Apomorphine in the treatment of Parkinson's disease

Cognition and serotonin in Parkinson's disease

Cognitive impairment affects up to 80% of patients with Parkinson's disease (PD) and is associated with poor quality of life. PD cognitive dysfunction includes poor working memory, impairments in executive function and difficulty in set-shifting. The pathophysiology underlying cognitive impairment in PD is still poorly understood, but there is evidence to support involvements of the cholinergic, dopaminergic, and noradrenergic systems. Only rivastigmine, an acetyl- and butyrylcholinesterase inhibitor, is efficacious for the treatment of PD dementia, which limits management of cognitive impairment in PD. Whereas the role of the serotonergic system in PD cognition is less understood, through its interactions with other neurotransmitters systems, namely, the cholinergic system, it may be implicated in cognitive processes. In this chapter, we provide an overview of the pharmacological, clinical and pathological evidence that implicates the serotonergic system in mediating cognition in PD.

Safety considerations when using non-ergot dopamine agonists to treat Parkinson's disease

INTRODUCTION

Nonergot dopamine agonists (NEDA) represent an excellent treatment option for Parkinson's disease (PD) patients, in both early and advanced stages of the disease. The post-marketing phase of NEDA has highlighted, though, the occurrence of important long-term adverse events.

AREAS COVERED

This review reports recent updates on NEDA adverse events, analyzing neurobiological bases and risk factors of these complications. A literature search has been performed using Medline and reviewing the bibliographies of selected articles.

EXPERT OPINION

NEDA represents a very important option in the treatment of PD. Criticisms on their use can be overcome through a better knowledge of these molecules and of the risk factors for adverse events which allow specialists to prevent the occurrence of undesired complications and consent a tailor-based approach. Abbreviations: PD: Parkinson's disease, DA: dopamine agonists, NEDA: non-ergot dopamine agonists, ICD: impulse control disorders, DAWS: dopamine agonist withdrawal syndrome, CYP: Cytochrome P, PK: pharmacokinetic, AUC: area under the curve, HRT: hormone replacement therapy, AV: atrioventricular, HF: heart failure, OH: orthostatic hypotension, RBD: REM behavior disorders, PDP: Parkinson's disease psychosis, DRT: dopamine replacement therapy, DDS: dopamine dysregulation syndrome, MMSE: Mini-Mental state examination, EDS: excessive daytime somnolence.

Apomorphine in the treatment of Parkinson's disease: a review

Optimizing idiopathic Parkinson's disease treatment is a challenging, multifaceted and continuous process with direct impact on patients' quality of life. The basic tenet of this task entails tailored therapy, allowing for optimal motor function with the fewest adverse effects. Apomorphine, a dopamine agonist used as rescue therapy for patients with motor fluctuations, with potential positive effects on nonmotor symptoms, is the only antiparkinsonian agent whose capacity to control motor symptoms is comparable to that of levodopa. Subcutaneous administration, either as an intermittent injection or as continuous infusion, appears to be the most effective and tolerable route. This review summarizes the historical background, structure, mechanism of action, indications, contraindications and side effects, compares apomorphine infusion therapy with other treatments, such as oral therapy, deep brain stimulation and continuous enteral infusion of levodopa/carbidopa gel, and gives practical instructions on how to initiate treatment.

Sustained efficacy of apomorphine in Japanese patients with advanced Parkinson's disease

BACKGROUND

This report presents data from one of the first trials of apomorphine rescue treatment for advanced Parkinson's disease (PD) conducted in Japan. This 3 month trial aimed to evaluate the sustainability of efficacy of intermittent apomorphine rescue treatment.

METHODS

A phase III, double-blind, placebo-controlled trial was conducted in PD patients (n = 31) with motor fluctuations in spite of individually titrated treatment with levodopa and other anti PD. Intermittent treatment was titrated to the maintenance dose with a subsequent unblind 12-week outpatient phase. At the week-12 visit, response to apomorphine or placebo was assessed as primary efficacy endpoint using the Unified Parkinson's Disease Rating Scale (UPDRS) part III (Motor Examination) under double-blind crossover conditions.

