Choudhury et al. (2011, 2012) reported that ZNS protected dopamine-producing cells in MPTP-treated models of PD, which may be related to increased S100beta secretion by astrocytes. ZNS also increased dopamine turnover, as surviving dopaminergic neurons recovered the production of dopamine after ZNS prescription (Choudhury et al., 2011). Yokoyama et al. (2010) reported that treatment with ZNS significantly prevented the dopaminergic neuronal damage and proliferation of microglia and increased TH protein levels. Furthermore, they found that ZNS significantly attenuated the motor syndrome a week after MPTP treatment in the behavioral study. Additionally, Ikeda et al. (2018) demonstrated that ZNS treatment delayed the reduction of striatal presynaptic dopamine transporter levels, suggesting that ZNS may play an essential role in slowing the deterioration of early stage PD. They examined the average specific binding ratio (SBR) of dopamine by dopamine transporter single-photon emission computed tomography at the baseline. The SBR decline rate was significantly reduced in the ZNS group, whereas the SBR was significantly reduced in the placebo at the endpoint.

Notably, Yano et al. (2009) mentioned that ZNS could prevent MPTP-induced increases in the levels of glial fibrillary acidic protein in striatal neurons, but Choudhury et al. (2011) came to the opposite conclusion. Further studies are needed to resolve this contradiction.

Sano et al. (2015) reported that ZNS could reduce the death of nigrostriatal dopamine-producing neurons via signal transduction by neurotrophic factors. There were increased levels of brain-derived neurotrophic factors in the substantia nigra and striatum in the ZNS group compared with placebo. Choudhury et al. (2012) found that ZNS increased the expression of neurotrophic factors in astrocytes.

An experimental study has reported that ZNS could enhance neurite elongation after nerve injury in primary motor neurons in vitro (Yagi et al., 2015). They also found that ZNS could enhance neurite regeneration and protect primary motor neurons against oxidative stress by inducing increased expression of neurotrophic factors (neurotrophin-4/5, brain-derived neurotrophic factors, and nerve growth factors) and their receptors. Further studies are needed to demonstrate whether ZNS has similar effects in nigrostriatal dopaminergic cells.

Some findings have suggested that ZNS exerts neuroprotective effects mainly via antioxidant action. Ueno et al. (2018) reported that long-chain acylcarnitine (LCAC) levels were raised in PD patients after ZNS treatment. As an increase in LCACs implies improvement in mitochondrial beta-oxidation, ZNS may have antioxidant properties. Condello et al. (2013) reported that ZNS could reduce intracellular ROS levels and restore mitochondrial membrane potential. Costa et al. (2010) found that ZNS could protect the striatum against irreversible mitochondrial impairment through a γ-aminobutyric acid (GABA)–mediated mechanism. Choudhury et al. (2012) and Kawajiri et al. (2010) reported that ZNS could increase levels of manganese/copper/zinc superoxide dismutase (Mn/Cu/Zn-SOD) in astrocytes. Yurekli et al. (2013) demonstrated that ZNS could induce modulating effects that increased glutathione (GSH) and glutathione peroxidase levels. Concentrations of cytosolic free Ca²⁺ and lipid peroxidation were also lower in ZNS treatment groups (Yurekli et al., 2013; Ueno et al., 2018). Asanuma et al. (2008, 2010) found that ZNS prevented levodopa-induced dopamine quinone formation in the striatum. They also found that ZNS treatment significantly increased GSH levels by enhancing the expression of cystine/glutamate exchange transporter (xCT). However, Bentea et al. (2017) reached the opposite conclusion. In their experiments, ZNS did not alter GSH levels or influence the expression of xCT both in vivo and in vitro. More experiments are needed to resolve the contradiction. Moreover, ZNS exerted antiapoptotic effects, as demonstrated by increased cell viability and attenuated caspase-3 activity in human neuroblastoma (SH-SY5Y) cells (Kawajiri et al., 2010; Condello et al., 2013).Tsujii et al. (2015a,b) reported that ZNS had neuroprotective effects against endoplasmic reticulum (ER) stress. ZNS inhibited SH-SY5Y cell death in vitro and suppressed ER stress–related cell death in vivo via inhibiting the expression of ER stress–induced factors. Furthermore, ZNS suppressed ER stress–related neuron death by inhibiting caspase-3 activation (Yurekli et al., 2013; Tsujii et al., 2015b). Omura et al. (2012, 2013) reported that ZNS inhibited caspase-3 activation via increases in HMG-CoA reductase degradation 1 protein levels. These studies demonstrated that inhibition of ER stress, especially the mechanism via caspase-3, was involved in the neuroprotective actions of ZNS.

