Stereoselectivity of (R)‐ and (S)‐hexahydro‐difenidol binding to neuroblastoma M₁, cardiac M₂, pancreatic M₃, and striatum M₄ muscarinic receptors

Abstract

ABSTRACT(R)-Hexahydro-difenidol has a higher affinity for M 1 receptors in NB-OK 1 cells, pancreas M 3 and striatum M 4 receptors (pKi 7.9 to 8.3) than for cardiac M 2 receptors (pKi 7 .0). (8)-Hexahydro-difenidol, by contrast, is nonselective (pKi 5.8 to 6.1). Our goal in the present study was to evaluate the importance ofthe hydrophobic phenyl, and cyclohexyl rings of hexahydro-difenidol for the stereoselectivity and reeeptor selectivity of hexahydro-difenidol binding to the four muscarinic receptors. Our results indieated that replacement of the phenyl ring of hexahydro-difenidol by a cyclohexyl group <~ dicyclidol) and ofthe cyclohexyl ring by a phenyl moiety <~ difenidol) indueed a !arge (4-to 80-fold) decrease in binding affinity for all musearlnie receptors. Difenidol had a signifieant preference for M 1 , M 3 , and M 4 over M 2 receptors; dicyclidol, by eontrast, had a greater affinity for M 1 and M 4 than for M 2 and M 3 receptors. The binding free energy deerease due to replacement ofthe phenyl and the cyelohexyl groups of(R)-hexahydro-difenidol by, respectively, a eyclohexyl and a phenyl moiety was almostadditive in the ease of M 4 (striatum) binding sites. In the ease ofthe cardiac M 2 , pancreatic M 3 , or NB-OK 1 M 1 receptors the respective binding free energies were not eompletely additive. These results suggest that the four (R)-hexahydro-difenidol ''binding moieties" (phenyl, cyclohexyl, hydroxy, and protonated amino group) cannot simultaneously form optimal interaetions with the M 1 , M 2 , and M 3 muscarinic receptors. When eaeh of the hydrophobic groups is modified, the position of the whole molecule, relative to the four subsites, was changed to allow an optimal overall interaction with the musearlnie receptor.

Stereoselective recognition of the enantiomers of phenglutarimide and of six related compounds by four muscarinic receptor subtypes

“…In this study, and in contrast with the procyclidine and hexahydro-diphenidol distomers (Waelbroeck et al, 1990a;, we obtained evidence that most distomers did not recognize…”

“…As shown in Tables I and 3, this modification resulted ina 6 to 60 fold affinity decrease. Further experiments are needed to determine whether the affinity decrease is due to steric hindrance or to the loss of a strong hydrogen bond between one of the two possible enol-like hydroxy groups and the receptor (Waelbroeck et al, 1990a;. Structure-activity relationships: distomers As previously stated in the Results section, we are confident that we measured the intrinsic affinity of most distomers, rather than the effect of a (very small) contamination by the eutomer.…”

Characterization of postjunctional muscarinic receptors mediating contraction in rat anococcygeus muscle

“…Comparison of the enantioselectivity ratio for the two enantiomers of HHD in RAM [ Table 2; (R)/(S) = 288] with data obtained in radioligand binding studies at native M 1 to M 4 receptors (M 1 = 158, M 2 = 16, M 3 = 158, M 4 = 100; Waelbroeck et al 1991b) clearly excludes the M 2 subtype for mediating contraction in RAM (see below). In addition, the stereoselectivity ratio for HHD in RAM [(R)/(S) = 288] is in very good agreement with that obtained in functional experiments at M 3 receptors in guinea-pig ileum longitudinal smooth muscle (= 191;Feifel et al 1990) and submucosal arterioles (= 229-275; Bungardt et al 1992) as well as in rat duodenum longitudinal smooth muscle (= 186;Pfaff et al 1995a).…”

“…In addition, muscarinic receptors may be differentiated on the basis of their stereoselective recognition of chiral antagonists such as hexahydrodifenidol (Feifel et al 1990;Dörje et al 1991;Waelbroeck et al 1992Waelbroeck et al , 1996Pfaff et al 1995b). In the present study, all of the above mentioned key antagonists, including the (R)-and (S)-enantiomers of hexahydro-difenidol [(R)-and (S)-HHD] (Waelbroeck et al 1991b), were used to identify the muscarinic receptor subtype mediating contraction in RAM.…”

SAR Studies on the Potent and Selective Muscarinic Antagonist 2‐Ethylthio‐2,2‐diphenylacetic Acid N,N‐Diethylaminoethyl Ester

“…The reaction pathways used to synthesize compounds 1-26 are reported in Scheme 1. n-Butyl (la), n-pentyl(5-10), and n-hexyl (12)(13)(14)(15)(16) analogs of the lead compounds were obtained from the corresponding thioacids (3%35), synthesized according to ref. [l].…”