Type-B monoamine oxidase inhibitors in neurological diseases: clinical applications based on preclinical findings

Abstract

Type-B monoamine oxidase inhibitors, encompassing selegiline, rasagiline, and safinamide, are available to treat Parkinson’s disease. These drugs ameliorate motor symptoms and improve motor fluctuation in the advanced stages of the disease. There is also evidence supporting the benefit of type-B monoamine oxidase inhibitors on non-motor symptoms of Parkinson’s disease, such as mood deflection, cognitive impairment, sleep disturbances, and fatigue. Preclinical studies indicate that type-B monoamine oxidase inhibitors hold a strong neuroprotective potential in Parkinson’s disease and other neurodegenerative diseases for reducing oxidative stress and stimulating the production and release of neurotrophic factors, particularly glial cell line-derived neurotrophic factor, which support dopaminergic neurons. Besides, safinamide may interfere with neurodegenerative mechanisms, counteracting excessive glutamate overdrive in basal ganglia motor circuit and reducing death from excitotoxicity. Due to the dual mechanism of action, the new generation of type-B monoamine oxidase inhibitors, including safinamide, is gaining interest in other neurological pathologies, and many supporting preclinical studies are now available. The potential fields of application concern epilepsy, Duchenne muscular dystrophy, multiple sclerosis, and above all, ischemic brain injury. The purpose of this review is to investigate the preclinical and clinical pharmacology of selegiline, rasagiline, and safinamide in Parkinson’s disease and beyond, focusing on possible future therapeutic applications.

Introduction

Monoamine oxidases (MAOs) are flavin-dependent enzymes located in the outer mitochondrial membranes. Two different isoforms have been identified, MAO_A and MAO_B. MAO_A is localized in the gastroenteric cells, heart myocytes, platelets, and aminergic neurons, whereas MAO_B has been found in platelets and astrocytes. MAO_A and MAO_B participate in the catabolism of amine neurotransmitters, including dopamine (Bainbridge et al., 2008; Tripathi et al., 2018; Tan et al., 2022). In particular, MAO_A metabolizes amines in presynaptic neuronal terminals, whereas MAO_B is involved in the degradation of dopamine uptaken into astrocytes from the synaptic cleft. In clinical practice, MAO_A and MAO_B inhibitors are, respectively, helpful in treating neuropsychiatric disorders such as depression and Parkinson’s disease (PD) (Carradori et al., 2018; Tan et al., 2022).

The symptomatic efficacy of MAO_B inhibitors in PD is mostly dependent on enzyme inhibition in the brain, which, in turn, increases the extracellular levels of dopamine (Alborghetti and Nicoletti, 2019). Three MAO_B inhibitors are available, selegiline, rasagiline, and safinamide. These drugs improve motor symptoms and reduce the severity and duration of motor fluctuations in PD patients undergoing levodopa (LD) treatment. Selegiline and rasagiline also display symptomatic efficacy as monotherapy in the early stage of the disease. Moreover, as discussed below, MAO_B inhibitors may be of benefit to some non-motor symptoms of PD (Alborghetti and Nicoletti, 2019; Armstrong and Okun, 2020). Therefore, MAO_B inhibitors could relieve symptoms and reduce LD dosage in patients with early or mild PD. In advanced PD, MAO_B inhibitors are useful add-ons to LD in patients experiencing motor fluctuations by reducing time spent OFF, increasing time ON, and significantly improving quality of life (Alborghetti and Nicoletti, 2019).

Despite the commonality of the primary mechanism of action, MAO_B inhibitors display peculiar differences that may be of interest to the clinical practice. Thus, selegiline and rasagiline are irreversible inhibitors forming a covalent bond within the active site of MAO_B. Conversely, safinamide is a reversible MAO_B inhibitor and, at the high daily dose (100 mg), inhibits voltage-sensitive sodium channels (VSSCs) and voltage-sensitive calcium channels (VSCCs), which, in turn, inhibit glutamate release (Blair and Dhillon, 2017; Alborghetti and Nicoletti, 2019; Tan et al., 2022). Beyond the primary mechanism of action, preclinical studies indicate additional consequences of MAO_B inhibitors that may represent a further therapeutic potential for these drugs in PD as well as other pathological conditions. Among anti-parkinsonian agents, MAO_B inhibitors have the most significant neuroprotective potential through inhibition of dopamine metabolism and stimulation of neurotrophic factors. In the case of high-dose safinamide, inhibition of glutamate neurotransmission. Preclinical studies suggest, indeed, additional therapeutic prospects of these drugs in PD as well as other neurological disorders characterized by either chronic progressive neurodegenerative damage or acute metabolic/oxidative stress (Figure 1). Given these premises, we sought interest in discussing the preclinical and clinical pharmacology of selegiline, rasagiline, and safinamide in PD and beyond, focusing on possible therapeutic advances. To this end, we focused on current preclinical evidence that may suggest the further therapeutic application of MAO_B inhibitors in PD as well as other neurological illnesses, with the aim of translationally expanding the use of these drugs.

