Stimuli-Responsive Photofunctional Rhenium(I) and Iridium(III) Polypyridine Complexes for Bioimaging and Phototherapeutic Applications


Student thesis: Doctoral Thesis

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Award date3 Jan 2024


Luminescent transition metal complexes with unique photophysical and photochemical behavior and tunable cytotoxicity have emerged as promising candidates for the design of on-demand activatable reagents for bioimaging and anticancer therapy. This thesis describes the design of photofunctional tricarbonylrhenium(I) and cyclometalated iridium(III) polypyridine complexes as stimuli-responsive reagents for bioimaging and phototherapeutic applications.

Glutathione (GSH) plays a crucial role in regulating redox homeostasis in living systems, and the endogenous level of GSH in cancer cells is much higher than that of normal cells. The thiol group of the cysteine residue is one of the main functional groups of GSH, which makes GSH a strong reducing agent and nucleophile. Thus, several strategies have been developed for the design of GSH-sensitive reagents for site-specific bioimaging and therapy, such as Michael addition and nucleophilic substitution. However, GSH-sensitive phosphorogenic rhenium(I) polypyridine complexes have rarely been examined. In Chapter 2, the synthesis and characterization of four rhenium(I) complexes appended with a dinitrophenylsulfonamide (DNPS) moiety [Re(N^N)(CO)3(py-DNPS)](CF3SO3) (py-DNPS = 3-((2,4-dinitrophenylsulfonyl)aminomethyl)pyridine; N^N = 4-N-((p-toluenesulfonylamin-o)ethyl)aminomethyl-4’-methyl-2,2’-bipyridine (bpy-tosylamide) (1), 3,4,7,8-tetramethyl-1,10-phenanthroline (Me4-phen) (2), 4,7-diphenyl-1,10-phenanthroline (Ph2-phen) (3), and 1,10-phenanthroline (phen) (4)) as GSH-sensitive photocytotoxic agents are described. Upon photoexcitation, the DNPS complexes showed very weak luminescence due to photoinduced electron transfer from the excited rhenium(I) diimine moiety to the quenching DNPS unit. However, upon treatment with GSH, the DNPS moiety departed, resulting in emission enhancement of the solutions (I/Io = 12.6 – 22.2). After the reaction of the DNPS complexes with GSH in live cells, intense intracellular emission and potent photocytotoxicity were observed. Additionally, modification of the diimine ligand with a tosylamide unit bestowed endoplasmic reticulum (ER)-targeting ability to the complex, which can be exploited for selective bioimaging and photocytotoxic applications.

Bioorthogonal reactions have emerged rapidly as a promising approach for on-demand drug activation in complex living systems because of their high reaction specificity, kinetics, and biocompatibility. These reactions include tetrazine ligation, strain-promoted azide–alkyne cycloaddition, and copper(I)-catalyzed azide–alkyne cycloaddition. In particular, bioorthogonal dissociative reactions using tetrazine as a bioorthogonal trigger have recently been discovered as a novel strategy for releasing payloads in a highly controlled manner. Tetrazine moieties can also efficiently quench the emission of luminescent metal complexes. Thus, modification of luminescent transition metal complexes with a bioorthogonal dissociable tetrazine unit is expected to modulate their emission intensities, reactive oxygen species generation efficiencies, and (photo)cytotoxicity. In Chapter 3, the synthesis and characterization of three rhenium(I) complexes modified with a tetrazine unit via a cleavable carbamate linker [Re(N^N)(CO)3(py-Tz-NHBoc)](CF3SO3) (py-Tz-NHBoc = 3-(1-(6-(4-(N-Boc-aminomethyl)-phenyl)-1,2,4,5-tetrazin-3-yl)ethyloxycarbonylaminomethyl)pyri-dine; N^N = phen (1), Me4-phen (2), and Ph2-phen (3)) are reported. The complexes displayed increased emission intensities and singlet oxygen (1O2) generation efficiencies upon reaction with trans-cyclooct-4-enol due to the separation of the quenching tetrazine unit from the rhenium(I) core. Additionally, one of the tetrazine complexes appended with a polyethylene glycol (PEG) group [Re(Ph2-phen)(CO)3(py-Tz-PEG5000)](CF3SO3) (4) (py-Tz-PEG5000 = 3-(1-(6-(4-(N-(3-(ω-methoxypoly(1-oxapropyl))-propanoyl)-aminomethyl)-phenyl)-1,2,4,5-tetrazin-3-yl)-ethyloxycarbonylaminomethyl)pyridine) exhibited negligible dark cytotoxicity but showed greatly enhanced (photo)cytotoxic activity toward TCO-OH-pretreated cells upon light irradiation. The reason is that the treatment with TCO-OH allows the synergistic release of the more cytotoxic rhenium(I) aminomethylpyridine complex and increased 1O2 generation. Importantly, the treatment could induce a cascade of events, including lysosome dysfunction, autophagy suppression, and ICD. This is the first example of using bioorthogonal dissociation reactions as a trigger to realize photoinduced ICD, opening up new avenues for the development of innovative photoimmunotherapeutic agents.

The development of intelligent immunotherapeutics with controllable activity and high cancer selectivity has been regarded as a promising strategy for advancing the safety and effectiveness of immunotherapy. Photoactivatable immunotherapeutic agents, which use light as an external stimulus to selectively convert noncytotoxic species into their bioactive counterparts to boost immunogenic response, have received increasing attention. Although the utilization of photoactivatable transition metal complexes for anticancer therapy has been widely reported, the exploration of their performance in immunotherapy is very rare. In Chapter 4, the synthesis and characterization of three photoactivatable iridium(III) complexes with the insertion of a photolabile S,S-tetrazine unit between the complex core and PEG pedant [Ir(N^C)2(bpy-S-Tz-S-PEG5000)](PF6) (bpy-S-Tz-S-PEG5000 = 4-(S-((6-methoxy poly(ethylene glycol)thio-1,2,4,5-tetrazine)-3-yl)-N-mercaptoethylaminocarbonyloxymethyl)-4’-methyl-2,2’-bipyridine); HN^C = 2-phenylpyridine (1), 2-phenylquinoline (2), and 2-phenylquinoline-4-carboxylic acid methyl ester (3)) as photoactivatable immunomodulators for photo-immunotherapy are presented. Due to the emission quenching effect of the tetrazine unit, the complexes displayed weak emission intensities and insufficient 1O2 generation. Upon photoirradiation, the S,S-tetrazine complexes undergo efficient photodissociation to give highly cytotoxic SCN complexes with significantly enhanced emission intensities and ROS photosensitization. This synergistic action triggered ER stress, subsequently induced immunogenic cell death, and stimulated systemic immune response, efficiently inhibiting tumor growth. This study paves the way for the design of novel therapeutics for precise immunotherapy.

    Research areas

  • Iridium, Rhenium, Stimuli responsive, Bioimaging, photodynamic therapy