The Many Faces of Apomorphine: Lessons from the Past and Challenges for the Future.

Apomorphine is now recognized as the oldest antiparkinsonian drug on the market. Though still underused, it is increasingly prescribed in Europe for patients with advanced Parkinson's disease (PD) with motor fluctuations. However, its history is far from being limited to movement disorders. This paper traces the history of apomorphine, from its earliest empirical use, to its synthesis, pharmacological development, and numerous indications in human and veterinary medicine, in light of its most recent uses and newest challenges. From shamanic rituals in ancient Egypt and Mesoamerica, to the treatment of erectile dysfunction, from being discarded as a pharmacological tool to becoming an essential antiparkinsonian drug, the path of apomorphine in the therapeutic armamentarium has been tortuous and punctuated by setbacks and groundbreaking discoveries. Throughout history, three main clinical indications stood out: emetic (gastric emptying, respiratory disorders, aversive conditioning), sedative (mental disorders, clinical anesthesia, alcoholism), and antiparkinsonian (fluctuations). New indications may arise in the future, both in PD (palliative care, nonmotor symptoms, withdrawal of oral dopaminergic medication), and outside PD, with promising work in neuroprotection or addiction.

Mechanistic analyses of the suppression of amyloid ß42 aggregation by apomorphine.

(R)-Apomorphine (1) has the potential to reduce the accumulation of amyloid ß-protein (Aß42), a causative agent of Alzheimer's disease (AD). Although the inhibition of Aß42 aggregation by 1 is ascribable to the antioxidative effect of its phenol moiety, its inhibitory mechanism at the molecular level remains to be fully elucidated. LC-MS and UV analyses revealed that 1 is autoxidized during incubation to produce an unstable o-quinone form (2), which formed a Michael adduct with Lys 16 and 28 of Aß42. A further autoxidized form of 1 (3) with o-quinone and phenanthrene moieties suppressed Aß42 aggregation comparable to 1, whereas treating 1 with a reductant, tris(2-carboxyethyl)phosphine diminished its inhibitory activity. 1H-15N SOFAST-HMQC NMR studies suggested that 1 interacts with Arg5, His13,14, Gln15, and Lys16 of the Aß42 monomer. These regions form intermolecular ß-sheets in Aß42 aggregates. Since 3 did not perturb the chemical shift of monomeric Aß42, we performed aggregation experiments using 1,1,1,3,3,3-hexafluoro-2-propanol-treated Aß42 to investigate whether 3 associates with Aß42 oligomers. Compounds 1 and 3 delayed the onset of the oligomer-driven nucleation phase. Despite their cytotoxicity, they did not exacerbate Aß42-mediated neurotoxicity in SH-SY5Y neuroblastoma cells. These results demonstrate that extension of the conjugated system in 1 by autoxidation can promote its planarity, which is required for intercalation into the ß-sheet of Aß42 nuclei, thereby suppressing further aggregation.

Design and Synthesis of Dopaminergic Agonists.

The use of dopaminergic agonists is key in the treatment of Parkinson's disease and related central nervous system (CNS) neurodegenerative disorders. Despite there are a number of commercialized dopaminergic agonists that are currently being used successfully in the first stages of the disease, they often fail to provide sustained clinical benefit for a long period due to the appearance of side-effects such as augmentation, sleepiness, nausea, hypothension, and compulsive behaviors among others. New dopaminergic agonists with less side effects are being developed. These novel compounds offer an alternative when the disease progresses and patients fail to respond to standard dopaminergic treatments or side-effects increased. Chemistry, and in particular chemical synthesis, has played a major role in bringing synthetic dopaminergic agonists to the clinic and continues to be crucial for the development of new and necessary drugs for long-term treatments with less undesired side effects. A number of structural modifications of parent compounds have led to enhanced agonism but also partial agonism or even antagonism of one or more dopamine receptors. In some cases, these activities are accompanied by agonist effect at serotonin receptors which suggests a potential clinical application in the treatment of schizophrenia In this review, chemical synthesis of dopaminergic agents, their affinity, and the corresponding agonist/antagonist effects will be highlighted.

Convenient synthesis of 18F-radiolabeled R-(-)-N-n-propyl-2-(3-fluoropropanoxy-11-hydroxynoraporphine.