Figure 1.

MAO_B inhibitors in neurological disorders.

DA: Dopamine; DOPAC: 3,4-dihydroxyphenylacetic acid; MAO_B: type B onoamine oxidase; NFT: neurofibrillary tangles.

Literature Search Strategy

A search was performed on the PubMed database for all studies according to the following keywords: “selegiline” OR “rasagiline” OR “safinamide” OR “MAOB” AND “neuroprotection” AND “Parkinson’s disease” OR “neurological disorders” AND “basal ganglia” AND “glutamate” OR “NMDA receptor antagonists”. Only articles in the English language published in the period between January 1978 and November 2022 were considered. We included preclinical studies as well as clinical trials and observational studies in humans. Two authors (MA and DR) examined each report independently with respect to methodologies, the relevance of the results, and potential implications. Discrepancies in the evaluation were discussed with the remaining Authors. Moreover, the search was enlarged to the references of the originally selected papers, and seminal studies were included with no age limit.

MAO_B Inhibitors for the Treatment of Parkinson’s Disease

To date, LD remains the gold standard for the symptomatic treatment of motor symptoms of PD. LD is administered orally together with L-aromatic-amino-acid-decarboxylase inhibitors (carbidopa or benserazide) to reduce peripheral metabolism and allow adequate crossing of the blood-brain barrier (LeWitt, 2015). Despite L-aromatic-amino-acid-decarboxylase inhibition, the rather short plasma half-life of LD is a significant determinant for causing pulsatile stimulation of central dopaminergic receptors. Such phenomenon, together with progressive neuronal loss and maladaptive changes in the response of postsynaptic neurons, plays a relevant role in developing LD-related complications, such as motor (and non-motor) fluctuations and dyskinesia (Nutt, 2001; Fox et al., 2018). Therapeutic approaches for managing fluctuations include add-ons with direct dopamine agonists, catechol-O-methyltransferase inhibitors, or MAO_B inhibitors (Fox et al., 2018; Alborghetti and Nicoletti, 2019; Gray et al., 2022). Conversely, amantadine is the only drug displaying anti-dyskinetic activity (Rascol et al., 2021).

Selegiline

Selegiline is an irreversible and selective MAO_B inhibitor with a therapeutic indication in PD as monotherapy or in combination with LD, at daily doses of 5 or 10 mg (Knoll, 1989). It is rapidly absorbed from the gastroenteric tract, with peak plasma concentrations of about 2.7 ng/mL and a T_max value of 0.5 hours, and prompt crossing of the blood-brain barrier. Selegiline is metabolized in the liver, mostly through CYP2B6 and in the lower part by CYP2A6 and CYP3A4, in desmethylselegiline and methamphetamine, which are both pharmacologically active (Magyar, 2011; Alborghetti and Nicoletti, 2019). Selegiline is safe and well-tolerated in PD patients, with rare side effects being characterized by sexual dysfunction, sleep difficulty, and weight gain.

The beneficial antiparkinsonian effects of early monotherapy with selegiline were demonstrated in the DATATOP study (Shoulson, 1989), showing a significant delay in the need for add-on with LD. Moreover, findings from the SINDEPAR trail showed that selegiline 10 mg reduced the severity of parkinsonism, as measured by the Unified Parkinson’s Disease Rating Scale (UPDRS) score when combined with either LD or bromocriptine (Stocchi and Olanow, 2003). In a head-to-head, real-life, retrospective study, Cereda and collaborators (Cereda et al., 2017) reported that long-term use of selegiline or rasagiline was associated with a reduction of LD requirement and LD-induced dyskinesia with respect to controls in moderate PD patients. In more advanced cases, an add-on with selegiline to LD treatment improved tremor, rigidity, and bradykinesia, significantly reducing the frequency and severity of motor fluctuations (Dashtipour et al., 2015; Tan et al., 2022).

Rasagiline

Rasagiline is an irreversible and selective MAO_B inhibitor with a therapeutic indication in PD at the daily dose of 1 mg as monotherapy or add-on to LD in fluctuating subjects. Higher daily doses (10 mg), not used in humans, also inhibit MAO_A activity. Rasagiline is readily absorbed following oral administration and has a bioavailability of about 35%, a peak plasma concentration of 2.5 ng/mL, and a Tmax value of 0.5 hours. The drug undergoes hepatic metabolism by CYP1A2, with a production of inactive metabolites (Chahine and Stern, 2011; Alborghetti and Nicoletti, 2019).

