sch-23390 and dihydrexidine

We report the synthesis of trans-2,3-dihydroxy-6a,7,8,12b-tetrahydro-6H-chromeno[3,4-c]isoquinoline hydrochloride 6 and the resolution of its enantiomers. This new compound is an oxygen bioisostere of the potent dopamine D1-selective full agonist dihydrexidine. The initial synthetic approach involved, as a key step, a Suzuki coupling between a chromene triflate and a boronate ester, followed by isoquinoline formation and reduction of the resulting isoquinoline. Subsequently, a more efficient route was developed that involved conjugate addition of an aryl Grignard reagent to a 2-nitrochromene. The title compound possessed high affinity (Ki=20-30 nM) for porcine D1-like receptors in native striatal tissue and full intrinsic activity at cloned human dopamine D1 receptors but had much lower affinity at dopamine D2-like receptors (Ki=3000 nM). The binding and functional properties of this compound illustrate again the utility of constructing dopamine D1 agonist ligands around the beta-phenyldopamine pharmacophore template.

Topics: Animals; Benzopyrans; Binding, Competitive; Cell Line; Corpus Striatum; Crystallography, X-Ray; Cyclic AMP; Humans; In Vitro Techniques; Isoquinolines; Molecular Structure; Phenanthridines; Psychotropic Drugs; Radioligand Assay; Rats; Receptors, Dopamine D1; Stereoisomerism; Structure-Activity Relationship; Swine

In an effort to define further the active geometry of the beta-phenyldopamine pharmacophore of certain dopamine D1 agonists, the title compounds have been synthesized as conformationally restricted homologues of the potent benzophenanthridine dopamine D1 agonist dihydrexidine 4a. The dihydroxy secondary amine 5b was evaluated as a potential agonist, whereas the N-methyl compounds 5a and 5c were hypothesized to be antagonists. Surprisingly, none of the three compounds had high affinity for dopamine D1 or D2 receptors. A comparison of the low-energy conformations of these molecules shows that the pendant phenyl ring of 5b is twisted about 28 degrees relative to that of the corresponding ring of 4a. Further, the additional methylene used to expand the C ring of 5b projects toward the alpha face of the molecule, perhaps suggesting that steric protrusion in this region of the molecule is not tolerated. Finally, the phenethylamine fragment incorporated into these molecules deviates about 30 degrees from the antiperiplanar conformation postulated to be necessary for agonist activity. On the other hand, the potential antagonist molecules 5a and 5c were compared with the dopamine D1 antagonist SCH 39166 2. The conformations of the former two structures differ quite dramatically from that of 2. The most notable differences lie in the relative orientations of the pendant phenyl rings in the two series, as well as the fact that the ethylamine fragment in 2 approximates a gauche conformation, while the comparable orientation in 5a and 5c more nearly approaches an antiperiplanar conformation. These findings will be used to refine further the model of the dopamine D1 agonist receptor that we have previously developed.

Topics: Animals; Computer Simulation; Corpus Striatum; Dopamine Agonists; Indicators and Reagents; Models, Molecular; Molecular Conformation; Molecular Structure; Phenanthridines; Rats; Receptors, Dopamine D1; Software; Spiperone; Structure-Activity Relationship

Compute-aided conformational analysis was used to characterize the agonist pharmacophore for D1 dopamine receptor recognition and activation. Dihydrexidine (DHX), a high-affinity full agonist with limited conformational flexibility, served as a structural template that aided in determining a molecular geometry that would be common for other more flexible, biologically active agonists. The intrinsic activity of the drugs at D1 receptors was assessed by their ability to stimulate adenylate cyclase activity in rat striatal homogenates (the accepted measure of D1 receptor activation). In addition, affinity data on 12 agonists including six purported full agonists (dopamine, dihydrexidine, SKF89626, SKF82958, A70108, and A77636), as well as six less efficacious structural analogs, were obtained from D1 dopamine radioreceptor-binding assays. The active analog approach to pharmacophore building was applied as implemented in the SYBYL software package. Conformational analysis and molecular mechanics calculations were used to determine the lowest energy conformation of the active analogs (i.e., full agonists), as well as the conformations of each compound that displayed a common pharmacophoric geometry. It is hypothesized that DHX and other full agonists may share a D1 pharmacophore made up of two hydroxy groups, the nitrogen atom (ca. 7 A from the oxygen of m-hydroxyl) and the accessory ring system characterized by the angle between its plane and that of the catechol ring (except for dopamine and A77636). For all full agonists (DHX, SKF89626, SKF82958, A70108, A77636, and dopamine), the energy difference between the lowest energy conformer and those that displayed a common pharmacophore geometry was relatively small (< 5 kcal/mol). The pharmacophoric conformations of the full agonists were also used to infer the shape of the receptor binding site. Based on the union of the van der Waals density maps of the active analogs, the excluded receptor volume was calculated. Various inactive analogs (partial agonists with D1 K0.5 > 300 nM) subsequently were used to define the receptor essential volume (i.e., sterically intolerable receptor regions). These volumes, together with the pharmacophore results, were integrated into a three-dimensional model estimating the D1 receptor active site topography.

