Eight-year immunogenicity and safety of interferon beta-1a-Avonex treatment in patients with multiple sclerosis.

An open-label extension study of the phase III trial of intramuscular interferon beta-1a (IFNβ-1a-Avonex) was conducted to evaluate the immunogenicity and safety of IFNβ-1a-Avonex over six years in patients with relapsing multiple sclerosis (MS). Patients who participated in the pivotal phase III study were offered enrolment; entry was also open to patients who had not participated. All patients received IFNβ-1a-Avonex 30 mg intramuscularly once weekly for six years, for a treatment duration of up to eight years in patients who received IFNβ-1a-Avonex in the phase III trial. Serum levels of IFNβ antibodies were measured every six months using a screening enzyme-linked immunosorbent assay (ELISA) followed by an antiviral cytopathic effect assay to detect neutralizing antibodies (NAbs) in serum samples positive on ELISA. The incidence of adverse events and laboratory test results assessed safety. Of 382 total patients, 218 had participated in the phase III study (103 placebo, 115 IFNβ-1a-Avonex) and 164 had...