The preclinical discovery and development of opicapone for the treatment of Parkinson’s disease

INTRODUCTION
Opicapone (OPC) is a well-established catechol-O-methyltransferase (COMT) inhibitor that is approved for the treatment of Parkinson's disease (PD) associated with L-DOPA/L-amino acid decarboxylase inhibitor (DDI) therapy allowing for prolonged activity due to a more continuous supply of L-DOPA in the brain. Thus, OPC decreases fluctuation in L-DOPA plasma levels and favors more constant central dopaminergic receptor stimulation, thus improving PD symptomatology.


AREAS COVERED
This review evaluates the preclinical development, pharmacology, pharmacokinetics and safety profile of OPC. Data was extracted from published preclinical and clinical studies published on PUBMED and SCOPUS (Search period: 2000-2019). Clinical and post-marketing data are also evaluated.


EXPERT OPINION
OPC is a third generation COMT inhibitor with a novel structure. It has an efficacy and tolerability superior to its predecessors, tolcapone (TOL) and entacapone (ENT). It also provides a safe and simplified drug regimen that allows neurologists to individually adjust the existing daily administration of L-DOPA. OPC is indicated as an adjunctive therapy to L-DOPA/DDI in patients with PD and end-of-dose motor fluctuations who cannot be stabilized on those combinations.


Introduction
Idiopathic Parkinson's disease (PD) is a progressive neurodegenerative disorder clinically characterized by the presence of loss of motor control, slowness of movement, tremors at rest and rigidity [1]. Age is the most important risk factor, affecting approximately 1% of the population over 60 years. In addition to causing loss of motor control, patients show non-motor symptoms such as mood disorders, constipation, cognitive impairment, sleep disorders and difficulties with speech and swallowing [1][2][3][4]. Therefore, PD has generally been considered a motor disorder. The clinical manifestations are subject to the severity of neuron loss in the striatum region, specifically in the substantia nigra, resulting in neurophysiological changes in basal ganglia (BG) activity, dose deterioration and morning bradykinesia are generally the first clinical changes to appear.
The ¨off ¨ phase is defined as a lack of mobility (bradykinesia, akinesia, or stiffness), while during the ¨on¨ phase, the patient responds to L-DOPA, regardless of the presence of dyskinesias [1][2][3][4].
Motor fluctuations, the primary side effect of long-term L-DOPA therapy, may be a consequence of the short half-life of oral L-DOPA. Likewise, several alterations of dopamine receptors have been described. These are mainly associated with the involved molecular signalling pathways.
The modification of L-DOPA dosage and changes to its pharmaceutical formulation are the most common therapeutic strategies used to manage these motor fluctuations [8][9][10][11][12][13]. Increasing the half-life of L-DOPA helps to prevent fluctuations in plasma L-DOPA levels and increases the amount of L-DOPA available to the brain.
This objective can be achieved by administering a COMT inhibitor together with L-DOPA / DDI Administration (FDA) accepted the new drug application of OPC as a potential add-on therapy to L-DOPA/carbidopa for PD in April 2020. OPC is a novel third generation, highly potent, once daily and effective COMT inhibitor that optimizes the pharmacokinetics and bioavailability of L-DOPA therapy. Given the safety issues with the use of TOL, OPC may be considered a safer alternative for PD patients that have not responded to treatment with other COMT inhibitors [14][15][16][17][18][19][20][21][22].
Until recently, TOL and ENT were the only COMT inhibitors commercially available COMT inhibitors. In clinical practice, TOL was the first used, but it was withdrawn from Europe in 1998 because there were reported cases of liver damage [23]. Nevertheless, it was reincorporated in European clinical practice with a mandatory control during the first year of treatment of the liver function.
TOL is currently administered in PD treatment, at a dose of 100 mg three times a day, for patients who have not been treated with ENT. ENT is better tolerated than TOL and is administered as a supplement to each daily dose of L-DOPA / DDI [17][18][19][20]. This compound extends the half-life of L-DOPA, increasing the bioavailability of the drug and prolonging its effect. While it inhibits COMT centrally, ENT acts peripherally and produces a potent and reversible inhibition [14-20].

