Synthesis and radioreceptor binding activity of N-0437, a new, extremely potent and selective D2 dopamine receptor agonist

The complex molecular pharmacology of the dopamine D2 receptor: Implications for pramipexole, ropinirole, and rotigotine

With L-DOPA, dopamine agonists such as pramipexole, ropinirole and rotigotine constitute key therapeutic options for the management of motor symptoms of Parkinson's disease. These compounds exert their beneficial effect on motor behaviours by activating dopamine D₂-class receptors and thereby compensating for the declining dopaminergic transmission in the dorsal striatum. Despite a strong similarity in their mechanism of action, these three dopamine agonists present distinct clinical profiles, putatively underpinned by differences in their pharmacological properties. In this context, this review aims at contributing to close the gap between clinical observations and data from molecular neuropharmacology by exploring the properties of pramipexole, ropinirole and rotigotine from both the clinical and molecular perspectives. Indeed, this review first summarizes and compares the clinical features of these three dopamine agonists, and then explores their binding profiles at the different dopamine receptor subtypes. Moreover, the signalling profiles of pramipexole, ropinirole and rotigotine at the D₂ receptor are recapitulated, with a focus on biased signalling and the potential therapeutic implications. Overall, this review aims at providing a unifying framework of interpretation for both clinicians and fundamental pharmacologists interested in a deep understanding of the pharmacological properties of pramipexole, ropinirole and rotigotine.

Optical Control of Dopamine Receptors Using a Photoswitchable Tethered Inverse Agonist

Family A G protein-coupled receptors (GPCRs) control diverse biological processes and are of great clinical relevance. Their archetype rhodopsin becomes naturally light sensitive by binding covalently to the photoswitchable tethered ligand (PTL) retinal. Other GPCRs, however, neither bind covalently to ligands nor are light sensitive. We sought to impart the logic of rhodopsin to light-insensitive Family A GPCRs in order to enable their remote control in a receptor-specific, cell-type-specific, and spatiotemporally precise manner. Dopamine receptors (DARs) are of particular interest for their roles in motor coordination, appetitive, and aversive behavior, as well as neuropsychiatric disorders such as Parkinson's disease, schizophrenia, mood disorders, and addiction. Using an azobenzene derivative of the well-known DAR ligand 2-(N-phenethyl-N-propyl)amino-5-hydroxytetralin (PPHT), we were able to rapidly, reversibly, and selectively block dopamine D1 and D2 receptors (D1R and D2R) when the PTL was conjugated to an engineered cysteine near the dopamine binding site. Depending on the site of tethering, the ligand behaved as either a photoswitchable tethered neutral antagonist or inverse agonist. Our results indicate that DARs can be chemically engineered for selective remote control by light and provide a template for precision control of Family A GPCRs.

In vitro mimicry of metabolic oxidation reactions by electrochemistry/mass spectrometry

The aim of these studies was to investigate the scope and limitations of electrochemistry on-line with mass spectrometry as a quick and convenient way to mimic phase I oxidative reactions in drug metabolism. A compound with previously reported in vitro and in vivo metabolism, the dopamine agonist 2-(N-propyl-N-2-thienylethylamino)-5-hydroxytetralin, was examined in an electrochemistry/mass spectrometry (EC/MS) system. The previously reported N-dealkylation was mimicked by the electrochemical cell while the oxidation of the phenol function was not fully mimicked by the EC/MS system, since the catechol and p-hydroquinone formed were immediately oxidized to the corresponding quinones. Since cytochrome P450 isoenzymes are the most important enzymes in phase I oxidative metabolism, two standard substrates used for the characterization of those enzymes, lidocaine and 7-ethoxycoumarin, were tested in the EC/MS system. The electrochemical cell was capable of mimicking the N-dealkylation of lidocaine but, under the conditions used in our experiments, the O-deethylation of 7-ethoxycoumarin could not be simulated in the electrochemical cell.

