Synthesis and Characterization of an Epidermal Growth Factor Receptor selective Ru(II) Polypyridyl-Nanobody Conjugate as a Photosensitizer for Photodynamic Therapy

There is currently a surge for the development of novel photosensitizers (PSs) for photodynamic therapy (PDT) since those currently approved are not completely ideal. Among the tested compounds, we have previously investigated the use of Ru(II) polypyridyl complexes with a [Ru(bipy)2(dppz)]2+ and [Ru(phen)2(dppz)]2+ scaffold (bipy = 2,2'-bipyridine; dppz = dipyrido[3,2-a:2′,3′-c]-phenazine, phen = 1,10-phenanthroline). These complexes selectively target DNA. However, since DNA is ubiquitous, it would be of great interest to increase the selectivity of our PDT PSs by linking them to a targeting vector in view of targeted PDT. Herein, we present the synthesis, characterization and in-depth photophysical evaluation of a nanobody containing Ru(II) polypyridyl conjugate selective for the epidermal growth factor receptor (EGFR) in view of targeted PDT. Using ICP-MS and confocal microscopy, we could demonstrate that our conjugate had a high selectivity for the EGFR receptor, which is a crucial oncological target as it is overexpressed and/or deregulated in a variety of solid tumors. However, contrary to expectations, this conjugate was found to not produce reactive oxygen species (ROS) in cancer cells and to be therefore not phototoxic.