Evaluation of the potassium channel tracer [¹⁸F]3F4AP in rhesus macaques

Abstract

Demyelination causes slowed or failed neuronal conduction and is a driver of disability in multiple sclerosis and other neurological diseases. Currently, the gold standard for imaging demyelination is MRI, but despite its high spatial resolution and sensitivity to demyelinated lesions, it remains challenging to obtain specific and quantitative measures of molecular changes involved in demyelination. To understand the contribution of demyelination in different diseases and to assess the efficacy of myelin-repair therapies, it is critical to develop new in vivo imaging tools sensitive to changes induced by demyelination. Upon demyelination, axonal K+ channels, normally located underneath the myelin sheath, become exposed and increase in expression, causing impaired conduction. Here, we investigate the properties of the K+ channel PET tracer [ 18 F]3F4AP in primates and its sensitivity to a focal brain injury that occurred three years prior to imaging. [ 18 F]3F4AP exhibited favorable properties for brain imaging including high brain penetration, high metabolic stability, high plasma availability, high reproducibility, high specificity, and fast kinetics. [ 18 F]3F4AP showed preferential binding in areas of low myelin content as well as in the previously injured area. Sensitivity of [ 18 F]3F4AP for the focal brain injury was higher than [ 18 F]FDG, [ 11 C]PiB, and [ 11 C]PBR28, and compared favorably to currently used MRI methods.

Radiochemical synthesis and evaluation in nonhuman primates of 3-[11C]methoxy-4-aminopyridine: a novel PET tracer for imaging potassium channels in the CNS

“…Given these limitations, the use of PET radioligands that target proteins differentially expressed in demyelinated lesions appears to be a highly promising approach 6 as these channels significantly increase in expression and in accessibility upon demyelination 12,13,25 . In our previous work, we showed that [ 18 F]3F4AP can detect demyelinated lesions in rodent models of MS 8 and, more recently, that it has favorable pharmacokinetic properties in non-human primates and is highly sensitive to a focal brain injury 16 . In a parallel but related effort, we also investigated four novel derivatives of 4AP as potential candidates for imaging and/or therapy, among which 3MeO4AP was found to possess good binding affinity as well as suitable basicity (essential for binding) and lipophilicity (essential for brain penetration) 17 .…”

“…However, in this work 90 min of data was used as the reference scan duration for the kinetic analyses because of increased statistical noise in late PET images as well as in the parent fraction measurements. According to the Akaike information criterion (AIC) 19 16 . We did not attempt coadministration of higher doses of nonradioactive 3MeO4AP as previous studies have shown that high doses of 4AP and related compounds can cause seizures 8,20 .…”

“…We recently reported the pharmacokinetic evaluation of [ 18 F]3F4AP in the same two animals 16 . Compared to (Figure 6A), 2.58 for Monkey 2 ( Figure 6B) and 2.78 when combining Monkey 1 and 2 on the same graph ( Figure 6C), thus suggesting that [ 11 C]3MeO4AP has higher in vivo affinity than [ 18 F]3F4AP for K v channels.…”

“…Eluent was collected in 1-minute intervals and assayed for radioactivity using a Wallac Wizard gamma counter (1470 model). The procedure adopted for these measurements was similar to the one described in Guehl et al 16 analysis MA1 graphical methods 29,30 were tested. V T parametric maps were generated using the Logan method.…”

“…Standardized uptake value (SUV) radioactivity time courses in WB and in PL were generated by correcting radioactivity concentrations (C [kBq/ml]) for injected dose (ID [MBq]) and animal body weight (BW [kg]) and were calculatedas SUV(t) = C(t)/(ID/BW). Measured time courses of percent parent in plasma (%PP(t)) were fitted with a single exponential decay plus a constant and individual metabolite-corrected arterial input function were derived as previously described16 . The plasma free fraction f p of [ 11 C]3MeO4AP was measured in triplicate by ultrafiltration following the procedure described in Guehl et al16 .Image registration and processing: Structural MEMPRAGE and PET images were aligned into the MRI National Institute of Mental Health Macaque Template (NMT)21 following the procedures extensively described in our previous work with [18 F]3F4AP16 .…”