Current Options and Future Possibilities for the Treatment of Dyskinesia and Motor Fluctuations in Parkinson s Disease

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1 670 CNS & Neurological Disorders - Drug Targets, 2011, 10, Current Options and Future Possibilities for the Treatment of Dyskinesia and Motor Fluctuations in Parkinson s Disease M.A. Cenci *,1, K.E. Ohlin 1 and P. Odin 2 1 Basal Ganglia Pathophysiology Unit, Dept. Experimental Medical Science, Lund University, BMC F11, Lund, Sweden 2 Department of Neurology, University Hospital, SE Lund, Sweden and Department of Neurology, Central Hospital, D Bremerhaven, Germany Abstract: Dyskinesia and motor fluctuations affect up to 90% of patients with Parkinson s disease (PD) within ten years of L-DOPA pharmacotherapy, and represent a major challenge to a successful clinical management of this disorder. There are currently two main treatment options for these complications, namely, deep brain electrical stimulation or continuous infusion of dopaminergic agents. The latter is achieved using either subcutaneous apomorphine infusion or enteric L-DOPA delivery. Some patients also benefit from the antidyskinetic effect of amantadine as an adjunct to L-DOPA treatment. Ongoing research in animal models of PD aims at discovering additional, novel treatment options that can either prevent or reverse dyskinesia and motor fluctuations. Alternative methods of continuous L-DOPA delivery (including gene therapy), and pharmacological agents that target nondopaminergic receptor systems are currently under intense experimental scrutiny. Because clinical response profiles show large individual variation in PD, an increased number of treatment options for dyskinesia and motor fluctuations will eventually allow for antiparkinsonian and antidyskinetic therapies to be tailor-made to the needs of different patients and/or PD subtypes. Keywords: Parkinson's disease, motor fluctuations, dyskinesia, motor complications, basal ganglia, glutamate, serotonin, rodent, non-human primate. INTRODUCTION The dopamine (DA) precursor, L-DOPA, is still the most effective symptomatic medication against the motor symptoms of Parkinson s disease (PD) [1, 2], and it is also the least expensive. Furthermore, L-DOPA treatment increases the life expectancy of PD patients [3, 4]. Standard L-DOPA pharmacotherapy consists of tablets for oral administration, which are taken from three to eight times per day depending on the individual response and disease stage. L-DOPA is rapidly absorbed from the small intestine, although its absorption depends on the rate of gastric emptying, and on the ph and amino acid concentration of the gastric contents [5]. Plasma concentrations usually peak between 1 and 2 hours after an oral dose, and the plasma half-life is between 1 and 3 hours [6, 7]. To reduce the extracerebral conversion of L-DOPA, standard L- DOPA preparations also contain a peripheral inhibitor of DOPA decarboxylases, such as carbidopa or benserazide [8]. Despite its cost-effectiveness and ease of administration, L- DOPA pharmacotherapy has some major limitations. A first, inescapable limitation depends on the fact that the response to L- DOPA changes during the progression of PD. As the disease becomes more severe, the need for symptomatic treatment becomes larger. Thus, both the total dosage and the number of L-DOPA doses per day are usually increased a few years after treatment initiation [9]. At this point, many patients start to exhibit abnormal involuntary movements (dyskinesia) and motor fluctuations. These complications affect approximately 40% of the patients after 4-6 years of L-DOPA therapy [10], and up to 90% of the patients by 10 years of treatment [11, 12]. The most common pattern of L-DOPAinduced dyskinesia consists of choreiform movements that are most severe at the time when the drug is producing the maximal relief of parkinsonian motor symptoms, hence the term peak-of-dose or on dyskinesia [13]. In some patients, involuntary movements are most prominent at the beginning and the end of the L-DOPA dosing *Address correspondence to this author at the Basal Ganglia Pathophysiology Unit, Dept. Experimental Medical Science, Lund University, BMC F11, Lund, Sweden; Tel: ; Fax: ; Angela.Cenci_Nilsson@med.lu.se cycle, a pattern referred to as diphasic dyskinesia. This form of dyskinesia typically manifests as stereotypic or ballistic movements mixed with dystonia, and it is particularly severe in the legs [14]. Motor fluctuations appear as rapid transitions from good motor function ( on phase) to severe parkinsonian immobility ( off phase) [15, 16]. The earliest and most common type of motor fluctuation consists in a decreased duration of the effect of single L- DOPA doses, termed wearing-off phenomenon or end-of-dose deterioration. This can appear following the administration of daily L-DOPA doses, but also in the form of late night or early morning off periods. In more advanced stages of PD, fluctuations between on and off time can become unpredictable [9]. Unpredictable responses to single L-DOPA doses may partly depend on pharmacokinetic problems [17]. The development of L- DOPA-induced motor complications is however attributed to dysfunctional plastic changes that occur in the brain [18-22]. The plastic alterations involved are multiple and complex, but they all seem to converge on two general mechanisms: presynaptic dysregulation of DA release and clearance and abnormal postsynaptic responses in dopaminoceptive cells [22, 23], striatal neurons in particular [21]. Dyskinesia and motor fluctuations are not the only problems associated with the use of L-DOPA in PD. The stimulation of DA receptors in mesocorticolimbic regions can contribute to psychoticlike symptoms (hallucinations, vivid dreams, paranoia and confusion) [24] and to altered behavioural patterns bearing some resemblance to compulsive drug use [25]. A matter of even greater concern is the limited efficacy of L-DOPA against a range of severe motor and non-motor deficits that plague patients in an advanced disease stage [26]. Non-motor problems that are either not improved or only partly improved by L-DOPA include cardiovascular symptoms (in particular, orthostatic hypotension), sleep problems, mood alterations, cognitive deficits, gastrointestinal symptoms (e.g. obstipation and dysphagia), urological problems (urgency, frequency and nocturia), and pain [27]. Finally, L-DOPA is unable to halt the progression of neurodegeneration [24, 28]. Despite all these limitations, L-DOPA remains the goldstandard treatment to alleviate symptoms of PD [1, 24]. There is no realistic prospect that L-DOPA pharmacotherapy will be radically /11 $ Bentham Science Publishers

