Introduction

Parkinson’s disease (PD) is a multifaceted neurological disorder comprised of both motor and nonmotor symptoms at all stages of the disease. PD is the second most common neurodegenerative disorder in prevalence, consisting in a progressive condition caused by degeneration of dopaminergic neurons in substantia nigra pars compacta and the presence of Lewy bodies abnormal protein aggregates, that include alfa-synuclein and ubiquitin. However, the precise pathogenic mechanisms of the disease are still unknown. Some of the common motor symptoms caused by this disorder are bradykinesia, postural instability, rigidity and tremor. As life expectancy rises, PD incidence is increasing and over 1 % of population above age 60 is affected, with overall life-time risk approaching 4 % (Barbosa et al. 2006; Schapira 2013).

Despite the prevalence of the disease, no cure or definitive therapeutic option is available to this date. The non-invasive gold standard treatment for motor symptoms of PD is the dopamine precursor levodopa (L-DOPA; L-3,4-dihydroxyphenylalanine), a highly effective strategy. Unfortunately, long-term treatment with L-DOPA is associated with motor fluctuations, dyskinesia (abnormal involuntary movements) and psychiatric problems (Stocchi 2005). Dyskinesia affects the vast majority of patients at some point during L-DOPA treatment. Because of its side-effects, L-DOPA treatment has to be suspended in many cases (Stocchi 2005). Together with the natural progression of the disease, it makes managing of advanced PD hard task. Hence, there is great interest for the synthesis of new drugs and novel therapeutic strategies.

Nanotechnology is a science broadly defined by the manipulation of compounds in the nanometric scale. One nanometer is the equivalent to one billionth of a metre. The first reports on the theoretical bases of the field started early in 1959 in a talk given by Richard Feynman (Feynman 1959), although the term nanotechnology itself was only coined in 1986 (Drexler 1986). In nanotechnology, molecular or even atomic level manipulation, aided by very specific techniques, enable enhancement of the physicochemical properties of many compounds. Nanoparticles vary in size, but tend to be much smaller than a red blood cell (which has around 7000 nm in diameter) and sometimes, smaller than a virus (Human Immunodeficiency Virus—HIV—for instance, has a diameter close to 120 nm). Although the exact definition varies, the European Medicines Agency considers any structure under 1000 nm designed to have specific properties as nanotechnology. Within the field, one of the most known areas is the nanotechnology of materials applied to the production of lighter, more efficient, tougher, overall enhanced materials. Production of magnetic nanoparticles, nanosensors, nanoconductors, nanodevices and beyond, are other examples of applications. More than that, nanotechnology products are used in many areas of research, industrial improvement, creation and upgrade of novel materials and energy production.

Nanomedicine can be defined as applying nanotechnology to medical and health-related purposes. One of the most explored approaches has being refine imaging tests, e.g. using nanoparticles that function as contrast to enhance tumours detection in magnetic resonance (Iv et al. 2015). The production of nanobiosensors for precise detection of specific molecules, like dopamine (Mercante et al. 2015), glucose (Scognamiglio 2013) or many others, is another promising field. Similarly, nanotechnologies may improve the delivery of drugs. This is achieved by manipulating a nanoparticle to exhibit ideal properties and associating it to a drug. The goal of this association is to improve, for instance, the drug bioavailability, half-life, and to achieve more sustained levels in circulation. A number of recent reviews discuss the use of nanotechnology in neurodegenerative diseases (Re et al. 2012; Spuch et al. 2012) including in PD (Garbayo et al. 2013; Pathak and Akhtar 2016) and PD models (Huang et al. 2009). Our objective is to review the translational potential of nanotechnology drug delivery in PD.

Regarding the intersection between PD and nanomedicine, it is important to note that drug delivery is not the only area in which studies are focusing on. There are very consistent works covering delivery of neuroprotective factors, enzymes and anti-oxidants (e.g. Haney et al. 2011; Ksendzovsky et al. 2012; Da Rocha et al. 2015), growth factors (e.g. Fletcher et al. 2011; Herrán et al. 2014), α-Synuclein aggregation inhibition (e.g. Joshi et al. 2015), imaging and detection (e.g. Park et al. 2014; Agarwal et al. 2015) and deep brain stimulation (e.g. Yue et al. 2012), among many others. After our screening for this work, there were at least 250 studies worldwide in the databases researched (for details, check methods) involving PD and nanomedicine. The majority of these studies were realized in the last 10 years, showing that nanomedicine and nanotechnology are becoming an important part of the scientific advances involving many aspects of PD. Some of those works explore specific physiological mechanisms like alpha-synuclein folding and biomarkers of the disease, opening many options for further studies. It is likely that nanotechnologies will continue playing an important role in future breakthroughs in many areas, including neurodegenerative diseases like PD.

To the best of our knowledge, this is the first review on how nanomedicine and nanotechnology are searching for solutions to PD treatment in a systematic way. At least, part of it may be explained by the freshness of the field, as most of the papers reviewed here were published less than 5 years ago. Our aim is to present a well-based picture of how nanomedicine is being applied in different drug formulations to help overcoming barriers in PD treatment.

Methods

This systematic review was conducted considering PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) methodology (Moher et al. 2009). Our methodology is summarized in Fig. 1.

