[A]s 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....

A half-dozen years ago, in the heat of political and scientific excitement over hES [human embryonic stem] cells, Parkinson's disease was regarded as one of the prime candidates for stem cell therapy. But even as iPS [induced pluripotent stem] 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....

Stem cell treatment "looked most hopeful when people were treating [Parkinson's] just as a dopamine disease," says [neurologist C. Warren] Olanow. Degeneration of dopamine-producing cells is not the first or the only symptom of Parkinson's, however. It's become increasingly clear that, as neurologist J. William Langston of the Parkinson's Institute and Clinical Center in Sunnyvale, California, has put it, "Parkinsonism (that is, dopamine-related movement problems) is just the tip of the iceberg."...

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. ...

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 naïve 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."