2 Treatment of Dyskinesia and Motor Fluctuations in Parkinson s Disease CNS & Neurological Disorders - Drug Targets, 2011, Vol. 10, No replaced by newer treatments in the nearest future. There is however a need to devise improved methods of L-DOPA delivery that can minimize the risk of motor complications. Moreover, intense research efforts strive to develop non-dopaminergic treatments that can be added to L-DOPA in order to reduce the occurrence of dyskinesia and response fluctuations and/or to alleviate non-motor symptoms in PD. In this article, we review the current clinical management of motor complications with peroral drug treatment and advanced therapies based on continuous dopaminergic stimulation (CDS) or deep brain stimulation (DBS). Thereafter, we summarize and discuss the current status of preclinical research aimed at defining non-dopaminergic drug targets for the treatment of dyskinesia and motor fluctuations. CURRENT TREATMENT OPTIONS Current Management of Motor Complications with Peroral Drug Therapy In the early phases of motor complications, modifications of peroral L-DOPA therapy might improve treatment efficacy. These modifications include a fragmentation of the daily L-DOPA dosage into more frequent and smaller doses, the use of long-acting dopamine agonists, as well as the use of enzyme inhibitors that can prolong the effect of each L-DOPA dose [29]. Modified release L- DOPA preparations, which prolong the plasma half-life and the pharmacodynamic effect of L-DOPA, were originally proposed as an option [30] (NICE guidelines; The clinical experience gained over the years indicates that modified release preparations are useful to provide symptomatic control over the night (when given as a late evening dose). However, they do not reduce the severity of motor complications in day-time therapy [31]. A relatively large number of compounds acting as direct DA receptor-agonists have been introduced in the treatment of PD during the past 35 years. The prototype of these agonists is bromocriptine, an ergot derivative with agonist activity at the D2 receptor, but additional DA agonists (both ergot and non-ergot derivatives) are available (Tables 1 and 2). These DA agonists differ between each other in terms of pharmacokinetics and sideeffect profile, but have similar DA receptor specificity, acting foremost on dopamine D 2 /D 3 receptors. Moreover, they all have a much longer duration of action than L-DOPA. Initial treatment of PD with these agonists has been consistently reported to reduce the incidence of dyskinesia, dystonia and motor fluctuations [32]. Dopamine agonists have therefore become first-line agents for de novo treatment of young PD patients in many countries (see e.g. German National Guidelines for treatment of Parkinson s disease, Unfortunately, all DA agonists have inferior symptomatic efficacy compared to L-DOPA. Most patients starting their PD treatment on DA agonist monotherapy will therefore need additional treatment with L-DOPA at some point. It is unclear whether the reduced incidence of motor complications associated with the use of DA agonists will persist after adding L-DOPA to the treatment, and the results of different investigations are controversial in this regard [33]. Nevertheless, peroral DA agonist treatment can be useful to patients with advanced PD and motor fluctuations, because the addition of a DA agonist to L-DOPA reduces the time spent in the off condition [34, 35]. While entailing a lower incidence of motor complications, the DA agonists are not devoid of untoward effects. The incidence of oedema, somnolence, constipation, dizziness, nausea and psychiatric side effects (hallucinations, delusions, confusion or impulse-control disorders) is overall larger for these compounds compared to L-DOPA [32, 36]. Daytime tiredness occurs in 5-10% of DA agonist-treated patients, but can occur also under other types of dopaminergic therapy. Ergot-derived DA agonists can induce pulmonary or retroperitoneal fibrosis [37]. Moreover, treatment with pergolide or cabergoline has been found to be associated with an increased risk of cardiac valve regurgitation [38-40]. The effect was attributed to the agonist activity exerted by pergolide and cabergoline at the 5-hydroxytryptamine 2B (5-HT 2B ) receptor. Indeed, stimulation of 5-HT 2B receptors has mitogenic properties on cardiac fibromyoblasts, potentially leading to valvular fibroplasia [41, 42]. Many of the patients with cardiac valve regurgitation do not, however, suffer from clinical symptoms related to this condition, and the relevance of this side effect is still debated [43]. Table 1. Dopamine Agonists Currenty Used in the Treatment of PD Substance Type T 1/2 [h] Elimination Apomorphine Non-Ergot 0.5 Bromocriptine Ergot 6 Hepatic Cabergoline Ergot 65 Hepatic -Dihydroergocriptine Ergot 15 Hepatic Lisuride Ergot 2-3 Hepatic/renal Pergolide Ergot 7-16 Hepatic/renal Piribedil Non-Ergot 12 Hepatic/renal Pramipexole Non-Ergot 8-12 Renal Pramipexole, prolonged release Non-Ergot 8-12 Renal Ropinirole Non-Ergot 6 Renal Ropinirole, prolonged release Non-Ergot 6, delayed release Renal Table 2. Rotigotine (patch) Non-Ergot 5-7 Renal Comparison Between Equivalent Antiparkinsonian Doses of L-DOPA and Dopamine Agonists in Human PD Patients Equivalence Doses L-DOPA Apomorphine Bromocriptine Cabergoline -Dihydroergocriptine Lisuride Pergolide Pramipexole Piribedil Ropinirole Single Dose 100 mg 3-5 mg mg 1,5-2 mg mg 1 mg 1 mg mg mg 3-5 mg Rotigotine (patch) 4 mg / 24 h L-DOPA dose equivalents: The dose of the respective drug clinically estimated to have a similar effect compared to 100 mg L-DOPA [251]. Based on experimental as well as clinical results, it has been speculated that DA agonists might have diseasemodifying/neuroprotective effects. Some trials using imaging biomarkers of presynaptic DA fiber integrity (i.e. 18 F-DOPA-PET and beta-cit-spect) have demonstrated a slower loss of signal in patients treated with DA agonists compared to L-DOPA [44, 45].

