An Ir(III)-Peptide Nucleic Acid (PNA) conjugate as a Singlet Oxygen Photosensitizer for Photodynamic Therapy

Photodynamic therapy (PDT) is a noninvasive medical treatment used against skin diseases, various types of infections and even cancer. It is based on the excitation of molecular oxygen from its triplet ground-state (3O2) to the toxic singlet excited state (1O2) using a photosensitizer (PS) which, after exposure to suitable light, is excited to the singlet state (S1) and then undergoes intersystem crossing (ISC) to the triplet excited state (T1), where the energy transfer to 3O2 takes places (Type II PDT). Cyclometallated Ir(III) complexes are suitable as PSs for 1O2 production in PDT due to the accessibility of triplet excited states having usually metal-to-ligand charge transfer character (3MLCT).2 The positive charge that many of these kind of organometallic compounds possess promotes their solubility in water, allows an interaction with the cellular membrane and enhances their cellular uptake. In our ongoing study, we have synthetized a novel tris cyclometallated Ir(III) complex (Ir-COOH), containing a carboxyl group, and the corresponding conjugate with a peptide nucleic acid (PNA) tetramer (Fig. 1A) The final aim will be to combine the Ir(III) complex properties with an antisense strategy promoted by specific sequences of PNA. Both the complex and the corresponding Ir-PNA conjugate have been characterized with UV-vis absorption and emission spectroscopy, displaying absorptions in the blue and a moderate emission (F ≈ 3%) at ca 580 nm. The capability to produce 1O2 has been also tested for both species in cuvette, employing the indirect reporter DHN (Fig. 1B, ΦΔ= 0.54 for Ir-PNA vs. ΦΔ= 0.42 for Ir-COOH). Moreover, preliminary in vitro cytotoxicity test were carried out on mouse motor neuronal cell line (NSC34). Since the emission is not completely quenched by O2 interaction, the cellular uptake will be followed in vitro by confocal microscopy even using a low-energy light by exploiting the two-photon excitation. This last feature, together with the possibility to shift toward the NIR region their emission by using different ligands of the Ir(III) complex will be also investigated for in vivo treatments.