Fig. 1
figure 1

The methodology used in this systematic review. The top red boxes show details on the search strategy used, and bottom red box shows details of the exclusion criteria. The blue box contains the fluxogram that comprises search on databases, removal of duplicates, screening and the final inclusion of 28 studies

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Sources of Information

Three databases were searched (PUBMED, Lilacs and Central [Cochrane]) through May and June. In the final screening date, June 19, 2015, PubMed provided 291 results, Lilacs provided 2 results and Central provided 4 results. No data restraints were imposed. Therefore, all papers found were evaluated for their relevance to our objective. Before starting our screening, Web of Science database was evaluated but had fewer results compared to PubMed, so PubMed was selected as our main database and Lilacs and Central complemented our search. No other sources of information were used.

Search Strategy

A combination of keywords such as nanomedicine, nanotechnology, nanoparticle, nano and Parkinson’s was used in PubMed (the algorithm was “nanotechnology and Parkinson’s OR nanomedicine and Parkinson’s OR nanoparticle and Parkinson’s OR nano and Parkinson’s”). For the two other bases, Lilacs and Central, we used the algorithm “Parkinson’s AND nano*”. There were no duplicates between the different bases, since the vast majority of studies came from PubMed.

Article Selection and Exclusion Criteria

Titles, abstracts and full texts were evaluated considering strategies that explored improvement on current PD treatment using nanotechnology. We excluded every study that, besides using nanotechnology, did not focused on drug treatment. For instance, we excluded studies that focused on α-Synuclein physiological mechanisms or interactions, neuroprotective characteristics of enzymes or molecules, growth factors, imaging agents or deep brain stimulation. Finally, 28 studies were selected and included in this systematic review, evaluating strategies regarding dopamine delivery and the drugs L-DOPA, ropinirole, bromocriptine, apomorphine, urocortin, rasagiline and selegiline.

Overall Approaches

Different nanoparticles can be manipulated to show a very large spectre of properties. Additionally, a nanoparticle can be associated to drugs in many different ways, leading to plenty of applications. For example, an adhesive nanoparticle may be manipulated for one purpose (e.g. residing longer in nasal cavity), a nanoparticle associated with a sugar can be used to another (e.g. help blood–brain barrier [BBB] crossing) and trapping drugs among a lipid structure can serve other function (e.g. improve uptake by the gastrointestinal tract). Some nanoparticles have a protective function, inhibiting degradation, and others serve to aid sustained drug release. Therefore, it is no surprise that nanomedicine allowed solid improvement of drug delivery in the past few years. There are satisfactory reviews that cover this theme in many different areas (e.g. Hamidi et al. 2013; Singh et al. 2012; Fonseca-Santos et al. 2015); our aim is to review this improvement on PD treatment.

The drugs that will be discussed here are L-DOPA, ropinirole, bromocriptine, apomorphine, urocortin, rasagiline and selegiline, based on common and viable PD treatment strategies (Nyholm 2006). Most of the studies described here presented data from the association of the drugs above highlighted with nanoparticles using animal models. Articles collected here show how successful these efforts have been both individually and as a collective, something that becomes very noticeable when those works are put side by side. At this point, there are promising results with more than five marketed drugs.

For quicker assessment, the critical points of all the works described in this review are summarized on Table 1, with exception of the study conducted by Demirel et al. (2001), which was excluded for focusing mainly on microparticles instead of nanoparticles.

Table 1 Summary of the PD studies using nanomedicine
Full size table

Some main approaches described here were used in drug delivery and justify deeper examination at this point. The treatment of PD symptoms requires drugs that should be able to interact with the brain. The first approach widely used by the studies reviewed is to use nanoparticles to ameliorate drug cross through BBB, hence elevating drug bioavailability to the brain and leading to increased efficacy. As we know, some drugs such as L-DOPA cross BBB by use of saturable transport systems. However, conversion of L-DOPA to dopamine is not exclusive to the brain, occurring in peripheral tissues and causing unwanted side-effects. This also reduces the amount of prodrug that reaches the brain. A variety of approaches have been found to increase L-DOPA transport to the brain, like associating L-DOPA with nanoparticles and galactose, allowing passage through GLUT-1 (glucose transporter 1) receptors in BBB cells (Malvindi et al. 2011). Another example is associating L-DOPA with iron, which has its own receptors in BBB, enabling better L-DOPA uptake (Hu et al. 2011). Another approach even suggests the use of ultrasound waves to temporarily disrupt the integrity of the BBB, thus allowing the passage of the nanoparticles (Hwang et al. 2009).

To this date, however, there is no precise explanation of how those different nanoparticles are able to cross BBB. There is a hypothesis that some kind of nanoparticles could open tight junctions between BBB cells (Mittal et al. 2014). On the other hand, in some works, it was demonstrated that the mechanism of BBB crossing by the nanoparticle was through endocytosis (Malvindi et al. 2011; Hu et al. 2011). The precise mechanism enrolled on nanoparticles crossing to BBB is currently on intense discussion.

Intranasal delivery is another commonly used approach by researchers in this field. It consists in a non-invasive way through which toxins (in experimental conditions) or drugs may be administered to exert local or systemic effect. In experimental conditions, it appears that the intranasal delivery compounds travel directly via the olfactory tract or the trigeminal pathways to the brain, thus evading BBB (Kulkarni et al. 2015). The nasal cavity is very accessible and has great surface area; drugs introduced this way avoid first-pass metabolism (Costantino et al. 2007). Although not only nanoparticles could be absorbed in this way, employing nanoparticles usually accelerate the absorption rate. Also, some desirable properties can be manipulated using them. For instance, Shadab et al. (2014) explored the mucoadhesive nature of chitosan nanoparticles to nasal mucosa and used it as a tool for longer adherence of bromocriptine in the loci, hence improving its absorption. For all these advantages, intranasal delivery appeared as a novel trend in studies concerning nanomedicine and PD treatment, as discussed later.