Aporphines are attractive candidates for imaging D2 receptor function because, as agonists rather than antagonists, they are selective for the receptor in the high affinity state. In contrast, D2 antagonists do not distinguish between the high and low affinity states, and in vitro data suggests that this distinction may be important in studying diseases characterized by D2 dysregulation, such as schizophrenia and Parkinson's disease. Accordingly, MCL-536 (R-(-)-N-n-propyl-2-(3-[(18)F]fluoropropanoxy-11-hydroxynoraporphine) was selected for labeling with (18)F based on in vitro data obtained for the non-radioactive ((19)F) compound. Fluorine-18-labeled MCL-536 was synthesized in 70% radiochemical yield, >99% radiochemical purity, and specific activity of 167 GBq/µmol (4.5 Ci/µmol) using p-toluenesulfonyl (tosyl) both as a novel protecting group for the phenol and a leaving group for the radiofluorination.

1-substituted apomorphines as potent dopamine agonists.

A novel set of 1-substituted apomorphines as dopaminergic agonists were synthesized according to our new strategy employing the acid-catalyzed rearrangement of diversely functionalized 5ß-substituted-6-demethoxythebaines. The activities of new compounds for dopamine receptors subtypes were evaluated using HEK293 based stable cell lines expressing D1, D2L or D3 receptor subtypes. All studied compounds had affinities in nanomolar range for D2L and D3 receptors and the change of the nature of substituent in position 1 had only moderate effect. D1 receptors were sensitive to the introduction of the 4-OH-benzyl function resulting in an increased affinity. The small hydrophilic group (hydroxymethyl) highly reduced the agonist affinity and potency thereby increasing subtype selectivity. This strategy for selective modulation of affinities and potencies of 1-substituted apomorphines gives essential hints for future design of subtype selective dopaminergic ligands.

New 2-thioether-substituted apomorphines as potent and selective dopamine D2 receptor agonists.

A set of novel apomorphine derivatives were synthesized with diversely functionalized side chains in the proximity of position 2 of the aporphine skeleton. Amino and/or carboxylic functions were introduced to this region of the backbone to test their pharmacological effects. During the synthesis of 2-(S-3-mercaptopropionic acid)-derivative a heteroring-fused congener was also isolated. The structural elucidation confirmed that the formation of this product was in accordance with our previous observations on the reaction of thebaine (2) with thiosalycilic acid. All the novel apomorphine congeners 4a-g were neuropharmacologically characterized to discover their dopaminergic profiles. Two derivatives were identified as D(2) full agonists equipotent with apomorphine (1) having significantly increased D(2)/D(1) selectivity ratios.

Radiosynthesis and ex vivo evaluation of (R)-(-)-2-chloro-N-[1-11C-propyl]n-propylnorapomorphine.

INTRODUCTION: Several dopamine D(2) agonist radioligands have been used with positron emission tomography (PET), including [(11)C-]-(-)-MNPA, [(11)C-]-(-)-NPA and [(11)C]-(+)-PHNO. These radioligands are considered particularly powerful for detection of endogenous dopamine release, but they either provide PET brain images with limited contrast or have affinity for both D(2) and D(3) receptors. We here present the carbon-11 radiolabeling and ex vivo evaluation of 2-Cl-(-)-NPA, a novel PET-tracer candidate with high in vitro D(2)/D(3) selectivity. METHODS: 2-Cl-[(11)C]-(-)-NPA and [(11)C]-(-)-NPA were synthesized by a two step N-acylation-reduction process using [(11)C]-propionyl chloride. Awake rats were injected with either tracer, via the tail vein. The rats were decapitated at various times, the brains were removed and quickly dissected, and plasma metabolites were measured. Radioligand specificity, and P-glycoprotein involvement in brain uptake, was also assessed. RESULTS: 2-Cl-[(11)C]-(-)-NPA and [(11)C]-(-)-NPA were produced in high specific activity and purity. 2-Cl-[(11)C]-(-)-NPA accumulated slower in the striatum than [(11)C]-(-)-NPA, reaching maximum concentrations after 30 min. The maximal striatal uptake of 2-Cl-[(11)C]-(-)-NPA (standard uptake value 0.72+/-0.24) was approximately half that of [(11)C]-(-)-NPA (standard uptake value 1.37+/-0.18). Nonspecific uptake was similar for the two compounds. 2-Cl-[(11)C]-(-)-NPA was metabolized quickly, leaving only 17% of the parent compound in the plasma after 30 min. The specific binding of 2-Cl-[(11)C]-(-)-NPA was completely blocked and inhibition of P-glycoprotein did not alter the brain uptake. CONCLUSION: Ex vivo experiments showed, despite a favorable D(2)/D(3) selectivity, that 2-Cl-[(11)C]-(-)-NPA is inferior to [(11)C]-(-)-NPA as a PET tracer in rat, because of slower brain uptake and lower specific to nonspecific binding ratio.

