SCIENTISTS have discovered three existing drugs - used for cancer, hepatitis C and for parasitic infections - that they say appear promising against the Zika virus.

The experiments were conducted only in lab-grown human cells in petri dishes, but the results were dramatic.

Zika is so devastating that the damage it does has been thought to be irreversible. But the researchers said some of the compounds that the group tested not only allowed cells to live longer in the face of infection - but also in some cases fully recover from them.

The news, reported in Nature Medicine on Monday, is exciting but only a very preliminary step toward a treatment.

The researchers will have to test the drugs in animal models to see if they can replicate the results, and if these tests are successful, they will have to start the long process of trying to test the drugs' effectiveness in humans.

Co-author Hongjun Song, director of the stem cell program at Johns Hopkins, said that the drugs "are very effective against Zika in the dish, but we don't know if they can work in humans in the same way."

He said, for instance, that it's unknown whether niclosamide, which is used to treat patients with parasites in the gut, can even penetrate the central nervous system to get to an unborn baby's brain.

The researchers also don't have any idea whether the drugs could treat the whole spectrum of damage done by the virus, from microcephaly (abnormally small heads) in infants to paralysis of the gastrointestinal tract.

Scientists have not yet determined if the drug can even penetrate the central nervous system of adults or a fetus inside a carrier's womb to treat the brain cells targeted by Guillain-Barré syndrome in adults.

The identification of the drugs was based on a large-scale screening of 6,000 existing drugs.

Given that it can take decades for scientists to come up with and test medications, looking at those that are already approved by regulators or in clinical trials can cut the development time a great deal. This technique has also been used in the hunt for an Ebola treatment.

The three drugs identified are PHA-690509, which is investigational and is being used on cancer patients; emricasan, which is being used in clinical trials to determine whether it might be able to help reduce liver damage from the hepatitis C virus; and niclosamide.

The experiments were conducted in what the researchers described as "two- and three-dimensional cell cultures called 'mini-brains,' " which share some structures with the human brain.

They measured indicators of cell death and found that there seemed to be two classes of drugs with potential to help stop or hinder the virus.

The first group includes neuroprotective drugs, which might stop cell death, and antiviral drugs, which might slow or stop infection or replication of the virus in the brain.