Subthalamic GAD gene transfer in Parkinson disease patients who are candidates for deep brain stimulation

Abstract

This gene transfer experiment is the first Parkinson's Disease (PD) protocol to be submitted to the Recombinant DNA Advisory Committee. The principal investigators have uniquely focused their careers on both pre-clinical work on gene transfer in the brain and clinical expertise in management and surgical treatment of patients with PD. They have extensively used rodent models of PD for proof-of-principle experiments on the utility of different vector systems. PD is an excellent target for gene therapy, because it is a complex acquired disease of unknown etiology (apart from some rare familial cases) yet it is characterized by a specific neuroanatomical pathology, the degeneration of dopamine neurons of the substantia nigra (SN) with loss of dopamine input to the striatum. This pathology results in focal changes in the function of several deep brain nuclei, which have been well-characterized in humans and animal models and which account for many of the motor symptoms of PD. Our original approaches, largely to validate in vivo gene transfer in the brain, were designed to facilitate dopamine transmission in the striatum using an AAV vector expressing dopamine-synthetic enzymes. Although these confirmed the safety and potential efficacy of AAV, complex patient responses to dopamine augmenting medication as well as poor results and complications of human transplant studies suggested that this would be a difficult and potentially dangerous clinical strategy using current approaches. Subsequently, we and others investigated the use of growth factors, including GDNF. These showed some encouraging effects on dopamine neuron survival and regeneration in both rodent and primate models; however, uncertain consequences of long-term growth factor expression and question regarding timing of therapy in the disease course must be resolved before any clinical study can be contemplated. We now propose to infuse into the subthalamic nucleus (STN) recombinant AAV vectors expressing the two isoforms of the enzyme glutamic acid decarboxylase (GAD-65 and GAD-67), which synthesizes the major inhibitory neurotransmitter in the brain, GABA. The STN is a very small nucleus (140 cubic mm or 0.02% of the total brain volume, consisting of approximately 300,000 neurons) which is disinhibited in PD, leading to pathological excitation of its targets, the internal segment of the globus pallidus (GPi) and substantia nigra pars reticulata (SNpr). Increased GPi/SNpr outflow is believed responsible for many of the cardinal symptoms of PD, i.e., tremor, rigidity, bradykinesia, and gait disturbance. A large amount of data based on lesioning, electrical stimulation, and local drug infusion studies with GABA-agonists in human PD patients have reinforced this circuit model of PD and the central role of the STN. Moreover, the closest conventional surgical intervention to our proposal, deep brain stimulation (DBS) of the STN, has shown remarkable efficacy in even late stage PD, unlike the early failures associated with recombinant GDNF infusion or cell transplantation approaches in PD. We believe that our gene transfer strategy will not only palliate symptoms by inhibiting STN activity, as with DBS, but we also have evidence that the vector converts excitatory STN projections to inhibitory projections. This additional dampening of outflow GPi/SNpr outflow may provide an additional advantage over DBS. Moreover, of perhaps the greatest interest, our preclinical data suggests that this strategy may also be neuroprotective, so this therapy may slow the degeneration of dopaminergic neurons. We will use both GAD isoforms since both are typically expressed in inhibitory neurons in the brain, and our data suggest that the combination of both isoforms is likely to be most beneficial. Our preclinical data includes three model systems: (1) old, chronically lesioned parkinsonian rats in which intraSTN GAD gene transfer results not only in improvement in both drug-induced asymmetrical behavior (apomorphine symmetrical rotations), but also in spontaneous behaviors. In our second model, GAD gene transfer precedes the generation of a dopamine lesion. Here GAD gene transfer showed remarkable neuroprotection. Finally, we carried out a study where GAD-65 and GAD-67 were used separately in monkeys that were resistant to MPTP lesioning and hence showed minimal symptomatology. Nevertheless GAD gene transfer showed no adverse effects and small improvements in both Parkinson rating scales and activity measures were obtained. In the proposed clinical trial, all patients will have met criteria for and will have given consent for STN DBS elective surgery. Twenty patients will all receive DBS electrodes, but in addition they will be randomized into two groups, to receive either a solution containing rAAV-GAD, or a solution which consists just of the vector vehicle, physiological saline. Patients, care providers, and physicians will be blind as to which solution any one patient receives. All patients, regardless of group, will agree to not have the DBS activated until the completion and unblinding of the study. Patients will be assessed with a core clinical assessment program modeled on the CAPSIT, and in addition will also undergo a preop and several postop PET scans. At the conclusion of the study, if any patient with sufficient symptomatic improvement will be offered DBS removal if they so desire. Any patients with no benefit will simply have their stimulators activated, which would normally be appropriate therapy for them and which requires no additional operations. If any unforeseen symptoms occur from STN production of GABA, this might be controlled by blocking STN GABA release with DBS, or STN lesioning could be performed using the DBS electrode. Again, this treatment would not subject the patient to additional invasive brain surgery. The trial described here reflects an evolution in our thinking about the best strategy to make a positive impact in Parkinson Disease by minimizing risk and maximizing potential benefit. To our knowledge, this proposal represents the first truly blinded, completely controlled gene or cell therapy study in the brain, which still provides the patient with the same surgical procedure which they would normally receive and should not subject the patient to additional surgical procedures regardless of the success or failure of the study. This study first and foremost aims to maximally serve the safety interests of the individual patient while simultaneously serving the public interest in rigorously determining in a scientific fashion if gene therapy can be effective to any degree in treating Parkinson's disease.