By Sharon Begley

NEW YORK, April 22 (Reuters) - An experimental Ebola drug from Tekmira Pharmaceuticals Corp cured all three monkeys intentionally infected with the virus, scientists reported on Wednesday, the first such success against the strain of Ebola in West Africa's 2014-2015 outbreak.

Although other experimental treatments appeared to help Ebola patients last year, especially in the United States, those one-time uses cannot prove efficacy against the "Makona" strain, since patients' recovery might be due to other causes. Similarly, drugs, including Mapp Biopharmaceutical's ZMapp, cured monkeys in lab experiments, but in a strain of Ebola different from that responsible for the current outbreak, the worst ever recorded.

"We can't say for certain that an experimental drug that works against one strain will work in another, even if they're almost identical genetically," said Thomas Geisbert of the University of Texas Medical Branch, senior author of the study published in the journal Nature.

In the experiment, six rhesus monkeys were infected with huge doses of the Makona strain. Three days later, three received an infusion of the Tekmira product, a cocktail of "small-interfering RNAs" (siRNAs) encapsulated in a fat droplet called a lipid nanoparticle.

The siRNAs bind to two of the virus's seven genes, silencing them and thereby preventing the virus from replicating.

All three treated monkeys survived despite fevers and enormous blood levels of virus. Three untreated monkeys became so ill they were euthanized within nine days.

The drug can be adapted to target any strain of Ebola and produced in as little as eight weeks (compared with the months required for ZMapp), Geisbert said, in what he called "plug and play": sequence any Ebola genome and custom-make a siRNA cocktail.

"There is a need for treatments that can be quickly modified if the strain changes," he said.

A clinical trial of the new cocktail began last month in Ebola patients in Sierra Leone. A study of an earlier version was partly halted after healthy volunteers developed side effects at high doses.

The study continues a string of encouraging news in Ebola research.

This month scientists reported that an improved version of a vaccine from NewLink Genetics Corp and Merck & Co Inc protected all eight lab monkeys injected with Ebola. The improved version, from Profectus BioSciences, was altered to improve safety.

In March, scientists reported that both the NewLink-Merck vaccine and one from GlaxoSmithKline PLC appear safe and triggered an anti-Ebola immune response in volunteers in Liberia.

(Editing by Jonathan Oatis)