Protein Scaffolds For Light‐Activated Delivery Of Toxic Iron To Cancer Cells

Abstract

An iron storage protein called bacterioferritin (Bfr) has been adapted for a novel approach to cancer therapy. Tumor‐targeting peptides (TTPs) have been fused to the protein and photosensitizers, like zinc protoporphyrin (ZnPPIX), have been substituted into the native heme binding sites. After binding to cancer cell receptors, photo‐excitation would trigger release of iron to generate a flux of toxic hydroxyl radicals that would overwhelm a cell's defenses.The gene encoding the TTP‐Bfr was synthesized and inserted into an E. coli expression plasmid and was readily expressed with ZnPPIX and purified by standard methods. The protein was then loaded by anaerobic incubation of iron. Bfr solutions were irradiated for various time points. Ferrozine was added to quantitate the iron release. These procedures reproducibly resulted in soluble fully loaded [Fe~1,000ZnPPIX12TTP24‐Bfr]. After 3 hours irradiation, ~30% of the iron was released. At longer irradiation times, 65–70% of the iron was released. No iron was released from dark identical control solutions. We have established that [Fe~1,000ZnPPIX12TTP24‐Bfr] releases significant amounts of iron upon irradiation. Current work is aimed at assessing binding of this protein to melanoma cells, and assessing inhibition of cell growth or apoptosis upon irradiation. Results of these investigations could provide proof of principle for a new approach to cancer therapy.