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.

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.

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.

Rasagiline (N-propargyl-1-(R)-aminoindan) is a selective, irreversible monoamine oxidase B (MAO B) inhibitor, which has been developed as an anti-Parkinson drug and was sold as a mesylate salt under brand name AZILECT. AZILECT is indicated for the treatment of the signs and symptoms of idiopathic Parkinson’s disease (PD) as initial monotherapy and as adjunct therapy to levodopa. The effectiveness of AZILECT was demonstrated in patients with early Parkinson’s disease who were receiving AZILECT as monotherapy and who were not receiving any concomitant dopaminergic therapy. The effectiveness of AZILECT as adjunct therapy was demonstrated in patients with Parkinson’s disease who were treated with levodopa. PD is a progressive neurodegenerative, dopamine deficiency disorder. The main therapeutic strategies for PD treatment relies on dopamine precursors (levodopa), inhibition of dopamine metabolism (monoamine oxidase [MAO] B and catechol-O-methyl transferase inhibitors), and dopamine receptor agonists. In contrast to selegiline, rasagiline is not metabolized to potentially toxic amphetamine metabolites. The precise mechanisms of action of rasagiline is unknown. One mechanism is believed to be related to its MAO-B inhibitory activity, which causes an increase in extracellular levels of dopamine in the striatum.

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".

Tolcapone is a potent, selective, and reversible inhibitor of catechol-O-methyltransferase (COMT). In humans, COMT is distributed throughout various organs. COMT catalyzes the transfer of the methyl group of S-adenosyl-L-methionine to the phenolic group of substrates that contain a catechol structure. Physiological substrates of COMT include dopa, catecholamines (dopamine, norepinephrine, epinephrine) and their hydroxylated metabolites. The function of COMT is the elimination of biologically active catechols and some other hydroxylated metabolites. COMT is responsible for the elimination of biologically active catechols and some other hydroxylated metabolites. In the presence of a decarboxylase inhibitor, COMT becomes the major metabolizing enzyme for levodopa catalyzing it to 3-methoxy-4-hydroxy-L-phenylalanine (3-OMD) in the brain and periphery. When tolcapone is given in conjunction with levodopa and an aromatic amino acid decarboxylase inhibitor, such as carbidopa, plasma levels of levodopa are more sustained than after administration of levodopa and an aromatic amino acid decarboxylase inhibitor alone. It is believed that these sustained plasma levels of levodopa result in more constant dopaminergic stimulation in the brain, leading to greater effects on the signs and symptoms of Parkinson's disease in patients as well as increased levodopa adverse effects, sometimes requiring a decrease in the dose of levodopa. The precise mechanism of action of tolcapone is unknown, but it is believed to be related to its ability to inhibit COMT and alter the plasma pharmacokinetics of levodopa, resulting in an increase in plasma levodopa concentrations. The inhibition of COMT also causes a reduction in circulating 3-OMD as a result of decreased peripheral metabolism of levodopa. This may lead to an increase distribution of levodopa into the CNS through the reduction of its competitive substrate, 3-OMD, for transport mechanisms. Sustained levodopa concentrations presumably result in more consistent dopaminergic stimulation, resulting in greater reduction in the manifestations of parkinsonian syndrome. Tolcapone is used as an adjunct to levodopa/carbidopa therapy for the symptomatic treatment of Parkinson's Disease. This drug is generally reserved for patients with parkinsonian syndrome receiving levodopa/carbidopa who are experiencing symptom fluctuations and are not responding adequately to or are not candidates for other adjunctive therapies. Tolcapone is sold under the brand name Tasmar.

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.

Pramipexole 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 precise mechanism of action of Pramipexole as a treatment for Parkinson's disease is unknown, although 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 Pramipexole 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. Pramipexole is used for the treatment of signs and symptoms of idiopathic Parkinson's disease.

Cabergoline is a long-acting dopamine receptor agonist with a high affinity for D2 receptors. Results of in vitro studies demonstrate that cabergoline exerts a direct inhibitory effect on the secretion of prolactin by rat pituitary lactotrophs. It is FDA approved for the treatment of hyperprolactinemic disorders, either idiopathic or due to pituitary adenomas. Common adverse reactions include constipation, nausea, dizziness, headache and fatigue. Cabergoline should not be administered concurrently with D-antagonists, such as phenothiazines, butyrophenones, thioxanthenes, or metoclopramide.

Bromocriptine is an ergot derivative with potent dopamine receptor agonist activity, which activates post-synaptic dopamine receptors. Bromocriptine is indicated for the treatment of dysfunctions associated with hyperprolactinemia. Bromocriptine therapy is indicated in the treatment of acromegaly and in the treatment of the signs and symptoms of idiopathic or postencephalitic Parkinson’s disease. It is approved as an adjunct to diet and exercise to improve glycemic control in adults with type 2 diabetes mellitus. Some commonly reported adverse reactions include nausea, fatigue, dizziness, vomiting and headache. Bromocriptine may interact with dopamine antagonists, butyrophenones and certain other agents.