Opicapone (Ongentys®), a potent, oral, third-generation, long-acting, peripheral catechol-O-methyltransferase (COMT) inhibitor, is approved as the adjunctive treatment to levodopa (L-Dopa)/dopa-decarboxylase inhibitor (DDCI) therapy in adults with Parkinson's disease (PD) and end-of-dose motor fluctuations who cannot be stabilized on those combinations. Opicapone is a hydrophilic 1,2,4-oxadiazole analog with a pyridine N-oxide at position 3, with these modifications enhancing its potency and extending its duration of action, whilst avoiding cell toxicity. In preclinical animal studies, Opicapone-induced inhibition of peripheral (but not central) COMT activity was associated with a prolonged increase in systemic and central exposure to L-Dopa, with a corresponding reduction in 3-OMD exposure. Following single or multiple doses of Opicapone (5–1200 mg) in healthy adult volunteers or patients with PD, Opicapone inhibited COMT activity in ex vivo erythrocyte assays in a reversible dose-dependent manner, with the duration of Opicapone-induced COMT inhibition independent of dose. Adjunctive Opicapone was generally well tolerated during more than a year of treatment in BIPARK I and BIPARK II (double-blind plus extension phases). The recommended dosage is 50 mg once daily, which should be taken at bedtime at least 1 h before or after L-Dopa combinations.

Istradefylline is a first-in-class adenosine A2A receptor antagonist antiparkinsonian agent and has been marketed as the brand name NOURIAST® in Japan since May 30, 2013. NOURIAST is indicated for the improvement of wearing-off phenomena in patients with Parkinson’s disease on concomitant treatment with levodopa-containing products.

Pitolisant (INN) or tiprolisant (USAN) is a histamine receptor inverse agonist/antagonist selective for the H3 subtype. It has stimulant and nootropic effects in animal studies and may have several medical applications, having been researched for the treatment of narcolepsy, for which it has been granted orphan drug status in the EU and US. It is currently in clinical trials for schizophrenia and Parkinson’s disease. Pitolisant hydrochloride was approved by European Medicine Agency (EMA) on Mar 31, 2016. It was developed and marketed as Wakix® by Bioprojet in EU. Wakix® is available as the tablet for oral use, containing 4.5 mg and 18 mg of Pitolisant hydrochloride. The initial dose of 9 mg (two 4.5 mg, tablets) per day, and it should be used at the lowest effective dose, depending on individual patient response and tolerance, according to an up-titration scheme, without exceeding the dose of 36 mg/day. Pitolisant was the first clinically used H3 receptor inverse agonist.

Safinamide (FCE 26743, NW 1015, PNU 151774, PNU 151774E, trade name Xadago) combines potent, selective, and reversible inhibition of MAO-B with blockade of voltage-dependent Na+ and Ca2+ channels and inhibition of glutamate release. Safinamide is under development with Newron, Zambon and Meiji Seika Pharma for the treatment of Parkinson's disease. Safinamide has been launched in the EU, Iceland and Liechtenstein. Safinamide was well tolerated and safe in the clinical development program that demonstrated the amelioration of motor symptoms and OFF phenomena by safinamide when combined with dopamine agonists or levodopa.

Pimavanserin, marketed under the trade name Nuplazid, a non-dopaminergic atypical antipsychotic developed by Acadia Pharmaceuticals is the first and only medication approved by the U.S. Food and Drug Administration (FDA) for the treatment of hallucinations and delusions associated with Parkinson’s disease psychosis. The mechanism of action of pimavanserin in the treatment of hallucinations and delusions associated with Parkinson’s disease psychosis is unknown. However, the effect of pimavanserin could be mediated through a combination of inverse agonist and antagonist activity at serotonin 5-HT2A receptors and to a lesser extent at serotonin 5-HT2C receptors. In vitro, pimavanserin acts as an inverse agonist and antagonist at serotonin 5-HT2A receptors with high binding affinity (Ki value 0.087 nM) and at serotonin 5-HT2C receptors with lower binding affinity (Ki value 0.44 nM). Pimavanserin shows low binding to sigma 1 receptors (Ki value 120 nM) and has no appreciable affinity (Ki value >300 nM), to serotonin 5-HT2B, dopaminergic (including D2), muscarinic, histaminergic, or adrenergic receptors, or to calcium channels. Pimavanserin was approved by the FDA to treat hallucinations and delusions associated with psychosis experienced by some people with Parkinson's disease on April 29, 2016.

