Recent Developments in the Drug Treatment of Parkinson’s Disease

Parkinson's disease (PD) is a common neurodegenerative disorder caused by a spread of misfolded α-synuclein and ascending neuronal degeneration mainly in dopaminergic neurons, leading to progressive nigrostriatal dysfunction and disruption of other pathways. It is clinically defined by its cardinal motor features of bradykinesia, rest tremor and rigidity, which are usually accompanied by a variety of non-motor symptoms. Here we provide a review on recent developments in the pharmacological treatment of PD with a focus on recently approved drugs, new modes of delivery and agents that are in advanced stages of clinical development. Pharmacological dopamine substitution remains the mainstay of symptomatic treatment approaches to control PD motor symptoms. While levodopa is still considered the gold-standard of symptomatic efficacy, its chronic use is associated with the development of response oscillations and drug-induced dyskinesias in a majority of patients. Thus, much effort has been put into developing more continuous ways of levodopa delivery. Novel formulations and modes of application include extended-release (ER) oral levodopa (IPX203), levodopa powder for inhalation, and levodopa infusion therapies through subcutaneous (foslevodopa/foscarbidopa, ND0612) or intrajejunal pumps (levodopa [entacapone] carbidopa intestinal gel). Additionally, different preparations for subcutaneous and sublingual administration of apomorphine, a dopamine agonist with equivalent efficacy to levodopa, are available for the treatment of PD motor fluctuations. Novel dopamine agonists have been developed and tavapadon, a selective dopamine D1/D5 receptor partial agonist, was shown to be efficacious as monotherapy in early PD and adjunct to levodopa in patients with motor fluctuations. In addition, new data on the early use of the COMT-inhibitor opicapone have emerged. This expanding drug armamentarium is complemented by an increasing number of drugs targeting non-dopaminergic pathways with increasing evidence for amantadine's symptomatic efficacy in treating levodopa-induced dyskinesia and motor fluctuations. Beyond the approval of different botulinum toxin preparations for the treatment of sialorrhea, efforts to address the plethora of non-motor symptoms of the disease have not translated to novel PD-specific approvals over the last few years. PD symptoms can usually be satisfactorily controlled in the early to mid-stages, but the progressive course of the disease inevitably leads to increasing functional disability underlining the yet unmet need for disease-modifying therapies. While there is currently no conclusive evidence for disease-modifying efficacy on any of the numerous efforts, we summarize recent late-stage clinical trial evidence focusing on glucagon-like peptide 1 (GLP1) agonists, approaches targeting the glucocerebrosidase (GBA) pathway and inhibition of the leucine-rich repeat kinase 2 (LRRK2), as well as α-synuclein based treatments.