One last technique requires attention for being largely employed in the field of nanotechnology overall. It consists in masking the chemical and physical characteristics of a specific drug by associating it with a lipid-based manipulated nanomolecule, like a membrane, that traps the compound inside itself or adhere the drug to its structure. With this approach, some benefits like lower systemic degradation or better penetration through a specific tissue can be obtained. A good demonstration of this mechanism can be found in the study conducted by Tsai et al. (2011) involving apomorphine (further reviewed here), where lipid nanoparticles taken via oral stimulate chylomicron formation. Hence, there is better systemic absorption of apomorphine through the lymphatic system and systemic uptake is enhanced.

On the next sections, we discuss each of the 28 selected works that are summarized in Table 1, including a quick introduction of the drugs and their liabilities.

The Use of Nanoparticles to Improve Drugs Intranasal Absorption

L-DOPA

As discussed before, low dopamine levels secondary to dopaminergic loss provokes poor movement control and originates PD symptoms. To correct this dysfunction, dopamine levels are replaced with drugs such as dopamine agonists and dopamine precursors. Dopamine cannot cross BBB since it is a highly hydrophobic particle, and there is no specific transport system to allow its passage to the brain parenchyma (Bonina et al. 2003). L-DOPA is nowadays the main option in PD treatment, in use for at least three decades worldwide. This drug penetrates the BBB through an active process mediated by a carrier of aromatic amino acids where is converted to dopamine by the enzyme aromatic L-amino acid decarboxylase (AADC; Nyholm 2006). L-DOPA can be converted to dopamine in peripheral body regions. For this reason, L-DOPA is associated with peripherally acting inhibitors of AADC like carbidopa or benserazide that are able to reduce the peripheral conversion of the drug into dopamine (Nyholm 2006).

Apart from satisfactory initial effects, its effectiveness often declines and various L-DOPA-related side-effects appear after long-term treatment. The disabling side-effects of L-DOPA therapy include motor fluctuations such as the wearing-off or on–off phenomena, dyskinesia and psychiatric symptoms (Stocchi 2005). Gradually, treatment gets less viable, and because PD is a progressive disease, patient’s functionality gets severely compromised. The precise mechanisms for the appearance of these treatment-related fluctuations are not clear. A new L-DOPA formulation that guarantees sustained release is desirable for its potential to lessen oscillations in dopamine concentration, reducing peak and valley variations while enabling less drug intakes per day. Importantly, dyskinesia could be also reduced through sustained release of small dopamine concentrations, although the precise mechanisms are still unknown (Kim et al. 2006; Smith et al. 2005).

Sharma et al. (2013) used chitosan nanoparticles intranasally, to assess if brain delivery of dopamine could be improved and peripheral degradation could be avoided. To achieve this, those nanoparticles were incorporated with L-DOPA in a specific gel (thermo-reversible Pluronic F127 gel) that facilitates administration and increases the formulation residence time in nasal cavity. Their results were positive, as sustained release of L-DOPA from the particles could be observed. Also, there was great L-DOPA recovery in the brain, demonstrating that intranasal delivery of the drug was viable and could have great potential with further studies.

In 2014, Gambaryan et al. (2014) also experimentally tested the efficacy of L-DOPA intranasally. After a neurotoxin 6-hydroxydopamine (6-OHDA) infusion in the right medial forebrain bundle, rats were tested for apomorphine-induced rotations to access the level of dopaminergic lesion. The apomorphine-induced rotational movements contralateral to the side of the brain that had been damaged are found in the case of severe imbalance in dopaminergic innervation in the brain hemispheres (So et al. 2012). In this study, marketed L-DOPA was compared with L-DOPA associated with nanoparticles, called nano-DOPA (approximate size of 250 nm). Drugs were administered daily for 13 weeks, and motor coordination of animals tested with several tests during this period (placing task, open field test, vertical grid holding test, footfault asymmetry test). About 30 min after the first administration, nano-DOPA-treated rats had approximately double motor coordination efficiency compared with conventional L-DOPA-treated rats after 4 weeks. Differences between the two approaches could be seen after only 1 week of treatment and persisted even after discontinuation of the treatment (lasting about a week). The intranasal delivery of nanoparticles was confirmed, together with their longer half-life and bigger efficacy compared with L-DOPA alone. This approach is a very promissory option to overcome L-DOPA current treatment limitations. As authors suggested, considerable reduction of dose and administration frequency may be possible using this nanodrug in clinical patients.

In the next year, Di Gioia et al. (2015) tested a combination of nanoparticles, dopamine and another substances with the goal of delivering dopamine to the brain, also via intranasal. The most promissory formulation, as related, contained glycol chitosan and sulfobutylether-b-cyclodextrin within the structure. Applying the particles to one nostril alone, they could compare two different results in the brain of the same rat. They were able to demonstrate via fluorescence microscopy that particles successfully reached the brain, and no evident damage to the brain tissue was found. As related, acute administration of their special dopamine nanoparticle could not modify dopamine levels in the striatum, but this effect was achieved by repeated administration. Authors pointed the potential of this nanocarrier for non-invasive delivery of dopamine to the brain.

