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1. A modelling approach allowing different nonspecific uptake in the reference region and regions of interest – Opening up the possibility to use white matter as a reference region in PET occupancy studies.M Kågedal(1,2), MO Karlsson(2) AC Hooker(2)(1) AstraZeneca R&D Södertälje, Sweden (2) Department of Pharmaceutical Biosciences, Uppsala University, Uppsala, Sweden Objectives Analyses of receptor occupancy studies are often performed using the concentration in a reference region in the brain as input function (independent variable). These analyses assume identical non-specific concentration (CND) in the reference region and in the region of interest (ROI) at equilibrium. This assumption may not always be valid between gray matter regions. Additionally, when no grey matter region void of receptors exists, white matter (WM) is sometimes considered as a reference region, but CND of WM can not á priori be assumed to be the same as for grey matter. The objectives of the present work were: A. To investigate if an apparent difference in occupancy between regions in observed data could be explained by different non-specific uptake. B. To investigate the power to differentiate between difference in CND and ID50 as the reason for regional differences by means of a simulation experiment C. To investigate if WM can be used as a reference region by relaxing the assumption of identical CND at equilibrium in the ROI and the reference region. Methods Nonlinear mixed effects modelling of data from PET scans with the 5-HT1B radioligand [11C] AZ10419369 was applied. The simplified reference tissue model (SRTM) with concentrations in a reference region (CREF) as input function was applied as described previously [2]. The study included six healthy subjects with PET-scans at baseline and after different oral doses of a 5-HT1B antagonist. The analysis was based on data from a study published previously [1]. A. Initial analyses suggested that he dose corresponding to 50% occupancy (ID50) was lower in Caudate compared to other regions (Fig1, Fig 2). In order to elucidate whether this difference was a result of an actual difference in ID50 or in the non-specific concentration of the radioligand (CND), a simultaneous analysis, including all regions was performed. Data was analysed based on the following models with CER as reference region: Base: The ID50 and CND at equilibrium is assumed to be the same in all regions. Alt 1: ID50 in caudate (CAU) differ from other regions by estimation of ID50 in CAU relative to ID50 in other regions (RID50). Alt 2: The CND in CAU at equilibrium differ from the reference region by the use of CREF *RND as input function where RND (estimated as a parameter) is the CND in CAU relative to CREF at equilibrium. B. Stochastic simulation and estimation (SSE) was performed in order to investigate the power to differentiate between these models when 1: the ID50 in caudate is 61% of the ID50 in other regions (RID50=0.61) and 2: the CND in Caudate is 90% of that in the reference region (RND=0.90). C. In order to investigate if white matter could be used as a reference region by accounting for any differences in nonspecific uptake, the Base and Alt 2 models were also applied to data using WM as input function. Results Figure 2. Lower ID50 in CAU based on estimation including each ROI separately (95% CI) . Table A. Lower ID50 in Caudate appears to agree better with data as compared to different CND based on objective function value (OFV). Table B. The simulation experiment show that the ability to differentiate between the models is good, and that the ID50 will be biased if differences in CND between ROI and reference region is not accounted for. * OFVdrop > 3.84 vs alternative models ** All other parameters identical to Alt1, except RID50 which was fixed to 1. Table C. WM could be used as a reference region by accounting for differences in nonspecific uptake (CND). When not doing so, the estimated ID50 was slightly higher as compared to the Base model with cerebellum as reference region. Figure 1. Example showing displacement of radioligand with increasing doses of AZD3783 in caudate and occipital cortex but not in cerebellum.Relative reduction in specific conc appears larger in Cau compared to Occ . • Discussion and Conclusions • Difference in ID50 (occupancy) rather than in non-specific uptake between regions was concluded. This evaluation was possible by a simultaneous analysis of several regions of interest and all PET-measurements. • It is shown that a model based approach can have a high power in discriminating between regional differences in terms of ID50 or CND. • Differences in non-specific binding between the reference region and the regions of interest can bias the occupancy estimate if not accounted for. • Estimation of occupancy by the use of white matter is possible by accounting for differences in non-specific uptake. Mean References [1] Dose-dependent binding of AZD3783 to brain 5-HT1B receptors in non-human primates and human subjects: a positron emission tomography study with [11C]AZ10419369. Varnas K. et al. Psychopharmacology. 213(2-3):533-45, 2011 Feb. [2] Estimate the time varying brain receptor occupancy in PET imaging experiments using non-linear mixed effects modelling approach. Stefano Zamuner, Roberto Gomeni, Alan Bye. Nuclear Medicine and Biology 29 (2002) 115-123. Occ specific Cau specific