Hossain et al. (2018) reported that ZNS may potentially modify the disease by inhibition of microglial voltage-gated sodium channels 1.6 (Nav1.6), as well as neuroinflammation. Nav1.6 is expressed in microglial cells of MPTP-induced PD mice and postmortem PD brains, and it contributes to neuroinflammation. ZNS also inhibited the MPTP-induced expression of tumor necrosis factor α and gp91 (phox). Together, these findings suggest that ZNS can reduce neuroinflammation and has neuroprotective potential.

It is also possible that ZNS has an effect on synaptic transmission. Experimental studies have suggested that ZNS affects synaptic transmission related to the kynurenine pathway in astroglia (Fukuyama et al., 2014). ZNS could increase the release of metabotropic glutamate-receptor (mGluR) agonists from astrocytes, and activated mGluR agonists inhibited neurotransmission in both direct and indirect pathways, which may explain the efficacy and tolerability of ZNS in PD (Fazio et al., 2012; Copeland et al., 2013; Fukuyama et al., 2014). Yamamura et al. (2009) found that ZNS could influence the level of extracellular neurotransmitters in a dose-dependent manner. Striatal perfusion of ZNS increased the extracellular levels of dopamine in striatal cells and attenuated extracellular levels of GABA in both the subthalamic nucleus and globus pallidus. Extracellular levels of glutamate in the substantia nigra pars reticulata (SNr) were also decreased. Furthermore, they demonstrated that ZNS inhibited the indirect pathways of movement. Sano and Nambu (2019) examined neuronal activities in the SNr and found that ZNS administration showed longer inhibition and reduced late excitation, which suggested that ZNS may enhance the direct pathway, while inhibiting the indirect pathway (Tachibana et al., 2008; Sano et al., 2013).

Arawaka et al. (2014) demonstrated that ZNS had neuroprotective effects in a genetic model of PD by attenuating A53T α-synuclein–induced neurotoxicity. They found that ZNS could prevent the loss of dopamine-producing cells by inhibiting a cell death pathway or cellular damage caused by α-synuclein. Furthermore, Ueda et al. (2012) found that ZNS upregulated the expression of several genes associated with nervous system function and development, such as Gpr143 and Bdnf, and of some genes associated with metabolism, including Aldh1a7, Akr1b7, Dhdh, and Agmat. Moreover, ZNS downregulated the expression of a few genes associated with inflammation and the immune system.

Recently, Cha et al. (2020) identified a ZNS-responsive gene in PD patients. Carriers of mouse double-minute 4 (MDM4) gene had higher ZNS sensitivity, improved motor fluctuation, and reduced “OFF” time. Inhibition of p53 is associated with upregulation of MDM4. The association between higher MDM4 expression and “OFF” time reduction suggested that p53 downregulation may prevent the reduction of dopamine-producing cells and deterioration of PD.

An OL trial on nine patients with advanced PD showed that ZNS has beneficial effects in improving the primary symptoms of PD, including motor fluctuations (Murata et al., 2001). The Unified Parkinson’s Disease Rating Scale (UPDRS) score, Hoehn and Yahr stage (H-Y stage), and “OFF” time per day were assessed after 12 weeks of ZNS treatment. Both the UPDRS score and H-Y stage (ON/OFF) improved, and unexpectedly, the duration of the “OFF” time markedly improved. These effects continued for more than a year, even for advanced-PD patients. Additionally, ZNS has a long half-life (25 mg: 90 h), and its beneficial effects could be maintained throughout a day with a single dose of the drug per day (Murata et al., 2015).

Murata et al. (2007) conducted an RCT in Japan to assess the efficacy of a daily dose of 25, 50, and 100 mg of ZNS in 347 advanced-PD patients. ZNS 25 and 50 mg resulted in significant decreases in UPDRS III total scores from baseline (ZNS 25 mg, −6.3 ± 0.8; ZNS 50 mg, −5.8 ± 0.8, as compared with −2.0 ± 0.8 in placebo; P = 0.001 and P = 0.003, respectively; Dunnett test). Likewise, the percentage of responders, defined as patients with a reduction of ≥ 30% in UPDRS part III scores, was significantly higher with ZNS 50 mg (38.8%, P = 0.018) than with placebo (22.0%). ZNS 50 mg and ZNS 100 mg groups achieved significant improvements in reducing the total “OFF” time [the mean changes were −1.30 h (P = 0.014) and −1.63 h (P = 0.013), respectively] compared with the placebo group. Importantly, ON time with annoying dyskinesia did not increase in any ZNS group as compared with placebo; in contrast, a reduction in disabling dyskinesia was observed in the ZNS 50 mg group.

Two phase 3 multicenter RCTs (Murata et al., 2015, 2016) have investigated the efficacy of ZNS among 618 advanced-PD patients: the Murata et al. (2015) and Murata et al. (2016) studies.

