Duopa - Scientific Information
|Class:||Dopaminergic antiparkinsonism agents|
|Ingredients:||Levodopa, Carbidopa, Carboxymethylcellulose Sodium, Water|
DUOPA is a combination of carbidopa, an inhibitor of aromatic amino acid decarboxylation, and levodopa, an aromatic amino acid.
Carbidopa is a white, crystalline compound, slightly soluble in water, with a molecular weight of 244.2. It is designated chemically as (2S)-3-(3,4-dihydroxyphenyl)-2-hydrazino-2-methylpropanoic acid monohydrate. Its empirical formula is C10 H14 N2 O4 •H2 O, and its structural formula is:
The content of carbidopa in DUOPA is expressed in terms of anhydrous carbidopa which has a molecular weight of 226.3. The 4.63 mg/mL of anhydrous carbidopa is equivalent to 5.0 mg/mL of carbidopa.
Levodopa is a white, crystalline compound, slightly soluble in water, with a molecular weight of 197.2. It is designated chemically as (2S)-2-Amino-3-(3,4-dihydroxyphenyl) propanoic acid. Its empirical formula is C9H11 NO4 , and its structural formula is:
The inactive ingredients in DUOPA are carmellose sodium and purified water.
Mechanism of Action
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.
Levodopa is the metabolic precursor of dopamine, does cross the blood-brain barrier, and presumably is converted to dopamine in the brain. This is thought to be the mechanism whereby levodopa treats the symptoms of Parkinson’s disease.
Because its decarboxylase inhibiting activity is limited to extracerebral tissues, administration of carbidopa with levodopa makes more levodopa available to the brain. The addition of carbidopa to levodopa reduces the peripheral effects (e.g., nausea and vomiting) due to decarboxylation of levodopa; however, carbidopa does not decrease the adverse reactions due to the central effects of levodopa.
The pharmacokinetics of carbidopa and levodopa with 16-hour intrajejunal infusion of DUOPA was evaluated in 18 patients with advanced Parkinson’s disease who had been on DUOPA therapy for 30 days or longer. Patients remained on their individualized DUOPA doses.
The plasma concentrations versus time profile for levodopa with DUOPA 16-hour intrajejunal infusion is presented in Figure 1.
Figure 1. Plasma Concentrations (mean ± standard deviation) versus Time Profile of Levodopa with DUOPA (levodopa, 1580 ± 403 mg; carbidopa, 366 ± 92 mg) 16-Hour Infusion
Absorption and Bioavailability
Following initiation of the 16-hour intrajejunal infusion of DUOPA, peak plasma levels of levodopa is reached at 2.5 hours. The absorption of levodopa may be decreased in patients on a high-protein diet because levodopa competes with certain amino acids for transport across the gut wall. The gastric emptying rate does not influence the absorption of DUOPA since it is administered by continuous intestinal infusion. In a cross-study population pharmacokinetic analysis, DUOPA had comparable bioavailability to the oral immediate-release carbidopa-levodopa (25/100 mg) tablets (over-encapsulated tablets). The estimated bioavailability for levodopa from DUOPA relative to oral immediate-release carbidopa-levodopa tablets was 97% (95% confidence interval; 95% to 98%).
In the controlled clinical trial, the intra-subject variability in carbidopa and levodopa plasma concentrations were lower for patients treated with DUOPA (N=33, 25% and 21%, respectively) than in patients treated with oral immediate-release carbidopa-levodopa (25/100 mg) tablets (N=28, 39% and 67%, respectively).
Carbidopa is approximately 36% bound to plasma proteins. Levodopa is approximately 10-30% bound to plasma proteins.
Metabolism and Elimination
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. The elimination half-life of carbidopa is approximately 2 hours.
Levodopa is mainly eliminated via metabolism by the aromatic amino acid decarboxylase (AAAD) and the catechol-O-methyl-transferase (COMT) enzymes. Other routes of metabolism are transamination and oxidation. The decarboxylation of levodopa to dopamine by AAAD is the major enzymatic pathway when no enzyme inhibitor is co-administered. O-methylation of levodopa by COMT forms 3-O-methyldopa. When administered with carbidopa, the elimination half-life of levodopa is approximately 1.5 hours (see Figure 1).
