A growing number of doctors, including some in the United States, have begun to use the tissue-based treatments described by the Food and Drug Administration as a possible replacement for surgery.
The technology is being used to treat both cancers and cardiovascular diseases, and has already been approved for use in people with cancer and HIV.
But the FDA says it is not ready to give the drug the green light for clinical trials.
While tissue-derived stem cells can be used to produce new organs and tissues, some experts question whether they are truly regenerative.
“If you’re just trying to create a new organ, that’s going to be problematic,” said Dr. Steven Novella, chief of the division of cardiovascular medicine at Boston Children’s Hospital.
He and his colleagues have been working to develop an alternative approach that uses a different set of cells to produce a tissue-like layer of blood vessels, a type of blood vessel called endothelial.
“We want to find out how to do it right,” Dr. Novello said.
The key to making such a technique work, he said, is to avoid creating a new tissue-specific type of cell that may react to a particular molecule and cause the cell to die or break down.
The goal is to develop a “reprogramming” technique that mimics the process of creating new blood vessels in the body by replacing the damaged cells.
This could potentially be done by taking out one of the damaged cell types and replacing it with a more mature cell, he added.
The FDA said it would be more than a year before the agency will be able to review whether tissue-generated stem cells are ready for human clinical trials, but it has begun to take the steps to evaluate potential therapies.
This includes issuing a provisional application for a drug called T2S-2 to treat glioblastoma, a rare, aggressive type of brain cancer.
It is being tested on a group of patients with the disease.
A similar drug, T2B-2, has been approved in clinical trials in patients with multiple myeloma, also known as multiple myeloencephalomyelitis, or multiple sclerosis.
But it is still unclear whether the drugs will work in people.
The company that made T2s, Episetech, said it is testing the drugs in people in China and other countries where they are already being used.
T2 is expected to be available for sale by the end of this year.
Other companies have begun using T2 therapies in people, including a company in Japan called Regeneron.
But while some researchers are cautiously optimistic about the technology, others say the drugs are too risky and potentially dangerous.
“I think they should be kept away from anyone who doesn’t have a good record of clinical trials,” said Robert H. Kostelnick, an associate professor of medicine at the University of Michigan School of Medicine.
In addition to the potential risks, T4B-3, the compound in T2A-1, has not been approved by the FDA.
“That compound is extremely toxic, very potent, and it’s a very difficult compound to make,” said Daniel D. Hodge, a professor of molecular medicine at Northwestern University Feinberg School of Engineering.
He said that while the compound is safe for some people, it is a concern that some people may have trouble tolerating it.
“It is really hard to take a drug like this and say it’s safe for everyone,” Dr Hodge said.
One study showed that patients who received T2-2 did not benefit from the drug.
The study, published in the journal Neurochemistry in April, concluded that patients with T2 had worse brain function than those who received the drug in the group of people with relapsed brain cancer who did not have relapsed tumors.
“The results do not support the hypothesis that this treatment improves cognitive functioning,” Dr Kostellnick said.
“They have no effect on the rate of relapse in people who have relived the disease and have relapses, but the rate is higher for those who are cured.”
For now, T3A-3 has been available in the U.S. for some time.
But in the meantime, it has been blocked by the U,S.
Drug Enforcement Administration, which said in March it was blocking the drug because it was not approved by a regulatory body.
The agency also said that T3B-4, the same compound that has been licensed in the past to treat patients with gliomas, had not been studied in human studies.
“This compound is an FDA-approved drug, but because it is in limited use, we cannot provide FDA approval for its use for treatment of glioma,” the agency said in a statement.
Some doctors say that the FDA’s stance may be a reflection of its failure to regulate stem cells.
“You’ve got this really big industry, and yet the FDA just doesn’t want to regulate this stuff,” said Charles