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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