OPC
[2,5-dichloro-3-(5-(3,4-dihydroxy-5-nitrophenyl)-1,2,4-oxadiazol-3-yl)-4,6dimethylpyridine 1-oxide], formerly known as BIA 9-1067, is a peripheral, selective and reversible inhibitor of COMT ( Figure 1). OPC inhibits with a high affinity binding that results in a slower rate of complex dissociation and a longer and more constant duration of action in vivo (> 24 hours) [23]. This enzyme catalyzes the transfer of a methyl group from S-adenosyl-Lmethionine to substrates with a catechol group, resulting in the product of O-methylation and a stoichiometric amount of 5-adenosyl homocysteine [24,25]. This reaction takes place in the presence of the divalent cation Mg 2+ . Although the existence of O-methylation processes in animal tissues were observed for the first time in 1951, this enzyme was not described until 1958, when it was partially purified [15]. The same year, Axelrod and co-workers showed that this enzyme catalyzes the O-methylation of catecholamines such as dopamine and adrenaline. Studies carried out thereafter showed that COMT plays an important physiological role in the extraneuronal inactivation of endogenous catecholamines [26].
In the presence of a DDI, COMT becomes the main metabolizing enzyme of L-DOPA, catalysing its conversion to 3-O-methyldopa (3-OMD) in the brain and the periphery. In patients taking L-DOPA and a peripheral DDI, such as carbidopa or benserazide, OPC increases plasma L-DOPA levels [25,27]. The recommended dose of OPC is 50 mg orally, given once daily at bedtime, at least one hour before or after the combination of L-DOPA [23,27]. It may be necessary to adjust the dose of L-DOPA during the initial course of treatment with OPC, by either extending the treatment intervals or decreasing the dose. Although hepatotoxic effects have not been demonstrated, it is not recommended to administer OPC to patients with severe liver impairment, and caution should be taken with moderate hepatic impairment [22,23,25,27]. Precaution is also recommended in aged patients.

Pharmacodynamics of Opicapone
In a set of preclinical research studies in vitro and in vivo, OPC was examined for its mechanism of action, the affinities of binding to peripheral COMT and the effect on the availability of L-DOPA after concomitant use with L-DOPA and DDI [28][29][30]. Experiments were performed in rats and monkeys. The first preclinical research studies of OPC as a COMT inhibitor in PD treatment were published by Kiss and colleagues in 2010 [19]. They reported that dose-dependent inhibition of rat liver COMT is determined 3h after oral administration of increasing doses (from 0.03 to 10 mg/kg) of OPC in rats. In addition, OPC increased 2-fold the plasma L-DOPA levels at 2 h. This increase was constant and maintained over a 24 h period. The ability of OPC to deliver a consistent increase in L-DOPA levels over time was related to a prolonged peripheral COMT inhibition, suggesting a substantial advantage over the other COMT inhibitors that were on the market.
Moreover, it was reported that the administration of OPC in rats (3 mg/kg, p.o.) produced an 80% COMT inhibition in liver and kidney from 1 to 8h post-administration. Therefore, OPC increased peripheral and central bioavailability of L-DOPA in rats. Gonçalves and colleagues studied a 30 mg/kg dose in rats, reporting that the effect of COMT inhibit ion produced by OPC was rapid, within 1 h post-dosing, and nearly complete, with 97% inhibit ion, in erythrocytes, liver and kidney. In addition, authors compared the duration of action of the three COMT inhibitors OPC, TOL and ENT [30]. In the case of OPC, at 24 h following oral administration, greater than 50% of COMT activity was still inhibited. In the case of TOL and ENT, at 12 h following administration, the percentage of liver COMT inhibition was 50% and 15%, respectively.
Bonifácio and colleagues investigated the effect of OPC on brain availability and disposition of L-DOPA by means of simultaneous microdialysis of the substantia nigra and other brain areas in cynomolgus monkey [31]. They found that erythrocyte COMT activity that was inhibited by 75%.
In addition, in the OPC-treated substantia nigra dialysates, there was a 1.4-fold increase in L-DOPA levels. At the peripheral level, animals treated with intracerebroventricular (ICV) OPC significantly increased two-fold the plasma L-DOPA bioavailability. Kitajima and colleagues examined the effect of acute and repeated administration of OPC (1, 10 and 100 mg/kg) on erythrocyte COMT activity in cynomolgus monkey [32,33]. Their results showed that COMT activity was significantly inhibited by OPC at 10 and 100 mg/kg, with a COMT inhibition mean of 76.4% and 93.2% from baseline, respectively, and a maximal effect at 2 h with both doses.
After 24 hours, COMT activity was evaluated again and results exhibited an inhibition percentage of 42.6% and 60.2% in both groups, respectively [32,33].
Moura and colleagues demonstrated that in a hemi-parkinsonian model of PD, achieved through injection of 6-OHDA, OPC administration to rats (3 mg/kg, p.o.) once daily for three consecutive days prior to L-DOPA/ benserazide produced an improvement in rotational behaviour [36] (Table   1). This effect was also associated with a significant peripheral COMT inhibition in OPC treated animals.