3 672 CNS & Neurological Disorders - Drug Targets, 2011, Vol. 10, No. 6 Cenci et al. The results are, however, much debated and a neuroprotective effect cannot presently be regarded as proven [46]. In addition to DA receptor agonists, other dopaminergic treatments for PD include inhibitors of enzymes that inactivate DA, i.e. monoamine oxidase B (MAO-B) [47] and catechol-o-methyltransferase (COMT) [48]. There are two different COMT-inhibitors presently available, entacapone and tolcapone. Tolcapone has caused hepatotoxicity in a few patients, and its use should therefore be associated with regular laboratory controls. The COMTinbibitors are administered together with L-DOPA in patients with motor fluctuations in order to prolong the effects of single drug doses and reduce the time spent in the off -state. It is still unclear whether the early addition of COMT-inhibitors to L-DOPA in patients who do not yet have motor complications can prevent the development of motor fluctuations and dyskinesias. The clinical studies thus far have not been able to demonstrate such effects [49, 50]. In addition to prolonging the effect of L-DOPA, MAO-B inhibitors also have mild efficacy as a monotherapy and can delay the need for L-DOPA by several months [51]. Interestingly, the MAO-B inhibitor rasagiline can activate neurotrophic factor signalling [52], raising hopes for a disease-modifying action in human PD patients [53]. In the so called ADAGIO trial, early PD treatment with rasagiline at a dose of 1 mg per day provided benefits that were consistent with a possible disease-modifying effect [54]. Several studies provide indications of diseasemodifying effects also with the MAO-B inhibitor, selegiline. When selegiline was added to L-DOPA, a slower development of symptoms, a reduced need for L-DOPA, and a reduced incidence of dyskinesias were observed in one trial [55]. The weak non-competitive NMDA receptor antagonist, amantadine has a mild to moderate symptomatic antiparkinsonian effect, both when used as a monotherapy and in combination with other PD drugs. In addition, amantadine reduces L-DOPA-induced dyskinesias (LID) and is now mainly prescribed as an antidyskinetic drug [56]. Current investigations aim at establishing the antidyskinetic efficacy of additional NMDA receptor antagonists. In this regard, memantine, a compound chemically related to amantadine [57], appears particularly promising because it has the potential to improve both cognitive function [58] and dyskinesia in PD [59]. Anticholinergic drugs belong to the oldest class of anti- Parkinsonian medicines. These might have beneficial effects against tremor and dystonia, in particular, but there is little published evidence to support such effects (NICE Guidelines; Because of their relatively strong tendency to produce autonomic side effects and cognitive impairment anticholinergics are normally used as second-tier therapies, and their prescription is limited to patients without cognitive problems. Management of Severe Motor Complications by Continuous Dopaminergic Stimulation Optimizations of peroral dopaminergic pharmacotherapy are usually insufficient to control severe dyskinesias and motor fluctuations. In the advanced complicated phase of PD, three types of interventions currently represent the most effective options, i.e. continuous duodenal L-DOPA administration, subcutaneous apomorphine infusion, or deep brain stimulation (Fig. 1). Continuous dopaminergic stimulation treatments (CDS) for PD were developed based on the proposal that a continuous supply of L-DOPA and/or a continuous occupancy of DA receptors by longacting agonists would be required to adequately reproduce the physiological features of nigrostriatal DA transmission [60-62]. In addition to the experimental data, in vivo imaging studies in PD patients strongly support the association between rapid and large changes in striatal DA release and the occurrence of dyskinesia and motor fluctuations [63, 64]. Moreover, it has been proposed that the lower dyskinesiogenic potential of the DA agonists compared to L- DOPA depends on their longer duration of action [62, 65]. Continue oral drug therapy YES NO NO Pronounced dementia Severe tremor with insufficient effect of medication Motor complications Cognitive/psychiatric impairments Biological age >70-75 yrs Disabling dyskinesia Contraindications for brain surgery Contraindications for abdominal surgery All advanced treatment options possible - discuss individual risk/benefit of DBS and pump Advanced treatments Fig. (1). Treatment decision flow chart for patients in an advanced stage of idiopathic Parkinson s Disease. LCIG, L-DOPA-carbidopa intestinal gel (continuous infusion). The first studies describing intravenous delivery of L-DOPA were published in 1975 [66], and were soon followed by several other reports showing an improvement of motor fluctuations [67]. In most cases the patients were, however, only treated for a few days. It proved practically difficult to give L-DOPA intravenously over longer times. The first experiences with intraduodenal L- DOPA infusion were published in 1986 [68], where effects comparable to intravenous L-DOPA delivery were reported. This was later confirmed in several other studies (reviewed in [69-71]). L-DOPA-Carbidopa Intestinal Gel (LCIG; Duodopa ) is a combination of L-DOPA (20 mg/ml) and carbidopa (5 mg/ml) constituted in a pseudoplastic gel that is delivered via portable infusion pumps. Short-term therapy can be achieved using a nasoduodenal catheter, but long-term treatment is achieved by means of a duodenal catheter. This is inserted by gastroenterological intervention (most often a so-called percutaneous endoscopic gastrostomy, PEG, or, in some cases, jejunostomy). Controlled studies have shown that changing from peroral L-DOPA therapy to LCIG infusion results in a stabilization of both plasma L-DOPA concentrations