Cyclic Ruthenium-Peptide Conjugates as Integrin-Targeting Phototherapeutic Prodrugs for the Treatment of Brain Tumors

Abstract

To investigate the potential of tumor-targeting photoactivated chemotherapy, a chiral ruthenium-based anticancer warhead, Λ/Δ-[Ru(Ph₂phen)₂(OH₂)₂]²⁺, was conjugated to the RGD-containing Ac-MRGDH-NH₂ peptide by direct coordination of the M and H residues to the metal. This design afforded two diastereoisomers of a cyclic metallopeptide, Λ-[<b>1</b>]Cl₂ and Δ-[<b>1</b>]Cl₂. In the dark, the ruthenium-chelating peptide had a triple action. First, it prevented other biomolecules from coordinating with the metal center. Second, its hydrophilicity made [<b>1</b>]Cl₂ amphiphilic so that it self-assembled in culture medium into nanoparticles. Third, it acted as a tumor-targeting motif by strongly binding to the integrin (<i>K</i>_d = 0.061 μM for the binding of Λ-[<b>1</b>]Cl₂ to α_IIbβ₃), which resulted in the receptor-mediated uptake of the conjugate <i>in vitro</i>. Phototoxicity studies in two-dimensional (2D) monolayers of A549, U87MG, and PC-3 human cancer cell lines and U87MG three-dimensional (3D) tumor spheroids showed that the two isomers of [<b>1</b>]Cl₂ were strongly phototoxic, with photoindexes up to 17. Mechanistic studies indicated that such phototoxicity was due to a combination of photodynamic therapy (PDT) and photoactivated chemotherapy (PACT) effects, resulting from both reactive oxygen species generation and peptide photosubstitution. Finally, <i>in vivo</i> studies in a subcutaneous U87MG glioblastoma mice model showed that [<b>1</b>]Cl₂ efficiently accumulated in the tumor 12 h after injection, where green light irradiation generated a stronger tumoricidal effect than a nontargeted analogue ruthenium complex [<b>2</b>]Cl₂. Considering the absence of systemic toxicity for the treated mice, these results demonstrate the high potential of light-sensitive integrin-targeted ruthenium-based anticancer compounds for the treatment of brain cancer <i>in vivo</i>.