RESULTS

In the crossover phase (n = 28), least squares mean changes in the UPDRS part III score from pre-dose were -24.5 points with apomorphine and -2.3 points with placebo, showing that apomorphine, compared with placebo, provided a significantly greater improvement in the UPDRS part III score change (difference between treatments: -22.1 [95% confidence interval, -27.8, -16.4]; P < 0.001). The most frequently reported adverse events during the study were increased eosinophil count (8 patients), nausea (7), somnolence (6), dyskinesia (5), yawning (5), and decreased blood pressure (3).

CONCLUSIONS

Our results indicate that a 3-month use of intermittent apomorphine is an effective rescue therapy for "off" episodes in advanced PD patients.

[Apomorphine in advanced Parkinson disease]

Apomorphine is the most potent dopamine receptor agonist and its symptomatic effectiveness is comparable to levodopa. Subcutaneous apomorphine is rapidly and completely absorbed. Plasma peak concentrations are achieved after 5-15 minutes and onset of clinical effect is within 20 minutes. Apomorphine intermittent subcutaneous injections are effective as rescue therapy for unpredictable off periods in advanced Parkinson disease (PD). More often apomorphine is administered as a subcutaneous infusion which secures the continuous dopaminergic stimulation. The benefit on 'off' periods is consistent across all studies, but dyskinesia improvement is not so obvious. Two infusion therapies (apomorphine and intraduodenal levodopa) and deep brain stimulation (DBS) are effective in advanced PD patients with untreatable motor complications. Apomorphine infusions should be considered in patients unable to undergo DBS because of cognitive impairment and neurosurgical contraindications.

The utility of (11)C-arachidonate PET to study in vivo dopaminergic neurotransmission in humans

We developed a novel method to study dopaminergic neurotransmission using positron emission tomography (PET) with [1-(11)C]arachidonic acid ([1-(11)C]AA). Previous preclinical studies have shown the utility of [1-(11)C]AA as a marker of signal transduction coupled to cytosolic phospholipase A(2) (cPLA(2)). Using [1-(11)C]AA and [(15)O]water PET, we measured regional incorporation coefficients K(*) for AA and regional cerebral blood flow (rCBF), respectively, in healthy male volunteers given the D(1)/D(2) agonist (10 or 20 μg/kg subcutaneous) apomorphine. We confirmed a robust central dopaminergic response to apomorphine by observing significant increases in the serum concentration of growth hormone. We observed significant increases, as well as decreases in K(*) and increases in rCBF in response to apomorphine. These changes remained significant after covarying for handedness and apomorphine dosage. The magnitude of increases in K(*) was lower than those in our previous animal experiments, likely reflecting the smaller dose of apomorphine used in the current human study. Changes in K(*) may reflect neuronal signaling downstream of activated D(2)-like receptors coupled to cPLA(2). Changes in rCBF are consistent with previous studies showing net functional effects of D(1)/D(2) activation. [1-(11)C]AA PET may be useful for studying disturbances of dopaminergic neurotransmission in conditions such as Parkinson's disease and schizophrenia.

Investigation of D₁ receptor-agonist interactions and D₁/D₂ agonist selectivity using a combination of pharmacophore and receptor homology modeling