and clinical status, with a pronounced reduction of motor fluctuations and time spent in off, and an increased time spent in a good on state [69]. A blinded randomized cross-over study comparing LCIG monotherapy with individually optimized peroral therapy has demonstrated an increase in daily on time from 81% to 100% and an improvement in health-related quality of life following LCIG infusion [72]. In a German study, 13 patients treated with LCIG experienced a mean 82% reduction of time spent in off per day, whereas the time spent in on without dyskinesia increased from 30% to 90%. Peakdose L-DOPA-induced dyskinesia virtually disappeared during a mean follow-up time of 6 months [73]. Similar effects on motor fluctuations and dyskinesias have been reported in a number of other studies [74-78]. We have verified the antidyskinetic effect of continuous duodenal L-DOPA infusion in a dedicated study on 9 patients changing from peroral therapy to LCIG. The mean dyskinesia severity (as detected with repeated scoring over 3 days) was reduced by 90% over a period of 6 months. Upon drug challenge tests performed before and after 6 months of LCIG therapy, an identical dose of L-DOPA produced significantly lower dyskinesia scores (-70%) following pump treatment (Odin, unpublished results). The improvement in dyskinesia in spite of unchanged (or even increased) daily L-DOPA equivalent doses indicates that what causes dyskinesia is not L-DOPA itself, but NO YES YES YES YES YES YES DBS Apomorphine pump or LCIG Apomorphine pump or LCIG DBS or LCIG Apomorphine pump or LCIG DBS or apomorphine pump

4 Treatment of Dyskinesia and Motor Fluctuations in Parkinson s Disease CNS & Neurological Disorders - Drug Targets, 2011, Vol. 10, No rather the pulsatile nature of DA receptor stimulation resulting from peroral L-DOPA treatment [73-75, 77]. Initially, duodenal LCIG infusion is most commonly given only during day-time. In patients experiencing night-time problems with Parkinson symptoms and suboptimal sleep, a 24-hour treatment can bring significant improvement of sleep without inducing further side effects or tolerance [79]. A recent investigation on LCIG infusion and the non-motor aspects of PD demonstrated pronounced improvements on several non-motor symptoms, including gastrointestinal, urological, cardiovascular and cognitive problems, as well as sleep and pain, when switching from peroral to pump therapy [77]. Compared with motor symptoms, the improvement in non-motor symptoms showed a stronger correlation with the amelioration of health-related quality of life. The adverse events of LCIG therapy are mainly related to the infusion method, including dislocation or occlusion of the duodenal catheter, leakage in the infusion system and problems related to the PEG establishment. Apomorphine Treatment The efficacy of apomorphine in treating Parkinson symptoms was first demonstrated in 1951 [80]. A broader clinical application of apomorphine became possible after discovering that domperidone (a peripherally acting DA-receptor antagonist) could be coadministered with apomorphine to block its peripheral adverse reactions (nausea, vomiting, orthostatic hypotension) [81]. Since the late nineteen-eighties, apomorphine has been in use for PD treatment, mostly in the form of subcutaneous (s.c.) infusions or injections [82, 83]. Together with L-DOPA, apomorphine is the pharmacological treatment exerting the strongest effects on PD motor symptoms. Following s.c. injection, apomorphine has a halflife in distribution phase of about 5 minutes, leading to a clinical effect after 5-10 minutes. The biological half-life in elimination phase is around 33 minutes and the effect duration is about 45 minutes [84]. Because of its short duration of action, apomorphine is currently administered by continuous subcutaneous infusion via a portable infusion pump. This treatment is best suited for the severely disabled patient who has a good L-DOPA response, but whose condition is dominated by prolonged or frequent off periods and/or peak-dose dyskinesias despite optimized oral drug treatment [85-87]. In a review of clinical outcomes of continuous apomorphine infusion therapy [88], including 11 published studies (mainly [84, 87, 89-94]), the treatment resulted in an average 61% reduction of the time spent in the off phase after a mean followup period of 21 months. The daily L-DOPA dosage could be reduced by about 39%, and dyskinesias were significantly improved [91, 95]. When L-DOPA and apomorphine tests were performed before and after 6 months of apomorphine infusion, identical doses of L-DOPA/apomorphine produced 40% less dyskinesias following pump treatment [95]. The effects of apomorphine infusions seem to be stable over long-term follow-up [96]. The most pronounced clinical improvements using apomorphine infusion are seen in patients who manage with this treatment as a monotherapy [88]. However, in most cases apomorphine needs to be combined with peroral L-DOPA therapy to achieve a full clinical effect. The most frequent problem associated with apomorphine infusion is the formation of subcutaneous nodules. This occurs in almost all treated patients, and may lead to therapy discontinuation. The problem can be partly prevented by avoiding apomorphine concentrations higher than 5 mg/ml, and by changing the infusion area at least twice per day. The prevalence of psychotic complications is not higher with apomorphine compared to other dopaminergic therapies. Hemolytic anemia has been reported in about 3% of the treated patients [90, 93]. Besides pump treatment, apomorphine can also be given as s.c. bolus injections in order to terminate off periods occurring in spite of an optimized peroral therapy. The effective dose is the lowest apomorphine dose producing a full antiparkinson effect (typically 2-4 mg), and this has to be titrated individually in each