Several randomized clinical trials have shown the therapeutic efficacy of monotherapy with rasagiline in terms of motor symptoms and quality of life in early PD subjects (Parkinson Study Group, 2002, 2004; Hauser et al., 2009, 2016). Specifically, in placebo-controlled multicenter studies, TEMPO (TVP-1012 in Early Monotherapy for PD Outpatients) and ADAGIO (Attenuation of Disease Progression with Azilect Given Once-daily), rasagiline improved UPDRS part III score and quality of life to placebo over 36 weeks of treatment (Parkinson Study Group, 2002, 2004; Hauser et al., 2009, 2016; Stocchi and ADAGIO investigators, 2014).

Two other studies, LARGO (Lasting effect in Adjunct therapy with Rasagiline given Once-daily) and PRESTO (Parkinson’s Rasagiline: Efficacy and Safety in the Treatment of ‘Off’) trials, demonstrated the efficacy of add-on with rasagiline to LD in reducing the “off” periods in fluctuating PD subjects (Rascol et al., 2005; Olanow et al., 2009). In particular, in the PRESTO study, rasagiline showed efficacy in the reduction of motor fluctuation in PD patients treated with LD and/or dopamine agonist or COMT inhibitors showing an increase in dyskinesia than placebo (Olanow et al., 2009).

Moreover, a recent retrospective case-control study showed that an add-on with either rasagiline or selegiline was associated with lower daily LD dose and lower frequency of LD-induced dyskinesia (Cereda et al., 2017). A further cohort study confirmed the reduction of daily LD dose following add-on with rasagiline (Rascol et al., 2005). In this double-blind, multicenter study, the Authors found a significant improvement in CGI score, daily living activities during off-time, and motor function during on-time in the rasagiline group compared to placebo.

Safinamide

Safinamide is the prototype of new-generation MAO_B inhibitors. It is a selective, reversible MAO_B inhibitor approved as an add-on to LD in PD patients experiencing fluctuations. Two dosages are available, 50 mg and 100 mg, which display similar inhibitory effects on MAO_B. At the daily dose of 100 mg, however, safinamide also blocks VSSCs and VSCCs, thus inhibiting glutamate transmission at overactive synapses (Blair and Dhillon, 2017; Schapira et al., 2017; Alborghetti and Nicoletti, 2019). Safinamide is readily absorbed after oral administration and shows a > 80% bioavailability with a T_max value of 1.8–2.8 hours, distribution volume of 150 L, plasma albumin binding of 92%, and elimination half-life of 21–24 hours. It is not metabolized by CYP450, thus avoiding significant drug interactions (Alborghetti and Nicoletti, 2019). The drug is metabolized by MAO_A and amide hydrolases into pharmacologically inert acidic and N-dealkylated products.

Efficacy and safety profiles of safinamide were confirmed in a recent multicenter, observational, retrospective cohort study (SYNAPSES trial) (Abbruzzese et al., 2021) as well as in elderly people (Rinaldi et al., 2021a). Safinamide reduced “Off” time and improved “On” time without troublesome dyskinesia, as reported in two phase-III double-blind, randomized clinical trials (Borgohain et al., 2014a; Cattaneo et al., 2016). Interestingly, in a subgroup of patients with moderate-severe dyskinesia, treatment with a daily dose of 100 mg safinamide was accompanied by the reduction of the Dyskinesia Rating Scale score (Study 018) (Borgohain et al., 2014b). These latter, preliminary findings suggest that the anti-glutamatergic activity of high-dose safinamide may find clinical application for the management of LD-induced dyskinesia. This motor complication arises from maladaptive synaptic plasticity at synapses between corticostriatal fibers and GABAergic striatopallidal neurons of the direct pathway (Nutt, 2001; Alborghetti and Nicoletti, 2019), and, as mentioned previously, is poorly responsive to available treatments, amantadine being the unique drug with verified activity (Rascol et al., 2021).

MAO_B inhibitors and non-motor symptoms of Parkinson’s disease

Beyond motor symptoms of parkinsonism, people with PD frequently experience a number of non-motor symptoms (Schapira et al., 2017), which are often disabling and rather challenging to manage. Moreover, dopamine replacement therapy prescribed for the treatment of parkinsonism may exert rather controversial effects on some non-motor symptoms (Schapira et al., 2017; Armstrong and Okun, 2020).

As to MAO_B inhibitors, selegiline displayed antidepressant effects (Knoll, 1989; Magyar, 2011, Ishikawa et al., 2019), possibly due to its amphetamine-like metabolic products. A transdermal patch of selegiline is marketed in some Countries for the treatment of depression (Thomas et al., 2015) and may represent an option in subjects affected by PD with comorbid depression.

The impact of rasagiline on depressive symptoms is somewhat controversial. In a double-blind, placebo-controlled trial, 1-month treatment with rasagiline showed significant efficacy (Olanow et al., 2009), whereas there was no difference between rasagiline and placebo in terms of depression outcome in the ACCORDO study (Parkinson Study Group, 2004). However, a sub-analysis of data from the ADAGIO trial showed the benefit of rasagiline on fatigue (Stocchi and Olanow, 2003; Stocchi and ADAGIO investigators, 2014). Eventually, both selegiline and rasagiline may ameliorate prefrontal executive functions in the early-moderate disease stage (Rinaldi et al., 2018, 2022), while in a more advanced stage, they appear to worsen prefrontal inhibitory control, possibly as the consequence of the excessive dopaminergic drive to prefrontal cortical regions (Rinaldi et al., 2018, 2022).

