The Epidermal Growth Factor (EGF) family of receptors, also called ErbB or HER family, is a group of tyrosine kinase transmembrane proteins that have many regulatory purposes including regulating cell proliferation and survival. Members of the HER family rely on forming dimers upon ligand binding to promote downstream signaling. Gene mutations can result in the deregulation of the HER receptors, further resulting in cancer. HER3, a receptor that is deregulated in many cancers including ovarian, breast, and lung cancer, has been found to be responsible for drug resistance to therapeutics that currently exist to target other members of the HER family. This can occur through increased phosphorylation and overexpression of the HER3 receptor. There are many HER3 targeted therapeutics, including monoclonal antibodies (mAbs), that are currently in phase 1 and 2 of clinical studies; however, no HER3 targeted therapeutics have been approved by the FDA. In addition to this, previous studies have demonstrated that not every patient will respond to a specific treatment plan or therapeutic; therefore, the development of various treatment options is essential.

An engineered protein known as the affibody, which in previous studies has shown to be highly soluble, thermally stable, and small in size allowing for effective tissue penetration, has emerged as a potential therapeutic agent for cancer. In this study, it was found that multivalent affibodies, which are affibodies with more than one binding domain, are more effective at inhibiting HER3 activation, also known as phosphorylation, and inducing HER3 downregulation than monovalent affibodies in multiple cell lines. Inhibiting receptor activation can be effective at reducing cell proliferation and survivability. In addition, other modifications were made to optimize the affibodies, such as altering the length of the linker that tethers the binding domains in a multivalent affibody together, and to test for their efficacy. Finally, an albumin binding domain was incorporated into the affibody design to help increase affibody half-life, which would be essential for in vivo testing.