Trail-blazed by dozens of laboratory rats and two rhesus monkeys over the last decade, the first-ever clinical trial for treating Parkinson's disease by gene therapy expects to enroll its first 12 human patients by February at the latest.
The trial's two principals are neuroscientist Matthew During, at Thomas Jefferson University's Medical College in Philadelphia, and neurosurgeon Michael Kaplitt, at Cornell University's Weill Medical College hospital in New York.
"At the end of July this year," During told BioWorld Today, "we got approval for the IND [investigational new drug] application we submitted last year. We hope to get approval for clinical gene transfer this month by the hospital's Institutional Biosafety Committee. So our hope is we'll start enrolling patients as early as November."
During is senior author of a paper in Science dated Oct. 11, 2002, titled: "Subthalamic GAD gene therapy in a Parkinson's disease rat model." Kaplitt is a key co-author.
"The primary finding of the paper," During pointed out, "is that we could reprogram neuronal circuitry in the brain. And by doing so we came up with a new treatment for Parkinson's disease [PD]. Its novelty," he added, "is that a single transferred gene was sufficient to change the phenotypic function of the pathway from excitatory to inhibitory. Thus, using this rat model, we protected the inhibitory cells from dying off in PD. That's the critical finding that persuaded Science to accept this paper.
"The implications are that such a strategy can be used in the clinic," During continued, "not just to provide symptomatic relief, but actually slow down progression of the disease. The subthalamic nucleus cells - which are overactive in PD - produce glutamate, an excitatory neurotransmitter. It enters the substantia nigra, which coordinates bodily movement, and makes the brain chemical dopamine. PD is caused by deterioration of dopamine-producing nerve cells.
"We had been working for almost 10 years on gene therapy for PD," During observed, "and used different viral vectors to deliver the gene. The main thing hindering gene therapy development in the clinic these days is fear for safety. AAV [adeno-associated virus] is the safest vector. We improved the DNA sequences, so I think we are finally at a stage where we can be pretty confident that the AAV vector will deliver the gene, which will stay put in the brain for the life of the organism."
PD Neurons: From Excitation To Inhibition
"The gene in this Science study," During explained, "is called GAD - glutamic acid decarboxylase. It converts to GABA - gamma aminobutyric acid - the brain's major inhibitory neurotransmitter. GABA acts like a brake in the brain, damping down the firing and excitation of the neurons. Almost all cells have glutamate inside them, which provides them with the ability to convert glutamate into GABA. It quiets those nerve cells. By putting the GAD gene into the specific PD-sensitive target region in the brain, those neurons became capable of making GABA, and inhibiting the activity of the cells downstream.
"Not only could this be a symptomatic palliative treatment," During suggested, "but it also could be a preventative - a treatment that actually affects the underlying process in the brain, and slows down the degeneration. So that - in laboratory rats so far - was the major advance here."
During recounted the co-authors' pivotal in vivo experiment. "We took a bunch of normal laboratory rats and broke them into five treatment cohorts: One group just got saline injections; two marker-gene groups, variants of the GAD gene, and a fifth group was destined to make up a parkinsonian-model lesion."
Rodents Respond To Gene Therapy Drill
"Three weeks later we lesioned the neuronal system on the same side of the rats' brain. We did that by injecting a chemical - 6-hydroxydopamine - which is a variant of dopamine itself. It's a well-established model that generates hemiparkinsonian rats. They're PD-symptomatic on one side of the brain. If you make it on both sides, the animals stop eating, and just die. But treated with the GAD gene transfer therapy produced significant recovery of protection from dopamine behaviors. Performance tests included spontaneous motor activity, limb-touching, spontaneous movement and no spinning in circles when given dopamine. One test showed that nearly 70 percent of the rats with PD lesions, which received the GAD gene therapy, had no PD symptoms after getting chemicals that mimicked dopamine in the brain.
"The AAV viral vectors," he noted, "were made at the University of Auckland, New Zealand, and the rat studies done at Jefferson Medical College, where they finished approximately six months ago. We're just winding up two primate experiments for FDA's approval of our Phase I clinical trial. Those larger, blinded rhesus monkey tests will be completed next month.
"My colleague, Mike Kaplitt, will be doing the surgery at Cornell hospital," During said. "The first protocol involves 12 patients in an open-label design. They will have MRI [magnetic resonance image] stereotaxic frames attached to their skulls. Then under local anesthetic and MRI guidance, microelectrodes will map exactly where in the brains the gene transfer will be introduced. That will be infused over the course of 45 minutes to an hour. The whole procedure might take four hours or a little bit more. The gene payload will be a combination of two variants - GAD65 and GAD67.
As for the intellectual property position, During observed, "The most important patent in this area is one that myself and Mike Kaplitt wrote back in '93 when I was at Yale and he at Rockefeller. Those two universities jointly own that patent, which is sublicensed to companies we consult for. Kaplitt and I sit on the advisory board" of Neurologix Inc. in Newark, Del.