The dual mechanism of action of high-dose safinamide stimulated significant research on non-motor symptoms in PD. Indeed, simultaneous modulation of dopaminergic and glutamatergic neurotransmission appears extremely promising for several non-motor domains. Thus, safinamide improved painful cramps or spasms and allodynia in PD patients and allowed a 25% reduction in concomitant pain treatment use (Grigoriou et al., 2021). Other studies demonstrated that safinamide improves cognition and fatigue and is safe in elderly people (Bianchi et al., 2019; Rinaldi et al., 2021a; De Micco et al., 2022). Differently from other MAO_B inhibitors, high-dose safinamide improved executive functions, including inhibitory control (Rinaldi et al., 2021b), an effect possibly secondary to the modulation of glutamatergic neurotransmission. Moreover, the results of a recent Spanish study showed that safinamide improves urinary symptoms, including urgency, incontinence, frequency of micturition, and nocturia (Gómez-López et al., 2021). Eventually, treatment with safinamide was beneficial to sleep and daytime sleepiness in fluctuating PD patients (Santos García et al., 2022).

Additional Properties of MAO_B Inhibitors

Anti-oxidant and neurotrophic properties

Beyond the indirect stimulation of amine transmission, MAO_B inhibitors bear additional pharmacological effects that may be of interest in PD and other neurological disorders. Among all anti-parkinsonian drugs, MAO_B inhibitors hold the strongest neuroprotective potential, at least on the basis of preclinical studies, and administration of these drugs in the early stage of illness might be of benefit to disease progression (Riederer and Müller, 2017).

The etiology of PD and other synucleinopathies depends on multiple pathogenetic factors, including oxidative stress, mitochondrial dysfunction, alteration of the ubiquitin-proteasome protein degradation system, and neuroinflammation. In this context, MAO_B inhibitors may be of potential benefit as they reduce the production of reactive oxygen species, prevent mitochondrial damage, induce the expression of antiapoptotic proteins Bcl-2, downregulate the pro-apoptotic FAS and Bax protein families, and the production of neurotrophic factors. They also prevent aggregation and abnormal oligomerization of α-synuclein (see below).

The neuroprotective potential of MAO_B inhibitors has been investigated preclinically in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) model of parkinsonism. MPTP is a lipophilic compound that readily crosses the blood-brain barrier and is converted into its toxic metabolite 1-methy-4-phenylpyridinium (MPP⁺) by MAO_B in the astrocytes. MPP⁺ is then uptaken by dopaminergic terminals and interacts with complex I of the mitochondrial transport chain, thus producing oxygen-free radicals and leading to cell death. Consistently with this cascade, several reports indicate that MAO_B inhibitors protect nigral dopaminergic neurons against acute or subacute MPTP administration in mice and monkeys (Meredith and Rademacher, 2011; Zhao et al., 2013).

Several reports, however, suggest that the protective effect of MAO_B inhibitors on MPTP-induced dopaminergic cell damage may be, at least partially, independent of enzyme inhibition. Thus, MAO_B inhibitors showed rescuing properties on nigral dopaminergic cells also when injected after MPTP or after intracerebral infusion of MPP⁺ (Wu et al., 2000; Szökő et al., 2018), and low-dose selegiline, ineffective on MAO_B activity in vivo, already protected against MPTP toxicity (Zhao et al., 2013). Moreover, the protective effects of pretreatment with selegiline on MPP⁺ neuronal damage in the rat striatum were accompanied by increased superoxide dismutase and catalase activities (Vizuete et al., 1993). This observation is, indirectly, in line with microdialysis findings from Wu et al. (2000) showing that selegiline exerts stronger protective activity on MTPT or MPP⁺ dopaminergic cell damage in the highly-metabolic-demanding nigrostriatal compared with the mesocorticlimbic neurons. Moreover, safinamide suppressed microglial activation and protected dopaminergic neurons from degeneration in the 6-hydroxydopamine model of parkinsonism (Sadeghian et al., 2016). In these experiments, safinamide 50 or 150 mg/mL was administered for seven days after injection of 6-hydroxydopamine into the medial forebrain bundle and caused a significant reduction of activated microglia MHC-II+ and the protection of dopaminergic neurons.

