[¹⁸F]-3-Fluoro-4-amynopyridine, a novel radiotracer surrogate of myelin content in nonhuman primates

Introduction: Specific measurement of demyelination of the central nervous system (CNS) with in vivo imaging techniques is difficult. Magnetic resonance imaging (MRI) is the prevailing technique to study in vivo brain disorders; it has excellent spatial resolution and high sensitivity to myelin damage but lacks molecular specificity [1]. Positron emission tomography (PET) yields excellent biochemical specificity but suffers from relatively poor spatial resolution. Therefore, combining MRI and PET can be a powerful imaging strategy, benefiting from the complementary information provided by both techniques. We evaluated a new PET radiotracer ([¹⁸F]3-fluoro-4-amynopyridine, 3F4AP) targeting shaker-type potassium channels (K_v1.1 and K_v1.2), which are located in the juxtaparanodal region of myelinated axons and known to be exposed by demyelination [2-3].

Methods: Animals: 5 healthy marmosets (Callithrix jacchus; 3 males, 2 females) were imaged on a PET/CT scanner (Inveon, Siemens, Germany) and the following week on a 7-tesla MRI. Two of the animals were rescanned on both systems in order to measure experimental reproducibility. PET: [¹⁸F]3F4AP was synthesized as previously reported [2]. A one-hour dynamic scan was performed immediately after intravenous tail injection of 1 mCi of [¹⁸F]3F4AP. The images were then averaged (window 10-60 min) to increase the signal-to-noise ratio and facilitate co-registration with MRI (figure 1a). MRI: High-resolution proton density-weighted (PD-w) scans were acquired for each monkey using the following parameters (TE/TR: 16/2300 ms; in-plane resolution: 150 x 150 μm, slice thickness: 1 mm). A high-resolution T₁-Magnetization-Prepared 2 Rapid Acquisition Gradient Echoes (MP2RAGE) was also acquired for each monkey using the following parameters (TE/TR: 3.5/6000 ms; in-plane resolution: 150 x 150 μm, thickness: 1 mm). T₁-maps were computed by post-processing using MP2RAGE raw data. Post-processing: MRI and PET images were post-processed according to the pipeline in figure 1b. Standardized uptake value ratio (SUVR) was calculated for 54 cortical gray matter (GM) and 35 white matter (WM) regions of interest (ROI) [4] using the following formula: SUVR = SUV (ROI brain) / SUV (ROI liver). Signal in the liver was found to be consistent across animals and was used for normalization. Statistical analysis was then performed. Results: The scan-rescan data in the first monkey showed a mean difference of 5% for the average SUVR across the 54 GM ROI (range: 1%-11%) and 3% across the 35 WM ROIs (range: 0%-5%). The scan-rescan difference in the second monkey was 13% for average SUVR of GM ROI (9%-17%) and 13% for WM ROI (8%-16%) (figure 1c). Multivariable linear regression adjusting for voxel number per ROI, and bodyweight showed significant differences in SUVR values and T₁ values between GM and WM ROIs for each monkey (p<0.05, figure 1d). Discussion/Conclusion: Similar to a previous study in other species [3], a higher uptake of 3F4AP in GM was observed as compared to WM in the brains of healthy nonhuman primates. The lower amount of myelin in GM and the higher number of potassium channels available for binding may explain the signal difference between the two types of cerebral tissues. A higher T₁ value was also observed in GM as compared to WM, which is expected based on its known sensitivity to myelin content [5]. Overall, there was an excellent correspondence between the spatial distribution of 3F4AP and T₁ map-derived intensity values, highlighting the potential of this combined approach for probing biological variations related to demyelination. There was also reasonable reproducibility of radiotracer uptake, which is promising for future quantification of 3F4AP uptake. In conclusion, our study illustrates the use of 3F4AP as a promising radiotracer surrogate of myelin integrity in the Callithrix jacchus, a relevant nonhuman primate model to study demyelinating diseases such as multiple sclerosis [6].