From the research paper:
"The results presented herein reveal potential protective side effects for doxycycline in the pathogenesis cycle of synucleinopathies that could be exploited repurposing an old safe drug."
"This data strongly suggests that doxycycline in subantibiotic doses (20–40 mg/day) would be enough to exert neuroprotection."

From the ScienceDaily article:
"This treatment could stop Parkinson's from progressing, and we therefore plan to start a clinical trial shortly."

Could this be another clinical trial worthy of a fast-track process?

Repurposing doxycycline for synucleinopathies: remodelling of α-synuclein oligomers towards non-toxic parallel beta-sheet structured species : Scientific Reports

(this is an open-access research paper)