Background
Synaptic vesicle glycoproteins (SV) are critical to proper nervous system function and have been demonstrated to be involved in vesicle trafficking. They belong to the Major Facilitator Superfamily (MFS) of transporters and consist of a 12-transmembrane glycoprotein and a cytoplasmic N-terminal region containing a long sequence that varies among the three SV2 isoforms (SV2A, SV2B and SV2C) [
1,
2]. SV2A and -B are highly homologous to each other, with SV2A showing ubiquitous expression in both excitatory and inhibitory synapses throughout the entire brain [
3,
4]; in contrast, SV2B and SV2C are present in a more restricted pattern in the brain, and in only a subset of synapses [
2,
5]. The hypothesis of SV2 as vesicular transport protein is based on a significant homology to other transport proteins, however no endogenous substrate has been reported [
3], neither has any transport activity been demonstrated [
2]. Another hypothetical function is vesicle trafficking and exocytosis, and the modification of the synaptic function [
4‐
6].
Lynch and coworkers identified SV2A as the brain-binding site of the anti-epileptic drug levetiracetam (LEV, Keppra®, UCB Pharma Ltd., Slough, Berkshire, UK) [
7]. Seizure protection by LEV and other SV2A ligands strongly correlates with the degree of SV2A occupancy in vivo [
3,
8]. However, the site of SV2A-LEV interaction and the mechanism of action remain unclear. LEV does not cause a SV2A conformational state change and it is assumed that SV2A transports LEV or LEV prevents transport of the endogenous substrate [
4], as one of the functional consequences of LEV binding to SV2A in brain slices is reduced exocytosis [
6]. With the aim of a better understanding of the role of SV2A in epilepsy and of studying SV2A in diseases of the central nervous system, several SV2A-specific ligands have been developed [
9], [
18F]UCB-H being one of the first to be labelled [
10], subsequently characterized in the rodent [
10,
11], and in the human brain [
12]. The demonstration of the co-localization of SV2A with other synaptic markers using [
11C]-UCB-J [
13], showed the potential of in vivo imaging of the synaptic density using Positron Emission Tomography (PET), and led recently to the development of new [
18F]-labelled ligands [
9,
14‐
19]. Preclinical characterization of [
18F]UCB-H has mostly been done in the rodent brain [
20‐
22], while in non-human primates (NHP) limited data is available [
23,
24]. In humans a preliminary study was performed on four healthy subjects [
12], preceding a clinical study in Alzheimer’s patients [
25]. Here, we aimed to characterize pharmacodynamics properties of [
18F]UCB-H in non-human cynomolgus primates in complement to these existing data and to discuss the obtained results in the light of the current state of SV2A PET ligands.
Discussion
The current study evaluates [
18F]UCB-H pharmacokinetics in healthy NHPs. We demonstrated similar metabolism in NHP as previously described in the rodent, human and rhesus monkey [
11,
12,
23]. We have shown that [
18F]UCB-H equilibrates rapidly between whole blood and plasma, with high availability in plasma (
fp of 42.6 ± 1.6%). Similar to data obtained in rodent and human [
11,
12], we observed in the NHP a good brain penetrance with ubiquitous brain uptake. Tissue influx parameters of [
18F]UCB-H were comparable to other SV2A radioligands for grey matter regions [
13,
24]. Even though a better goodness of fit of the TACs was obtained with 2TCM compared to 1TCM, not all TACs could be fitted with 2TCM. Previously, this has been the criteria to preconize the graphical method of Logan [
12]. Here we demonstrate that Logan provides reliable estimates of V
T with a relatively low aTRV (~ 12%), although larger than that reported for [
11C]UCB-J in humans (~ 4%)[
35], and a small bias relative to 2TCM of ~ -3% compared to ~ -9% for 1TCM. In a preliminary rhesus NHP study, Zheng and colleagues alternatively proposed 1TCM and the multilinear analysis method (MA1) to obtain reliable estimates of V
T for [
18F]UCB-H [
23]. Here, in the cynomolgus NHP, we demonstrated an aTRV above 10% for 1TCM and a poor reliability. For MA1 we found aTRV of V
T (~ 11%), but a larger negative bias (~ -8%) compared to Logan (
data not shown).