Topics: Adenylyl Cyclases; Animals; Brain; Computer Simulation; Dopamine; Dopamine Agonists; Ligands; Molecular Conformation; Molecular Structure; Phenanthridines; Rats; Receptors, Dopamine D1; Software; Structure-Activity Relationship

Racemic trans-10,11-dihydroxy-5,6,6a,7,8,12b- hexahydrobenzo[a]phenanthridine (2, dihydrexidine) was shown previously to be the first bioavailable full efficacy agonist at the D1 dopamine receptor. In addition to its full D1 agonist properties, 2 also is a good ligand for D2-like dopamine receptors. The profound anti-Parkinsonian actions of this compound make determination of its enantioselectivity at both D1 and D2 receptors of particular importance. To accomplish this, the enantiomers were resolved by preparation of diastereomeric (R)-O-methylmandelic acid amides of racemic trans-10,11-dimethoxy-5,6,6a,7,8,12b- hexahydrobenzo[a]phenanthridine 4 that were then separated by centrifugal chromatography. An X-ray analysis of the (-)-N-(R)-O-methylmandel diastereoamide revealed the absolute configuration to be 6aS,12bR. Removal of the chiral auxiliary and O,O-deprotection afforded enantiomeric amines that were then tested for biological activity. In striatal membranes, the (6aR,12bS)-(+)-enantiomer 2 had about twice the affinity of the racemate and 25-fold greater affinity than the (-)-enantiomer at the D1 receptor labeled by [3H]SCH23390 (K0.5s of 5.6, 11.6, and 149 nM, respectively). Similarly, the (+)-enantiomer 2 had about twice the affinity of the racemate for human D1 receptors expressed in transfected Ltk- cells. Functionally, the (+)-enantiomer of 2 was a full agonist, with an EC50 of 51 nM in activating striatal dopamine-sensitive adenylate cyclase versus 2.15 microM for the (-)-enantiomer. With respect to D2-like receptors, (+)-2 had a K0.5 of 87.7 nM in competing with [3H]spiperone at D2 binding sites in rat striatal membranes versus about 1 microM for the (-)-enantiomer. Together, these data demonstrate that both the D1 and D2 activities of dihydrexidine reside principally in the (6aR,12bS)-(+)-enantiomer. The results are discussed in the context of structure-activity relationships and conceptual models of the D1 receptor.

Topics: Animals; Benzazepines; Binding Sites; Corpus Striatum; Dopamine Agents; Humans; Male; Mice; Phenanthridines; Radioligand Assay; Rats; Rats, Sprague-Dawley; Receptors, Dopamine D1; Stereoisomerism; Transfection

trans-10,11-Dihydroxy-5,6,6a,7,8,12b-hexahydrobenzo[a]phenan thridine (4a, dihydrexidine) has been found to be a highly potent and selective agonist of the dopamine D1 receptor in rat brain. Dihydrexidine had an EC50 of approximately 70 nM in activating dopamine-sensitive rat striatal adenylate cyclase and a maximal stimulation equal to or slightly greater than that produced by dopamine. Dihydrexidine had an IC50 of 12 nM in competing for [3H]SCH23390 (1a) binding sites in rat striatal homogenate, and of 120 nM versus [3H]spiperone. These data demonstrate that dihydroxidine has about ten-fold selectivity for D1/D2 receptors. More importantly, however, is the fact that dihydrexidine is a full agonist. Previously available agents, such as SKF38393 (1b), while being somewhat more selective for the D1 receptor, are only partial agonists. The isomeric cis-dihydroxybenzo[a]-phenanthridine neither stimulated cAMP synthesis nor inhibited the cAMP synthesis induced by dopamine. The cis isomer also lacked appreciable affinity for [3H]-1a binding sites. N-Methylation of the title compound decreased affinity for D1 sites about 7-8-fold and markedly decreased ability to stimulate adenylate cyclase. Addition of an N-n-propyl group reduced affinity for D1 sites by about 50-fold and essentially abolished the ability to stimulate adenylate cyclase. However, this latter derivative had twice the affinity of the D2-selective agonist quinpirole for the D2 receptor. The results are discussed in the context of a conceptual model for the agonist state of the D1 receptor.

Topics: Adenylyl Cyclases; Animals; Computer Simulation; Dopamine Agents; In Vitro Techniques; Molecular Conformation; Phenanthridines; Rats; Receptors, Dopamine; Receptors, Dopamine D1; Receptors, Dopamine D2; Structure-Activity Relationship