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A Canadian company testing an experimental drug against Ebola has closed down the trial and concluded they cannot say whether the drug helped anyone.

Tekmira Pharmaceuticals Corporation had been testing the drug in patients in Sierra Leone, but said it was clear the drug wasn’t showing any benefit to patients. The testing was scheduled to end anyway.

"The Phase II clinical trial of TKM-Ebola-Guinea has reached a predefined statistical endpoint and enrollment has been closed,” the company said in a statement.

Thomas Geisbert, an expert in infectious diseases at the University of Texas Medical Branch, who has himself tested the drug on monkeys, says this doesn’t mean the drug cannot work. Instead, Geisbert told NBC News, it demonstrates just how difficult it is to test drugs on real patients during epidemics.

“It’s not the best product that they have. It’s not the best formulation."

For one thing, he says, the drug tested in Sierra Leone is not the same version that had completely protected monkeys from a lethal dose of virus.

"It’s not the best product that they have. It’s not the best formulation," Geisbert said. "This is not what we tested in monkeys."

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Related: Ebola Cases Rise Again in West Africa

Ebola has infected more than 27,000 people in an ongoing epidemic in West Africa and it’s killed more than 11,000 of them. There’s no approved treatment, other than keeping patients hydrated, breathing and comfortable, and it has a death rate of 50 percent or more.

When the West African epidemic got bad, companies and governments rushed various experimental treatments and vaccines into the field, from simple blood transfusions from survivors to patients, to more sophisticated plasma transfusions, to drugs such as Tekmira’s, another treatment called ZMapp, and existing antiviral pills.

No one could say which treatment worked because none of the therapies were given in a controlled clinical trial — one set up to show whether patients did better with the treatment than without it.

Now some clinical trials have begun. including this study of Tekmir'a product. But the epidemic has dropped off. That makes it harder to enroll enough people to test a drug.

The company says it will release full details later.

Geisbert said the company would have been hard-pressed to show that the drug was helping patients in Liberia. "Do you give this product because it’s better than nothing or do you take the safe approach and say sorry, we don’t want to play?” he asked.

"We all want to do the right thing. Everybody wants to save lives."

Dr. Rick Sacra, an American medical missionary infected in Liberia last year, got Tekmira’s drug and survived. But he also got many other treatments.

“We all want to do the right thing. Everybody wants to save lives.”

The drug uses bits of genetic material called small interfering RNAs, or siRNAs for short.

Viruses kill cells by invading them and taking over their machinery to force them to pump out copy after copy of virus. This drug doesn’t prevent infection, but stops the virus from replicating by attaching to its RNA.

Geisbert says it should be a successful approach. He’s afraid other companies testing drugs against Ebola will be scared away from moving forward.