PET study design
In this single centre, open-label study
(EudraCT 2017-000670-11), male
participants (25–60 years old, body weight 55–100 kg, BMI 18.5–30
kg/m2) received a single oral dose of MR309 across 4
dosing cohorts (200–800 mg). All participants were considered healthy
and subject to similar inclusion and exclusion criteria as described for
the PK study, with the additional exclusion of any contraindications to
radial artery cannulation and magnetic resonance imaging (MRI).
Prior to study treatment, participants underwent a single brain MRI scan
to confirm suitability to take part in the study and obtain anatomical
information to support PET scan analysis.
Each subject underwent three PET scans with
[11C]SA-4503 at baseline, 2h and 8h after a single
oral dose of MR309. Participants fasted for ≥8 hours prior to receiving
MR309. Arterial blood samples were collected from a radial artery
cannula throughout each PET scan to estimate the time-course of total
blood and plasma radioactivity and the concentration of unmetabolized
[11C]SA-4503.
PET data quantification
The primary objectives were to evaluate brain σ1R occupancy following a
single oral dose of MR309 and to describe the relationship between σ1R
occupancy and MR309 plasma concentration at the time of the PET scan.
The primary quantification parameter was the total volume of
distribution (VT) for [11C]SA-4503
at each PET scan. VT represents the partition
coefficient for SA-4503 between brain and plasma at equilibrium, and is
proportional to the total binding of SA-4503 (displaceable and
non-displaceable) in the relevant brain region (Innis et al., 2007). σ1R
occupancy was calculated by comparing VT at baseline and
post-dose [11C]SA-4503 for each subject, and σ1R
occupancy for each scan was related to the plasma concentration of
MR309.
Brain MRI images underwent brain extraction, grey matter segmentation
and co-registration to a standard reference space (MNI152 template brain
image and associated CIC atlas), (Grabner, Janke, Budge, Smith,
Pruessner & Collins, 2006; Tziortzi et al., 2011) and were nonlinearly
warped to each participant’s MRI image to enable automated definition of
regions of interest (ROIs). ROIs included the frontal, temporal and
occipital lobes, caudate nucleus, putamen, thalamus, hippocampus and
cerebellum. Representative orthogonal cross sections of co-registered
PET and MRI images are shown in Figure 1.
Dynamic PET images were corrected for motion using a frame-to-frame
registration process with a normalized mutual information cost function,
and co-registered to each participant’s MRI scan. ROIs defined on the
MRI images were applied to the dynamic PET data to derive regional
time-activity curves (TACs), with activity concentrations expressed as
standardised uptake values (SUV), calculated by normalising the measured
radioactivity concentration to the injected radioactivity and the
participant’s bodyweight.
Regional TACs were analysed using the alternative multilinear analysis 1
(MA1) kinetic model with a metabolite-corrected arterial plasma input
function, to generate regional VT values. σ1R occupancy
for each post-dose PET scan was calculated from the regional
VT estimates using an occupancy plot analysis
(Cunningham, Rabiner, Slifstein, Laruelle & Gunn, 2010). The occupancy
plot method makes the assumption that the volume of distribution of the
non-displaceable component (VND) is uniform across an
individual’s brain, and that the fractional occupancy of the target in
each post-dose scan is uniform across all ROI. VND was
constrained to be the same for each post-dose PET scan in each
individual.
The relationship between σ1R occupancy and drug exposure was then
explored using data-driven model fitting. To characterize the
relationship between MR309 plasma concentration and σ1R occupancy, the
σ1R occupancy estimates were plotted against MR309 plasma concentration
measured at the start of each post-dose PET scan. Four models were
fitted and compared: the optimal model to describe the relationship
between MR309 plasma concentration and σ1R occupancy was selected based
on goodness of fit and AIC values (Table 1). For each model,
EC50 , Emax, Hill slope and Akaike
information criterion (AIC) values (measure of goodness of fit) were
obtained. (Bozdogan, 1987)
PK of single-dose MR309 and its metabolites were assessed as a secondary
objective. Plasma concentrations of MR309 and its metabolites were
assessed as described in the PK study with the endpoints of Cmax, Tmax,
AUCt (area under the plasma concentration time curve from the time of
dosing to the last measurable concentration), AUCINF (area under the
plasma concentration time curve from the time of dosing to infinity),
LambdaZ (estimate of terminal elimination rate constant) and t1/2Z
(apparent terminal phase half-life). Safety and tolerability were
assessed pre-dose and at regular intervals up to Day 4 post-dose, as
described in the PK study.