http://www.touchbriefings.com/pdf/3009/Mouradian.pdf

Where to Infuse
Levodopa is poorly soluble in water, making it impractical for outpatient
or ambulatory use. Furthermore, it is irritating to veins and soft tissues,
ruling out long-term intravenous or subcutaneous administration. The
current solution is the gel formulation, used in Duodopa, which reduces
the volume of infusate 10-fold, and as it includes carbidopa eliminates
the need to take additional oral medications. By delivering straight to the
duodenum, Duodopa also avoids the problems of erratic gastric emptying
and interference from food boluses.
Conclusion
After more than two decades of research into continuous dopaminergic
stimulation, the original data have stood the test of time and the hypotheses
are still valid – it is the intermittent method of administration of levodopa,
coupled with the underlying neural degeneration, that causes motor
fluctuations and dyskinesias. More data are generated each year that further
support these hypotheses. In parallel, there have been significant advances
in developing both a workable formulation of levodopa for infusion and a
practical way to deliver it continuously, thus reducing or even eliminating the
off periods and improving patients’ quality of life.