Pharmacokinetic profile
Preclinical studies reported that following oral administration of OPC, it was rapidly absorbed, reaching its T max in less than 4 hours. The drug possesses a short half-life in plasma and binds strongly to rat plasma proteins (99.7%).
The prolonged inhibition of COMT, observed in the rat and the cynomolgus monkey after OPC administration was not, however, due to the presence of the compound in the circulation [23,[27][28][29][30].
However, clinical studies reported that the percentage of the active metabolite BIA 9-1079 was less than 15 % of total OPC derivatives [47][48][49][50]. Thus, authors suggest that the potential contribution of the active metabolite to the therapeutic effect of OPC should be much lower than expected for drugs.
The most abundant plasma peaks after a single dose of 100 mg of 14  More than 68% of OPC and metabolite radioactivity was excreted via the biliary route in animals, and the same results were reported in humans. In monkeys, very low levels (less than 10%) were excreted by the kidneys [23. 44]. In rats and humans, renal excretion accounted for approximately 10% and 12% of total excretion, respectively [44,48]. This percentage of BIA 9-1106 in the urine suggests that kidney is not the main route of excretion for OPC and other metabolites. The authors concluded that OPC is mostly excreted via the biliary route. In Cynomolgus monkey and Wistar rats, faecal route is the main route of excretion [23]. Furthermore, there are differences between the formations of OPC metabolites between species [23].

Toxicologic profile
One of the major concerns with the use of COMT inhibitors is hepatoxicity caused by TOL administration [21]. The use of TOL requires liver function monitoring, due to the risk of liver In summary, preclinical studies suggest that OPC is a safe and well-tolerated drug which possesses a reduced risk of hepatic and cardiac toxicity, and adverse reactions are in line with those described for ENT [23].  Table 2). OPC 50 mg once a day (QD) achieved the same therapeutic effects as ENT, and was found to be superior to placebo [58,69].

Two
The study included 600 patients from 106 study centers in Europe. Patient criteria were as follows: The patients were aged between 34 and 83 years old and carried the diagnosis of idiopathic PD for a minimum of 3 years. In the ¨on¨ phase, their modified Hoehn and Yahr score was of ≤3. All subjects had received optimal treatment with L-DOPA and had been clinically stable for at least 4 weeks. Signs of end-of-dose deterioration were present for at least 4 weeks, with an average of daily ¨off ¨time of 1.5h, excluding AM periods prior to the first daily dose. Finally, subjects were required to demonstrate an ability to keep a precise 24-hr diary [58].
The study showed that the OPC-treated patients experienced an increase in average total ¨on ¨time of 119 minutes, as compared with 47 OPC treatment effectively reduced ¨off ¨ time and increased ¨on ¨ time without increasing the frequency of dyskinesia, and this benefit was maintained for at least 1 year of therapy without increasing the dose of L-DOPA [23,27,53,59]. 50mg OPC daily is thus considered the most efficacious dose, providing an average of one additional hour of ¨off ¨time reduction [53].