Rotigotine is an agonist at all 5 dopamine receptor subtypes (D1-D5) but binds to the D3 receptor with the highest affinity. It is also an antagonist at α-2-adrenergic receptors and an agonist at the 5HT1A receptors. Rotigotine also inhibits dopamine uptake and prolactin secretion. It is FDA approved for the treatment of Parkinson's disease, restless legs syndrome. Dopamine antagonists, such as antipsychotics or metoclopramide, may diminish the effectiveness of Rotigotine. Common adverse reactions include nausea, vomiting, somnolence, application site reactions, dizziness, anorexia, hyperhidrosis, insomnia and dyskinesia.

Entacapone is a selective, reversible catechol-O-methyl transferase (COMT) inhibitor for the treatment of Parkinson's disease. It is a member of the class of nitrocatechols. When administered concomittantly with levodopa and a decarboxylase inhibitor (e.g., carbidopa), increased and more sustained plasma levodopa concentrations are reached as compared to the administration of levodopa and a decarboxylase inhibitor. The mechanism of action of entacapone is believed to be through its ability to inhibit COMT in peripheral tissues, altering the plasma pharmacokinetics of levodopa. When entacapone is given in conjunction with levodopa and an aromatic amino acid decarboxylase inhibitor, such as carbidopa, plasma levels of levodopa are greater and more sustained than after administration of levodopa and an aromatic amino acid decarboxylase inhibitor alone. It is believed that at a given frequency of levodopa administration, these more sustained plasma levels of levodopa result in more constant dopaminergic stimulation in the brain, leading to a greater reduction in the manifestations of parkinsonian syndrome. Entacapone is used as an adjunct to levodopa / carbidopa in the symptomatic treatment of patients with idiopathic Parkinson's Disease who experience the signs and symptoms of end-of-dose "wearing-off".

Ropinirole (INN; trade names Requip, Repreve, Ronirol, Adartrel) is a dopamine agonist of the non-ergoline class of medications, used in the treatment of Parkinson's disease and restless legs syndrome. Although the precise mechanism of action of ropinirole as a treatment for Parkinson's disease is unknown, it is believed to be related to its ability to stimulate dopamine receptors in the striatum. This conclusion is supported by electrophysiologic studies in animals that have demonstrated that ropinirole influences striatal neuronal firing rates via activation of dopamine receptors in the striatum and the substantia nigra, the site of neurons that send projections to the striatum. Ropinirole is a nonergot dopamine agonist with high relative in vitro specificity and full intrinsic activity at the D2 subfamily of dopamine receptors, binding with higher affinity to D3 than to D2 or D4 receptor subtypes. The relevance of D3 receptor binding in Parkinson's disease is unknown. The mechanism of ropinirole-induced postural hypotension is presumed to be due to a D2 -mediated blunting of the noradrenergic response to standing and subsequent decrease in peripheral vascular resistance. Ropinirole can cause nausea, dizziness, hallucinations, orthostatic hypotension, and sudden sleep attacks during the daytime. Unusual side effects specific to D3 agonists such as ropinirole and pramipexole can include hypersexuality, punding, and compulsive gambling, even in patients without a history of these behaviors.

Perindoprilat is a metabolite of perindopril. Perindopril is a long-acting angiotensin converting enzyme (ACE) inhibitor and it is used to treat high blood pressure, heart failure or stable coronary artery disease. Perindopril is designed to allow oral administration as perindoprilat is poorly absorbed from the gastrointestinal tract.

Selegiline, also known as L-deprenyl, is a substituted phenethylamine, a selective, irreversible inhibitor of Type B monoamine oxidase. Selegiline is available in pill form under many brand names (Eldepryl, Carbex, Atapryl) and is used to reduce symptoms in early-stage Parkinson's disease. Selegiline delays the time point when the L-DOPA (levodopa) treatment becomes necessary from about 11 months to about 18 months after diagnosis, which is beneficial despite not being definitive evidence of neuroprotection. The rationale for adding selegiline to levodopa is to decrease the required dose of levodopa and thus reduce the motor complications of levodopa therapy. Selegiline is also delivered via a transdermal patch (brand name, Emsam) and in this form, Selegiline is used as a treatment for the major depressive disorder. Selegiline (brand name Anipryl) is also used (at extremely high dosages relative to humans) in veterinary medicine to treat the symptoms of Cushing's disease and cognitive dysfunction (Canine Cognitive Dysfunction) in dogs. Side effects of the pill form include, in decreasing order of frequency, nausea, hallucinations, confusion, depression, loss of balance, insomnia, increased involuntary movements, agitation, arrhythmia, slow heart rate, delusions, hypertension, new or increased angina pectoris, and syncope. The main side effects of the patch form for depression included application site reactions, insomnia, diarrhea, and sore throat.