As part of a recent crackdown on stem cell clinics, the FDA found an operation using Sanofi's smallpox vaccine in an illegitimate cancer therapy.

U.S. Marshals seized five vials of the vaccine, a controlled substance known as ACAM2000, which contains live virus. The drug regulator said the vaccine, in "excess amounts," was mixed with stem cells derived from body fat to create a potentially dangerous treatment.

The unapproved therapy was marketed by San Diego-based StemImmune and administered to patients at two California Stem Cell Treatment Centers facilities.

The only FDA-approved smallpox vaccine, ACAM2000 has been stockpiled by the federal government for emergencies; it is strictly controlled for biodefense purposes and is not commercially available, according to an FDA spokesperson. Therefore, the FDA has "serious concerns" about how the clinic got its hands on the product.

"The FDA will not allow deceitful actors to take advantage of vulnerable patients by purporting to have treatments or cures for serious diseases without any proof that they actually work," FDA Commissioner Scott Gottlieb said in a statement. "I especially won't allow cases such as this one to go unchallenged, where we have good medical reasons to believe these purported treatments can actually harm patients and make their conditions worse."

Not only was the vaccine used in an unapproved way, it could pose serious dangers to cancer patients who may have compromised immune systems, the FDA said in a release.

Sanofi Pasteur makes and sells the smallpox vaccine, but in July, Maryland-based Emergent BioSolutions agreed to pay $125 million for the product. The deal comes with an existing CDC stockpile contract set for renewal in 2018, an FDA-cleared bulk manufacturing facility and some 100 employees. That deal is expected to close by year-end.

Because the FDA is investigating the case, Sanofi said in a statement to FiercePharma that it can offer no further comment other than that it "remains fully committed to public health and the appropriate, safe and ethical use of its vaccines."

The FDA has stepped up its oversight and enforcement of unregulated stem cell practices in Florida and California in the last few days. In a separate case, the FDA sent a warning letter to US Stem Cell Clinic of Sunrise, Florida, which had used stem cells to purportedly treat such diseases as Parkinson’s, amyotrophic lateral sclerosis, chronic obstructive pulmonary disease and pulmonary fibrosis, the agency said.

In a separate statement, Gottlieb said the FDA plans to continue its crackdown. The agency will "establish clearer lines around when these regenerative medicine products have sufficient complexity to fall under the agency's current authority, and then define an efficient process for how these products should be evaluated for safety and effectiveness," he said.