Through a Facebook friend I found this link to a New Scientist article suggesting the medication for Diabetes may help slow the progression of PD.
The drug, FDA-approved since 2005, is called exenatide and is used to treat type 2 diabetes. It works by protecting neurones from toxins. One potential downside is that the drug is currently injected twice daily though a once-weekly injection was approved in 2012. The major side-effects appear to be gastrointestinal.
If I have a major criticism with the article it is with the title which implies that there are no current ways to slow PD progression. In fact exercise, diet, social interaction and at least one drug, Azilect, are all known to or suspected of slowing PD progression to varying degrees.