Effect of covalent photoconjugation of affibodies to epidermal growth factor receptor (EGFR) on cellular quiescence

Cellular quiescence is a reversible state of cell cycle arrest whereby cells are temporarily maintained in the nondividing phase. Inducing quiescence in cancer cells by targeting growth receptors is a treatment strategy to slow cell growth in certain aggressive tumors, which in turn increases the efficacy of treatments such as surgery or systemic chemotherapy. However, ligand interactions with cell receptors induce receptor‐mediated endocytosis followed by proteolytic degradation, which limits the duration of cellular quiescence. Here, we report the effects of targeted covalent affibody photoconjugation to epidermal growth factor receptors (EGFR) on EGFR‐positive MDA‐MB‐468 breast cancer cells. First, covalently conjugating affibodies to cells increased doubling time two‐fold and reduced ERK activity by 30% as compared to cells treated with an FDA‐approved anti‐EGFR antibody Cetuximab, which binds to EGFR noncovalently. The distribution of cells in each phase of the cell cycle was determined, and cells conjugated with the affibody demonstrated an accumulation in the G1 phase, indicative of G1 cell cycle arrest. Finally, the proliferative capacity of the cells was determined by the incorporation of 5‐ethynyl‐2‐deoxyuridine and Ki67 Elisa assay, which showed that the percentage of proliferative cells with photoconjugated affibody was half of that found for the untreated control. A novel strategy is reported here for reducing cancer cell growth for extended periods in vitro by chemically conjugating small proteins called affibodies to cell receptors overexpressed on cancer cells. The authors have demonstrated that reacting photo‐cross‐linkable affibodies to cells prevents them from endosomal degradation. In addition, using micromolar affibody concentrations leads to reduced cellular ERK activity, decreased proliferation rates and an enhanced number of cells arrested in the G1 phase.