California-based Mapp Biopharmaceutical Inc is making progress in efforts to boost production of the experimental Ebola treatment ZMapp, as the deadly virus continues to spread through West Africa and beyond, the company said Friday.

The drug, which hasn't been tested in humans and was available only in very limited quantities, won the spotlight earlier this summer when two American aid workers, who contracted Ebola in Liberia, were cured after being treated with it. Still, their physicians do not know if it was the drug that helped them.

Other Ebola patients treated with ZMapp, including a Spanish missionary priest and a Liberian doctor, did not survive.

ZMapp is a mix of three antibodies designed to bind to proteins on the Ebola virus, triggering the immune system to destroy them. The drug is manufactured in tobacco plants at Kentucky BioProcessing, a unit of cigarette maker Reynolds American.

The company, which is based in San Diego, said in a statement it was seeking to improve yield from the tobacco plants. Mapp said it was finetuning the dose needed for each patient, and had begun manufacturing the drug using traditional methods of cultivating the antibodies from mammalian cells in stainless steel vats.

This would allow the company to produce more of the drug so that human testing can be carried out to evaluate ZMapp's safety and efficacy.

"While offering a slower route than plant production, the infrastructure for manufacturing in CHO (Chinese Hamster Ovary) cells is well established, which means the potential scale of drug production is greater than the production capacity of existing PMP (plant-made pharmaceutical) facilities," the company said.

Mapp said it was working on the expansion efforts with the Bill and Melinda Gates Foundation and military agencies, but didn't say which pharmaceutical company it had partnered with to produce the drug in a traditional biotech facility.

The company, one of several that are developing drugs for Ebola, received federal funding early in September to speed up testing and production of the treatment.

Lots manufactured under that contract would be used in early-stage clinical studies evaluating the safety and efficacy of ZMapp in humans, Mapp said.