Several engineered protein scaffolds have been developed recently to circumvent 
particular disadvantages of antibodies such as their large size and complex 
composition, low stability, and high production costs. We previously identified 
peptide aptamers containing one or two disulfide-bonds as an alternative ligand 
to the interleukin-6 receptor (IL-6R). Peptide aptamers (32 amino acids in 
length) were screened from a random peptide library by in vitro peptide 
selection using the evolutionary molecular engineering method "cDNA display". In 
this report, the antagonistic activity of the peptide aptamers were examined by 
an in vitro competition enzyme-linked immunosorbent assay (ELISA) and an 
IL-6-dependent cell proliferation assay. The results revealed that a 
disulfide-rich peptide aptamer inhibited IL-6-dependent cell proliferation with 
similar efficacy to an anti-IL-6R monoclonal antibody.