Carbidopa 37.5 mg, Entacapone 200 mg, Levodopa 150 mg Tablets

Levodopa — Current evidence indicates that symptoms of Parkinson’s disease are related to depletion of dopamine in the corpus striatum. Administration of dopamine is ineffective in the treatment of Parkinson’s disease because it does not cross the blood-brain barrier. However, levodopa the metabolic precursor of dopamine, does cross the blood-brain barrier, and is presumably converted to dopamine in the brain. This is thought to be the mechanism whereby levodopa relieves the symptoms of Parkinson’s disease.

Carbidopa — When levodopa is administered orally, it is rapidly decarboxylated to dopamine in extracerebral tissues so that only a small portion of a given dose is transported unchanged to the central nervous system. Carbidopa inhibits the decarboxylation of peripheral levodopa, making more levodopa available for delivery to the brain.

Entacapone — Entacapone is a selective and reversible inhibitor of catechol-O-methyltransferase (COMT).

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, and epinephrine) and their hydroxylated metabolites. When decarboxylation of levodopa is prevented by carbidopa, COMT becomes the major metabolizing enzyme for levodopa, catalyzing its metabolism to 3-methoxy-4-hydroxy-L-phenylalanine (3-OMD).

The pharmacokinetics of carbidopa, levodopa and entacapone tablets has been studied in healthy subjects (age 45 years to 75 years). Overall, following administration of corresponding doses of levodopa, carbidopa and entacapone as carbidopa, levodopa and entacapone tablets or as carbidopa and levodopa product plus Comtan (entacapone) tablets, the mean plasma concentrations of levodopa, carbidopa, and entacapone are comparable.

Absorption and Distribution — Both levodopa and entacapone are rapidly absorbed and eliminated, and their distribution volume is moderately small. Carbidopa is absorbed and eliminated slightly more slowly compared with levodopa and entacapone. There are substantial inter- and intra-individual variations in the absorption of levodopa, carbidopa and entacapone, particularly concerning its Cmax.

The food-effect on the carbidopa, levodopa and entacapone tablet has not been evaluated. Because levodopa competes with certain amino acids for transport across the gut wall, the absorption of levodopa may be impaired in some patients after eating a high protein meal. Meals rich in large neutral amino acids may delay and reduce the absorption of levodopa [see Patient Counseling Information (17) ].

Levodopa — The pharmacokinetic properties of levodopa following the administration of single-dose carbidopa, levodopa and entacapone tablets are summarized in Table 3.

Levodopa is bound to plasma protein only to a minor extent (about 10% to 30%).

Carbidopa — Following administration of carbidopa, levodopa and entacapone tablets as a single dose to healthy male and female subjects, the peak concentration of carbidopa was reached within 2.5 hours to 3.4 hours on average. The mean Cmax ranged from about 40 nanogram per mL to 225 nanogram per mL and the mean AUC from 170 nanogram•h per mL to 1,200 nanogram•h per mL, with different carbidopa, levodopa and entacapone tablets strengths providing 12.5 mg, 25 mg, 37.5 mg, or 50 mg of carbidopa.

Carbidopa is approximately 36% bound to plasma protein.

Entacapone — Following administration of carbidopa, levodopa and entacapone tablets as a single dose to healthy male and female subjects, the peak concentration of entacapone in plasma was reached within 0.8 hour to 1.2 hours on average. The mean Cmax of entacapone was about 1,200 nanogram per mL to 1,500 nanogram per mL and the AUC 1,250 nanogram•h per mL to 1,750 nanogram•h per mL after administration of different carbidopa, levodopa and entacapone tablets strengths all providing 200 mg of entacapone.

The plasma protein binding of entacapone is 98% over the concentration range of 0.4 mcg per mL to 50 mcg per mL. Entacapone binds mainly to serum albumin.

Levodopa — The elimination half-life of levodopa, the active moiety of antiparkinsonian activity, was 1.7 hours (range 1.1 hours to 3.2 hours).

Levodopa is extensively metabolized to various metabolites. Two major pathways are decarboxylation by dopa decarboxylase (DDC) and O-methylation by COMT.

Carbidopa — The elimination half-life of carbidopa was on average 1.6 hours to 2 hours (range 0.7 hour to 4.0 hours).

Carbidopa is metabolized to two main metabolites (α-methyl-3-methoxy-4-hydroxyphenylpropionic acid and α-methyl-3,4-dihydroxyphenylpropionic acid). These 2 metabolites are primarily eliminated in the urine unchanged or as glucuronide conjugates. Unchanged carbidopa accounts for 30% of the total urinary excretion.

Entacapone — The elimination half-life of entacapone was on average 0.8 hour to 1 hour (0.3 hour to 4.5 hours).

Entacapone is almost completely metabolized prior to excretion with only a very small amount (0.2% of dose) found unchanged in urine. The main metabolic pathway is isomerization to the cis-isomer, the only active metabolite. Entacapone and the cis-isomer are eliminated in the urine as glucuronide conjugates. The glucuronides account for 95% of all urinary metabolites (70% as parent and 25% as cis-isomer glucuronides). The glucuronide conjugate of the cis-isomer is inactive. After oral administration of a 14C-labeled dose of entacapone, 10% of labeled parent and metabolite is excreted in urine and 90% in feces.

Due to short elimination half-lives, no true accumulation of levodopa or entacapone occurs when they are administered repeatedly.