The Murata, 2015 study (Murata et al., 2015) examined the effectiveness of ZNS in the reduction of “OFF” time in advanced-PD patients with motor fluctuation as compared with placebo. The primary endpoint was the variation in daily “OFF” time from baseline to the end of a 12-week prescription, according to diarized information at the final assessment. ZNS 50 mg met the primary endpoint and had the most significant efficacy, having a significantly longer decrease in daily “OFF” time at week 12 than placebo (ZNS 50 mg daily “OFF” time decrease: −0.719 ± 0.179 h, compared with placebo: −0.011 ± 0.173 h; P = 0.005). Moreover, the percentage of participants with a reduction in “OFF” time ≥ 20% (responders) from the beginning of treatment to week 16 was markedly higher in those receiving ZNS 50 mg than those receiving placebo (40.5% for ZNS 50 mg vs. 20.9% for placebo; P < 0.001). ZNS 50 mg also resulted in statistically significant improvements in the UPDRS II (OFF) score (P = 0.021) and UPDRS III score at week 12 vs. placebo. However, UPDRS I, UPDRS II (ON), UPDRS IV, and Parkinson’s Disease Questionnaire-39 scores did not show statistically significant differences in improvements between ZNS 50 mg and placebo. Moreover, the increase in the dyskinesia duration at week 12 was not statistically significant in those receiving ZNS 25 mg and 50 mg as compared with placebo (P = 0.103 and P = 0.235, respectively). This study demonstrated that ZNS 50 mg could significantly reduce the “OFF” time and improve the “OFF” status of activity in advanced-PD patients who exhibited the “wearing-off phenomenon.”

The Murata, 2016 study (Murata et al., 2016) explored the effectiveness and safety of ZNS 25 and 50 mg taken orally once daily, as compared with placebo. The primary endpoint was the variation from the beginning of the treatment to the final assessment in UPDRS part III scores. ZNS 25 mg met the primary endpoint at the final assessment, showing a marked reduction in UPDRS III scores than placebo at week 14 (−5.09 ± 0.9 for ZNS 25 mg vs. −2.9 ± 0.9 for placebo, P = 0.029). Likewise, ZNS 50 mg resulted in a significant reduction in UPDRS III total scores at week 12 (−6.1 ± 1.0, P = 0.049). The percentage of participants with a reduction in UPDRS III scores ≥ 30% from baseline to week 14 (responders) was markedly higher with ZNS 50 mg than with placebo (45.8% for ZNS 50 mg vs. 27.0% for placebo; P < 0.038). Furthermore, the UPDRS II (OFF) scores were significantly improved with ZNS 25 mg than with placebo (P = 0.039).

Impulse control behaviors (ICBs) belong to impulsive–compulsive disorders, which are associated with dopamine replacement therapy in PD (Evans et al., 2006; Raja and Bentivoglio, 2012). Recently, Kon et al. (2018) found that patients who developed ICB at final evaluation were prescribed ZNS earlier and at a higher dosage than other patients, suggesting that ZNS may be associated with ICB. In another study, ZNS increased novelty-seeking behaviors in mice, which were risk factors for ICB (Voon et al., 2011; Uemura et al., 2017). Thus, ZNS might be related to the development of impulsive–compulsive disorders (ICDs) in PD. However, an OL trial (Bermejo et al., 2010) performed by Bermejo et al. suggested that ZNS could play an essential positive role in ICDs. The severity of ICBs was significantly reduced, from −5.8 to −4.8 (mean change). It is important that further RCTs estimate the effectiveness of ZNS on ICDs in patients with PD.

Rapid eye movement sleep behavior disorder (RBD) is considered one of the most prevailing non-motor symptoms in PD patients (Mollenhauer et al., 2016). It may occur several years before the first symptoms of PD or during the progression of PD (Claassen et al., 2010). Recently, Kataoka and Ueno (2012) reported a case in which treatment with ZNS resolved dream-enacting behaviors and vivid nightmares in an early stage PD patient, suggesting that ZNS might be efficacious for the management of RBD. Further studies are required to estimate the effectiveness of ZNS on RBD in early- and advanced-stage PD patients.

Dementia is described as an essential complication in PD (Iwaki et al., 2019). Iwaki et al. (2019) and Murata et al. (2007) demonstrated that ZNS may reduce the development of cognitive impairment better than other antiparkinsonian drugs. Although the vast majority of trials were performed in the Japanese population, Tombini et al. (2013) reported the case of a 78 years old Caucasian male patient with PD and dementia, who later developed epilepsy. ZNS significantly improved his seizure and extrapyramidal symptoms without affecting his cognitive status. Further RCTs are needed to evaluate the efficacy of ZNS in preventing cognitive decline among PD patients of different ethnicities (Grover et al., 2013; Tombini et al., 2013).