Peripheral Decarboxylase Inhibitors

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Chapter: Essential pharmacology : Antiparkinsonian Drugs

Carbidopa and benserazide are extracerebral dopa decarboxylase inhibitors; they do not penetrate bloodbrain barrier and do not inhibit conversion of levodopa to DA in the brain.


PERIPHERAL DECARBOXYLASE INHIBITORS

 

Carbidopa and benserazide are extracerebral dopa decarboxylase inhibitors; they do not penetrate bloodbrain barrier and do not inhibit conversion of levodopa to DA in the brain. Administered along with levodopa, they increase its t½ in the periphery and make more of it available to cross bloodbrain barrier to reach its site of action.

 

Benefits of the combination are

 

1.    The plasma t½ of levodopa is prolonged and its dose is reduced to approximately 1/4th.

2.   Systemic concentration of DA is reduced, nausea and vomiting are not prominent— therapeutic doses of levodopa can be attained quickly.

3.   Cardiac complications are minimized.

4.   Pyridoxine reversal of levodopa effect does not occur.

5.   ‘Onoff’ effect is minimized since cerebral DA levels are more sustained.

6.   Degree of improvement may be higher; some patients, not responding adequately to levodopa alone, also improve.

 

Problems not resolved or accentuated are

 

1.   Involuntary movements  

2.   Behavioral abnormalities

3.   Postural hypotension.

 

Currently, levodopa is practically always used along with a decarboxylase inhibitor, except in patients who develop marked involuntary movements with the combination.

 

Combination of levodopa with carbidopa has been given the name ‘Cocareldopa’.

 

BENSPAR, MADOPAR: Benserazide 25 mg + levodopa 100 mg cap.

 

Usual daily maintenance dose of levodopa is 0.4–0.8 g along with 75–100 mg carbidopa or 100–200 mg benserazide, given in 3–4 divided doses. Therapy is started at a low dose and suitable preparations are chosen according to the needs of individual patients, increasing the dose as required.

 

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