Gene Therapy For Advanced Parkinson's Disease
Neurodegeneration of dopaminergic neurons underlies the motor manifestations of Parkinson’s disease. When mild, Parkinson’s disease is generally well controlled by drugs; however, as the disease progresses, pharmacotherapy often fails to provide adequate symptom relief and sometimes causes disabling complications, such as motor fluctuations. Additional treatment approaches, such as deep brain stimulation (DBS) and pharmacological interventions at sites beyond the nigrostriatal dopaminergic pathway have been used to manage problems of advanced Parkinson’s disease. In vivo gene therapy is a new approach. Despite promising results in animal models of parkinsonism and in several open-label clinical investigations, the efficacy of gene therapy has yet to be confirmed in a randomized double-blind clinical trial. In Parkinson’s disease, loss of nigrostriatal dopaminergic neurons alters striato-pallidal circuitry such that decreased GABA input to the subthalamic nucleus renders this structure disinhibited. Treatments that diminish or modulate the activity of the subthalamic nucleus, such as subthalamotomy and DBS, can help with some parkinsonian symptoms. Like dopaminergic treatment, however, DBS can fail to improve some parkinsonian features such as freezing of gait, imbalance, dysphagia, cognitive and psychiatric problems, and speech difficulties. Furthermore, this technique necessitates implantation of devices and much effort to adjust electrical stimulation variables.
Gene therapy consisting of insertion of the glutamic acid decarboxylase gene (GAD) into the subthalamic nucleus may offer an alternative therapeutic strategy. GAD is the rate-limiting enzyme for GABA production, and the activity of both GABA efferents to the subthalamic nucleus and its targets within the basal ganglia circuitry are affected in Parkinson’s disease. During DBS surgery in patients with this disease, an infusion of the GABAergic agonist muscimol into the subthalamic nucleus suppressed its neuronal firing rates and temporarily improved parkinsonian symptoms, suggesting that improvement of GABA transmission within the subthalamic nucleus could be beneficial in Parkinson’s disease. Similar results in animal models of parkinsonism were achieved with gene transfer of GAD. This strategy uses an adeno-associated viral vector (AAV2) to deliver GAD to the subthalamic nucleus to both restore local GABA transmission within the nucleus and to normalize output from the nucleus (by adding an inhibitory GABA outflow, thereby reducing excessive excitatory glutamate output to key targets such as the globus pallidus interna and the substantia nigra reticulata). An open-label clinical trial of AAV2-GAD injected unilaterally into the subthalamic nucleus showed this procedure to be safe and associated with improvements of parkinsonism. Although studies of other gene, cell, and biological therapies in patients with Parkinson’s disease have also shown promise in small, open-label studies, subsequent randomized double-blind clinical trials have not substantiated their initial findings. Consequently, progress in the assessment of CNS gene therapy needs careful attention to all aspects of study methods, including sham procedures, effective blinding, and successful delivery of the experimental therapy to the intended targets. This trial was done to assess the effect of bilateral delivery of AAV2-GAD into the subthalamic nucleus compared with bilateral sham surgery in patients with advanced Parkinson’s disease.
LeWitt PA, Rezai AR, Leehey MA, et al.
AAV2-GAD gene therapy for advanced Parkinson's disease: a double-blind, sham-surgery controlled, randomised trial. The Lancet Neurology.
AAV2-GAD gene therapy for advanced Parkinson's disease: a double-blind, sham-surgery controlled, randomised trial : The Lancet Neurology
Background - Gene transfer of glutamic acid decarboxylase (GAD) and other methods that modulate production of GABA in the subthalamic nucleus improve basal ganglia function in parkinsonism in animal models. We aimed to assess the effect of bilateral delivery of AAV2-GAD in the subthalamic nucleus compared with sham surgery in patients with advanced Parkinson's disease.
Methods - Patients aged 30—75 years who had progressive levodopa-responsive Parkinson's disease and an overnight off-medication unified Parkinson's disease rating scale (UPDRS) motor score of 25 or more were enrolled into this double-blind, phase 2, randomised controlled trial, which took place at seven centres in the USA between Nov 17, 2008, and May 11, 2010. Infusion failure or catheter tip location beyond a predefined target zone led to exclusion of patients before unmasking for the efficacy analysis. The primary outcome measure was the 6-month change from baseline in double-blind assessment of off-medication UPDRS motor scores.
Findings - Of 66 patients assessed for eligibility, 23 were randomly assigned to sham surgery and 22 to AAV2-GAD infusions; of those, 21 and 16, respectively, were analysed. At the 6-month endpoint, UPDRS score for the AAV2-GAD group decreased by 8•1 points (SD 1•7, 23•1%; p<0•0001) and by 4•7 points in the sham group (1•5, 12•7%; p=0•003). The AAV2-GAD group showed a significantly greater improvement from baseline in UPDRS scores compared with the sham group over the 6-month course of the study (RMANOVA, p=0•04). One serious adverse event occurred within 6 months of surgery; this case of bowel obstruction occurred in the AAV2-GAD group, was not attributed to treatment or the surgical procedure, and fully resolved. Other adverse events were mild or moderate, likely related to surgery and resolved; the most common were headache (seven patients in the AAV2-GAD groupvs two in the sham group) and nausea (six vs two).
Interpretation - The efficacy and safety of bilateral infusion of AAV2-GAD in the subthalamic nucleus supports its further development for Parkinson's disease and shows the promise for gene therapy for neurological disorders.