Synthesis and pharmacological evaluation of thiazole and isothiazole derived apomorphines.

We have presented the synthesis of novel thiazolo- and isothiazolo-apomorphines 12-17 resulting-in part-from an unexpected isomerization step occurred during the acid-catalyzed rearrangement of precursor thiazolo-morphinandienes 3-5. These 2,3-disubstituted apomorphines represent a new group of A-ring substituted aporphines. The receptor binding studies revealed that with the exception of two derivatives all the tested compounds have limited affinity for dopamine-receptor subtypes. Functional calcium assay for the most active isothiazolo-apomorphine showed higher affinities for D(1) and D(2L) subtypes. The docking of these ligands has been modelled to human D(2) and D(3 )receptors. On the basis of the predicted models, we identified an important cation-p interaction for the binding of isothiazolo-apomorphine 16.

N-Substituted-2-alkyl- and 2-arylnorapomorphines: novel, highly active D2 agonists.

Two synthesis routes have been elaborated for the preparation of novel N-substituted-2-alkyl- and 2-arylnorapomorphines. The first one utilizes the traditional methodology of N-substitution of morphinans before acid-catalyzed rearrangements into aporphinoids, while our new approach involves the N-substitution directly on the aporphine backbone. The aimed compounds were obtained in similar overall yields in different synthesis routes and were investigated with respect to their binding affinities and activities to dopamine D(2) and D(1) receptors. These studies revealed remarkable affinity and selectivity of some compounds for D(2) over D(1) receptor subtypes. Partial or full agonist properties were confirmed for all tested compounds.

Prolonged modulation of FGF-2 expression in astrocytic cultures induced by O,O'-diacetyl-apomorphine.

Apomorphine (APO) is an anti-parkinsonian drug currently in use, which provides relief of Parkinson's symptoms. However, the utility of APO is greatly hampered by its poor bioavailability and rapid metabolism. In the present study, O,O'-diacetyl-apomorphine, a prodrug of apomorphine, was synthesized and its biological activity was examined. The prodrug induced fibroblast growth factor-2 production in astrocytic cultures similarly to apomorphine. However, its duration of action was significantly prolonged, and its resistance to oxidation was markedly enhanced compared to APO. O,O'-Diacetyl-apomorphine also induced MEK/MAPK signaling. These results suggest that O,O'-diacetyl-apomorphine can efficiently counteract oxidation and thereby enhance FGF-2 production in astrocytes.

Synthesis and neuropharmacological characterization of 2-O-substituted apomorphines.

We have synthesized novel 2-O-substituted apomorphines with both different lengths of lipophilic alkyl chains and alkyl chains carrying free hydroxyl groups. Two bis-apomorphines formed as side products of the reactions with diols were isolated and characterized as well. The neuropharmacological profile of all these new compounds were investigated with respect to their binding affinities and activities to dopamine D(2) and D(1) receptors. The obtained data pointed to the fact that, in the examination of dopaminergic activities of 2-substituted apomorphines, the lipophilicity of the substituent is more important than its spatial parameters.

Synthesis and neuropharmacological evaluation of 2-aryl- and alkylapomorphines.

A novel synthesis has been elaborated for the pharmacologically remarkable 2-arylapomorphines described and characterized in the last few years. This new procedure contains two alternative synthetic routes and has allowed the preparation of several hitherto unknown compounds as well. The pharmacological profile of the previously published and the novel 2-alkyl- and arylapomorphines has been determined with the application of in vitro and in vivo techniques. For 2-phenyl- (2) and 2-(4-hydroxyphenyl)apomorphines (3) the superior dopamine agonist profile has been confirmed and for the novel compounds some remarkable results have been observed.

Synthesis and binding studies of 2-arylapomorphines.