Ropinirole

Dopamine agonists, as ropinirole, exert anti-parkinsonian effects by acting directly on dopamine receptors. Ropinirole is an agonist at dopamine D2 receptors (D2R) that mimics the endogenous dopamine action. For that reason, it is highly useful in the treatment of PD. Ropinirole can be used alone or in combination with L-DOPA, helping reducing L-DOPA dosage. Nonetheless, the drug has poor bioavailability and short half-life, requiring intake of three doses a day (Kaye and Nicholls 2000).

Pardeshi et al. (2013a, b) tried an intranasal delivery approach that associated ropinirole with solid lipid nanoparticles. No severe signs of loss of integrity of the nasal mucosa were observed as the drug was delivered in a sustained manner. They found reduced tremors in rats where the intranasal delivery was used in comparison to their control matches, who had received the marketed oral formulation. The research introduced the overall feasibility of the intranasal administration of ropinirole, which brought benefits like sustained-release (thus reducing peak-to-valley fluctuations) and could help reducing dosing frequency.

In a study published by Patil and Surana (2013), the use of ropinirole-loaded poly-lactic-co-glycolic acid-based biodegradable nanoparticles was assessed. The particle could release ropinirole in a sustained manner in vitro for up to 24 h, contrasting the marketed drug (released in about 2 h). Almost 98 % of this compound was able to penetrate freshly excised sheep nasal mucosa in 24 h (ex vivo), in comparison to 75 % of the original drug. The mucosa also showed no severe signs of damage.

Another very interesting approach was made by Jafarieh et al. (2014) now in 2014, applying ropinirole hydrochloride-loaded chitosan nanoparticles in rats via intranasal administration. The mucoadhesive nature of the particles allowed more bioavailability, as particles had more residence time in nasal cavity. Sustained-release profiles up to 18 h were demonstrated, and concentrations were higher compared with the free drug solution. Indeed, the drug was able to reach the brain, indicating BBB bypassing and high uptake. Gamma scintigraphic studies showed that while the presence of the drug in the brain decreased at 1 h with the use of free ropinirole, a sustained concentration could be maintained up to 2 h with the association of ropinirole with nanoparticles. Overall, bioavailability increased, indicating the superiority of such system over conventional approaches.

In 2015, Mustafa et al. (2015) tested chitosan-coated oil in water nanoemulsion combined with ropinirole via intranasal route in PD models. Rats administered with the ropinirole nanoparticle had partial recovery of muscular coordination and were able to swim better. Furthermore, the particle yielded better results than solid ropinirole administered intranasally (or intravenous). Ropinirole in nanoemulsion applied intranasally had better mucoadhesion, resisting longer before being washed away by nasal mucus and thus enhancing its brain bioavailability. Together, these studies show very noticeable advantage of the ropinirole delivery systems associated with nanoparticles compared with the actual marketed drug. New options have been presented, and certainly, will be explored in a near future. The expectation is that these new nanoparticle formulations could help lowering the doses needed for effect, better resolving peek and valley fluctuations, and reduce the number of daily doses. The associated use of L-DOPA with ropinirole would probably also benefit, yielding even more positive advances.

Bromocriptine

Bromocriptine is a potent ergot-derived agonist of D2R and a partial agonist of D1 receptors (D1R). Bromocriptine was the first dopamine receptor agonist on market, approved since 1974 for clinical use. Today, it can be used as an adjuvant to L-DOPA therapy or alone (especially in early stages of PD), with an onset of action of 1–2 h and a half-life of about 5 h (Radad et al. 2005). However, bromocriptine therapy has some known limitations like other treatments for PD, including short half-life (although higher than levodopa) and unstable plasmatic concentrations.

In 2014, Shadab et al. tested intranasal delivery of bromocriptine combined with chitosan nanoparticles. They used haloperidol-induced pharmacological PD in a mice model and, after intranasal administration; authors observed that use of bromocriptine nanoparticles increased brain targeting and absorption compared with bromocriptine solution alone via intranasal or even bromocriptine nanoparticles given intravenous. Intranasal bromocriptine nanoparticles had the highest concentration at all-time points after dosing and showed a sustained-release pattern. Authors argued that the mucoadhesive nature of the nanoparticles made them adhere for a longer period to nasal mucosa, hence improving penetration. Additionally, histopathology study of mice brain showed that the use of bromocriptine nanoparticle could still prevent some damage caused by haloperidol-induced parkinsonism if the drugs were administered together.

Rasagiline

Rasagiline is a selective and irreversible second-generation inhibitor of monoamine oxidase type B (MAO-B). Inhibiting MAO-B blocks dopamine metabolism, prolonging its overall activity. This function allows rasagiline to be used as monotherapy or as adjunctive therapy for PD, associated with L-DOPA (Perez-Lloret and Rascol 2011). Clinical trials suggested that rasagiline has neuroprotective and disease-modifying properties (Olanow et al. 2009). On the other hand, MAO-B inhibition causes several collateral effects like nausea, headache and dizziness, limiting rasagiline application. Also, problems like low oral bioavailability (around 36 %) and short half-life (around 2 h) do exist. But, in 2014, rasagiline-loaded chitosan glutamate nanoparticles have been developed by Mittal et al. (2014) in attempt to overcome some of those problems. In mice, the nanoparticles associated with rasagiline were intranasally administrated and were compared with either the nanoparticles via intravenous route or with single drug intranasally applied. Able to surpass hepatic first-pass metabolism and with better bioavailability, the superiority of the rasagiline-associated nanoparticles was attested compared to other strategies in trials. This new approach could enable the expansion of rasagiline use in PD treatment, although more pre- and clinical studies are needed.

