16 OCTOBER 2009 VOL 326 SCIENCE www.sciencemag.org

In 1987, an American neurologist told The New York Times, “I think I witnessed history,” after watching Mexican surgeons perform a transplant of human adrenal gland cells into the brains of four people with Parkinson’s disease. The physicians anticipated that the cells would turn into producers of the neurotransmitter dopamine, replacing the striatal neurons whose degeneration was clearly at the heart of the condition. At about the same time, Swedish doctors were experimenting with trans-planting cells from fetal brains, a controversial strategy that produced such striking results in some cases that by 1997 about 200 patients around the world had received the treatment.

But two fetal tissue trials in the United States in the 1990s that used sham surgeries as controls burst the balloon, indicating that any benefits may be little more than a placebo effect. Moreover, many of the patients developed “dyskinesias”—uncontrolled movements from excess dopamine from the brain grafts. It was a huge step backward; cell therapy for Parkinson’s was all but abandoned.

Then this decade, human embryonic stem (hES) cells came onto the scene, and scientists began trying to convert these malleable cells into dopamine-producing neurons that might pro-vide a safer, more abundant and less controversial source of transplantable tissue than aborted fetuses. More recently, induced pluripotent stem (iPS) cells have been added to the mix. But as the lab research proceeds apace, there’s growing doubt in some quarters about whether cell transplants will ever show a clear benefit for Parkinson’s disease beyond what can be achieved by existing therapies. And researchers are increasingly realizing that there’s much more to Parkinson’s than a dopamine shortage.

The debate about cell therapy for Parkinson’s may soon become more intense as scientists in Europe, in collaboration with colleagues in NorthAmerica, are in final negotiations for a large European Commission grant to conduct anew fetal cell transplant trial. The trial will refine clinical methods and be a “stepping-stone” to therapies with cells derived from iPS or hES cells, says the principal investigator, neurologist Roger Barker of the University of Cambridge in the United Kingdom. Yet news of the plans dismays some. “I think it’s a step backwards; ... all the double-blind trials have failed,” says C. Warren Olanow, a neurologist at Mount Sinai School of Medicine in New York City, who headed one of those trials.

A half-dozen years ago, in the heat of political and scientific excitement over hES cells, Parkinson’s disease was regarded as one of the prime candidates for stem cell therapy. But even as iPS cells have opened new vistas, the prospect of cell therapy trials has been steadily receding as scientists have gained new appreciation of both the difficulties of cell culture and the complexity of the disease itself

All over the world, researchers are trying to develop dopamine-producing cells from animal and human ES cells and from iPS cells.

Although many have generated human dopamine-producing neurons in a dish, no one has proven that they have exactly the right kind: differentiated enough so the cells will not develop tumors but young enough to grow into the proper type of cell and make connections.

So far, says Lorenz Studer of Memorial Sloan-Kettering Cancer Center in New York City, researchers have gotten things right, with mouse cells. A big push now is testing human-derived cells in primates to find out what type is most likely to survive and build connections within the brain without forming tumors. For example, Evan Snyder of the Burnham Institute for Medical Research in San Diego, California, and colleagues are testing a half-dozen human cell types—including fetal brain cells, adult neural stem cells from cadavers, and ES and iPS cells at various stages of development—to see if they will engraft successfully in monkeys. “Nobody’s ever compared them head to head,” says Snyder, who wants to ascertain the optimal stage of differentiation, the best place in the brain to insert cells, and whether the trans-planted tissue needs to just chum out dopamine or reconstruct the whole striatal pathway, the neural circuitry that degenerates in Parkinson’s.

Studer points out that criteria have become very demanding for any such cell treatment: “To have a clinical trial, you have to have a treatment that promises to be better than any-thing now available.” Deep-brain stimulation, adopted in the 1990s, has raised the bar considerably: It can help patients when L-dopa medication loses its impact and can curb dyskinesias from long-term exposure to the drug (Science, 20 March, p. 1554). Studer predicts that in optimal circumstances^—following no-glitch studies treating Parkinsonian monkeys with human dopamine-producing cells—it would be at least 5 years before any clinical trial.

As these challenges have become clearer, some scientists have spent the past 3 years discussing a way to push the field ahead while human ES and iPS culture techniques are being perfected: a return to fetal tissue trans-plants. Despite the failure of the U.S. trials, which were funded by the National Institutes of Health (NIH), two uncontrolled trials held in Sweden in the 1990s appeared to produce striking improvements—with a handful of patients doing well up to 14 years later.

Taking the plunge. Surgeon Ivar Mendez
readies a needle containing fetal brain cells.

These results have convinced some scientists that it’s time to try again. If all goes as planned for the European Commission–sponsored trial, the first patients in a preliminary safety trial will receive injections of cells from the midbrains of 6-to-9-week-old fetuses in 2012. After that, a double-blind treatment trial—which Barker says would include sham surgeries “when we feel we have an optimal therapy for grafting”—will recruit patients at several centers in Europe and North America.

Although Parkinson’s disease researchers in Europe are generally enthusiastic about the plan, reaction among U.S. scientists is mixed. Ted Dawson, professor of neurodegenerative diseases at Johns Hopkins University in Baltimore, Maryland, finds it “surprising.... I had thought there was going to be a moratorium [on fetal tissue transplants] until we had a bet-ter understanding of dopamine neurons in transplantation.” Some scientists also point to long-term follow-up results reported last year in Nature Medicine, which revealed that the disease process had begun to affect a small portion of the fetal grafts in some patients.