The aim of this study was to use a combined structure and pharmacophore modeling approach to extract information regarding dopamine D₁ receptor agonism and D₁/D₂ agonist selectivity. A 3D structure model of the D₁ receptor in its agonist-bound state was constructed with a full D₁ agonist present in the binding site. Two different binding modes were identified using (+)-doxanthrine or SKF89626 in the modeling procedure. The 3D model was further compared with a selective D₁ agonist pharmacophore model. The pharmacophore feature arrangement was found to be in good agreement with the binding site composition of the receptor model, but the excluded volumes did not fully reflect the shape of the agonist binding pocket. A new receptor-based pharmacophore model was developed with forbidden volumes centered on atom positions of amino acids in the binding site. The new pharmacophore model showed a similar ability to discriminate as the previous model. A comparison of the 3D structures and pharmacophore models of D₁ and D₂ receptors revealed differences in shape and ligand-interacting features that determine selectivity of D₁ and D₂ receptor agonists. A hydrogen bond pharmacophoric feature (Ser-TM5) was shown to contribute most to the selectivity. Non-conserved residues in the binding pocket that strongly contribute to D₁/D₂ receptor agonist selectivity were also identified; those were Ser/Cys³·³⁶, Tyr/Phe⁵·³⁸, Ser/Tyr⁵·⁴¹, and Asn/His⁶·⁵⁵ in the transmembrane (TM) helix region, together with Ser/Ile and Leu/Asn in the second extracellular loop (EC2). This work provides useful information for the design of new selective D₁ and D₂ agonists. The combined receptor structure and pharmacophore modeling approach is considered to be general, and could therefore be applied to other ligand-protein interactions for which experimental information is limited.

Selective pharmacophore models of dopamine D(1) and D(2) full agonists based on extended pharmacophore features

This study is focused on the identification of structural features that determine the selectivity of dopamine receptor agonists toward D(1) and D(2) receptors. Selective pharmacophore models were developed for both receptors. The models were built by using projected pharmacophoric features that represent the main agonist interaction sites in the receptor (the Ser residues in TM5 and the Asp in TM3), a directional aromatic feature in the ligand, a feature with large positional tolerance representing the positively charged nitrogen in the ligand, and sets of excluded volumes reflecting the shapes of the receptors. The sets of D(1) and D(2) ligands used for modeling were carefully selected from published sources and consist of structurally diverse, conformationally rigid full agonists as active ligands together with structurally related inactives. The robustness of the models in discriminating actives from inactives was tested against four ensembles of conformations generated by using different established methods and different force fields. The reasons for the selectivity can be attributed to both geometrical differences in the arrangement of the features, e.g., different tilt angels of the pi system, as well as shape differences covered by the different sets of excluded volumes. This work provides useful information for the design of new D(1) and D(2) agonists and also for comparative homology modeling of D(1) and D(2) receptors. The approach is general and could therefore be applied to other ligand-protein interactions for which no experimental protein structure is available.

Pharmacological MRI (phMRI) Monitoring of Treatment in Hemiparkinsonian Rhesus Monkeys

There is a great need for the development of noninvasive, highly sensitive, and widely available imaging methods that can potentially be used to longitudinally monitor treatment of Parkinson's disease (PD). Here we report the monitoring of GDNF-induced functional changes of the basal ganglia in hemiparkinsonian monkeys via pharmacological MRI measuring the blood oxygenation level-dependent (BOLD) response to a direct dopamine agonist (apomorphine, APO). After testing BOLD responsiveness to APO in their normal state, two additional scans were taken with the same dose of APO stimulation after induced parkinsonism. Then all animals were chronically treated with GDNF for 18 weeks by a programmable pump and catheter system. The catheter was surgically implanted into the right putamen and connected to the pump via flexible polyurethane tubing. phMRI scans were taken at both 6 and 18 weeks while they received 22.5 μg of GDNF per day. In addition, behavioral changes were monitored throughout the entire study. The primary finding of this study was that APO-evoked activations in the DA denervated putamen were attenuated by the chronic intraputamenal infusion of GDNF accompanied by improvements of parkinsonian features, movement speed, and APO-induced rotation compared to data collected before the chronic GDNF treatment. The results suggest that phMRI methods in combination with administration of a selective DA agonist may be useful for monitoring neurorestorative therapies in PD patients in the future.