Selegiline

Selegiline is also a MAO-B inhibitor that has efficacy similar to rasagiline (Marconi and Zwingers 2014). In 2014, Gulati prepared selegiline hydrochloride-loaded chitosan nanoparticles and tested their potential in vitro. They found a sustained-release pattern of up to 28 h; like discussed before, this may contribute to reduce dosing frequency, enhancing treatment adhesion. It was also postulated that those particles would easily penetrate nasal mucosa for their small particle size. It is likely that this approach will be tested in rats soon, opening yet another avenue to overcome PD treatment limitations using the intranasal route.

Urocortin

Wen et al. (2011), described the association of urocortin with odorranalectin nanoparticles to prolong the resident time of the drug in nasal cavity, allowing better passage to the brain via intranasal route. Odorranalectin is one of the smallest members of the lectin family, sugar-bend proteins that can recognize sugar molecules and therefore adhere to the glycosylated nasal mucosa. Thus, the association of odorranalectin with nanoparticles improved adherence of nanoparticles into nasal cavity. This is an interesting approach, since the mucus flow of nasal cavity constantly washes the local compounds, a necessity so smell recognition can work perfectly. Urocortin was added to this nanoparticle complex, a member of the corticotrophin-releasing factor family which was shown to partially restore nigrostriatal function over long-term administration (Abuirmeileh et al. 2008). The peptide has neuroprotective effects but has no BBB penetration, hence the necessity of the nanoparticle association. Rats treated with those nanoparticles had their rotational number induced by apomorphine dramatically decreased compared with rats where only urocortin or only nanoparticles associated with urocortin (without odorranalectin) were administered.

The Use of Nanoparticles to Increase Drugs Passage Through the BBB

Malvindi et al. (2011) synthesized fluorescent CdSe/CdS (cadmium, selenium, and sulphur) quantum rods and nanoparticles, that were later integrated with galactose and dopamine. This clever association with the sugar enabled the particles to be internalized through the GLUT-1 receptor in vitro, a receptor expressed in the BBB endothelium, through an endocytic pathway. Besides the delivery of dopamine, the quantum nanoparticles had the unique property of being extremely good contrasting agents, overcoming some limitations on data acquisition and enabling better evaluation of the physiological mechanisms. Dopamine was released with success after crossing the artificial barrier in vitro.

In the same way, De Giglio et al. (2011) and Trapani et al. (2011) described the dopamine-loaded chitosan nanoparticles and the evaluation of their several characteristics. After immersed in a solution of dopamine, the nanoparticles absorbed the neurotransmitter and were able to deliver dopamine through BBB after intraperitoneal infusion (confirmed by analysis of striatum concentration of dopamine). A dose-dependent increase of dopamine levels was confirmed, and the administration produced a peak around 80 min.

Hu et al. (2011) reported the production of lactoferrin nanoparticles combined with urocortin. Lactoferrin is an iron-binding glycoprotein that has anti-inflammatory, immunomodulatory and antimicrobial proprieties. A receptor for this particle is present in the BBB, allowing its transport to the brain. Thus, associating nanoparticles with lactoferrin is a clever strategy to enhance brain delivery. The researchers associated urocortin within the structure, and therefore, the therapeutic effects of the formulation could be assessed on PD model in rats. After intravenous administration on hemilesioned rats, the group that received lactoferrin nanoparticles associated with urocortin presented significantly lower rotational behaviour induced by apomorphine, indicating a potential anti-parkinsonian activity. Furthermore, histological analysis showed an improvement in the tyrosine hydroxylase immunoreactivity in the striatum with the use of the nanoparticles, which was not reproduced by other control approaches. Dopamine level analyses showed increased dopamine concentration in the striatum of rats treated with urocortin lactoferrin nanoparticles. Thus, urocortin had been delivered to the brain by a non-invasive way, proving the lactoferrin nanoparticles efficiency. Once again, these promissory results demonstrate that there is plenty of room for improvement in PD treatment involving nanomedicine.

Other Approaches

This session covers studies that used different approaches to improve drug delivery from those previously described herein (enhanced BBB crossing or intranasal administration). Most of these studies used a nanoparticle association to improve half-life of the formulations, enable sustained release or enhance another pharmacological aspect of a given drug. There were also other approaches; some studies used platforms implanted directly into the brain, and others tested new hypothesis in vitro.

L-DOPA

Pillay et al. (2009) described the use of computer simulations to produce a platform (nano-enabled scaffold device) that contained dopamine-loaded nanoparticles (DA-loaded cellulose acetate phthalate nanoparticles) adhered in its structure. Later, they surgically implanted this platform in the brain of healthy rats and tested how dopamine was released to the brain parenchyma (frontal lobe). Their results showed that the neurotransmitter was successfully released from the structure, resulting in a stable concentration as the particles were released in a constant manner (up to 30 days). Dopamine concentration in blood was low, indicating a small systemic escape from the platform, which was a very desirable property.