But psychiatrist D. Eugene Redmond of Yale University, who has done fetal tissue transplants in a monkey model of Parkinson’s, points out that if you want to do a test of cell replacement, fetal brain tissue is currently “pretty much the gold standard” Those prenatal dopamine neurons are at just the right stage of development and don’t cause tumors, he explains. And they work better in monkeys (where scientists use a chemical to selectively kill off striatal dopamine cells) than do dopamine-producing cells grown from monkey ES cells, Redmond says.

The European group insists that the new trials will be better controlled in every way than were those held in the 1990s. “We think the NIH trials were held at a time when several aspects of the technique were not fully developed,” says Anders Bjorklund, a fetal trans-plant pioneer at Lund University in Sweden. Barker says the team has identified some of the “critical factors” responsible for the failures. One is the mix of cells in the fetal tissues that were transplanted. Recent animal work in Bjorklund’s lab has shown that if the fetal brain tissue contains a high proportion of dopamine neurons, they will offset possible dyskinesia-causing effects from other types of cells in the graft, principally serotonin neurons. To get the right mix, says Barker, there will be more “selective dissection” from the fetal brains (four to six are needed to treat each patient). Patients will also be younger (under 60), and with the aid of brain imaging, researchers will pick those whose dopamine loss is more restricted to the dorsal striatum.

Old and new. Above: one of the first
Swedish fetal cell transplant operations.
Below: cell insertion points
utlined by Mendez.

Ivar Mendez of Dalhousie University in Halifax, Canada, who has already reported success with implanting Parkinson’s patients with fetal dopamine neurons, has worked out a standardized surgical procedure with the “Halifax injector,” a computer-controlled system for administering precise amounts of cells at specific brain locations through a hole in the skull. Mendez says about 4 million fetal brain cells—equivalent to about a drop of tissue     will be injected deep in each side of the brain. The placement of the graft deposits will cover as much of the dopamine-depleted striatum as possible, says Bjorklund.

Neurologist Olle Lindvall of Lund University says the new trial will be able to address the “big question”: Why does the treatment bring “major recovery ... in some patients and not in others”? Barker says that the answer may come from new information revealing sub-types of Parkinson’s. Barker readily admits that cell therapy “is not a cure” but contends that it’s an “open question” whether cell grafts are superior to deep-brain stimulation.

Some U.S. observers agree that the trials are worth doing. “There’s a lot of [unwarranted] bias in the field because of two [U.S.] clinical trials,” says Ole Isacson of Harvard Medical School in Boston. Redmond agrees: “A whole lot more is known about fetal tissue transplants than was known [in the 1990s]. It makes sense to continue looking.”

To others, the return of fetal cell therapy fails to give enough heed to recent advances in the understanding of Parkinson’s disease. Stem cell treatment “looked most hopeful when people were treating [Parkinson’s] just as a dopamine disease,” says Olanow. Degeneration of dopamine-producing cells is not the first or the only symptom of Parkinson’s, how-ever. It’s become increasingly clear that, as neurologist J. William Langston of the Parkinson’s Institute and Clinical Center in Sunny-vale, California, has put it, “Parkinsonism [that is, dopamine-related movement problems] is just the tip of the iceberg.” In reality, says Olanow, there is “very extensive pathology” that covers many neurotransmitter systems as well as the autonomic nervous system. Non-dopamine symptoms include bowel and bladder problems, attacks of low blood pressure, falling, “freezing,” sleep disorders, pain, depression, speech difficulties, and dementia.

“If patients’ only problems were related to dopamine deficiency, we would be able to maintain [them] for decades” with drugs and deep-brain stimulation, says neurologist J. Eric Ahiskog of the Mayo Clinic in Rochester, Minnesota. But given the extensive nature of the disease, any cell therapy “would need to be broadly administered and not just in one of a few brain regions.” It may be that cell trans-plants ultimately are most useful in furnishing trophic factors, protective chemicals that stave off deterioration of existing neurons—as some researchers hope will work in the case of amyotrophic lateral sclerosis.

Olanow notes the limitations that pertain to dopamine-focused cell therapy also apply to current Parkinson’s disease gene-therapy efforts, which center on introducing one or more genes involved in dopamine synthesis. Two groups, including one reporting in the 14 October online issue of Science Translational Medicine, have evidence that such gene therapy can restore dopamine production without associated dyskinesias in monkeys. This strategy has now moved ⁱnto clinical testing. It’s “exciting work,” says Olanow, but he contends that “the near-term future of cell and gene therapies based on dopamine restoration don’t look particularly promising.”

The Michael J. Fox Foundation for Parkinson’s Research has also become much more cautious about the promise of cell therapy. The foundation is now placing its bets on new drug development and supports very little stem cell research. “I was totally naive when I came to the foundation” in 2002, says CEO Katie Hood. “All my exposure was pop media; I thought it was all about stem cells.” Now, she says, “I have not totally lost hope on cell replacement,” but “I just don’t think it’s a near-term hope.”

—CONSTANCE HOLDEN