Post-launch of Opicapone
In taken once daily at bedtime, at least 1 hour before or after L-DOPA.

Safety and pharmacovigilance
The safety, efficacy and tolerability of OPC was evaluated over the course of a year in the open extension phase [53,75] (Table 2) At 50mg OPC daily, no evidence for hepatic injury was noted. Furthermore, side effects that had been reported with both TOL and ENT, such as severe diarrhoea, were not found. The percentage of patients who interrupted the treatment due to adverse events mediated by OPC was low and similar in all treatment groups [53].
In a similar study carried out by Ferreira and colleagues, the same population groups were evaluated at doses of both 25 and 50mg OPC. Significant improvements in motor fluctuations were noted in the study groups, without a significant increase in dopaminergic side effects such as dyskinesia [75].
In addition, it was observed that when switching from placebo to OPC in the extension phase, the ¨off ¨ time was reduced by an average of 51.1 minutes. During the double-blind phase, a one-year follow-up showed that patients previously treated with OPC at 25 and 50mg demonstrated an average ¨off¨ time reduction of 35.1 minutes and 58.1 minutes, respectively, with a total ¨off ¨time reduction average of nearly 2 hours per day.
With regard to ¨on¨ time, in the patients originally receiving placebo, there was an average increase of 52.5 minutes, but no change in those patients that initially received 50mg OPC.
The most frequent side effect observed was dyskinesias at 16% in the treatment group.

Studies on market and main competitors
Currently, two COMT inhibitors, ENT and TOL, remain on the market for the treatment of PD.
While ENT is a safe product which is given several times a day, TOL requires close monitoring due to the potential risk of hepatotoxicity. However, TOL is more potent than ENT. Stalevo ® is a combination of L-DOPA / carbidopa in the same pill with production costs that are lower than OPC.

Conclusions
PD is a devastatingly progressive disease with only symptomatic therapy currently available.
In addition, in animal studies it has been observed that the active metabolite BIA 9-1079 contributes to the therapeutic effect, which does not occur in humans.
Moreover, preclinical data in rat and monkey models demonstrate that OPC enhances significantly the bioavailability of L-DOPA [23,[40][41][42][43][44][45]. In both MPTP and 6-OHD-induced PD animal models a significant improvement of movement-related disease symptoms can be demonstrated when OPC is used as an adjunct with L-DOPA / DDI. Preclinical results indicate that OPC is an effective compound with suitable pharmacokinetics, a potent inhibitor of COMT which produces few side effects.
Results of two-phase III studies conducted in patients with PD demonstrated a significant reduction in ¨off ¨ time following administration of 50 mg OPC as well as increased ¨on¨ time ( Table 2). The BIPARK-I study also showed greater efficacy of OPC over ENT [58]. OPC 50 mg appears to demonstrate a greater effect on ¨off ¨ time reduction suggesting a potentially greater clinical benefit.
The BIPARK-2 study confirms the presence of fewer adverse effect such as diarrhoea, a side effect characteristic of COMT inhibitors [59]. As suggested by the low rate of patient withdrawal due to side effects, OPC appears to be well tolerated.
When compared to other COMT, OPC is administered once daily, one hour before or after administration of L-DOPA, while ENT is administered up to 8 times a day along with L-DOPA.
Inasmuch as L-DOPA may be taken multiple times a day, the QD dosing of OPC does not necessarily simplify the dosing regimen. Presently an ENT/L-DOPA/DDI inhibitor combination tablet is available, facilitating dosing [79][80][81][82]. TOL is administered three times a day, but due to potential hepatotoxicity, TOL is recommended only for those patients unable to tolerate or have poor clinical response to ENT or OPC. In patients with mild hepatic impairment, OPC may be administered although dosing adjustments may need to be made.
Given the safety issues with the use of TOL, OPC may be considered a safer alternative in patients who have not responded to treatment with ENT.

