Carbidopa/levodopa

Carbidopa/levodopa is the combination of carbidopa and levodopa and is used to treat Parkinson's disease^[1] and dopamine-responsive dystonia (DRD). It is sold under several brand names, including Sinemet, Parcopa, and Atamet. The generic name under the British Approved Name system is Co-careldopa.

Benefits of the combination drug

Levodopa is converted to dopamine via the action of a naturally occurring enzyme called DOPA decarboxylase. This occurs both in the peripheral circulation and in the central nervous system after levodopa has crossed the blood brain barrier. Activation of central dopamine receptors improves the symptoms of Parkinson's Disease, however, activation of peripheral dopamine receptors causes nausea and vomiting. For this reason levodopa is usually administered in combination with a DOPA decarboxylase inhibitor (DDCI), in this case carbidopa, which is very polar (and charged at physiologic pH) and cannot cross the blood brain barrier, however prevents peripheral conversion of levodopa to dopamine and thereby reduces the unwanted peripheral side-effects of levodopa. Use of carbidopa also increases the quantity of levodopa in the bloodstream that is available to enter the brain.

History

In 1960 the Austrian biochemist Oleh Hornykiewicz, while at the University of Vienna, examined results of autopsies of patients who had died with Parkinson's disease. He suggested that the disease was associated with, or caused by, a reduction in the levels of dopamine in the basal ganglia of the brain. Since dopamine itself did not enter the brain, he tried treating twenty patients with a racemic mixture of dihydroxyphenylalanine (DOPA), which could enter the brain and be converted there to dopamine by the action of DOPA decarboxylase. His results were positive, as were those of another trial in Montreal run by André Barbeau. Unfortunately, other investigators were unable to replicate these early results, and the use of DOPA remained in question until 1967, when George Cotzias at the Brookhaven National Laboratories in Upton, New York, used megadoses of DOPA, up to 16 grams per day. Not long after these results became known, Curt Porter at Merck showed that L-DOPA was the active stereoisomer, thus reducing the effective dose to half.^[2]

With L-DOPA identified as the active form, Alfred Pletscher and his colleagues at Hoffman-LaRoche synthesized benserazide, an inhibitor of DOPA decarboxylase, which further reduced the required dose. A drug combining L-DOPA with benserazide was marketed under the brand name of Madopar. Independent work was carried out by Victor Lotti at Merck in West Point, Pennsylvania. Merck had already synthesized and patented carbidopa, another dopa decarboxylase inhibitor in 1962, and in 1971 Lotti showed that the use of the L-form of carbidopa, further reduced the therapeutic dose of L-DOPA into the range of 1 to 2 grams per day. The combination of L-carbidopa and L-DOPA was marketed under the brand name of Sinemet.^[2]

In 1991 the manufacture and sale of Sinemet was taken over by a new joint venture, Dupont Merck Pharmaceutical Company. That same year approvals for a sustained release formulation (Sinemet CR) which could be taken less frequently were also obtained.^[3] Dupont purchased Merck's share in the joint venture in 1998 and began operating the company as Dupont Pharmaceuticals (later Dupont Pharma), but Merck continued to manufacture the drug for Dupont.^[4] Starting in late 2009 and continuing into 2011 Merck stopped manufacturing the drug while awaiting regulatory approvals due to a change in the supplier of the active ingredient. This resulted in shortages of the brand name products Sinemet and Sinemet CR, although alternative generic versions were still available.^[5]

See also

• Carbidopa
• Levodopa
• Stalevo in combination with entacapone

References

1. ^ Nyholm D (October 2006). "Enteral levodopa/carbidopa gel infusion for the treatment of motor fluctuations and dyskinesias in advanced Parkinson's disease". Expert review of neurotherapeutics 6 (10): 1403–11. doi:10.1586/14737175.6.10.1403. PMID 17078781. http://www.future-drugs.com/doi/abs/10.1586/14737175.6.10.1403?url_ver=Z39.88-2003&rfr_id=ori:rid:crossref.org&rfr_dat=cr_pub%3dncbi.nlm.nih.gov. 
2. ^ ^{a b} Scriabine, Alexander (1999). "Discovery and Development of Major Drugs Currently in Use", pp. 222–223 in Pharmaceutical Innovation: Revolutionizing Human Health, edited by Ralph Landau, Basil Achilladelis, and Alexander Scriabine. Philadelphia: Chemical Heritage Press. ISBN 9780941901215.
3. ^ Sinemet at Dupont Heritage.
4. ^ SINEMET at listingdrugs.com
5. ^ Letter From MERCK About SINEMET Shortage at Bibmomma's Blog – Reflections of an early onset Parkinson's patient.

External links

• Levodopa and Carbidopa at MedlinePlus Drug Information
• Sinemet CR product information at Merck Frosst Canada
• Sinemet datasheet