GDNF and AADC Gene Therapy for Parkinson’s Disease

The use of viral vectors to express therapeutics genes in Parkinson’s disease (PD) trials has been hindered by a lack of understanding of the principles that guide effective distribution of vectors within the basal ganglia, even when we have a strong expectation of efficacy based on experimentation in animal models. The major problems we have faced include (1) scale-up from small rodent and nonhuman primates (NHP) brains to humans, (2) understanding how viral vectors distribute within the brain parenchyma, (3) prediction of how viral particles are disseminated by neuronal projections after direct delivery to the putamen and substantia nigra, (4) the mechanism of action of the therapeutic gene on dopaminergic system, and (5) the relevance of animal models to idiopathic PD. In this chapter, we will address these important issues and will try to put them into the context of data that has been obtained from current and recent clinical trials. In particular, we will address therapeutic strategies aimed at restoring dopaminergic function by either expressing genes that encode enzymes responsible for synthesis of dopamine (DA) or expressing growth factors capable of upregulating DA function in the degenerated neurons. We will not address inhibition of outflow innervation from the striatum in PD patients by expressing glutamic acid decarboxylase (GAD) as this strategy is described in the dedicated chapter in this book.