A neuroprotective or disease-modifying therapy that can slow or stop disease progression and prevent the development of intolerable disability is the major unmet medical need in the treatment of Parkinson's disease (PD). Many putative neuroprotective agents have been identified in the laboratory, but none has been unequivocally demonstrated to provide disease-modifying effects in PD patients, even when clinical trials are positive. Obstacles to defining a neuroprotective therapy in PD include: (1) uncertainty about the cause of PD and precisely what to target, (2) a reliable animal model in which to test putative neuroprotective agents that accurately predicts results in PD patients, (3) insight about which dose to employ in clinical trials and which patient group to study, (4) a clinical trial design that reliably differentiates disease-modifying and symptomatic effects and that is acceptable to regulatory authorities, and (5) the cost and time of the development program. Advances have been made in each of these areas, thereby increasing the prospects of developing a neuroprotective or disease-modifying therapy in the not-too-distant future. These issues are reviewed in the present article.
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