A high resolution gas chromatographic/mass spectrometric method for the determination of N-0923 in human and monkey plasma

A method that combines gas chromatography and mass spectrometry (GC/MS) for the determination of N-0923 in human and monkey plasma is detailed. This novel drug holds promise for the treatment of Parkinson's disease. Clinical use of N-0923 will result both in low peak plasma concentrations and in a short half-life; consequently, a highly sensitive method will be required. Previous methods using high-performance liquid chromatography with electrochemical detection have given reliable data in high dose studies in animals to 0.3-1.0 ng/mL in plasma but are not considered adequate for studies at clinical levels in humans. The data in this paper will show that GC/MS can offer a new direction in the determination of N-0923 in human and monkey samples, with reliable data to 25 pg/mL in plasma. This method is presented as a novel approach to the determination of N-0923, and as a starting point for further development.

Characterization and pharmacological responsiveness of dopamine release recorded by microdialysis in the substantia nigra of conscious rats

The extracellular concentration of dopamine (DA) and 3,4-dihydroxyphenylacetic acid in the substantia nigra (SN) and striatum was estimated by microdialysis. The dialysate content of DA from the SN was recorded during infusion of a DA uptake blocker (nomifensine; 5 mumol/L) dissolved in the perfusion fluid. Perfusion of tetrodotoxin (1 mumol/L) produced a virtually complete disappearance of nigral and striatal DA release. Dendritic as well as terminal release of DA was inhibited for several hours when the nerve impulse flow in dopaminergic neurons was blocked by systemic administration of gamma-butyrolactone (750 mg/kg, i.p.). The systemic administration (0.3 mg/kg, i.p.) as well as infusion (1 mumol/L) of the D2 agonist (-)-N-0437 [2-(n-propyl-N-2-thienylethylamino)-5-hydroxytetralin] produced a significant decrease in the release of DA in both the striatum and the SN. DA levels were recorded in the striatum both with and without addition of nomifensine to the perfusion fluid. The decrease in the striatum after (-)-N-0437 was suppressed in the presence of nomifensine. Infusion (1 mumol/L) as well as systemic administration (40 mg/kg) of sulpiride caused a similar increase in the release of striatal DA; this increase was, in both experiments, potentiated by nomifensine coinfusion. Sulpiride administration induced a small increase in the release of nigral DA. Infusion of (-)-N-0437 or (-)-sulpiride into the nigra caused a moderate decrease and increase, respectively, of striatal DA level.(ABSTRACT TRUNCATED AT 250 WORDS)

The metabolic fate of the dopamine agonist 2-(N-propyl-N-2-thienylethylamino)-5-hydroxytetralin in rats after intravenous and oral administration. II. Isolation and identification of metabolites

1. The in vivo metabolic pathways of 2-(N-propyl-N-2-thienylethylamino)-5-hydroxytetralin (I) in rats have been established, using in vitro metabolism as a complementary technique. 2. All identified metabolites were conjugates. Glucuronidation at the phenolic group yields the major metabolite, accounting for 50% (i.v.) or 65% (oral) of the dose. The corresponding sulphate conjugate of I is virtually absent (less than 0.2% dose). 3. Hydroxylation of I, at the ortho position to the phenolic hydroxy group, yields 6-hydroxy-I (II), accounting for about 13% (i.v.) or 9% (oral) dose. This catechol is excreted, as a glucuronide, almost exclusively into the bile. Both the 5- and the 6-glucuronide of II were detected in about equal amounts. 4. Metabolism of I in vitro showed that under oxidative conditions, depropylation of I occurred. Conjugation of 3H-I in the presence of UDPGA or PAPS, was successful in yielding the glucuronide and sulphate conjugates.

The metabolic fate of the dopamine agonist 2-(N-propyl-N-2-thienylethylamino)-5-hydroxytetralin in rats after intravenous and oral administration. I. Disposition and metabolic profiling

1. The disposition and metabolic profiling of 2-(N-propyl-N-2-thienylethylamino)-5-hydroxytetralin(I), a dopamine agonist, were studied in anaesthetized rats after i.v. administration and in non-anaesthetized rats after i.v. and oral dosing. No major differences due to narcosis were observed. 2. Independent of dosing route or anaesthetic, clearance of I was rapid. Bile was the main route of excretion, accounting for 88% dose, compared with 9% in urine. 3. Drug metabolic profiling revealed that I is almost completely metabolized before elimination; less than 0.5% total radioactivity in bile and urine was due to parent compound. 4. The biliary metabolic profiles after i.v. and oral administration were similar. One major metabolite was detected, accounting for 50% (i.v.) or 65% (oral) dose. The major biliary metabolite was identified as the glucuronide of I. 5. Urinary metabolic profiles were quantitatively different from those of bile. After i.v. administration one major metabolite was detected in urine, but this was not the major biliary metabolite. After oral administration, the major urine metabolite was the same as the major biliary metabolite. These differences can be explained by first-pass gastro-intestinal metabolism.

