Characterization of in vivo brain muscarinic acetylcholine receptor subtype selectivity by competition studies...

We have studied the in vivo rat brain muscarinic acetylcholine receptor (mAChR) m2 subtype selectivities of three quinuclidine derivatives: (R)-3-quinuclidinyl benzilate (QNB), E-(+,+)-1-azabicyclo[2.2.2]oct-3-yl alpha-hydroxy-alpha-(1-iodo-1-propen-3-yl)-alpha-phenylacetate (E-(+,+)-IQNP), and E-(+,-)-1-azabicyclo[2.2.2]oct-3-yl alpha-hydroxy-alpha-(1-iodo-1-propen-3-yl)-alpha-phenylacetate (E-(+,-)-IQNP), and two tricyclic ring compounds: 5-[[4-[4-(diisobutylamino)butyl]-1-phenyl]-10,11-dihydro-5H-dibenz o [b,e][1,4]diazepin-11-one [sequence: see text] (DIBD) and 11-[[4-[4-(diisobutylamino)butyl-1-phenyl]acetyl]-5,11-dihydro-6H- pyrido [2,3-b][1,4]benzodiazepin-6-one [sequence: see text] (PBID), by correlating the regional inhibition of (R,S)-[125I]IQNB with the regional composition of the m1-m4 subtypes. Subtle effects are demonstrated after reduction of the between-animal variability by normalization to corpus striatum. Substantial in vivo m2-selectivity is exhibited by QNB and DIBD, modest in vivo m2-selectivity is exhibited by E-(+,+)-IQNP, and little or no in vivo m2-selectivity is exhibited by PBID and E-(+,-)-IQNP. Surprisingly, the in vivo m2-selectivity is not correlated with the in vitro m2-selectivity. For example, QNB, which appears to be the most strongly in vivo m2-selective compound, exhibits negligible in vitro m2-selectivity. These examples indicate that a strategy which includes only preliminary in vitro screening may very well preclude the discovery of a novel compound which would prove useful in vivo.