left) and for blocking studies with progressively higher doses of the NET inhibitor desipramine (DMI).

FIGURE 6.

Compartmental modeling of ¹¹C-GMO kinetics using the simplified model shown in Fig. 1B. Myocardial ¹¹C-GMO kinetics (blue dots) and corresponding compartmental model fits (red lines) are shown for a control study (A) and three different DMI blocking doses (B-D). (E) Relationship between the net uptake rate constants K_i (mL/min/g) calculated from compartmental model parameter estimates and log [DMI dose (mg/kg)]. The decline in K_i values was well described by a sigmoidal dose response curve with variable Hill slope (n_H). Estimated parameter values for the dose-response curve fit are shown.

FIGURE 7.

Patlak plots of ¹¹C-GMO kinetics for all studies (A). Legend values are listed in order of highest Patlak slope to lowest, corresponding to values given in Table 2. (B) Relationship between Patlak slopes K_p (mL/min/g) and log [DMI dose (mg/kg)]. Reductions in measured K_p values vs. DMI dose were again well described by a sigmoidal dose response curve with variable Hill slope (n_H). Estimated parameter values for the dose-response curve are shown.