patient. The injections are delivered to the patient s lower abdomen or outer thigh upon the first signs of an "off" episode. Domperidone is given during the first days of treatment and can later be tapered off in most patients. The efficacy of this treatment has been demonstrated in a number of studies [97, 98]. Injections of apomorphine effectively interrupts off periods: the mean time in off states per day is reduced by around 50%, and the remaining off periods are less severe than those occurring before the start of apomorphine treatment [85]. Transdermal Drug Delivery Transdermal drug delivery is a relatively recent development tried for several DA agonists with the aim of providing a continuous drug supply. A transdermal patch formulation of the non-ergolinic DA receptor agonist, rotigotine is indicated either as a monotherapy in the treatment of early-stage PD or as an adjunct to L-DOPA across all disease stages. Transdermal rotigotine has been shown to be superior to placebo in patients with early-stage and advanced PD, although non-inferiority to the oral DA receptor agonists, ropinirole or pramipexole, was not consistently demonstrated [99, 100]. The patch delivery option is advantageous in several situations, such as, (i) when peroral delivery is contraindicated by specific medical conditions (dysphagia; gastrointestinal side effects of peroral drugs; perioperative conditions; gastrointestinal dysfunction with delayed gastric emptying); (ii) in patients with therapy compliance problems (due to e.g., cognitive difficulties); (iii) in patients with Parkinsonrelated sleep disturbances in late night/early morning. The most common side effects of rotigotine are skin reactions, which occur very frequently and lead to termination of the therapy in about 5% of the treated patients [99-101]. Lisuride and apomorphine are also being investigated regarding the possibility of a transdermal delivery [102, 103]. Current Surgical Management of Motor Complications Functional neurosurgery, including lesional surgery and deep brain stimulation (DBS), is now widely indicated as a treatment option for PD when conventional pharmacological treatments fail. Compared to high-frequency DBS, lesional neurosurgery is an irreversible intervention with a larger incidence of complications, and it is therefore scarcely used today. For DBS, macroelectrodes are stereotactically implanted in the brain structure of interest and connected to an electrical stimulator, which is positioned subcutaneously in the sub-clavicular region. The basic stimulation parameters (voltage and frequency) can thus be individually adapted. Currently, two brain nuclei are mostly commonly used as a target for DBS in the complicated stages of PD: the internal segment of the globus pallidus (GPi), and the subthalamic nucleus (STN). Stimulation of the STN has become the most widely used method. This intervention has been found to be more effective than medical treatment in patients with advanced PD and motor fluctuations in a randomized multicenter study with health-related quality of life as the primary outcome parameter [104]. When comparing both STN- and GPi-DBS with best pharmacological treatment in patients with advanced PD, DBS was found to be superior using either target [105]. Regarding long-term outcome, a slow worsening of hypokinesia has been demonstrated, the effects against tremor and rigidity being more stable [106]. STNstimulation is often regarded as having a stronger antiparkinsonian effect than GPi-stimulation, but this has not yet been proven in comparative clinical studies. Regarding STN-stimulation a meta-analysis has shown an improvement in UPDRS motor scores by 50-52% (comparable to L-DOPA) and a reduction of L-DOPA doses by 50-60% [107]. A reduction of dyskinesias by 54-74% has been demonstrated in controlled studies [104, 105], an effect obtained mainly through reduction of dopaminergic medications [108]. Procedure-related

5 674 CNS & Neurological Disorders - Drug Targets, 2011, Vol. 10, No. 6 Cenci et al. side effects, like intracerebral bleeding, infections and misplacement of electrodes (with need for replacement) occur in up to 4% of cases, with an estimated mortality of 0.4% and long-term morbidity of 1% [109]. Neuropsychological worsening is mainly seen with respect to verbal fluency and in the Stroop test (which measures executive functions, such as selective attention, cognitive flexibility and processing speed) [110, 111]. Older patients seem to have a greater risk of cognitive worsening compared to younger ones, and the prevalence of cognitive side effects shows a pronounced variation across studies. The procedure is associated with an increased risk of suicide (15-fold larger risk during the first postoperative year, with a slow normalization over time [112]). The indications for STN-DBS are, severe disease with motor fluctuations and/or tremor that cannot be adequately controlled with pharmacological therapy (Fig. 1). STN stimulation is normally only recommended in patients with an age below years. Cognitive decline and major psychiatric symptoms are commonly regarded as contraindications (Fig. 1), whereas a good response to L-DOPA is a prerequisite to this intervention. High-frequency stimulation of the GPi improves UPDRS motor scores by about 33% [113, 114] and reduces the time spent in off by 30% to 60%, without any substantial reduction in L-DOPA equivalent doses [115]. GPi-DBS exerts a potent effect on dyskinesias that is unrelated to reductions in L-DOPA dosage and stable over at least 3-4 years [116, 117]. The surgical risks are comparable to those of STN-DBS, but the stimulation-related side effects seem to be less common [117]. Stimulation of the GPi is mainly chosen when dyskinesias are the major problem and/or to minimize the risk of cognitive side effects in older patients who need functional neurosurgery. Thalamotomy