Pharmacological approaches to disease-modifying therapies in Parkinson's disease

Parkinson's disease is a movement disorder resulting from neurodegeneration of the basal ganglia. Parkinson's disease is usually diagnosed at approximately 55-60 years of age and affects approximately 1% of the population over 60 years. Dopaminergic cells located in the substantia nigra and whose terminals extend to the striatum degenerate slowly such that 60% of cells are already lost when clinical motor symptoms first become evident. In addition to the classic triad of Parkinson's disease symptoms, rest tremor, muscular rigidity and bradykinesia, abnormalities in postural reflexes, dementia and depression are important comorbid conditions. Current therapies are aimed primarily at replacing dopamine with the dopamine precursor L-dopa or by the use of direct acting dopamine receptor agonists. Adjunctive treatments with monoamine oxidase inhibitors, catechol-O-methyl transferase inhibitors and amantadine are also used. While providing very effective symptomatic therapy in early stages of the disease, these agents fail to halt disease progression. Thus, while these treatments generally provide excellent results for 2-5 years, quality of life for Parkinson's disease patients becomes increasingly poor 5-10 years after diagnosis. Symptoms that become increasingly problematic with disease progression include inconsistencies in motor control (response fluctuations), gait and balance abnormalities, cognitive loss, hypophonia and dysphagia. Therefore, in order to maintain an acceptable quality of life for patients with Parkinson's disease, therapies that provide not only symptomatic improvement, but also slow or stop disease progression are greatly needed. In this review, we will discuss possible mechanisms of cell death in Parkinson's disease and related potentially disease-modifying therapies. These therapies include dopaminergic cell tranplantation and the use of growth factors. Small molecules that may act as antioxidants, nicotinic receptor agonists, nitric oxide synthase inhibitors, immunophilins, excitatory amino acid-related (iGluR and mGluR agonists and antagonists) drugs and anti-inflammatory drugs will also be discussed.