Myocardial Kinetics of a Novel [¹⁸F]-Labeled Sympathetic Nerve PET Tracer LMI1195 in the Isolated Perfused Rabbit Heart

A radionuclide tracer approach is a unique assay to monitor the cardiac sympathetic nervous system in patients, and the most widely available tracer is the radiolabeled norepinephrine analogue [¹²³I]-metaiodobenzylguanidine ([¹²³I]-MIBG). A novel ¹⁸F-labeled positron emission tomography (PET) tracer, [¹⁸F]-LMI1195 (N-[3-bromo-4-(3-[¹⁸F]fluoro-propoxy)-benzyl]-guanidine), has been developed to overcome the limitations of conventional tracers (1). [¹⁸F]-LMI1195 shares similarities with [¹²³I]-MIBG based on its benzylguanidine structure, but higher sensitivity and specificity and more accurate quantification are expected via the general advantages of PET over single-photon emission computed tomography technology. The high specificity of [¹⁸F]-LMI1195 for the norepinephrine transporter was confirmed with a cell-binding assay and in vivo imaging (1,2). Most recently, a human volunteer study demonstrated promising results with uniform tracer uptake in the ventricular wall and acceptable radiation doses (3).

We aimed to evaluate further tracer kinetics at the nerve terminal. First-pass tracer extraction fraction (EF) and washout were measured in isolated rabbit hearts (New Zealand White) to avoid systemic recirculation and metabolism of the tracer.

The EF was determined at different flow values (n = 19). Additionally, the influence of desipramine hydrochloride (40 nM) added into the buffer (n = 5) and reserpine–pre-treated hearts (n = 5) were tested. Reserpine pre-treatment was performed with intravenous injection of reserpine (2 mg/kg) 3 h before the study.

A second protocol was designed to measure tracer washout. Hearts were perfused with [¹⁸F]-LMI1195 added to the buffer for 10 min (incubation phase) followed by 25 min of perfusion without tracer (washout phase). The tracer washout was examined by control Krebs-Henseleit bicarbonate buffer (n = 9), and desipramine was added (desipramine-chase; n = 5), with washout with electrical field stimulation (5 Hz, 5 V, 1 min × 5 times at the washout phase; n = 7).

There was a flow-dependent decrease in the EF, with 44.4 ± 5.1%, 28.1 ± 6.5%, and 21.5 ± 5.1% using flow values of 2, 4, and 8 ml/min/g, respectively. Desipramine, a specific neural uptake-1 blocker, decreased the EF significantly from 28.1 ± 6.5% (control) to 4.3 ± 3.8% (p < 0.001). Reserpine, an irreversible vesicular monoamine transporter blocker, pre-treatment did not change the EF (30.1 ± 3.4%; p = NS) (Figure 1).

• Download figure
• Open in new tab
• Download powerpoint

Figure 1

Myocardial Kinetics of Novel [¹⁸F]-LMI1195 Sympathetic Nervous Tracer

(A) In vivo [¹⁸F]-LMI1195 positron emission tomography images of a rabbit using a clinical positron emission tomography–cardiac magnetic resonance system. High-contrast left ventricular delineation is seen, with minimal tracer activity in the surrounding tissues. (B) Averaged extraction time activity curves from [¹⁸F]-LMI1195 cardiac uptake and washout studies in several isolated rabbit hearts. Flow-dependent decrease of tracer extraction is observed after bolus tracer administration (top). Desipramine decreased the tracer extraction, whereas reserpine pre-treatment did not change the extraction (bottom). BAT = brown adipose tissue; MIP = maximal intensity projection.

There was tracer washout with 0.73 ± 0.18 %/min in the first 20-min measurement. Desipramine added only during washout phase did not change the washout rate (0.94 ± 0.12 %/min). On the other hand, electrical field stimulation to enhance the vesicular release of norepinephrine increased the washout rate significantly (1.96 ± 0.48 %/min; p < 0.01).

In summary, we measured the myocardial kinetics of [¹⁸F]-LMI1195 with isolated perfused rabbit hearts. Specific uptake at the nerve terminal was confirmed by desipramine sensitive uptake. Furthermore, we observed enhanced tracer washout with electrical stimulation, as well as resistance for desipramine chase, suggesting that the tracer handling in the nerve terminals was consistent with norepinephrine vesicular storage and release.

The mechanistic insights of [¹⁸F]-LMI1195 uptake assessed in an isolated heart model might be relevant in the design of clinical imaging protocols, as well as the interpretation of imaging results in patients. However, further complementary in vivo animal experiments are needed to confirm and extended these findings for clinical studies.

• American College of Cardiology Foundation