and thalamic stimulation are very effective against parkinsonian tremor, but have little efficacy on other motor symptoms and are therefore seldom used in advanced PD patients. FUTURE TREATMENT OPTIONS Gene Therapy to Provide Continuous Dopaminergic Stimulation Before discussing non-dopaminergic drug targets, we first briefly review the most recent strategy for CDS, which consists in restoring the enzymatic machinery to produce DOPA and DA in the striatum either by implanting genetically modified cells (ex vivo gene transfer) or by viral vector-mediated gene delivery to resident cells (in vivo gene transfer). The precursor of DA synthesis in the brain is L-tyrosine, an essential amino acid supplied by a normal diet. L-tyrosine is converted to DA by two enzymatic steps. It is first converted to DOPA by the enzyme tyrosine hydroxylase (TH), which requires the co-factor tetrahydrobiopterin (BH4) in its reduced form. The reduced form of BH4 is synthetized from guanosine triphosphate (GTP) in a three step enzymatic reaction catalyzed by GTP cyclohydrolase 1 (GCH1), which is rate-limiting, and other enzymes that are ubiquitously expressed in all cell types (reviewed in [118]). The TH enzyme is exclusively expressed by dopaminergic cells, and GCH1 is mainly located in dopaminergic and serotonergic terminals of the striatum. The second step of DA synthesis is catalyzed by aromatic amino-acid decarboxylase (AADC). The majority of AADC is found in dopaminergic and serotonergic fibers, but the enzyme is expressed also in other cell types, such as glial cells, endothelial cells and striatal neurons (reviewed in [22]). This extranigral AADC activity explains why the capacity for decarboxylation of exogenous L-DOPA is maintained even after a severe nigrostriatal lesion (reviewed in [118]). The in vivo gene transfer approach has yielded very encouraging results in preclinical animal models of PD, and is now much closer to clinical application than any ex vivo gene transfer approach. In vivo gene transfer consists in supplying DOPA- or DA-synthetizing enzymes to DA-denervated brain regions. One successful strategy is to make multiple striatal injections of recombinant adeno-associated virus (raav) coding for TH and GCH1, which results in efficient DOPA synthesis that is associated with significant behavioural improvement in 6-OHDA-lesioned parkinsonian rats [119]. When gene transfer of TH and GCH1 was applied to already dyskinetic rats, the severity of L-DOPA-induced dyskinesia (provoked by peripheral drug injections) decreased by at least 80% over a 12 week period [120]. This effect was paralleled by reversal of maladaptive molecular changes associated with dyskinesia, such as the upregulation of FosB and opioid precursor genes in striatal neurons [120]. A more recent study using optimized gene delivery (raav serotype 5 vectors instead of raav serotype 2) has shown improved efficacy in terms of sensorimotor recovery and resistance to the induction of L-DOPA-induceddyskinesia in previously untreated animals [121]. Similar raavbased in vivo gene transfer approaches have been applied with success also to non-human primate models of PD [122]. Striatal AADC gene transfer using adeno-associated viral vectors has been proposed as a method to locally increase the capacity for DA production, with the aim of reducing the dose requirement for peripheral L-DOPA. The method has been well characterized in non-human primate models of PD [123]. This particular approach might be useful for reducing non-motor side effects of L-DOPA pharmacotherapy, in particular, the psychiatric complications related to high DA levels in mesolimbic regions. A phase I safety trial has shown that raav-aadc treatment is well tolerated and results in sustained putaminal transgene expression [124]. Analysis of the clinical data from this study pointed to a modest improvement, although the nonblinded assessments and the absence of a control group preclude a definite interpretation of these results [124]. A third option for viral vector mediated in vivo gene delivery is by triple enzyme replacement (TH, GHC1, AADC) to completely restore DA production in targeted cells. Encouraging results have come from preclinical studies using a type of lentiviral vector (equine infectious anemia virus, EIAV) with triple enzyme delivery, both in rats [125, 126] and in non-human primates [127]. These successful experimental results have prompted a phase I/II clinical trial in PD [128]. Bilateral injections of the EIAV virus carrying the three genes required for DA synthesis are made in the putamen of PD patients, and preliminary reports have presented encouraging results [129]. In summary, viral vector-mediated gene transfer of enzymes required for DA synthesis appears to be a very promising avenue to a CDS therapy in PD. This approach will however entail all the risks and potential problems associated with a neurosurgical intervention. Treatments Targeting Non-Dopaminergic Systems One proposed approach to prevent or treat motor complications consists in adding non-dopaminergic drugs to the standard L-DOPA pharmacotherapy (Table 3). This approach appears particularly promising to reduce the severity of peak-dose dyskinesia and/or to prolong the time spent in a good on condition upon L-DOPA dosing. In addition, some non-dopaminergic treatments may ameliorate psychiatric or cognitive symptoms that are L-DOPAresistant [58, 130]. To this day, the clinical evaluation of nondopaminergic drugs has not yet delivered any concrete, new therapeutic options for PD. However, several new compounds are now being evaluated for their antidyskinetic efficacy in phase I/II to phase III clinical trials with some promising results. Here we review the most important categories of non-dopaminergic targets, which are attracting great interest on the part of both basic scientists and pharmaceutical companies.

