This technology uses iterative affinity selection to screen for protein-protein interaction inhibitors. First, a library of short amino acid sequences is generated by randomizing residues in a peptide known to bind to the target protein’s interaction region. Next, the library is mixed with the target protein and binders are isolated and identified with high-pressure size exclusion chromatography followed by LC-MS/MS. The short amino acid sequences that successfully bind the target protein then undergo another round of library synthesis, isolation, and identification. In this second selection round, the amino acid randomization step additionally includes non-canonical amino acids, which increases the structural diversity of the libraries and leads to identification of novel, more efficient inhibitors. Finally, the top candidates from the selection scheme can be macrocyclized, which greatly increases cell permeability. Using this scheme, the inventors successfully generated macrocyclic inhibitors of the MDM2-p53 interaction that efficiently penetrate the cell membrane and rapidly kill the osteosarcoma cell line SJSA-1 at micromolar concentrations in vitro.