Measuring drug occupancy in the absence of a reference region: the Lassen plot re-visited

Quantitative estimation of neuroreceptor occupancy by exogenous drugs using positron emission tomography is based on the reduction in the total volume of distribution (V_T) of site-specific radioligands after drug administration. An estimate of the distribution volume of free and nonspecifically bound radioligand (V_ND) is also required to distinguish specific from total binding. However, a true reference region, devoid of specific binding, is often not available. We present a transformation of a graphical method, originally introduced by Lassen, using regional estimates of V_T alone to determine occupancy, together with an extension that does not require baseline data.