López et al. (2011) had a similar approach. Rats were stereotactically injected with 6-OHDA in the right substantia nigra pars compacta to produce a lesion in the nigrostriatal pathway. After the implant of nanosilica-dopamine reservoirs in the striatum, animals demonstrated a reduction in the rotational behaviour induced by apomorphine compared to control animals, (which were not implanted, or were implanted with an empty platform). In this study, rotational behaviour values in implanted rats reduced by half (57 % of the original values) compared to other strategies. It was postulated that dopamine was released from the reservoirs in two steps: (i) a fast-sustained release in the first day, and from then on, (ii) a constant slower release pattern. The implanted rats showed no signs of induced dyskinesia at any point, which was assumed as an indicative of slow and tonic dopamine release. This is a benefit over intermittent systemic drug administration strategies like the daily use of oral L-DOPA, which produce dyskinesia. They concluded that the nanosystem was biocompatible with cerebral tissue and had potential to be used in PD patients.

In 2012, Yang and cols. prepared L-DOPA methyl ester/benserazide-loaded nanoparticles (size of approximately 500 nm) that released L-DOPA and benserazide (a DOPA-descarboxylase inhibitor that reduces peripheral conversion of L-DOPA) in a constant pattern. They expected that drug concentration peaks existent in standard treatment could be lessened using these particles. Researchers used an experimental lesion model of PD in rats and then chronically treated the animals with L-DOPA to induce dyskinesia. Then, L-DOPA + benserazide classic combination given subcutaneously was tested in comparison with the nanoparticle associated treatment, also subcutaneously. Rats that received the L-DOPA/nanoparticles formulation presented less dyskinesia. They also measured molecular markers that are known to be elevated in dyskinetic individuals (such as phospho-dopamine and cAMP-regulated phosphoprotein of 32 kDa [p-DARPP-32], phospho-extracellular signal-regulated protein kinases 1 and 2 [p-ERK1/2] and the transcription factor-ΔfosB) and, as described before, the nanoparticle group had better results, since there was partial prevention of dyskinesia molecular markers expression.

Following the timeline, Kura et al. (2013), intercalated L-DOPA with zinc/aluminium-layered double hydroxides, resulting in a nanocompound that had the capacity to release L-DOPA in a sustained manner. In vitro studies demonstrated great viability of 3T3 cells (cell line derived from mouse embryonic fibroblast cells) exposed to the nanoparticles, showing the important fact that the particles had no important toxicity (being described as less cytotoxic than L-DOPA alone). Posterior studies from the same team suggested that this compound had potential to improve L-DOPA treatment. Kura (2014) tested the particle in PC12 cells (cell line derived from the rat adrenal medulla), also in vitro, and concluded that no toxic effects existed either. PC12 cells appear to be more suitable for Parkinson’s disease study estimations since they can be differentiated into a dopaminergic cell with use of growth factors.

Similar approach was reported by Tan et al. (2015). They managed to associate L-DOPA with single-walled carbon nanotubes. This nanoparticle showed a release pattern that lasted more than 20 h and was not toxic to PC12 cells in vitro. As the authors pointed out, in high concentrations, carbon nanotubes tend to form clusters that can harm cell growth and therefore need to be carefully evaluated. With further studies, this technology has potential.

Pahuja et al. (2015) prepared dopamine-encapsulated poly-lactic-co-glycolic acid (PLGA) nanoparticles, also used by Patil and Surana 2013. After 6-OHDA lesion, particles were given via intravenous infusion and crossed BBB. The particles could maintain higher dopamine levels in the rat striatum, with a sustained-release pattern. Significant less rotational scores and higher spontaneous locomotor activity were obtained, and no toxicity effects were observed.

Ropinirole

Azeem et al. (2012) tested a transdermal therapeutic system that would deliver ropinirole through the skin in a controlled and stable rate. The conventional use of the system has limitations such as drug inability to cross the skin barriers, but with the use of nanotechnology, represented here by nanoemulsion in the form of an oil base nanocarrier system, the absorption was greater. In fact, with this approach, bioavailability of ropinirole was enhanced more than twofold. As they applied their system in rats, the absence of histological injuries suggested the safety of the compound. Another important finding was that biochemical markers of parkinsonian rats were partially restored in comparison to control group, showing not only pharmacokinetic improvement but also a potential disease changing advantage.

Bromocriptine

Esposito et al. (2008) described an association of bromocriptine and solid lipid nanoparticles (nanostructured lipid carriers). After intraperitoneal injections, they compared the delivery of the nanodrug to the brain against the delivery of the free drug in hemilesioned rats. Their findings were that bromocriptine nanoparticles had faster onset and provided more stable plasmatic levels, with likely prolonged half-life and anti-parkinsonian action. In further investigations (Esposito et al. 2012), a comparison of nanostructured lipid carriers and monoolein aqueous dispersions was asessed. Besides both particle groups were capable of encapsulate bromocriptine, only the first was able to efficiently attenuate motor deficit in hemilesioned rats.