6 Treatment of Dyskinesia and Motor Fluctuations in Parkinson s Disease CNS & Neurological Disorders - Drug Targets, 2011, Vol. 10, No Table 3. Non-Dopaminergic Compounds Reducing L-DOPA-Induced Dyskinesia in Animal Models of PD (Partial Listing) Target System Compound Name Proposed Target and Mechanism Species Seminal References Amantadine NMDA-R antagonism Mouse, rat, macaque [168, 181, 252, 253] CP-101,606 NR2B-specific NMDA-R antagonism Macaque [145] MPEP mglur5 antagonism Rat, macaque [151, 153, 155] GLUTAMATE MTEP mglur5 antagonism Rat, macaque [151, 152, 155] Fenobam mglur5 antagonism Rat, macaque [156] Topiramate AMPA/Kainate-R antagonism Rat, macaque [134] IEM 1460 Calcium-permeable AMPA-R antagonism Rat, marmoset [133] Clozapine 5-HT2/6-R (and D4R) antagonism Rat, macaque [168, 181, 182] SEROTONIN Buspirone 5-HT 1A-R partial agonism Mouse, rat [168, 169, 252] Sarizotan 5-HT 1A-R partial agonism Macaque [254] 8-OH-DPAT and CP HT 1A- and 5-HT 1B-R agonism Rat, macaque [163, 173, 175, 255] NORADRENALINE Idazoxan 2C Adrenoceptor antagonism Rat, macaque [168, 191, 193] Fipamezole 2C Adrenoceptor antagonism Marmoset [192] ADENOSINE Istradefylline (KW-6002) Adenosine A 2A-R antagonism Macaque [217] HISTAMINE Immepip, Imetit Histamine H3-R agonism Marmoset [256] ACETYLCHOLINE Nicotine Nicotinic-R desensitization Mouse, rat, squirrel monkey [246, 247, 250, 257] Glutamate Receptors Glutamate neurotransmission is involved in L-DOPA-induced dyskinesia at multiple pathophysiological levels (reviewed in [22, 23, 131]). In particular, glutamate receptors are critically involved in the synaptic and molecular alterations of striatal neurons that have been found to occur in dyskinetic animals (reviewed in [21]). The possibility to treat L-DOPA-induced dyskinesia by inhibiting ionotropic glutamate receptors was proposed by Thomas Chase and collaborators more than 10 years ago [60, 132]. Indeed, compounds with antagonistic properties at N-methyl-D-aspartate (NMDA) or amino-3-hydroxy-5-methyl-4-isoxazole proprionic acid (AMPA) receptors have shown antidyskinetic efficacy in animal experiments [ ] and in small clinical trials in PD patients [ ]. As previously mentioned, amantadine, an anti-infectious agent that exerts weak non-competitive antagonism at the NMDA receptor [139], is the only non-dopaminergic drug currently used for the clinical management of L-DOPA-induced dyskinesia [ ]. The NR2B subunit of the NMDA receptor is abundantly expressed in striatal neurons, and shows an altered subcellular distribution in the rat model of L-DOPA-induced dyskinesia [144]. NMDA receptor antagonists that selectively interact with the NR2B subunit have been shown to prevent the development of dyskinesia in a non-human primate model of PD [145] and to alleviate dyskinesia in a small clinical trial in PD patients [146]. However, widely discrepant results have been produced by this category of compounds in the preclinical literature, ranging between improvement [147], to no effect [148], to aggravation [149] of L- DOPA-induced abnormal involuntary movements. These data would suggest that the clinical outcome of NR2B antagonist treatment is heavily influenced by the structure/activity profiles of the specific compounds used and by the particular types of dyskinesias that need to be treated. A growing number of studies in both rodent and non-human primates models of L-DOPA-induced dyskinesia have reported promising results using antagonists of metabotropic glutamate receptor type 5 (mglur5), which is abundantly expressed in striatal neurons and becomes upregulated following chronic dyskinesiogenic treatment with L-DOPA [150, 151]. The first reports of antidyskinetic efficacy came from the rat model of L- DOPA-induced dyskinesia, in which mglur5 antagonists were shown to both prevent the development of abnormal involuntary movements and acutely reduce their severity [148, ]. These effects were accompanied by a normalization of molecular and neurochemical changes that are closely associated with the movement disorder [148, ]. Antidyskinetic effects of different mglur5 antagonists were then reported also in parkinsonian monkeys [155, 156]. Interestingly, two phase-ii clinical trials of a Novartis mglur5 antagonist in patients with PD have reported a significant improvement of L-DOPA-induced dyskinesia [157]. Although modulation of other types of mglur (group II and III in particular) has been suggested to be beneficial, this pharmacological approach has not yet delivered any promising antidyskinetic effect in animal studies [148, 158]. Serotonin Receptors Growing experimental evidence implicates the brain serotonin (5-HT) system in the pathophysiology of L-DOPA-induced dyskinesia. Serotonin receptors are expressed both postsynaptically and presynaptically in striatal neurons, where they modulate signaling pathways downstream of DA receptors [159, 160]. Moreover, serotonin neurons provide the main route of L-DOPA uptake and conversion in the brain when the nigrostriatal DA projection is severely compromised [161, 162]. These neurons lack high-affinity DA reuptake mechanisms and DA autoreceptors in their axon terminals, thus turning into a source of unregulated DA release after the administration of L-DOPA (reviewed in [22]). The causal implication of serotonin neurons in LID is proven by the dramatic reduction in dyskinesia severity engendered by 5-HTspecific lesions either in the forebrain [163] or in the midbrain raphe nuclei [164].

7 676 CNS & Neurological Disorders - Drug Targets, 2011, Vol. 10, No. 6 Cenci et al. Agonists of 5-HT 1A autoreceptors blunt the increase in striatal extracellular DA levels induced by L-DOPA [165, 166] and attenuate the severity of dyskinesia [ ]. Substances with agonistic activity at the 5-HT 1A receptor have shown antidyskinetic efficacy also in small clinical trials in PD patients [170, 171]. However, these promising results were not replicated in a large double-blind, placebo-controlled trial of the 5-HT 1A agonist, sarizotan [172]. Negative results in the latter trial may have depended on insufficient drug dosage, which in turn may have been dictated by the need to avoid adverse events. Indeed, all the currently available 5-HT 1A agonists can potentially interfere with the antiparkinsonian action of L-DOPA if given at sufficiently high doses [166, 173]. To circumvent this problem, Carta and Björklund have proposed to treat dyskinesia using a combination of 5-HT 1A and 5-HT 1B receptor agonists [174]. Because of their synergistic antidyskinetic effect, the two classes of compounds can be given at relatively low doses, thus minimizing the risk of adverse effects that are due to a stimulation of post-synaptic 5-HT 1A and 5-HT 1B receptors (described in e.g. [166]). Combined antidyskinetic treatment