Dopamine receptor ligands. Part VII [1]: novel 3-substituted 5-phenyl-1, 2, 3, 4, 5, 6-hexahydro-azepino-[4, 5-b]indoles as ligands for the dopamine receptors

Advances in synthesis of novel annulated azecines and their unique pharmacological properties

Annulated azecines, mostly partially saturated benzo[d]azecine and dibenzo[c,g]azecine fusion isomers, constitute a unique class of alkaloids and nature-inspired azaheterocyclic compounds with interesting reactivity, physicochemical and biological properties. Due to difficulties associated with the synthesis of the benzazecine (or bioisosteric) scaffold they are not the focus of organic and medicinal chemists' consideration, whereas it is worth noting the range of their pharmacological activities and their potential application in medicinal chemistry. Herein, we reviewed the synthetic methodologies of arene-fused azecine derivatives known up to date and reported about the progress in disclosing their potential in drug discovery. Indeed, their conformational restriction or liberation drives their selectivity towards diverse biological targets, making them versatile scaffolds for developing drugs, including antipsychotic and anticancer drugs, but also small molecules with potential for anti-neurodegenerative treatments, as the recent literature shows.

Novel fused pyrrole heterocyclic ring systems as structure analogs of LE 300: Synthesis and pharmacological evaluation as serotonin 5-HT(2A), dopamine and histamine H(1) receptor ligands

LE 300 represents a structurally novel type of antagonists acting preferentially at the dopamine D(1)/D(5 )receptors and the serotonin 5-HT(2A )receptor. This compound consists of a ten-membered central azecine ring fused to an indole ring on one side and a benzene moiety on the other side. To estimate the importance of the indole and / or phenyl moieties in this highly active benz-indolo-azecine, both rings were removed and replaced with a 1H-pyrrole counterpart. Accordingly, some new analogs of LE 300 namely, pyrrolo[2,3-g]indolizine, pyrrolo[3,2-a]quinolizine rings and their corresponding dimethylpyrrolo[2,3-d]azonine, and dimethylpyrrolo[2,3-d]azecine were synthesized to be evaluated for their activity at the 5-HT(2A) and dopamine D(1), D(2L), D(4), D(5) receptors in relation to LE 300. In addition, their activity at the H(1)-histamine receptors was also determined. The results suggested that the rigid pyrrolo[2,3-g]indolizine 7 and pyrrolo[3,2-a]quinolizine 8 analogs lacked biological activity in the adopted three bioassays. However, their corresponding flexible pyrrolo[2,3-d]azonine 11 and pyrrolo[2,3-d]azecine 12 derivatives revealed weak partial agonistic activity and weak antagonistic potency at the serotonin 5-HT(2A )and histamine H(1 )receptors, respectively. Meanwhile, they showed no affinity to any of the four utilized dopamine receptors. Variation in ring size did not contribute to a significant influence on the three tested bioactivities. Removal of the hydrophobic moiety (phenyl ring) and replacement of the indole moiety with a 1H-pyrrole counterpart led to a dramatic alteration in the profile of activity of such azecine-type compounds.

Potential utility of histamine H3 receptor antagonist pharmacophore in antipsychotics

Histamine H3 receptor (H3R) antagonists have some antipsychotic properties although the clear molecular mechanism is still unknown. As actually the most effective and less side effective antipsychotics are drugs with multiple targets we have designed typical and atypical neuroleptics with an additional histamine H3 pharmacophore. The 4-(3-piperidinopropoxy)phenyl pharmacophore moiety has been linked to amitriptyline, maprotiline, chlorpromazine, chlorprothixene, fluphenazine, and clozapine. Amide, amine and ester elements have been used generally to maintain or slightly shift affinity at dopamine D(2)-like receptors (D2 and D3), to decrease affinity at histamine H(1) receptors, and to obtain H3R ligands with low nanomolar or subnanomolar affinity. Change of effects at D(1)-like receptors (D1) and (D5) were heterogeneous. With these newly profiled compounds different antipsychotic properties might be achieved.

Dopamine/serotonin receptor ligands. Part 15: Oxygenation of the benz-indolo-azecine LE 300 leads to novel subnanomolar dopamine D1/D5 antagonists

Relying on the high affinities of the benz-indolo-azecine LE 300 (1) and the hydroxylated dibenz-azecine LE 404 (2b) for the D1/D5 receptor subtypes, we synthesized methoxylated, hydroxylated and an indole-N methylated derivatives of 1 (Fig. 1). Hydroxylation of azecine derivatives is beneficial with regard to the affinities and selectivities for all the dopamine receptor subtypes. The 'serotonin-derived' 3-oxygenated target compounds but not the 11-oxygenated analogues were superior to the unsubstituted LE 300. 11-Methoxy-7,14-dimethyl-6,7,8,9,14,15-hexahydro-5H-indolo[3,2-f][3]benzazecine (3e) was found to be the most potent antagonist at D2/D3/D4 and D5 receptor subtypes (Ki for D5 = 0.23 nmol) of all known benz-indolo-azecines.