Taken together, all these findings suggest that mechanisms other than enzymatic inhibition may contribute to the neuroprotective potential of MAO_B inhibitors. Accordingly, treatment with selegiline was associated with the induction of neurotrophic factors such as glial cell line-derived neurotrophic factor and brain-derived neurotrophic factor, and anti-apoptotic genes in the subacute MPTP model of parkinsonism in mice (Zhao et al., 2013). In vitro and in vivo studies confirmed that both selegiline and rasagiline stimulate the synthesis and liberation of neurotrophic factors (Naoi et al., 2020). Among them, glial cell line-derived neurotrophic factor is considered a candidate target for disease-modifying therapy of PD since it promotes the survival and differentiation of dopaminergic neurons (Barker et al., 2020).

Furthermore, according to recent work, selegiline, and rasagiline also promote the expression of anti-apoptotic genes such as Bcl2 and genes coding for antioxidant enzymes. The induction of neuroprotective genes by MAO_B inhibitors was investigated in human glioblastoma U118MG cells. Interestingly, in MAO_B knockdown condition, mRNA levels of Bcl-2, brain-derived neurotrophic factor, and glial cell line-derived neurotrophic factor were higher compared to controls. These results indicate that MAO_B might function as a repressor of the constitutional expression of pro-survival genes, and selegiline and rasagiline might regulate different signal pathways to induce neuroprotective genes (Inaba-Hasegawa et al., 2017). The two MAO_B inhibitors also regulate the mitochondrial apoptosis system, sustain mitochondrial function, and suppress the oligomerization of alpha-synuclein in cellular and animal models (Naoi et al., 2020, 2022). In parkinsonian brains, apoptosis was demonstrated by the increase of active caspase-3 and decrease of Fas-associated protein with a death domain-immunoreactive DA neurons in the substantia nigra (Venderova and Park, 2012), and intrinsic apoptosis could be initiated by opening the mitochondrial permeability transition pore, leading to an increase in mitochondrial membrane permeability (Kroemer et al., 2007). Both rasagiline and selegiline prevented the mitochondrial permeability transition pore opening and the consequent apoptosis. In the pathophysiology of PD, there is probably a cross-talk between α-synuclein and MAOs. MAO_B expression is enhanced by α-synuclein, which may favor the loss of dopaminergic terminals. On the other hand, rasagiline and selegiline can bind to α-synuclein, change its conformation, turn it to non-fibril conformation, and reduce the aggregate accumulation (Braga et al., 2011; Kakish et al., 2015; Naoi et al., 2022).

Therefore, MAO_B inhibitors hold different molecular mechanisms of neuroprotection, involving mitochondrial homeostasis, pro-survival neurotrophic factors release, and prevention of toxic oligomerization and aggregation of α-synuclein.

The anti-glutamatergic activity of safinamide

High-dose safinamide inhibits depolarization-evoked glutamate and GABA release in the hippocampus, globus pallidus, subthalamic nucleus, and substantia nigra pars reticulata (Alborghetti and Nicoletti, 2019; Rinaldi et al., 2022). These effects are sustained by the blockade of VSSCs and VSCCs, reducing the excitatory overdrive in the basal ganglia motor circuitries in PD. Based on these pharmacological properties, safinamide was originally developed as an anti-epileptic drug and displayed a higher affinity for the batrachotoxin-sensitive site of VSCCs than riluzole, carbamazepine, phenytoin, and lamotrigine (Alborghetti and Nicoletti, 2019). Interestingly, zonisamide has a similar pharmacological profile to safinamide and these drugs are considered prototypes of a new generation of multi-active MAO_B inhibitors (Blair and Dhillon, 2017).

Preclinical studies confirm that excessive glutamatergic neurotransmission plays a fundamental role in several pathophysiological changes in the brain. As elegantly shown by Calabresi et al. (2022), several transmitter-receptor systems regulate synaptic plasticity in the striatum. In particular, long-term potentiation of excitatory synaptic transmission at corticostriatal synapses requires activation of D1 dopamine receptors, N-methyl-D-aspartate (NMDA) receptors, and metabotropic glutamate (mGlu) receptors, including mGlu1 and mGlu5 receptors. Thus, glutamate neurotransmission plays a crucial role in striatal long-term potentiation, and modulation of mGlu receptor activity represents a potential therapeutic target for basal ganglia circuitry dysfunction. Preclinical studies indicate that negative allosteric modulators of mGlu5 receptors (mavoglurant, dipraglurant, basimglurant) exert anti-dyskinetic effects in experimental animal models of PD (Ossowska et al., 2005; Samadi et al., 2008), findings confirmed by preliminary human studies (Rascol et al., 2014). Safinamide 100 mg reduces glutamate release but does not directly block glutamatergic receptors and is not considered an antidyskinetic drug. However, the second additional mechanism of action can reduce the excessive glutamatergic overdrive in the direct pathway underlying levodopa-induced dyskinesia. Morari et al. (2018) applied in vivo microdialysis to monitor the spontaneous and veratridine-induced glutamate and GABA release in naïve, awake rats. They demonstrated that safinamide at the maximum dosage inhibits the induced glutamate release in the hippocampus, in the subthalamic nucleus and substantia nigra pars compacta, but not in the dorsal striatum, and shows no effects on spontaneous glutamate release. Despite these negative findings, one cannot exclude that safinamide might counteract glutamatergic transmission in the striatum when a condition of excessive glutamatergic overdrive exists, as in PD.

