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apping the brakes on an effort to speed new stem cell treatments to patients by relaxing regulations, Congress this week is considering a modified proposal that is attracting cautious support from the research community.

Stem cell experts say they are still trying to tease apart how exactly the regenerative medicine sections of the 21st Century Cures Act, a behemoth bill that would expedite drug approval and increase funding for medical research, would affect the field. But a number of vocal critics of the original measure, the so-called Regrow Act, said some of their worries had been assuaged.

And exactly how Congress handles the issue matters, they say.

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“This is very, very important for all aspects of medicine but certainly for regenerative medicine,” said Sally Temple, the president of the International Society for Stem Cell Research.

The enthusiasm for stem cell-based treatments has been building for years. The cells, which can be found in tissues from fat to bone marrow, are the renaissance players in that they can develop into specialized cell types. As a result, doctors believe they can be used to replace damaged or dead cells. In small, early studies, scientists have found evidence that stem cells can help stroke victims regain bodily control and close holes in tissue.

But so far, only a tiny number of stem cell-related treatments exist. As exciting as the field is, many scientists warn that therapies derived from stem cells may not be ready for prime time yet. They fear that permitting therapies onto the market without rigorous testing could harm patients. Plus, they say, patients and insurers should not have to pay for treatments that haven’t yet been demonstrated to be safe and effective.

The Regrow Act was spearheaded by Republican Senator Mark Kirk of Illinois, who had a stroke in 2012 and said the bill would expand treatment options for people who suffered from strokes or had diseases including Alzheimer’s, Parkinson’s, and diabetes. (Kirk lost his reelection bid this month to Democratic Representative Tammy Duckworth.)

The measure would have allowed the Food and Drug Administration to approve stem cell treatments conditionally without the large final-stage clinical trials usually required of a therapy.

In its current form, the Cures Act does not allow new stem cell therapies to forgo those Phase 3 trials, when many drugs fail. Instead, it says that regenerative medicine products, which include stem cell therapies, can be granted what’s called accelerated approval by the FDA.

Under that program, regulators can green-light a therapy without waiting on a demonstrated clinical benefit. Rather, they can rely on symptoms or other measures — what are called surrogate endpoints — that might indicate the drug is working; a cancer drug, for example, could be approved if it shrinks a tumor, even if it hasn’t shown yet it extends survival.

Treatments that go to market under accelerated approval are required to be studied further to ensure that they do in fact show a real benefit as they become available to more patients. The designation is typically used for therapies for serious or threatening conditions for which there are few or no treatment options, according to the FDA.

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The changes have not mollified everyone. The advocacy group Public Citizen criticized the idea of allowing the therapies onto the market based on surrogate endpoints and then hoping the post-approval studies show that treatments actually work. Also, the group said, the regenerative medicine components were added to the bill late in the game.

“To throw this provision into a large packet without having time to fully vet it and hear from experts in the field and from patient safety advocates is an inappropriate way to advance legislation like this,” said Dr. Michael Carome, director of Public Citizen’s Health Research Group. “This is a field where we don’t know a lot about the value of these products and it would be best that we not create such expedited pathways until we have a better understanding of this field.”

Temple, the president of the international stem cell society, said she and her colleagues were still reviewing the measure, but that in principle, the group was open to using accelerated approval for stem cell treatments as long as the FDA had authority over that decision and as long as the FDA had oversight over the post-approval studies.

She said the stem cell field requires particular scrutiny because of the proliferation of largely unregulated “stem cell clinics” that claim to be able to treat everything from autism to heart failure. Critics say FDA regulations on stem cells need to be tightened, not loosened, if the agency is going to be able to push back on the clinics touting unproven and possibly unsafe products.

“Many people recognize the promise of stem cell research, including the public, including patients,” she said. “Unfortunately, this had led to a springing up of clinics that are offering unproven therapies, and patients who are persuaded to go to these. And what we’re seeing are really negative outcomes.

Temple added: “It’s very important to realize that stem cells are not like drugs. … You want to make sure that you’re really following whether or not the treatments are safe and efficacious.”

Despite the improvements over the Regrow Act, Paul Knoepfler, a stem cell scientist at the University of California, Davis, said he was still worried that an experimental therapy would reach patients and that it would turn out not to work, or worse. While he opposed the Regrow Act, he said he has mixed feelings about the regenerative medicine components of the Cures Act.

“Because Cures’ language pushes key requirements for data to be at the post-approval rather than pre-approval stage some unsafe, ineffective therapies would almost certainly get approved and go into patients in trials,” he said. “The billion dollar question is whether the potential risks or benefits would materialize more should Cures pass. No one has a crystal ball on this so it’d be a high-risk regulatory experiment.”

The Alliance for Regenerative Medicine — a group of companies, researchers, and patient advocacy groups — had also opposed the Regrow Act, but Michael Werner, who cofounded the group, said the language in the Cures Act avoids the worst parts of the original measure.

Werner, a partner with the Holland & Knight law firm, said the new regenerative medicine provisions would preserve the FDA’s approval standards and that the agency would work with companies pursuing therapies earlier in the process, offering meetings and faster review.

“We think it’s a good thing,” he said. “It strikes the right balance.”

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