The anorectic effect of SK&F 38393, a selective dopamine D1 receptor agonist: a microstructural analysis of feeding and related behaviour

An observational study was undertaken to provide a microstructural analysis of the effects of the selective dopamine D1 receptor agonist, SK&F 38393, on feeding and associated behaviours in the rat. Adult, male non-deprived rats were adapted to eating a meal of sweetened mash in a 30-min period. SK&F 38393 (3.0 and 10 mg/kg, SC) significantly reduced food consumption). It also reduced the frequency of feeding bouts and the local rate of eating, while there was an increase, at 10 mg/kg, in the mean duration of individual feeding episodes. The D1 receptor agonist also reduced the total duration of locomotor activity, but did not affect the total duration of rearing, grooming, sniffing, oral behaviours (other than feeding), or resting. The frequency of bouts of locomotion, rearing, grooming, and sniffing were reduced by SK&F 38393. Consideration of the time-courses for the several responses suggested that SK&F 38393 did not materially affect the form and sequence of behavioural responses in the test, although some changes occurred.

Ocular effects of a N,N-disubstituted 5-OH aminotetralin (N-0437): evidence for a dual mechanism of action

The selective DA2 agonist, N-0437, produced an acute reduction in intraocular pressure (IOP) and pupil diameter (PD) when applied topically to the eyes of normal monkeys. Ocular hypotension and miosis were primarily unilateral in nature, dose-related and lasted 3 to 5 hours following drug instillation. In normal monkeys, 24 to 48 hours following drug administration, a secondary chronic (greater than 24 hrs) reduction in pressure was observed. Once-a-day topical administration of N-0437 (250 micrograms), to normal monkeys, for 4 days produced a chronic unilateral reduction in IOP that persisted for 18 days. Associated with this reduction in pressure on day 2 through 6 were miosis and ptosis of the treated eye. Although topical administration did not lower IOP in rabbits, intracameral injection of N-0437 significantly depressed IOP for 3 days when compared to control injected eyes. Evaluation of ocular sympathetic innervation in N-0437 treated rabbits indicated that these fibers were not functional. In rabbits, intracameral administration of the active (S,-) and inactive (R,+) enantiomer of N-0437 produced equivalent reductions in pressure. These data provide further support for the hypothesis that DA2 receptor agonists can produce acute reductions in IOP. In addition, N-0437 appears to have a second non-receptor mediated mechanism of action that produces a chronic reduction in IOP. This chronic reduction in pressure appears, in part, to result from an interruption of ocular sympathetic nerves function.

D-2 dopamine-receptors regulate the release of [3H]dopamine in rat cortical regions showing dopamine immunoreactive fibers

Using an antibody raised against dopamine the occurrence of dopamine-containing fibers was demonstrated in the prefrontal cortex, anterior cingulate cortex, parietal neocortex, piriform cortex and entorhinal cortex. In extracts of these cortical regions significant amounts of dopamine, although approximately a 100-fold less than in the neostriatum or nucleus accumbens, were detected with high performance liquid chromatography. The release of [3H]dopamine from slices of all these cortical regions was studied in vitro in a superfusion system and desipramine was used to prevent the uptake of [3H]dopamine in noradrenergic nerve terminals. It appeared that the electrically evoked release of radioactivity was inhibited by drugs stimulating D-2 dopamine-receptors in all the regions studied. Cation-exchange column chromatography revealed that the radioactivity released consisted predominantly of [3H]dopamine, indicating that D-2 receptors mediate the inhibition of the release of [3H]dopamine from dopaminergic nerve terminals. Likewise, in the neostriatum as well as in the nucleus accumbens D-2 receptor stimulation inhibits the release of [3H]dopamine. Therefore it is our conclusion that D-2 receptors regulate the release of dopamine from dopaminergic neurons originating in the ventral tegmental area as well as in the substantia nigra.