Photofunctional Rhenium(I), Ruthenium(II), and Iridium(III) Polypyridine Complexes for Therapeutic and Concerted Bioorthogonal Applications

Abstract

In this thesis, the design of photofunctional rhenium(I), ruthenium(II), and iridium(III) polypyridine complexes for therapeutic and concerted bioorthogonal applications is described.

Mesoionic compounds are classified as 5-membered heterocyclics with a dipolar nature, with the positive charge localized primarily in the ring, which is counterbalanced with a negatively charged exocyclic atom. Due to their mesoionic properties, most of these compounds exhibit a wide spectrum of biological activity and have been utilized in various pharmacological pursuits. The combination of luminescent transition metal complexes and mesoionic ligands is a promising endeavor to develop a new class of luminescent systems as diagnostic and therapeutic tools. In Chapter 2, the synthesis, characterization, and photophysical and photochemical properties of a series of neutral mesoionic biscyclometalated N^S iridium(III) complexes [Ir(N^C)₂(p-ttqt)] (p-Httqt = 1-phenyl-5-thioxo-5,6-dihydro-[1,2,4]triazolo[1,5-c]quinazolin-1-ium-2-thiolate; HN^C = 2-phenylpyridine (Hppy) (1), 7,8-benzoquinoline (Hbzq) (2), 2-phenylbenzothiazole (Hbt) (3), and 2-(1-naphthyl)benzothiazole (Hbsn) (4)) are reported. X-ray crystallographic studies of complexes 1, 2, and 4 demonstrated that the direct coordination of the p-Httqt ligand to the metal center did not disrupt the mesoionic character of the ligand. The p-Httqt ligand was noncytotoxic in the dark, and this property could be bestowed on all the iridium(III) complexes. The iridium(III) complexes are extremely effective photosensitizers with high photocytotoxic activity against cancer cells with no adverse (photo)cytotoxicity toward normal cells. All the iridium(III) complexes showed specific localization in the endoplasmic reticulum of cancer cells, facilitating their use as photodynamic therapy (PDT)-activated Type II immunogenic cell death (ICD) inducers. The PDT-induced cell death pathways and ICD response effect of one of the complexes were also investigated.

Bioorthogonal chemistry arose as a versatile modality to enable chemical reactions in living systems to detect and visualize biomolecules in their native environments. Continued interest in new reaction partners to expand the functionality and applicability of bioorthogonal chemistry has led to recent interest in mesoionic compounds as bioorthogonal handles due to their 1,3-dipole nature. In particular, mesoionic sydnones have been exploited to develop fluorogenic-based imaging probes since the mesoionic ring can quench the fluorescence of the dyes, but upon 1,3-dipolar cycloaddition reactions with alkynes, can give rise to non-quenching pyrazole derivatives. This design could be adapted to luminescent transition metal complexes to yield new phosphorogenic probes with long-lived and environment-sensitive emissions to increase the signal-to-noise ratio in biological systems. In Chapter 3, the synthesis, characterization, and photophysical properties of three rhenium(I) polypyridine sydnone complexes [Re(N^N)(CO)₃(py-syd-ph)](CF₃SO₃) (py-syd-ph = 3-phenyl-4-(4-pyridyl)sydnone; N^N = 1,10-phenanthroline (phen) (1), 3,4,7,8-tetramethyl-1,10-phenanthroline (Me₄-phen) (2), 4,7-diphenyl-1,10-phenanthroline (Ph₂-phen) (3)) are described. The rhenium(I) complexes are π-conjugated to the sydnone moiety on the C-4 position of the mesoionic ring for efficient emission quenching. The rhenium(I) sydnone complexes displayed substantial emission enhancement and lifetime extension upon reaction with strained cyclooctyne derivatives due to the transformation of the quenching sydnone ring to a pyrazole derivative. Cyclooctyne-modified proteins were also labeled with the rhenium(I) sydnone complexes, highlighting their potential for bioconjugation applications. Specific labeling of lysosomes in live cells was achieved with the rhenium(I) sydnone complexes by modification of lysosome-targeting morpholine with a cyclooctyne unit.

Enzymes are important proteins that facilitate intricate and crucial metabolic reactions necessary for all life forms. Overexpression of enzymes can lead to detrimental effects such as overloading specific biological pathways or disrupting native processes. One important biomarker in clinical settings for identifying cancer is the abnormal overexpression of enzymes compared to normal cells due to the high energy and biosynthetic demand of proliferating cancer cells. Enzyme-instructed self-assembly (EISA) is a promising modality utilizing enzymatic cleavage of small molecular motifs appended with an enzyme substrate for the specific formation of well-defined supramolecular assemblies in cancer cells. A concerted EISA and bioorthogonal approach is a promising strategy for the selective accumulation and activation of diagnostic probes and photosensitizers in cancer cells with negligible effects on normal cells. In Chapter 4, the synthesis and characterization of an alkaline phosphatase (ALP) specific EISA motif modified with a cyclooctyne Nap-FFK(BCN)Yp (FFBCN1) are presented. The phosphate-free (Nap-FFK(BCN)Y (FFBCN2)) and its cyclooctyne-free (Nap-FFKYp (P1)) counterparts were also prepared for comparison studies. The bioorthogonal photosensitizers and reaction partners, three ruthenium(II) pyridyl-tetrazine complexes: [Ru(N^N)₂(py-tz-ph)](Cl)₂ (py-tz-ph = 6-phenyl-3-(2-pyridyl)tetrazine; N^N = 2,2’-bipyridine (bpy) (1), Ph₂-phen (2), and 2,2’-biquinoline (biq) (3)) were synthesized and characterized. The inverse electron-demand Diels-Alder (IEDDA) reaction analogs [Ru(N^N)₂(py-pyr-ph)](Cl)₂ (py-pyr-ph = ((6aR,7aS)-1-phenyl-4-(pyridin-2-yl)-6,6a,7,7a,8,9-hexahydro-5H-cyclopropa[5,6]cycloocta[1,2-d]pyridazin-7-yl)methanol; N^N = bpy (1-BCN), Ph₂-phen (2-BCN), and biq (3-BCN)) were also prepared for comparison studies. The cyclooctyne-modified FFBCN1 exhibited enhanced EISA properties and bioorthogonal capabilities compared to its cyclooctyne-free counterpart (P1). Dephosphorylation of FFBCN1 by ALP led to the formation of short nanofibers that could serve as bioorthogonal activation triggers for the ruthenium(II) pyridyl-tetrazine complexes. The morphology and size of the short nanofibers were significantly enhanced after the IEDDA reaction with the ruthenium(II) pyridyl-tetrazine complexes. Additionally, the complexes displayed further emission enhancement and lifetime extension even when the emission was turned on after the IEDDA reaction. Complex 3 with near-infrared emission was used to investigate the concerted EISA and bioorthogonal approach in high-expressing ALP cancer and low-expressing ALP normal cells. The concentration of the EISA motif could modulate the localization of the ruthenium(II) pyridyl-tetrazine complexes into extracellular or intracellular regions. The (photo)cytotoxicity of the complexes was manipulated based on the accumulation of the EISA assemblies and shown to be more selective and photoactive toward cancer cells than normal cells.

Date of Award	5 Mar 2024
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Kam Wing Kenneth LO (Supervisor)