Apomorphine

Apomorphine is a D1R and D2R agonist. The drug is used to treat PD symptoms and has an effect comparable to L-DOPA. However, apomorphine has certain disadvantages in clinical practice, considering its short half-life due to first-pass effect in the liver, resulting in oral bioavailability close to 2 % (Tsai et al. 2011). Therefore, the oral use of apomorphine requires large doses, which in turn can result in unpleasant side-effects such as nausea and vomiting. To avoid those problems, the drug is used today via subcutaneous route, but several applications are required (often over 10 per day). Although the fact that apomorphine is nowadays used in a pen or pump system in some locations (Henriksen 2014), it is still an invasive approach that limits the clinical potential of apomorphine. As a step to overcome these problems, researchers have been looking for novel formulations to associate with apomorphine. Hwang et al. (2009) developed apomorphine-loaded perfluorocarbon nanobubbles and tested them as delivery systems in vitro. Ultrasound pulses were used to temporary disrupts the integrity of a simulated membrane, allowing the passage of the nanoparticles and their successful release from the structure.

Tsai et al. (2011) went further, and using apomorphine solid lipid nanoparticles with emulsifiers via oral route, they were able to successfully delivery the drug in vivo to rat model of PD. The results indicated detectable apomorphine concentrations in different regions of the brain, including striatum, confirming the method efficacy. Bioavailability was increased more than 10-fold compared with controls, which received oral apomorphine alone. With this approach, frequency and dose of apomorphine intake could be reduced. The nanoparticle ability to enhance transport was attributed to an increased lymphatic transport (as the drug was associated with lipid nanoparticles), drug protection and sustained release, as reported by authors. These findings are a good demonstration of how nanomedicine may be an important ally in solving pharmacological problems and upgrade the clinical viable drug arsenal for PD treatment.

A Very Fresh Field

In the past 5 years, nanomedicine research in PD emerged strongly, possibly propelled by the increasing positive results. Proof of that is illustrated in Graph 1, where we can see that more than half of the works in the field were published in the past two and a half years (14 out of 27–51.85 %). Graph 1 also shows the number of in vitro and in vivo studies, and the movement of the field. Clearly, most of the approaches were tested in animals, not being restricted to in vitro analysis. But it is important to say that the vast majority of the in vivo works, if not all, evaluated several parameters of the nanoparticles in vitro before testing it in animals. Only seven studies (25.92 %) evaluated the nanoparticles in vitro alone, against 20 studies (74.08 %) that also made in vivo analysis.

Graph 1
figure 1

Studies distribution per year. In vitro studies are highlighted in blue, and in vivo studies are highlighted in red. Note that this is a compilation of the results found using a specific methodology and may not reflect all works already produced in the field

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The nanoparticles route of administration is showed in Graph 2. As we can see, among the in vivo group that totalize twenty studies, nine of then (45 %) utilized the intranasal approach. Regarding intranasal administration, interesting information is that eight of those nine works were published after 2013. This is a notable change, reflecting the evolution of the field. For comparison, of the six in vivo works conducted in 2011, only one used the intranasal via of administration. Therefore, intranasal route recently turned into the most used via of administration in studies, maybe for its known advantages, like great surface area, little invasiveness, and BBB and first-pass metabolism evasion. We have to wait for the outcomes, but there is a great chance that the first-based nanoparticles drugs to reach the market will use the intranasal administration. All the other administration routes, except from the proposed direct delivery via intranasal to brain and surgical implant, have to directly cross the BBB. Mechanisms of crossing this barrier and how nanoparticles may offer solutions should be a great discussion in the next years, not only involving delivery of drugs to PD but also to several other conditions.

Graph 2
figure 2

Administration route of the nanoparticles considering 20 in vivo studies

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Overall, our review compiles a great number of promissory studies. As a collective, nanomedicine is offering plenty of ideas to improve PD treatment, which remained unchanged for decades. As seen on Graph 1, a major part of those contributions are very recent.

The Next Step: Clinical Trials

Clinical trials with humans have already been suggested by some works (e.g. Mittal et al. 2014), and by observing the pace of progress, it’s likely that in a few years, Phase I trials will start involving nanomedicine and PD drugs. For this, however, more knowledge must be generated about the impact of those drugs on main systems (like cardiovascular and renal), providing more information about safety of the nanodrugs. Generally, for a study to proceed to Phase I trial, it requires toxicity studies in two animal species, with similar duration as the proposed to be used in humans. Also, pharmacokinetic values should be determined with great level of detail, and possible consequences to female fertility and foetal development should be established (Watt et al. 2013). Therefore, to reach clinical patients, many other steps have to be completed, requiring further basic and clinical studies. Regarding monetary investments, the move from preclinical phases to clinical trials implies significant increase in investment, which also needs to be considered. Since PD is currently incurable, patients tend to use medications chronically, and this may be of interest of laboratories and patent owners.

Phase 0 trials are done with extremely low doses, usually 1 % of the Phase I normal values (which are already conservative), so researchers can obtain pharmacological data with more safety. This may be an interesting option to a nanotechnology novel approach, since they have never been tested in humans before; regulation to conduct this kind of trial is less rigid, posing as a valuable option until the particle can match all Phase I requirements (Svendsen et al. 2015).

Currently, searching PubMed, Clinical Trials and Clinical trials EU databases looking for clinical trials that involve nanomedicine, nanotechnologies or nanoparticles and PD, no match can be found.

