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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.

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Four cyclic analogues of the psychoactive phenethylamine derivative 3,4-(methylenedioxy)amphetamine were studied. These congeners, 5,6- and 4,5-(methylenedioxy)-2-aminoindan (3a and 4a, respectively), and 6,7- and 5,6-(methylenedioxy)-2-aminotetralin (3b and 4b, respectively) were tested for stimulus generalization in the two-lever drug-discrimination paradigm. Two groups of rats were trained to discriminate either LSD tartrate (0.08 mg/kg) from saline, or (+/-)-MDMA.HCl (1.75 mg/kg) from saline. In addition, a 2-aminoindan (5a) and 2-aminotetralin (5b) congener of the hallucinogenic amphetamine 1-(2,5-dimethoxy-4- methylphenyl)-2-aminopropane (DOM) were also evaluated. None of the methylenedioxy compounds substituted in LSD-trained rats, while both 3a and 3b fully substituted in MDMA-trained rats. Compounds 4a and 4b did not substitute in MDMA-trained rats. Compounds 5a and 5b did not substitute in MDMA-trained rats, although 5a substituted in LSD-trained rats, but with relatively low potency compared to its open-chain counterpart. In view of the now well-established serotonin neurotoxicity of 3,4-(methylenedioxy)amphetamine and its N-methyl homologue 1, 3a and 3b were evaluated and compared to 1 for similar toxic effects following a single acute dose of 40 mg/kg sc. Sacrifice at 1 week showed that neither 3a nor 3b depressed rat cortical or hippocampal 5-HT or 5-HIAA levels nor were the number of binding sites (Bmax) depressed for [3H]paroxetine. By contrast, and in agreement with other reports, 1 significantly depressed all three indices of neurotoxicity. These results indicate that 3a and 3b have acute behavioral pharmacology similar to 1 but that they lack similar serotonin neurotoxicity.

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.