In vivo receptor occupancy of the angiotensin II receptor by nonpeptide antagonists: relationship to in vitro affinities and in vivo pharmacologic potency.

The affinities of 13 angiotensin II antagonists for the AT1 subtype determined in vitro with tissue homogenates were shown not to correlate well with in vivo pharmacologic potency. The addition of human serum albumin to the in vitro assay to mimic in vivo plasma protein interactions reduced the measured affinity by reducing the effective free concentrations of antagonists, but the resulting affinities were not predictive of the in vivo effects. Using an in vivo radioligand competition assay, in which receptor occupancy is demonstrated via competitive blockade of the in vivo binding of [125I][Sar1,Ile8]angiotensin II to AT1 receptors in rat kidney cortex, we demonstrated that the in vivo pharmacologic potencies reflect receptor occupancy. By comparing the effects of rat plasma and bovine serum albumin on the in vitro affinity of two antagonists, we suggest that the use of plasma would alter free plasma concentrations in a manner more consistent with in vivo measures of potencies.