A Challenge: Potential New Targets

Recently, novel molecules emerged with potential to be explored in PD treatment. In a general context, the nitric oxide (NO) system is considered a factor which may contribute for either neurodegeneration (Gomes et al. 2008) or neuroprotection. In the striatal pathways, NO interacts with dopamine. The pharmacological modulation of nitrergic transmission has been considered regarding the improvement of L-DOPA-induced dyskinesia. In hemiparkinsonian rats, L-DOPA-induced dyskinesia is associated with up-regulation of inducible and neuronal NO synthase in the dopamine-depleted striatum (Padovan-Neto et al. 2015; Bortolanza et al. 2015a, b). However, both NO synthase inhibitor and NO donor produce a reduction in dyskinesia in animal models of PD (Padovan-Neto et al. 2009; 2015; Takuma et al. 2012; Solís et al. 2015; Bortolanza et al. 2016). Evidence of the molecular effects of this intervention has been accumulating. The NO synthase inhibitor 7-nitroindazole (7NI) prevents Fos-B/deltaΔFosB, phosphoacetilation of histone 3 (pAcH3) and p-ERK up-regulation in the striatum of rodents model of dyskinesia (Padovan-Neto et al. 2013; Solís et al. 2015). Recently, Bortolanza and coworkers (2016) found that co-administration of sub-effective doses of both 7NI and amantadine (the only effective medication used to alleviate dyskinesia in PD patients) had a synergistic effect on reducing AIMs scores. Nevertheless, the precise mechanism of action through NO related compounds reduce L-DOPA-induced dyskinesia remains elusive. Since NO is implicated in many processes, as reviewed by Quinn et al. (2015), there are a great number of studies investigating the association of NO compounds and nanoparticles. The focus of these studies concern wound healing, antimicrobial application, cardiovascular treatment, migraine, erectile dysfunction and cancer. As far as we know, there is no study specifically addressed to the use of NO compounds associated with nanoparticles and PD.

Several other systems had been pointed as potential new anti-dyskinetic strategies. Two other biological systems directly enrolled to dopaminergic neurotransmission have been considered in L-DOPA-induced dyskinesia genesis. The postsynaptic density protein-95 (PSD-95), a membrane-associated guanylate kinase, is the major scaffolding protein in the excitatory postsynaptic density. PSD-95 interacts with D1R, regulating its function. An overexpression of PSD-95 has been described in the striatum of animal models of L-DOPA-induced dyskinesia (Porras et al. 2012). Interestingly, the disruption of PSD-95 interaction with D1R in the striatum attenuates L-DOPA-induced dyskinesia. In addition, it has been proposed that the regulation of the olfactory type G-protein α subunit (Gαolf) by dopamine could be part of the maladaptive response to chronic L-DOPA treatment in PD (Ruiz-DeDiego et al. 2015a). Gαolf levels are higher in dopamine-depleted mouse models, either after 6-OHDA denervation or in genetic models of dopamine depletion, such as the Aphakia (ak) mice deficient in Pitx3 (a transcription factor enriched in DA neurons; Ruiz-DeDiego et al. 2015a). Additionally, the same authors proposed that a calcium-binding protein that mediates cAMP-dependent transcriptional responses called DREAM (downstream regulatory element antagonistic modulator), located downstream of D1R-dependent cAMP/PKA activation, could play an important role in the cascade of molecular events leading to L-DOPA-induced dyskinesia (Ruiz-DeDiego et al. 2015b). In fact, DREAM knockout mice showed an increased expression of molecular markers associated with dyskinesia. DREAM not only attenuates the expression of these makers, but also decreases L-DOPA-induced dyskinesia (Ruiz-DeDiego et al. 2015b).

Recently, several studies point to the involvement of the cannabinoid system in PD and in the modulation of L-DOPA-induced dyskinesia. Cannabinoid receptors, such as cannabinoids type 1 (CB1) or type 2 (CB2), the transient receptor potential vanilloid type 1 (TRPV-1) and peroxisome proliferator-activated receptor (PPAR) have been described as modulatory targets for the treatment of motor dysfunctions such as L-DOPA-induced dyskinesia (dos-Santos-Pereira et al. 2016; González-Aparicio and Moratalla 2014; Martinez et al. 2015). It is worth to consider that Cannabidiol, the principal non-psychoactive phytocannabinoid constituent of Cannabis sativa, together with a TRPV-1 antagonist, reduces L-DOPA-induced dyskinesia by acting on CB1 and PPARγ receptors, also reducing the expression of inflammatory markers (dos-Santos-Pereira et al. 2016).

All of these biological systems mentioned represent a new platform to expand the knowledgement regarding nanomedicine benefits in PD. Nanoparticles can improve the delivery not only drugs, but proteins (e.g. Chaturvedi et al. 2014), small interfering RNA (e.g. Lee et al. 2015), and other molecules to the brain providing additional therapeutic benefits to PD patients. Under collaboration, new approaches would be developed to improve the PD patient’s quality of life.

Conclusion

In this review, we presented the state-of-the art and challenges of nanomedicine application to PD management, focusing on novel pharmacological strategies. Aided by nanotechnology, several strategies were able to sustain drug concentrations in circulation, improving their half-life, bioavailability and therapeutic effects. Studies highlight the potential of nanoparticles associated to drugs to lessen L-DOPA-induced dyskinesia compared to marketed drugs. As a novel strategy, nanomedicine has a high potential to improve current PD treatment. This is highly expected, since the disease is prevalent, debilitating, progressive and incurable. However, the safety of nanoparticles should be better explored.

Work Limitation

This review is a compilation of results obtained through a specific methodology and may not reflect all works produced in the field. PUBMED was the database with highest number of results, but results from other sources should be considered in further studies to assess the real impact of nanomedicine in PD.