Excessive glutamate transmission also has negative consequences on several non-motor symptoms of PD, belonging to the cognitive, neuropsychiatric, and sensory domains (Wang et al., 2020). In the early PD stage, cognitive impairment takes the features of the dysexecutive syndrome, whereas, in advanced stages, it may evolve into the condition of PD dementia with memory impairment, disorientation, and multiple deficits of cortical functions. Neuropsychiatric features, such as visual hallucinations, delusions, and psychosis, are also frequent. In such an advanced stage, multiple neurotransmitter/receptor systems contribute to cognitive impairment and neuropsychiatric features. Glutamate signaling regulates neuronal activity in the prefrontal cortex and modulates working memory and some neuropsychiatric signs. Memantine, a partial NMDA receptor antagonist, and amantadine, a low-affinity, non-competitive NMDA receptor antagonist, are frequently used to ameliorate attention and memory performances (Wesnes et al., 2015). Glutamate receptors are also widely expressed in the brain, spinal cord, and peripheral nerves involved in pain sensation and transmission, modulation of the glutamate system being an attractive target for pain therapies (Wozniak et al., 2012).

Eventually, pathological regulation of glutamate transmission is a major determinant of excitotoxicity and plays a role in both acute (Gasparini et al., 2013; Sciaccaluga et al., 2020) and chronic (Dong et al., 2009; Sciaccaluga et al., 2020) neuronal damage. Given the consequences of excessive glutamate transmission and the pathophysiological interactions between dopaminergic and glutamatergic pathways in the basal ganglia and cortical regions, the dual mechanism of action of high-dose safinamide is of particular interest for possible future studies.

Type-B Monoamine Oxidase Inhibitors and Parkinson’s Disease: Translating Preclinical Findings into Clinical Practice

Therapeutic approaches able to reverse, block, or even slow down neuronal loss are still an unmet need for the management of neurodegenerative disorders, including PD. With this respect, preclinical studies provide clear evidence that the therapeutic potentials of MAO_B inhibitors in PD extend much beyond the consequences of their primary mechanism of action, i.e., inhibition of dopamine metabolism. In particular, the neurotrophic and neuroprotective potentials of MAO_B inhibitors gained significant interest in the scientific and clinical communities. Despite this strong preclinical evidence, however, attempts to demonstrate a neuroprotective, or at least disease-modifying effects of MAO_B inhibitors in PD ended with negative results.

Back in the ‘90s, the results of the DATATOP trial (Shoulson, 1989) suggested a possible neuroprotective effect of selegiline 10 mg in early PD patients, based on the significant delay of the need for add-on with LD in selegiline-treated subjects with respect to a placebo-treated cohort. Later on, this effect was attributed to the symptomatic action of active metabolites of selegiline and the irreversible inhibition of MAO_B activity, which were found to last longer than the drug withdrawal period applied at the end of treatment. More than a decade after, the potential disease-modifying consequences of early treatment with rasagiline 1 mg were investigated in the TEMPO and ADAGIO trials using the “early vs. delayed start” design (Nayak and Henchcliffe, 2008; Hauser et al., 2009; Olanow et al., 2009). The such study design is of particular interest as it may contribute to identifying whether the early pharmacological intervention has a more pronounced chance of slowing down disease progression. Early treatment with rasagiline 1 mg, indeed, was associated with a more favorable outcome in terms of motor disability (UPDRS-III score at the end of the study), as if the early beginning of treatment in the early stage of the disease would reduce the progression of parkinsonism. These findings led to registering rasagiline as “disease-modifying treatment” for people with PD in the early stage. This enthusiastic interpretation was not confirmed, however, by long-term follow-up of participants and critical revaluation of the results. In particular, the minimal (< 2 points) difference in UPDRS-III score among groups, the low reliability of repeated administration of the outcome scale to the same subject, and the lack of effect of 2 mg rasagiline were considered significant biases to the original interpretation (Hauser et al., 2016).

