NAPLEX Practice Question # 31

NAPLEX Examination.

Practice Question # 31.





Levodopa/carbidopa  is the most eff ective drug for managing Parkinson disease; however, prolonged use decreases its therapeutic eff ects (there is a decline in effi cacy aft er 3 to 5 years) and increases adverse drug reactions. Dopamine does not cross the blood–brain barrier; therefore, a precursor is used. Peripheral conversion of levodopa to dopamine causes adverse reactions like nausea, vomiting, cardiac arrhythmias, and postural hypotension. To decrease the peripheral conversion and peripheral adverse eff ects, a peripheral dopa decarboxylase inhibitor (carbidopa) is added to levodopa.

Mechanism of action

  • Levodopa is converted to dopamine by the enzyme dopa decarboxylase, which elevates CNS levels of dopamine.
  • Th e sustained-release formulation is designed to release the drug over 4 to 6 hrs, thereby inhibiting variation in plasma concentration and decreasing motor fl uctuation “off ” time or improving overall dose response in patients with advanced disease.

Administration and dosage

  • It is necessary to give at least 100 mg daily of carbidopa to decrease the incidence of the peripheral conversion of levodopa and GI side eff ects (e.g., nausea) and increase the bioavailability of levodopa for the CNS.
  • If carbidopa is given in a separate dosage form, the dose of levodopa can be decreased by 75%.
  • Carbidopa given separately or in addition permits to lower dose of levodopa with decrease in GI side eff ect and more rapid dosage titration (max dose 200 mg/day).
  • Sustained-release preparations are approximately 30% less bioavailable as compared with levodopa/ carbidopa. Because of this lower bioavailability, the daily dosage should be higher.

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