Dopamine/Serotonin Receptor Ligands. 13: Homologization of a Benzindoloazecine-Type Dopamine Receptor Antagonist Modulates the Affinities for Dopamine D1−D5 Receptors

Enlarging the 10-membered ring of 7-methyl-6,7,8,9,14,15-hexahydro-5H-indolo[3,2-f][3]benzazecine (1, LE 300) yielded two homologue antagonists. Their affinities and inhibitory activities at D(1)-D(5) receptors were measured by radioligand binding experiments and a functional Ca(2+) assay. Compared to 1, phenylpropyl homologue 3 was superior in selectivity and affinity for the D(5) subtype (K(i) = 0.6 nM), whereas the affinity of the indolylpropyl homologue 2 for all subtypes decreased. Compounds 2, 3, 10, 11, 17, and 18 are derivatives of novel heterocyclic ring systems.

Dopamine/serotonin receptor ligands. 12(1): SAR studies on hexahydro-dibenz[d,g]azecines lead to 4-chloro-7-methyl-5,6,7,8,9,14-hexahydrodibenz[d,g]azecin-3-ol, the first picomolar D5-selective dopamine-receptor antagonist

Hydroxylated, methoxylated, and/or chlorinated 7-methyl-5,6,7,8,9,14-hexahydrodibenz[d,g]azecines were generally synthesized out of substituted 2-phenylethylamines and isochromanones by Bischler-Napieralski cyclization of the resulting benzamides to dibenzoquinolizines and the quaternization and cleavage of the central C-N bond under Birch conditions. Chlorination of 2-phenylethylamines was useful for the site direction of cyclization, but chlorine atoms were removed under Birch conditions so that chlorination had to be repeated to get the respective chlorinated dibenz[d,g]azecines. The target compounds were tested for their affinity at the different human-cloned dopamine-receptor subtypes (D1 family, D2 family). Generally, hydroxylation and chlorination of the dibenz-azecines increased affinities significantly. 1-Chloro-2-hydroxy-hexahydro-dibenz[d,g]azecine was a subnanomolar antagonist at both subtype families. 4-Chloro-3-hydroxy-7-methyl-5,6,7,8,9,14-hexahydro-dibenz[d,g]azecine was identified as the most potent and selective dopamine D5 receptor ligand described to date with Ki(D1)=0.83, Ki(D2L)=4.0, Ki(D3)=24.6, Ki(D4)=5.2 nM, and Ki(D5)=57 pM (radioligand binding experiments), respectively.

Dopamine/Serotonin Receptor Ligands. 10: SAR Studies on Azecine-type Dopamine Receptor Ligands by Functional Screening at Human Cloned D1, D2L, and D5 Receptors with a Microplate Reader Based Calcium Assay Lead to a Novel Potent D1/D5 Selective Antagonist

On the basis of the benz[d]indolo[2,3-g]azecine derivative 1 (LE300), structure-activity relations were investigated in order to identify the pharmacophore in this new class of ligands. Various structural modifications were performed and the inhibitory activities at human cloned D(1), D(2L), and D(5) receptors were measured by using a simple fluorescence microplate reader based calcium assay. Subsequently, the affinities of active compounds were estimated by radioligand binding experiments. Deleting one of the aromatic rings as well as replacing it by a phenyl moiety abolishes the inhibitory activities almost completely. Contraction of the 10-membered central ring decreases them significantly. The replacement of indole by thiophene or N-methylpyrrole reduces the inhibitory activity, whereas replacing the indole by benzene increases it. Finally, the hydroxylated dibenz[d,g]azecine derivative 11d (LE404) was found to be more active than the lead 1 in the functional calcium assay as well as in radioligand displacement experiments.

Dopamine/serotonin receptor ligands. Part VIII: the dopamine receptor antagonist LE300 - modelled and X-ray structure plus further pharmacological characterization, including serotonin receptor binding, biogenic amine transporter testing and in vivo testings

-LE300, a benz[d]indolo[2,3-g]azecine with nanomolar affinities to the hD(1) receptor family, has been further pharmacologically characterized and its modelled structure was compared to its X-ray structure in order to explain NMR data, that was not in accordance to the X-ray structure. Moderate affinity at the hD(3) receptor was determined, nanomolar affinities were found at the 5-HT(2A) and 5-HT(2C) receptors, micromolar affinity at the 5-HT(1A) receptor using binding assays. Functional studies indicate moderate antagonist activity at the 5-HT(2A) site. No activity was found at dopamine, serotonin and norepinephrine transporters. These results suggested the use of LE300 for cocaine addiction treatment. High activities were found using in vivo testing: LE300 suppressed spontaneous locomotor activity with an ID(50) of 1.24 mg/kg and attenuated locomotor activity induced by 20 mg/kg cocaine with an AD(50) of 1.50 mg/kg. It failed to substitute for the discriminative stimulus effects produced by cocaine.

Dopamine/serotonin receptor ligands. 9. Oxygen-containing midsized heterocyclic ring systems and nonrigidized analogues. A step toward dopamine D5 receptor selectivity

Eleven-membered heterocycles (dibenz[g,j]-1-oxa-4-azacycloundecenes) and open-chain analogues were synthesized and investigated for affinities to human dopamine receptor subtypes. The moderately rigidized rings displayed nanomolar and subnanomolar Ki values at D1-like receptors with a significant D1 to D2 and a slight D5 to D1 selectivity. The open-chain analogues showed lower affinities but significant D1 to D2 selectivities. Compound 3 (Ki(D5) = 0.57 nmol) showed antagonistic or inverse agonistic binding characteristics in a functional Ca assay.