We observed that the convergence issue with 2TCM was mainly due to instability of the k
4 estimate tending to zero in ~ 30% of all cases, independent of the regions. Brain kinetics of SV2A ligands [
11C]UCB-J [
35,
36] and [
18F]SynVesT-1 (a.k.a [
18F]SDM-8, [
18F]MNI-1126) [
14] have been reported as better described by 2TCM in human, but were better modeled with 1TCM in cynomolgus or rhesus monkeys [
24,
37,
38], and [
11C]UCB-A was better described with 1TCM at baseline in pigs but 2TCM was required after blocking of the specific signal [
39]. Interestingly, all authors reported issues with 2TCM similar to those reported here, namely lack of convergence or large standard error for V
T [
14,
35,
36,
39], with large uncertainty on k
4 and values close to 0 [
14,
35]. As a consequence, 1TCM for [
11C]UCB-J and [
18F]SynVesT-1 [
14,
35], and Logan graphical analysis for [
11C]UCB-A [
39] was selected as method of choice. We reported here similar issues with the use of 2TCM for [
18F]UCB-H. We obtained similar AIC for reversible or irreversible 2TCM, and spectral analysis indicated a component at the edge of the frequency range (data not shown), suggesting a small slow component for [
18F]UCB-H. Although this component could reflect penetrating metabolites, inaccuracy of metabolite correction or vascularity activity correction, its exact nature remains unclear. Given the similarity in chemical structures between the current SV2A ligands and the aforementioned results reported in the literature, we hypothesize that this slow component is likely common between all these ligands. Moreover, this component is problematic when the specific signal is low (low affinity ligands or blocking studies) and when kinetics can only be described with 2TCM. This was nicely illustrated with [
18F]MNI-1126 (high affinity of the (
R)-enantiomer), [
18F]MNI-1038 (racemate) and [
18F]MNI-1128 (low affinity of the (
S)-enantiomer) [
24,
40], where 1TCM was the best model for [
18F]MNI-1126, but 2TCM had to be used both for [
18F]MNI-1038 and [
18F]MNI-1128 (lower specific signal), with very low k
4 for [
18F]MNI-1128 [
24]. Here, similarly to [
11C]UCB-A [
39], the method of choice for [
18F]UCB-H was Logan graphical analysis, as 1TCM was not adequate due to its lower affinity compared to other SV2A radioligands.
We performed homologous (with [
19F]UCB-H) and heterologous (with reference compound LEV) displacement studies to evaluate the reversibility of [
18F]UCB-H binding. The total uptake of [
18F]UCB-H was clearly displaceable, and in a dose-dependent manner by [
19F]UCB-H up to 50% of the total uptake at the highest dose tested (5 mg/kg). Based on [
18F]UCB-H brain uptake curves and an f
ND of ~ 6%, we have a maximum free concentration of UCB-H of ~ 2.5 mM after administration 5.0 mg/kg. For UCB-H, in vitro K
i of 9 nM in human brain [
40] and in vivo K
D of 30 nM in NHP [
38] were reported, and conservatively considering a
KD of 30 nM, near full saturation of SV2A would be expected (> 98% occupancy) at a dose of 5 mg/kg and a free concentration of 2.5 mM. We have reported a maximum displacement of the total uptake of 50% with UCB-H, indicating that only about half of the total uptake is displaceable for [
18F]UCB-H; therefore, BP
ND would be expected close to 1.0. Although imperfect because of all the microparameters correlations, k
3/k
4 from 2TCM with coupled fit K
1/k
2 was taken here as a surrogate of BP
ND to estimate the size of the specific signal and compare to previously reported results with other ligands. The expected BP
ND close to 1.0 is in agreement with the average value for k
3/k
4 of 1.1 reported in Table
1. Lower BP
ND values were reported in humans [
12], however the binding potentials were derived relative to the centrum semiovale and would likely be higher if calculated using the true V
ND [
14]. Finally, the displacement study with LEV evidenced its slower brain penetrance compared to UCB-H, with a displacement of the total uptake of ~ 40% measured 2.5 h after 30 mg/kg i.v. LEV administration, which would correspond to an occupancy of ~ 75–80% of SV2A in agreement with values in the literature [
14,
24,
38]. We observed some displacement in the white matter both with [
19F]UCB-H and LEV. This is likely due to high spill-in from cortical regions rather than true specific signal in the white matter, as indicated by the higher than expected uptake and V
T [
12]. Here, we coregistered T
2-weighted MR-PET images to the Ballanger template [
31] and used the inverse transformation to extract TACs from the PET images. Cynomolgus brains have relatively small white matter regions, with some inter-animal variability. Therefore, this approach appeared less precise to segment white matter regions, and prone to partial volume effect; as such, the current data set did not allow to use or evaluate the utility of the white matter as a reference region.
Altogether, our data confirm similar brain penetration and non-displaceable uptake for [
18F]UCB-H as described for [
11C]UCB-J [
38], [
18F]MNI1126 [
16] (aka [
18F]Synvest-1 [
14]). In agreement with previous reports in rhesus NHP [
23,
38], V
T-estimates and consequently BP
ND of [
18F]UCB-H in cynomolgus NHP are considerably lower (~ 50% lower for V
T, and 3 to fourfold lower for BP
ND) compared to V
T and BP
ND measures with [
11C]UCB-J [
35,
38] and other [
18F]-labelled SV2A radioligands, [
18F]synvest1 [
14,
16] and [
18F]synvest2 [
17] (alias [
18F]SDM-2 [
18]). As a consequence, [
18F]UCB-H will be less sensitive to detect small changes compared to the latest SV2A ligands. Nevertheless, a clinical study using [
18F]UCB-H in Alzheimer’s Disease (AD) patients demonstrated a correlation between lower synaptic density and poorer awareness of memory functioning in Aβ-positive individuals [
25], confirming earlier data in AD patients using [
11C]UCB-J [
41]. Additionally, this [
18F]UCB-H clinical study [
25] suggested a widespread synaptic decline in AD patients across the neocortex and in some subcortical nuclei, including the basal forebrain, which was confirmed in a comparable but larger cohort of AD patients using [
11C]UCB-J [
42]. These and other data [
43] have demonstrated the potential of the [
18F]UCB-H radioligand.
Publisher's Note
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.