Estimation of occupancy in the absence of baseline data and reference regions

535 Objectives: In terms of the standard PET neuroreceptor model, receptor occupancy (RO) by an exogenous drug is measured by the fractional decrease in VS, the volume of distribution of the specifically bound radioligand. This can be derived from VT, the total volume of distribution, if an estimate of VN (free+non-specifically bound radioligand) is available. It is frequently the case that an ideal reference region (VS=0, VT=VN) does not exist for the target. Lassen et al. [1] overcame this problem by assuming VN and RO constant across regions. By comparing regional estimates of VT between a baseline scan in the absence of cold drug and after administration of cold drug, RO can be obtained graphically. This approach has previously been used to estimate the VN of an ideal reference region [1]. We now present a more general equation for graphical analysis of occupancy studies which does not assume the availability of a baseline scan. Methods: We show that; (VT1–VT2).C2/(C2-C1) = RO2.VT1–RO2.VN (eqn 1) for non-baseline scans where the number denotes the scan and C denotes the corresponding concentration/dose of the cold drug. The derivation assumes that the Hill coefficient is 1 and full occupancy is attainable in the limit. This gives an easily interpretable plot of the LHS of eqn 1 against VT1, with RO2 given by the slope, and VN by the x intercept. The special case, C1=0, in which scan 1 is a true baseline scan, gives: (VT1–VT2) = RO2.VT1–RO2.VN (eqn 2) corresponding to re-arrangement of equations in Lassen et al. [1]. Plots of (VT1–VT2) against VT1, are again easily interpretable with the slope a direct measure of occupancy. Results: Data are taken from an [11C]WAY100635 study of 5HT1a receptor occupancy. Scans were performed at baseline and at cold doses of a blocking agent at 1.5, 10 and 150ug/Kg. Analysis using the baseline scan with eqn 2 gave corresponding occupancies of 28.4%, 78.1% and 97.0%. Discarding the baseline scan and analyzing according to eqn 1 with the lowest dose as C1 gave occupancies of 81.4% and 96.8% at the higher doses. Estimates of the half saturation dose can then be derived, giving 31.5% occupancy at the lowest dose. Conclusions: All these latter estimates were thus obtained not only without a reference region but also without baseline data. [1] Lassen et al. JCBFM 15:152-65, 1995.