Something in the Way You Move - Drug Discovery and Development

Parkinson’s treatments make quantum leaps due to recently-improved gene therapy strategies.

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Transgenic human neural progenitor neurosphere with 75% of cells expressing the GFP transgene. Left: phase micrograph; right: fluurescent micrograph. Scale bar = 30 microns. (Source: Elizabeth E. Capowski, PhD, University of Wisconsin-Madison)

If growing older is the gradual, gentle theft of one’s vitality, then Parkinson’s disease is an act of perpetual vandalism: a slow, stealthy degradation of the mechanisms of neuronal control. While the wrecker remains unknown, recent scientific progress suggests that the damage need not remain unchecked.

“We still don’t know exactly what causes Parkinson’s disease,” says Paul Sandberg DSc, PhD, director of the Center for Aging and Brain Repair, University of South Florida, Tampa. “Some people think it could be related to autoimmunity, others see it as accelerated aging.” Also linked to Parkinson’s disease (PD) is repeated head trauma, such as that experienced by boxers.

Since PD remains largely idiopathic, treatments look to compensate for the damage done—the loss of inhibition of neuronal activity that results in the characteristically uncontrolled movements of the PD patient. One standard treatment is to supplement endogenous dopamine stores, having been diminished by neuronal damage, with the prodrug levodopa. While effective, off-target effects can be severe, and tolerance to the drug is quick to develop.

Seeing is believing

Key to determining the success or failure of a treatment is the criteria for disease assessment. The standard in Parkinson’s disease (PD) is changes in behavior—a subjective test at best. “You’re using a rather arbitrarily designed rating scale of one to four for each limb to assess disease severity,” says David Eidelberg, MD, of the Feinstein Institute for Medical Research, Manhasset, N.Y. “We found that we can do much better if you have a reliable imaging biomarker, to see possible changes from purely objective standpoints.” Working with the clinical trial team from Neurologix Inc., Fort Lee, N.J., Eidelberg was able to detect the effects of a GAD transgene using f-PET imaging. Similar to the use of f-PET in tumor detection, what’s being measured in PD is cellular metabolic activity. “In the case of the nervous system, ‘hot’ means abnormal increases in synaptic activity—neurons that are pathologically busy.”

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Left: Parkinson’s Disease-Related Pattern. This motor-related metabolic spatial covariance pattern was characterized by hypermetabolism in the thalamus, globus pallidus, pons, and motor cortex, associated with relative metabolic reductions in the lateral premotor and posterior parietal areas.

Right: Parkinson’s Disease-Related Cognitive Pattern. This cognition-related metabolic spatial covariance pattern was characterized by hypometabolism of dorsolateral prefrontal cortex, rostral supplementary motor area, and superior parietal regions, associated with relative metabolic increases in the cerebellum. (Source: David Eidelberg, MD)

Having a reliable, objective measure of treatment effect would be a boon for PD drug development. Almost all therapies that target the brain have a marked placebo effect, and particular to PD, the lesion itself caused by gene therapy injection may produce behavioral benefit. Though Eidelberg’s report concerns a Phase 1 open-label trial, he is certain his images can discern therapeutic activity. “We found that there were significant differences in the way gene therapy worked versus the lesion,” he says. And these differences are seen not only by brain location, but are correlated in time: lesioning effects happen acutely–patients in Eidelberg’s trial continue to get better over time, which makes sense. “Gene transfer takes a while to get in there,” Eidleberg says. f-PET will show, and time will tell.

Alternative strategies are being pursued with the use of stem cells, gene therapy, or both technologies combined. Commenting on one recent stem cell study by Redmond et al., in which human fetal cells were transplanted into the brains of PD-model primates, Sanberg relates that after transplant there was improvement, but, not from any spanking new neurons the fetal tissue may have spawned. “In fact, what the transplanted stem cells were doing is having a trophic effect by inducing the host brain to repair itself,” he says. The agents causing the observed effects—growth and repair factors–now command equal if not greater investigational attention than the replacement of lost neurons.

Some of the players in this stimulatory arena are GDNF (glial cell line-derived neurotrophic factor), GAD (glutamic acid decarboxylase) and neurturin (a member of the GDNF family). According to Sanberg, “Investigators are thinking that using a biological pump of cells transfected with these trophic factors may have a real advantage [over application of a drug] because then an implanted cell is localized.” While these gene products are presumed to be efficacious, it remains to be seen what a lifetime of constitutive expression will lead to; in theory, transformed neurons last as long as you do.

Outside the thinking box
Combining the somewhat mysterious advantages of stem cells with the reasoned power of recombinant gene technology is Elizabeth Capowski, PhD, assistant scientist, Stem Cell Research Program, Waisman Center, University of Wisconsin-Madison. “In neurodegenerative diseases, nobody is really doing ex vivo work—it’s usually direct viral injection,” she says. There are labs squeezing in more genes, and labs transplanting neurons, but only Capowski’s group wants to do both: put the genes in the cells, and then the cells in the brain.

The reasoning is this: rather than injecting a gene-loaded vector into an area of the brain that is, by definition, damaged, and then inducing the local cell population to express a protein not normally expressed, why not just use nature to greater advantage? “That way you get the double benefit of having healthy cells that can communicate with the sick ones—in all the ways we’ve yet to understand—and in addition, produce a growth factor which already has been shown to help promote the survival and [development] of dopamine neurons,” Capowski explains. That factor being, in this case, GDNF. “If you provide GDNF to the region where these neurons are degenerating, even if you just put a catheter in a person’s brain and drip it in, neurons live longer, and they tend to sprout, and you see improvements in function,” she says.

The latest progress in this work was a protocol development for the stable transfection of fetal-tissue-derived stem cells using lentivirus. “The vector choice was a matter of necessity,” Capowski says. “These cells are pretty refractory to being transduced with exogenous DNA.” With this technique in hand, Capowski is one step closer to clinical application. (For details, see J Neurosci Methods. 2007: 338-49.)

Gene genie
“The clinical trials I’m involved with all use AAV (adeno-associated virus) not lenti,” says Jeffery Kordower PhD, Director of the Research Center for Brain Repair, Rush University Medical Center, Chicago, and Scientific Advisor for Ceregene, San Diego, CA.. “From a scientific perspective, there’s not a great difference. It used to be that lentivirus expressed longer and with less inflammation but now, with improvements in the AAV, they both express equally well.” The main advantage to using AAV is that, since the FDA has already approved numerous trials using this technology, AAV represents the path of least regulatory resistance.

A further regulatory concern might be the matter of the destination—that being the site of gene insertion. This issue achieved prominence in 2002 when two of ten children being treated for severe combined immunodeficiency developed a leukemia-like illness after gene therapy. Kordower recognizes these events as a theoretical concern, “but all the empirical data indicates that there isn’t a problem in terms of mutagenesis or random integration.”

Regarding the issue of constitutive expression, and the possible use of promoters to as genetic watchdogs, Kordhower went so far as to author a related paper in the August issue of Experimental Neurology. “It’s not a sticking point at all from the regulatory standpoint,” he insists. In fact, as stated in his paper, “the mandatory use of regulatable vectors is not only unnecessary but, in some instances, misguided and potentially dangerous.”

Ceregene is now in the midst of Phase 2 trials using the transgene, neurturin, a protein meant to provide not only symptomatic benefit, but also be neuroprotective. The trial will be finished in October, 2008.

About the Author
Neil Canavan is a freelance journalist of science and medicine based in New York.

This article was published in Drug Discovery & Development magazine: Vol. 11, No. 1, January, 2008, pp. 42-44.