Receptor occupancy assays

A recent report on a NR2B selective NMDA receptor antagonist (9) supports the findings of Kalvass and Maurer [56], Rapid equilibration between plasma and CNS coupled with the lack of Pgp substrate activity led the authors to assume that plasma-free and brain-free drug concentrations were equivalent. An ex vivo receptor binding assay showed 50% occupancy at a total plasma concentration of 230 nM. Given a rat-free fraction of 15.3%, the authors concluded that 50% brain occupancy occurred at 35 nM unbound brain concentration, which was in reasonable agreement with the measured Ki of 3.4 nM versus the human receptor. 

FIGURE 14.14 The effects of various insurmountable antagonists in a hemi-equilibrium-sensitive assay (FLIPR) where slow offset from the receptor leads to a depression of the maximal response. In general, the slower the offset, the more depression is seen for a given degree of receptor occupancy. 

The value of biomarkers to establish the dose- and concentration response curves at the earliest stage of drug development cannot be overestimated. However, it should be recognised that the utility of any biomarker depends at least in part on the expertise of the experimentalists. Long before the study takes place a decision will need to be made about where the study will be placed and who precisely will perform the measurements. Whether assaying the concentrations of a hormone, performing respiratory function tests or measuring receptor occupancy... 

---