Gilead Sciences has suffered a setback in the development of its oral hepatitis C drug GS-7977.

The Electron trial of the drug, which was acquired during Gilead’s $11bn acquisition of Pharmasset, reported that the majority of HCV genotype 1 patients with a prior ‘null’ response to an interferon-containing regimen witnessed viral relapse within four weeks of completing treatment.

Ten patients were involved in this particular trial arm and of the eight that data were available for, six had experienced viral relapse. The two patients to not suffer relapse have only reached two weeks post-treatment.

Gilead Sciences executive vice president of research and development and chief scientific officer Norbert Bischofberger said that the data answer an important question about the drug and its treatment of genotype 1 null responder patients.

"We will continue to explore a number of therapeutic approaches to address this significant unmet medical need, including combinations with other oral antivirals," added Bischofberger.

Although the drug has demonstrated markedly more positive results in genotype 2 and 3 patients, the setback has led to shares in Gilead to tumble 14%.