Critical analysis of these trials is, in our opinion, fundamental for identifying and approaching difficulties and controversies in the attempt to translate findings from preclinical studies into clinical practice. The limitation of posing a correct clinical diagnosis early along the process of neurodegeneration in PD is, currently, a major limitation to neuroprotective or at least disease-modifying treatments. Unfortunately, attempts of validating instrumental markers of neurodegeneration, such as ¹²³I-fluopane scintigraphic scan, gave rather negative results (Cummings et al., 2011). Moreover, the application of such an approach as a screening tool on wide cohorts is ampered by economic burden. Furthermore, the current evidence of a long (even decades) premotor stage in PD further complicates attempts of early interventions. Hypothetically, the development and validation of algorithms combining familial/genetic features with premotor symptoms of PD and biological evidence of phosphorylated a-synuclein pathology in body fluids (Wang et al., 2012; Vivacqua et al., 2018) or peripheral biopsies (Delenclos et al., 2016; Campo et al., 2019) may help to identify subjects at high risk of being diagnosed with PD in the short future. These subjects might receive instrumental evidence of presymptomatic striatal dopamine denervation and become potentially eligible for enrollment in neuroprotective trials using safe and well-tolerated compounds. A further source of bias in clinical trials stems from the identification of correct outcome measures, scales measuring the progression of neural degeneration being lacking. Again, this problem should be faced by combining findings from available scales with biological/pharmacological data, including the request for symptomatic therapy. This is even more complicated if the case of differentiating disease-modifying from symptomatic effects of a drug (as in the instance of MAO_B inhibitors). Eventually, the long disease duration makes it difficult to get any conclusive finding within 3–5 years, but this latter observation does not support by itself any negative short-lasting finding. Future trials on PD subjects should face these difficulties with the aim of allowing promising advances.

Type-B Monoamine Oxidase Inhibitors and Other Neurological Disorders

MAO_B inhibitors also gained interest in managing neurodegenerative disorders other than PD. Several studies investigated the potential therapeutic application of selegiline and rasagiline in Alzheimer’s disease (AD). AD is the most common age-related neurodegenerative disorder worldwide. As in the case of PD, the pathophysiology of AD is complex and not completely defined at present. Progression of the neurodegenerative process leads to loss of memory and cognitive functions, with difficulties in managing complex activities and problem-solving, and a significant negative impact on disability and quality of life of affected individuals and caregivers. Neurotransmitter deficiency, dyshomeostasis of biometals, and oxidative stress play a relevant role in disease progression. The hallmarks of the disease are β-amyloid deposits and intracellular neurofibrillary tangles constituted by hyperphosphorylated tau protein (Rajmohan and Reddy, 2017). Many researchers favor therapeutic approaches that target the formation, deposition, and clearance of β-amyloid. MAO activity contributes to the formation of amyloid plaques and neurofibrillary tangles and to damage of cholinergic neurons and pathways (Behl et al., 2021), and there is evidence of early and sustained alteration of MAO_A and MAO_B in the AD brain (Behl et al., 2018). In particular, biochemical investigation in the post-mortem AD brain showed variations in MAO_A-B levels detected in the cortex of AD early phases and sustained alteration throughout the development of AD (Kennedy et al., 2003). MAO_B enzymatic activity increases in brain tissue, cerebrospinal fluid, and platelets from AD patients over time. Thus MAO_B activity may be viewed as a disease biomarker in AD (Muck-Seler et al., 2009; Behl et al., 2021).

Monoamine activity may affect the cleavage of the amyloid precursor protein, leading to the clinical features of the neurodegenerative disorder. Overactivity of MAO_A-B catalyzes the cleavage of amyloid precursor protein by directly activating the β-secretase and γ -secretase activity, causing an aberrant amyloid plaque generation (Behl et al., 2021). Selegiline and other antiparkinsonian agents were able to inhibit β-amyloid fibrils formation from β-amyloid 1–40 and β-amyloid 1–42 as well as their extension in vitro (Ono et al., 2006). Furthermore, MAO_B inhibition might be of benefit by reducing the synthesis of hydrogen peroxide and oxidative stress radicals, which, in turn, contribute to disease development and progression (Bainbridge et al., 2008; Meredith and Rademacher, 2011; Zhao et al., 2013). Thus, selegiline can inhibit apoptosis induced by oxidative stress and cell death caused by glutathione depletion (Naoi et al., 2022). In animal models of aging, selegiline is also effective in delaying memory impairment (Knoll, 1989). In AD mice, the drug limits the impairment of synaptic plasticity and cognitive dysfunction by inhibiting GABA release from astrocytes (Jo et al., 2014). Short-term administration of selegiline improves cognitive impairment in AD, derived from GABA/MAO_B interactivity, and long-term administration attenuates abnormal GABA measured (Tábi et al., 2020).

Therefore, the neuroprotective properties of MAO_B inhibitors in AD may depend on promoting anti-oxidant and iron chelating activity, preventing oxidative stress and tau protein hyperphosphorylation, regulation of β-amyloid plaques formation, and improvement of cognitive deficits (Behl et al., 2021).