From codeine, four different 2-aryl substituted apomorphines were synthesised in 6 steps each. Oxidation of codeine with IBX followed by acid catalysed rearrangement gave morphothebaine, which was selectively triflylated at the 2-position and subsequently O-acetylated at the 11-position. The resulting triflate was coupled in a Suzuki-Miyaura type reaction with a series of 4-substituted arylboronic esters which, after deprotection, gave the desired 2-aryl apomorphines. The analogues were tested for affinity towards a range of dopaminergic, serotonergic and adrenergic receptors. 2-(4-Hydroxyphenyl)-apomorphine exhibited high affinity for the dopamine D2 receptor. A putative ligand-receptor interaction was put forward.

(-)-N-[(11)C]propyl-norapomorphine: a positron-labeled dopamine agonist for PET imaging of D(2) receptors.

Imaging neuroreceptors with radiolabeled agonists might provide valuable information on the in vivo agonist affinity states of receptors of interest. We report here the radiosynthesis, biodistribution in rodents, and imaging studies in baboons of [(11)C]-labeled (-)-N-propyl-norapomorphine [(-)-NPA]. (-)-[(11)C]NPA was prepared by reacting norapomorphine with [(11)C]propionyl chloride and a lithium aluminum hydride reduction. [(11)C]Propionyl chloride was prepared by reacting [(11)C]CO(2) with ethylmagnesium bromide, followed by reacting with phthaloyl chloride. The radiochemical yield of (-)-[(11)C]NPA was 2.5% at end of synthesis (EOS), and the synthesis time was 60 min. The specific activity was 1700+/-1900 mCi/micromol ( N=7; ranged 110-5200 mCi/micromol at EOS). Rodent biodistribution studies showed high uptake of [(11)C](-)-NPA in D(2) receptor-rich areas, and the striatum/cerebellum ratios were 1.7, 3.4, and 4.4 at 5 min, 30 min, and 60 min postinjection, respectively. Pretreating the animals with haloperidol (1 mg/kg) decreased the striatum/cerebellum ratio at 30 min postinjection to 1.3. (-)-[(11)C]NPA was also evaluated via baboon positron emission tomography (PET) studies. Under control conditions ( N=4), rapid uptake of the tracer was observed and the striatum/cerebellum ratio reached 2.86+/-0.15 at 45 min postinjection. Following haloperidol pretreatment (0.2 mg/kg IV), the striatum/cerebellum ratio was 1.29 at 45 min postinjection. The result demonstrated the existence of specific binding of this new tracer to the D(2) receptor. To our knowledge, the current finding of a striatum/cerebellum ratio of 2.8 in baboon was the highest reported with a radiolabeled D(2) agonist. (-)-[(11)C]NPA is a promising new D(2) agonist PET tracer for probing D(2) receptors in vivo using PET.

Synthesis and in vivo distribution in the rat of several fluorine-18 labeled N-fluoroalkylaporphines.

A method is described for the rapid production and purification of new potential dopamine agonists. Via microwave heating 10,11-dihydroxy-N-(n-2-fluoroethyl)norapomorphine (FNEA), 10,11-dihydroxy-N-(n-3-fluoropropyl)norapomorphine (FNPA) and 2,10,11-trihydroxy-N-(n-3-fluoropropyl)norapomorphine (FTNPA) and their isotopic fluorine-18 derivatives were synthesized. The fluorine-18 label was introduced via N-fluoroalkylation of acylated noraporphine derivatives with no-carrier-added (n.c.a.) 18FCH2CH2I and 18FCH2CH2CH2I. Within 160 min (E.O.B.), radiochemical yields of 13-29% (corrected for decay) were achieved based on [18F]fluoroalkyliodide. The specific activity obtained, ranged from 15 to 75 GBq/mumol. The fluorine-18 labeled compounds were investigated for their in vivo binding potency to the D2-receptors. After i.v. injection, the distribution was studied in rats. High uptakes of the N-[18F]fluoroalkylaporphines were found in the lungs, liver, adrenals and kidneys. No significant different radioactive accumulation was observed in striatum, cerebellum and frontal cortex. Dopamine depletion with reserpine did not affect the striatum to cerebellum ratio at low dosage of N-[18F]fluoroalkylaporphines (10 nmol/kg).

Synthesis and in vivo distribution in the rat of a dopamine agonist: N-([11C]methyl)norapomorphine.

