Levodopa / Carbidopa (and Benserazide)

Mechanism

Chemical and Commercial Profile

Common trade names: Sinemet, Madopar, Duodopa.
Group: Immediate precursor of dopamine (left-handed isomer) combined with a peripheral inhibitor of the enzyme dopa-decarboxylase.

Mechanism of Action

Levodopa is a neutral amino acid immediate precursor of dopamine. It actively crosses the blood-brain barrier via the LAT-1 transporter, where it is centrally decarboxylated in presynaptic striatal nerve terminals by the enzyme L-amino acid aromatic decarboxylase (DDC), restoring dopaminergic tone and stimulating postsynaptic D₁ and D₂ receptors.

Effect of Peripheral Blockade of DDC (Carbidopa/Benserazide) \text{Levodopa alone} \longrightarrow \text{99\% peripheral metabolism by DDC and COMT} \longrightarrow \text{Severe nausea, arrhythmias and no central effect} \\ \text{Levodopa + Carbidopa} \longrightarrow \text{Peripheral inhibition of DDC} \longrightarrow \text{10% increase in free Levodopa crossing the BBB}

Carbidopa and benserazide are competitive inhibitors of peripheral DDC that **do not cross the blood-brain barrier**, specifically preventing the peripheral catabolism of levodopa and minimizing its autonomic adverse effects.

Pharmacokinetics

Pharmacokinetics

• Absorption: Rapid in the duodenum through active transport by the LAT-1 transporter. It presents **competition with dietary amino acids** (meals rich in protein drastically reduce its absorption and clinical effectiveness). The plasma peak is reached at the time of intake on an empty stomach.
• Distribution: Low binding to plasma proteins (10-30%). The volume of distribution is 1.6 L/kg. It easily crosses the blood-brain barrier.
• Metabolism: In the absence of peripheral inhibitors, levodopa is metabolized mainly by peripheral decarboxylation to dopamine (99%), and to a lesser extent by O-methylation catalyzed by the enzyme COMT to 3-OMD. When carbidopa is combined, the COMT pathway becomes peripherally predominant.
• Excretion: Renal (80%) in the form of conjugated homovanillic acid (HVA) and dihydroxyphenylacetic acid (DOPAC).
• Half-life (t_1/2): Plasma short of 1-1.5 hours, increasing to 2-2.5 hours in association with carbidopa or benserazide. Requires multiple daily feedings.

Indicators and dose

Indications

• Symptomatic treatment of choice for Idiopathic Parkinson's Disease (initial and advanced phase).
• Secondary symptomatic parkinsonism (postencephalitic, of atherosclerotic origin, or induced by carbon monoxide). (Note: Not effective in neuroleptic-induced parkinsonism).

Dosage and Settings

• Startup (Immediate-release tablets): 50-100 mg of levodopa combined with 12.5-25 mg of carbidopa three times a day (after meals initially to reduce nausea; then before meals to improve absorption).
• Titration: Increase 50-100 mg of levodopa weekly until optimal response is obtained. Usual range: 300-1000 mg of levodopa/day (divided into 3 to 6 doses).
• Advanced Systems: Continuous intestinal infusion of levodopa/carbidopa gel (Duodopa) via percutaneous gastrostomy tube (PEG-J) in patients with refractory motor fluctuations.
• Renal/Hepatic Failure: No special dosage adjustments are required, but clinical caution is recommended in Child-Pugh C liver failure.

Security

Contraindications

• Concurrent use of non-selective monoamine oxidase inhibitors (MAOIs) (risk of hypertensive crisis due to massive synaptic accumulation of catecholamines).
• Unstable acute narrow angle glaucoma.
• Personal history of metastatic malignant melanoma or undiagnosed suspicious lesions (levodopa is a precursor of melanin synthesis).

Adverse Effects (ADR)

Long-Term Motor Fluctuations

After a period of continuous treatment of 3-5 years ("honeymoon" of levodopa), 50% of patients experience severe motor fluctuations, such as the phenomenon of wearing-off, the phenomenon of sudden fluctuations in mobility/immobility on-off, and involuntary hyperkinesias or choreoathetotic dyskinesias of peak dose**. This responds to the progressive loss of the buffering capacity of the residual striatal presynaptic terminals.

• Gastrointestinal: Persistent nausea, vomiting, early-onset moderate stomach pain (mitigated by administering the drug with water or increasing the percentage of carbidopa).
• Cardiovascular: Severe orthostatic hypotension, postural dizziness, palpitations, transient sinus tachycardia, cardiac extrasystoles.
• Psychiatric / From the Control Field: Vivid visual hallucinations of late onset (common in the elderly), delusions of persecution, nightmares, impulse control disorder (due to mesolimbic hyperstimulation).

Clinical Interactions

• Typical neuroleptics / antipsychotics: Haloperidol, thioridazine or chlorpromazine block the D₂ receptors centrally, absolutely nullifying the therapeutic effect of levodopa. (Note: If treatment of induced psychosis is required, only clozapine or quetiapine is authorized).
• Vitamin B6 (Pyridoxine): Acts as a cofactor of the DDC enzyme, accelerating the peripheral catabolism of levodopa and reducing its effectiveness, unless carbidopa is associated.

Pregnancy and Breastfeeding

FDA Category: C. Animal models of pregnancy associate fetal visceral and skeletal anomalies. It is absolutely discouraged. Breastfeeding: Absolutely contraindicated. Levodopa potently inhibits pituitary prolactin secretion, suppressing lactogenesis reflexively, in addition to being excreted in breast milk.