Duchenne muscular dystrophy (DMD) is a severe form of inherited muscular dystrophy, leading to death from heart and respiratory failure. Therapy of DMD is rather ineffective, although gene therapies have been recently introduced for patients with specific mutations (Fairclough et al., 2013). Mitochondrial dysfunction and oxidative stress play a concrete role, at least in disease progression, and increased expression and activity of muscular MAO_B have been found in murine models of muscular dystrophy, mdx for DMD, and Col6a1^–/– mice for collagen VI-related myopathies (Menazza et al., 2010, 2012; Sorato et al., 2014). A study by Vitiello et al. (2018) evaluated the effects of safinamide on the skeletal muscles of mdx mice and cultured muscle cells from DMD patients, with good effects on myofiber damage, oxidative stress, and muscle functionality. By blocking skeletal muscle sodium channels, safinamide might represent a therapeutic option also in non-dystrophic myotonia because mutations in chloride and sodium channels occur, causing stiffness and prolonged muscle contraction (Cannon, 2015; Imbrici et al., 2016; Desaphy et al., 2020).

As mentioned earlier, safinamide was originally developed as an antiepileptic drug because of its ability to inhibit VSSCs and VSCCs. This mechanism is useful to limit the hyperexcitability of neuronal circuitries mediating seizure but may also be useful in other pathological instances. Thus, blockade of VSSCs can exert a protective action on white matter lesions in experimental models of demyelinating disorders and may position safinamide as a potentially protective agent in multiple sclerosis. Despite the original definition of multiple sclerosis as an immune-mediated, demyelinating disorder, evidence exists of long-term, degenerative damage producing axonal and neuronal degeneration over time (Haines et al., 2011), in turn determining permanent neurological disability. VSSCs blockers, such as phenytoin, carbamazepine, and lamotrigine, may limit axonal damage in experimental models of MS, the latter drug being also of benefit in preliminary trial on secondary progressive MS, despite questionable tolerability (Morsali et al., 2013). The high tolerability and safety profiles of high-dose safinamide in PD patients support its utilization in MS. With this respect, a study by Morsali et al. (2013) investigated the effects of safinamide and flecainide on axonal function and structure in two models of experimental autoimmune encephalomyelitis. Safinamide treatment was beneficial even when delayed to the onset of neurological symptoms. The authors concluded that the efficacy of safinamide on axonal survival likely depended on the reduction of microglial activation.

Epidemiologically, the most relevant perspective for using MAO_B inhibitors beyond PD refers to ischemic stroke. Stroke is a leading cause of death and disability in the United States and the European Community (Guzik and Bushnell, 2017). Despite the significant clinical benefit of pharmacological thrombolysis and thrombectomy in acute ischemic stroke, drug-induced neuroprotection and pharmacological strategies to improve post-stroke recovery are still unmet needs in stroke management. Profound neurorestorative processes occur in brain tissue following focal cerebral ischemia, and the post-acute phase is characterized by intense neuronal sprouting. Thus, the sprouting of pyramidal tract fibers that occurs after Wallerian degeneration is accompanied by the remodeling of trans-callosal projections that connect the cortical areas of the two hemispheres (Obi et al., 2018). Therefore, neural plasticity involves both the lesioned and contralateral hemispheres and may drive functional hemispheric imbalance, which may be targeted by therapeutic interventions. Modern strategies to stroke management should include functional- and structural-based approaches (Silasi and Murphy, 2014), including the effort to rebalance the activity of the two hemispheres through the modulation of glutamate neurotransmission. According to recent reports on animal models (Hermann and Chopp, 2012; Xu et al., 2020), drugs restraining glutamate neurotransmission may promote functional recovery even if administered days after transient or definite ischemic injury (Hermann et al., 2019; Salvalaggio et al., 2020). A recent work by Xu et al. (2020) demonstrates that safinamide has a protective effect in animal models of acute ischemic stroke and, in vitro, on endothelial cells and suggests that the drug may improve vascular integrity in cerebral ischemia. Indeed, safinamide might be a helpful treatment option for correcting abnormalities of brain connectivity secondary to focal ischemia because of its complex mechanism of action. Thus, the drug combines the potential benefit of increased dopamine transmission (Obi et al., 2018; Talhada et al., 2021) and reduced free oxygen radical species genesis due to the inhibition of MAO_B with the inhibition of glutamate release. Despite the more straightforward mechanism of action, the administration of selegiline reduced oxidative stress, cell death, and cognitive impairment following transient global ischemia in rodents (Maia et al., 2004; Ahmari et al., 2020).

Conclusions

Robust preclinical evidence indicates that MAO_B inhibitors may benefit PD and other neurological disorders by means of mechanisms different from the conventional increase of dopamine neurotransmission. In particular, the antioxidant and potentially neuroprotective effects of these drugs and the stimulating action on the synthesis of neurotrophic factors and gene products affecting neuronal survival may represent the basis for the disease-modifying activity of these drugs. Moreover, the dual mechanism of action of the recent MAO_B inhibitors, safinamide, may add further symptomatic and non-symptomatic benefits by combining dopaminergic stimulation with inhibiting the consequences of an excessive glutamatergic drive. Future clinical studies must be designed considering several biases that may hamper the evaluation of the results.