Expert opinion
Ideal treatment of PD would be to modify the course of the illness, but currently no such treatment exists. Another strategy used to modify or delay the disease has been the use of neurotrophic growth factors such as glial cell-line-derived neurotrophic factor (GDNF) and neurturin (NTN), which are members of the transforming growth factor β superfamily [86]. The objective of these strategies is to restore the viability of degenerated neurons. A clinical trial (ClinicalTrials.gov, number NCT00985517) has been conducted based on the direct administration of the gene therapy of an AAV2-neurturin vector (CERE-120) in the brain (to the putamen plus substantia nigra) [86][87][88]. However, the phase 2 was discontinued because the compound did not demonstrate statistically significant efficacy for an improvement in patient scores according to the Unified Parkinson's Disease Rating Scale. Glial derived neurotrophic factor (GDNF) was also developed as a therapy that would modify PD [89]. However, the results of the clinical trial were not satisfactory, but the authors suggest hopes for longer GDNF treatments that it is possible to restore damaged cells in PD [89,90]. Trophic factors may represent a future PD treatment, however this approach has not met with great success [88].
Since there is no cure for PD, the objective of the treatment consists of controlling the motor symptoms primarily through administration of L-DOPA/DDI and improving the quality of life of patients [2][3][4]. Although L-DOPA improves the control of PD symptoms, it is not able to halt disease progression, and, furthermore, the beneficial effects of L-DOPA eventually wear off, leading to worsening of symptoms and the appearance of motor fluctuations in patients.
In An important challenge in PD treatment is to minimize the ¨off ¨ time and to increase the ¨on¨ time, a property related to the efficacy and the pharmacokinetic properties of the drug. In the absence of disease-modifying treatments, the therapeutic strategy is based on improving treatment with L-DOPA by enhancing the ¨on ¨ phenomenon and decreasing the ¨off¨ effect. As demonstrated by clinical trials, the latter has been ameliorated with the administration of OPC .
Before OPC approval for the treatment of PD, only two COMT inhibitors were available for clinical L-DOPA/DDI association for the management of end-of-dose motor fluctuations: TOL and ENT. TOL was the first COMT inhibitor to be commercially available and it is a more potent COMT inhibitor than ENT, both in the periphery and CNS [20-23]; however, it has also been associated with severe liver injury. For this reason, it´s administration requires continuous liver function monitoring and must be restricted to patients who have failed to respond, or those patients that are intolerant to other COMT inhibitors. Conversely, ENT acts only in the periphery and is safer than TOL but has limited efficacy as well as a low to moderate oral bioavailability, which requires frequent dosing [76,77].
Accordingly, OPC represents an improvement in the treatment of PD compared to the other COMT inhibitors. The beneficial effects of OPC in PD therapy are currently thought to be due to a) dosing regimen, since it is administered once a day, b) lack of hepatotoxicity compared with TOL (ENT is not associated with hepatotoxicity, therefore, it does not require liver monitoring), c) higher mean reduction of ¨off ¨ time, with an average reduction of 116.8 min at the therapeutic dose of 50mg OPC.
Furthermore, ENT has the disadvantage of low oral bioavailability with a shorter duration of action as well as being less potent.
At the recommended therapeutic dose of 200 mg, ENT therefore has limited clinical efficacy [79,80]. OPC meets the need for a safer, more potent and long-acting COMT inhibitor [80]. As a selective COMT, reduction in ¨off ¨ time provides a new strategy that could well accommodate the therapeutic needs of PD patients. A small molecule such as OPC demonstrates proven advantage over medicines previously used for idiopathic PD and are welcome in the marketplace.
Of course, OPC represents symptomatic treatment of PD; a curative or preventative approach remains elusive.
In conclusion, despite its limitations, OPC constitutes a paradigm shift in the treatment of PD. It may represent a starting point for both a better understanding of PD´s pathogenesis as well as the development of new types of medical and pharmaceutical interventions to improve care for PD patients.
However, improvements in the design of COMT inhibitors should still be pursued, including, for example, an improved half-life and low toxicity whilst maintaining or increasing drug efficacy.
COMT inhibitors currently marketed (with the exception of TOL) are intended to act exclusively in the inhibition of peripheral COMT. We strongly believe that it would be of interest for new drugs to also exert a centrally acting COMT inhibitor effect.