Conformational analysis of D1 dopamine receptor agonists: pharmacophore assessment and receptor mapping.

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.

Dihydrexidine, a novel full efficacy D1 dopamine receptor agonist.

The present work provides a detailed pharmacological characterization of dihydrexidine (DHX) (trans-10,11-dihydroxy- 5,6,6a,7,8,12b-hexahydrobenzo[a]phenanthridine), the first high-potency, full efficacy, bioavailable D1 dopamine receptor agonist. DHX represents a new conformationally rigid structural class of dopamine receptor ligands. It competes stereoselectively and potently for D1 binding sites in rat striatal membranes labeled with [3H]SCH23390 with an IC50 of about 10 nM compared to about 30 nM for the prototypical D1 agonist SKF38393. Like other dopamine agonists, DHX has a shallow competition curve (nH = ca. 0.7) that can be fitted by a two-site model consisting of high-affinity (63%; KD = 3 nM) and low-affinity (37%; KD = 75 nM) sites. DHX was screened for activity against 40 other binding sites, and was inactive (IC50 greater than 10 microM) against all except D2 dopamine receptors (IC50 = 130 nM) and alpha 2 adrenoreceptors (IC50 = ca. 230 nM). Functionally, DHX is a full efficacy dopamine D1 agonist. In homogenates of rat striatum, DHX or dopamine doubles the rate of cyclic AMP synthesis, whereas SKF38393 only causes a maximal increase of about 50%. These effects of DHX are blocked by the selective D1 antagonist SCH23390, but are not affected by D2, 5-hydroxytryptamine2, muscarinic, or alpha or beta adrenergic antagonists. Because DHX is known to cause D2-like behavioral effects at high doses, the nature of its D2 activity was characterized using prolactin release as an end-point. DHX and the prototypical D2 agonist quinpirole both caused a significant inhibition of the prolactin release induced by 5-hydroxytryptophan. These effects of DHX are not due to "indirect" alterations at the presynaptic terminal, because DHX is essentially inactive at inhibiting the dopamine uptake system, and does not cause the release of dopamine. These data demonstrate the utility of DHX for probing the biochemistry and function of D1 dopamine receptors.

trans-10,11-dihydroxy-5,6,6a,7,8,12b-hexahydrobenzo[a]phenanthridine: a highly potent selective dopamine D1 full agonist.

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.