with 5-HT 1A and 5-HT 1B agonists was first tested in 6- OHDA-lesioned rats [163] and then in MPTP-lesioned macaques [175], achieving an antidyskinetic effect similar in magnitude to that of a lesion of forebrain serotonin projections [163]. Another possible option would consist in finding compounds that combine a 5-HT 1A agonistic profile with stimulatory properties on DA receptors. One compound fulfilling this profile seems to be piclozotan [176]. A phase II clinical trial of piclozotan in PD patients with motor complications (defining on time without dyskinesia as the primary outcome measure) has just been completed ( and results should be made publicly available soon. Among older drugs targeting 5-HT receptors, a particular mention should be given to the atypical antipsychotic drug, clozapine, which has shown antidyskinetic efficacy first in small open-labeled trials [177, 178] and then in a larger double-blind placebo-controlled clinical trial in PD [179]. Reductions in dyskinesia severity and duration were achieved by clozapine at doses that did not interfere with the therapeutic benefit provided by L-DOPA. This effect was thus attributed to the antagonistic properties of clozapine at 5-HT rather than DA receptors [177, 179]. Indeed, clozapine has low nanomolar affinity for serotonin 5- HT1A, 5-HT2 and 5-HT6 receptors [180]. From a translational viewpoint, it is worth noting that treatment with clozapine has been shown to reduce L-DOPA-induced abnormal involuntary movements in both rat [168, 181] and non-human primate models of PD [182]. A wider clinical application of clozapine may be limited by the risk of severe haematologic side effects, agranulocytosis in particular ( insert.pdf.ashx). Adrenergic Receptors Noradrenaline (NA) is an important neuromodulator in all brain regions, including the basal ganglia. It exerts its effects by interacting with adrenergic receptors (also called adrenoceptors), which are G-protein-coupled receptors belonging to two main groups, and, each including several subtypes. The 2C - adrenoceptor subtype is abundantly expressed in the basal ganglia, and particularly in the striatum, globus pallidus and substantia nigra pars reticulata (SNr) [183, 184]. In the striatum, alpha2cadrenoceptors are localized on GABAergic medium-sized spiny projection neurons (but not on interneurons) and they may modulate both the direct and indirect striatofugal pathways [185]. The important role played by alpha2c- and 1 adrenoceptors in the modulation of GABA release and neuronal excitability in the basal ganglia [186, 187] warrants focus on these receptors as targets for antidyskinetic and antiparkinsonian therapies. Another reason to be interested in this system is that L-DOPA is a precursor of both DA and NA. Hence brain levels of NA are expected to increase following L-DOPA administration, giving presynaptic 2A - adrenoceptors a false signal [188]. Furthermore, a recent study has shown that local infusion of NA in the 6-OHDA-lesioned rat striatum can by itself induce dyskinesia [189]. Several studies in rat and primate models of PD have shown that antagonists of 2B/C - adrenoceptors are effective in reducing dyskinesia and can prolong the antiakinetic effect of single L-DOPA doses [168, 181, ]. One potential underlying mechanism may involve a reduction of extracellular levels of DOPA and DA, which the 2C adrenoceptor antagonist idazoxan has been shown to achieve at a dose that significantly reduced the severity of dyskinesia in rats [193]. Antagonists of 1 -adrenoceptors have been recently evaluated in animal models of L-DOPA-induced dyskinesia, and found to be effective in rats [194], whereas they seem to merely attenuate L- DOPA-induced hyperactivity but not dyskinesia in MPTP-lesioned macaques [195]. The main limitation to the clinical use of adrenoceptor antagonists is given by potential adverse effects on the autonomic nervous system. Clinical trials with idazoxan in dyskinetic PD patients have indeed reported cardiovascular side effects, such as flushing, headache, tachychardia, and hypertension [196]. Adenosine Receptors Several neuromodulators within the basal ganglia are likely to be implicated in L-DOPA-induced motor complications. Among these, adenosine, opioids, and endocannabinoids are reviewed here because they are currently the focus of therapeutic development programs. Adenosine receptors represent attractive targets for nondopaminergic antiparkinsonian therapies (recently reviewed by [197, 198]). Adenosine A 1 and A 2A receptors are widely expressed throughout the brain. The expression of A 2A is particularly abundant in the striatum, where this receptor shows both presynaptic and postsynaptic localizations. In the striatum, postsynaptic A 2A receptors are expressed in GABAergic medium spiny projection neurons of the indirect pathway [199, 200]. This gives adenosine A 2A receptors an important regulatory influence on striatopallidal GABAergic transmission, which is mediated through multiple cellular mechanisms [201]. Functionally, the adenosine A 2A receptor is linked to both D2 and mglur5 receptors [202], with which it can heterodimerize [203, 204]. Indeed, high-resolution immunoelectron microscopy has revealed that the three receptors co-distribute within the extrasynaptic plasma membrane of the same dendritic spines of asymmetrical (putative glutamatergic) striatal synapses [205]. Adenosine A 2A receptor stimulation decreases the binding affinity of D2 receptors for DA [206, 207]. At the downstream signalling levels, A 2A stimulation counters D2 receptor-mediated inhibition of camp formation, activating nuclear signaling pathways and the expression of immediate early genes in striatopallidal neurons [159, 208, 209]. Preclinical studies have demonstrated that adenosine A 2A antagonists have anti-akinetic properties in both rodent [210, 211] and non-human primate models of PD [212, 213]. This anti-akinetic effect has been attributed to an inhibitory action on the striatopallidal pathway [214]. Great interest has been raised by the potential use of A 2A antagonists as an antiparkinsonian treatment. Moreover, antagonism of A 2A receptors has been proposed as a treatment for L-DOPA-induced dyskinesia by several authors (reviewed in [215]). Supporting such a proposal, a human post mortem study showed increased putaminal expression of the A 2A receptor mrna in dyskinetic PD patients compared to nondyskinetic cases [216]. Moreover, coadministration of an A 2A receptor antagonist (KW-6002) with apomorphine was shown to prevent the development of dyskinesias in MPTP-treated primates [217]. More recently, conditional ablation of forebrain A 2A receptors was found to attenuate L-DOPA-induced dyskinesia in 6- OHDA lesioned mice [218]. However, studies in 6-OHDA-lesioned