Serum interferon β-1a (Avonex) levels following intramuscular injection in relapsing-remitting MS patients

Background: The pharmacokinetics of IFN beta-1a in MS patients are poorly understood. We have previously reported an ELISA sensitive and specific for measuring serum IFN beta-1b levels in patients with MS. Objective: We describe an ELISA to measure interferon beta-1a (Avonex) in the serum of MS patients following IM administration. Methods: We have developed an ELISA for detecting serum IFN beta-1a in MS patients receiving 6 million units (MU) of IFN beta-1a, IM once weekly. The specificity of this ELISA was confirmed by the lack of cross-reactivity with other cytokines except for IFN beta-1b. Results: Serum IFN beta-1a levels were measured at 3 and 6 months after initiating treatment with IFN beta-1a in 10 MS patients. At 3 months, all 10 patients had detectable levels ranging from 68 to 86 IU/mL. At 6 months, IFN beta-1a could be detected in the serum of all but three patients, with levels ranging from 64 to 81 IU/mL. A kinetic study of IFN beta-1a serum levels in a separate group of six MS patients who had been receiving IFN beta-1a for several months was carried out. Blood was drawn before and 2, 4, 6, 8, and 24 hours after IFN beta-1a injection. Peak serum IFN beta-1a levels were observed at 8 hours and became undetectable at 24 hours after injection. Conclusion: The described ELISA may have useful clinical applications in examining the correlation between serum IFN beta-1a levels and clinical efficacy.