A method for the rapid production and purification of 10,11-dihydroxy-N-([11C]methyl)norapomorphine ([11C]APO), a dopamine agonist (DA), is described. The potency of this ligand for studying the D2-receptors was examined. The label was introduced by N-methylation of norapomorphine hydrobromide with no-carrier-added (n.c.a) [11C]CH3I, produced from cyclotron-produced [11C]carbon dioxide. In 60 min (EOB) a radiochemical yield of 15% (corrected for decay) was achieved, based on [11C]CH3I. The specific activity ranged from 5 to 11 GBq/mumol. The distribution, after intravenous injection, was studied in rats. The radioactivity level in the striatum was higher than in the cerebellum and frontal cortex and was decreased after D2-blockade. The highest uptake ratio (1.47) was found at 30 min after injection. Dopamine depletion with reserpine did increase the striatum/cerebellum ratio at a low dosage of [11C]APO (10 nmol/kg). High uptakes of [11C]apomorphine were found in the lungs, liver and kidneys.

Synthesis and dopamine receptor affinities of enantiomers of 2-substituted apomorphines and their N-n-propyl analogues.

Syntheses of (R)-(-)-2-methoxyapomorphine (R-8), its antipode S-8, and its (R)-(-)-N-n-propyl R-9 derivatives are described. The dopaminergic receptor affinities of these compounds and their 2-unsubstituted counterparts (R)-(-)-apomorphine (R(-)-APO, R-1), (S)-(+)-apomorphine (S(+)-APO, S-1), and (R)-(-)-N-n-propylnorapomorphine (R(-)-NPA, R-2), as well as those of (R)-(-)-2-chloroapomorphine (R(-)-2-Cl-APO, R-6), (R)-(-)-2-bromoapomorphine (R(-)-2-Br-APO, R-6), were determined with tissue membrane preparations of corpus striatum from rat brain. Contribution of both an N-n-propyl and a 2-hydroxy in (R)-(-)-2-hydroxy-N-n-propylnorapomorphine (R(-)-2-OH-NPA, R-7) or a methoxy group in (R)-(-)-2-methoxy-N-n-propylnorapomorphine (R(-)-2-OCH3-NPA, R-9) produced the highest D2 affinity (0.053 and 0.17 nM) and D2 over D1 selectivity (17,300 and 10,500 times) of the compounds evaluated. The structure-affinity relationships of these 2-substituted aporphines suggest that secondary binding sites of D2 receptors interact with 2-substituents on the A ring of aporphines through H-bonding.

Synthesis and structural requirements of N-substituted norapomorphines for affinity and activity at dopamine D-1, D-2, and agonist receptor sites in rat brain.

A series of N-substituted analogues of (R)-(-)-norapomorphine were synthesized to study the optimal structural requirements of the N-alkyl side chain to interact with D-1 and D-2 dopaminergic receptors as well as dopamine (DA) agonist binding sites. Evaluations included testing the affinity of these compounds for DA receptor sites in rat striatal tissue and assessing stereotypy as a behavioral index of dopaminergic activity. The electronic, steric, and lipophilic properties of the N-alkyl side chain were found to be related to affinity, D-2 selectivity, and dopaminergic activity. All 11 compounds evaluated had relatively low affinity at D-1 sites. Optimum D-2 and agonist-site affinity as well as agonist activity were exhibited by N-cyclopropylmethyl (7) greater than or equal to N-allyl (8) greater than or equal to N-propyl (4) or N-ethyl (3) substituted compounds. Branching of the N-alkyl side chain as in N-isopropyl (5) and N-isobutyl (6) markedly reduced the D-2 affinity and activity, presumably due to steric effects. The N-trifluoroethyl (10) and N-pentafluoropropyl (11) derivatives had low affinity for all their dopamine receptor sites and no agonistic activity; evidently, the highly electronegative F atoms decrease basicity of the N atom and therefore decrease the ability of the N atom to be cationic at physiological pH, a proposed requirement for high-affinity binding to DA receptors.

2-Haloaporphines as potent dopamine agonists.

The synthesis of 2-amino- and 2-halo-substituted aporphines is described. The key step is the substitution of a hydroxy group in the 2-position with an amino group effected by a Smiles rearrangement reaction of the 2-methylpropanamide derivative 6. The affinity of the new compounds for the dopamine D-2 receptor in the anterior pituitary gland was evaluated. 2-Fluoroapomorphine was the most potent compound, being 1.5 times more potent than (-)-apomorphine. The structure-activity relationships are discussed in relation to a previously proposed receptor model.