Shares in US biotechnology firm Avi BioPharma closed up 29% on Friday after the firm reported laboratory tests showing that an experimental drug was effective in treating avian influenza.

Avi BioPharma said three independent laboratories confirmed that its antisense oligonucleotide, designed to prevent replication of the flu virus, was effective against H5N1 strain of bird flu. The drug targets highly-conserved regions of the virus genome, and this should give it broad activity against influenza despite the virus’ tendency to mutate into different forms.

Avi BioPharma now says it plans to file for approval to begin human clinical trials of the drug, one of its range of NeuGene antisense compounds, for the treatment of strains of influenza A, including H5N1.

Antisense drugs, which work by binding to the messenger RNA strands made from DNA and preventing the creation of proteins, have so far failed to live up to their promise, mainly because of the difficulty in delivering them into cells.