Drug Interaction Studies
Systemic exposure of levodopa is expected to increase in the presence of entacapone.
Carcinogenesis, Mutagenesis, Impairment of Fertility
In rat, oral administration of carbidopa-levodopa for two years resulted in no evidence of carcinogenicity. DUOPA contains hydrazine, a degradation product of carbidopa. In published studies, hydrazine has been demonstrated to be carcinogenic in multiple animal species. Increases in liver (adenoma, carcinoma) and lung (adenoma, adenocarcinoma) tumors have been reported with oral administration of hydrazine in mouse, rat, and hamster.
Carbidopa was positive in the in vitro Ames test, in the presence and absence of metabolic activation, and the in vitro mouse lymphoma tk assay in the absence of metabolic activation but was negative in the in vivo mouse micronucleus assay.
In published studies, hydrazine was reported to be positive in in vitro genotoxicity (Ames, chromosomal aberration in mammalian cells, and mouse lymphoma tk) assays and in the in vivo mouse micronucleus assay.
Impairment of Fertility
In reproduction studies, no effects on fertility were observed in rats receiving carbidopa -levodopa.
The efficacy of DUOPA was established in a randomized, double-blind, double-dummy, active-controlled, parallel group, 12-week study (Study 1) in patients with advanced Parkinson’s disease who were levodopa-responsive and had persistent motor fluctuations while on treatment with oral immediate-release carbidopa-levodopa and other Parkinson’s disease medications.
Patients were eligible for participation in the studies if they were experiencing 3 hours or more of “Off” time on their current Parkinson’s disease drug treatment and they demonstrated a clear responsiveness to treatment with levodopa. Seventy-one (71) patients enrolled in the study and 66 patients completed the treatment (3 patients discontinued treatment because of adverse reactions, 1 patient for lack of effect, and 1 patient for non-compliance).
Patients enrolled in this study had a mean age of 64 years and disease duration of 11 years. Most patients (89%) were taking at least one concomitant medication for Parkinson’s disease (e.g., dopaminergic agonist, COMT-inhibitor, MAO-B inhibitor) in addition to oral immediate-release carbidopa-levodopa. Thirty nine percent of patients were taking two or more of such concomitant medications.
Patients were randomized to either DUOPA and placebo capsules or placebo suspension and oral immediate-release carbidopa-levodopa 25/100 mg capsules. Patients in both treatment arms had a PEG-J device placement. DUOPA or placebo-suspension was infused over 16 hours daily through a PEG-J tube via the CADD-Legacy 1400 model ambulatory infusion pump. The mean daily levodopa dose was 1117 mg/day in the DUOPA group and 1351 mg/day in the oral immediate-release carbidopa-levodopa group.
The clinical outcome measure in Study 1 was the mean change from baseline to Week 12 in the total daily mean “Off” time, based on a Parkinson’s disease diary. The “Off” time was normalized to a 16-hour awake period, based on a typical person’s waking day and the daily infusion duration of 16 hours. The mean score decrease (i.e., improvement) in “Off” time from baseline to Week 12 for DUOPA was significantly greater (p=0.0015) than for oral immediate-release carbidopa-levodopa. Additionally, the mean score increase (i.e., improvement) in “On” time without troublesome dyskinesia from baseline to Week 12 was significantly greater (p=0.0059) for DUOPA than for oral immediate-release carbidopa-levodopa. The treatment difference (DUOPA – oral immediate release carbidopa-levodopa) for decrease in “Off” time was approximately 1.9 hours and the treatment difference for the increase in “On” time without troublesome dyskinesia was approximately 1.9 hours. Results of Study 1 are shown in Table 1.
|LS Mean Change from|
Baseline at Week 12
|Oral immediate-release carbidopa-levodopa||6.9||-2.1|
|“On” time without troublesome dyskinesia|
|Oral immediate-release carbidopa-levodopa||8.0||2.2|
LS Mean Change from Baseline based on Analysis of Covariance (ANCOVA).
Figure 2 shows results over time according to treatment for the efficacy variable (change from baseline in “Off” time) that served as the clinical outcome measure at the end of the trial at 12 weeks.
Figure 2. Change in “Off” Time Over 12 Weeks.