<qa>  What is the DDC inhibitor?Carbidopa is a DOPA decarboxylase DDC inhibitor. At therapeutic doses carbidopa does not cross the blood-brain barrier and therefore should not inhibit CNS metabolism of droxidopa to NE. Decreasing the activity of DDC in the periphery enables droxidopa metabolism to be focused in the CNS. This CNS focus should increase CNS response while also limiting the increase in blood pressure associated with peripheral droxidopa metabolism. In addition, as DDC is an enzyme required for the conversion of droxidopa into its active metabolite norepinephrine, inhibition of peripheral DDC, utilizing Carbidopa, should result in a reduction of any side effects resulting from the peripheral production of NE, whilst allowing for increased central levels, and hence, increased centrally mediated benefits. 