An iridium(iii) 3-chloro-6-thio-1,2,4,5-tetrazine complex for cysteine conjugation, bioimaging and photoactivated therapy

Lili Huang; Justin Shum; Lawrence Cho-Cheung Lee; Guang-Xi Xu; Peter Kam-Keung Leung; Kenneth Kam-Wing Lo

doi:10.1039/d4cb00316k

An iridium(iii) 3-chloro-6-thio-1,2,4,5-tetrazine complex for cysteine conjugation, bioimaging and photoactivated therapy

Lili Huang, Justin Shum, Lawrence Cho-Cheung Lee, Guang-Xi Xu, Peter Kam-Keung Leung, Kenneth Kam-Wing Lo^*

^*Corresponding author for this work

Research output: Journal Publications and Reviews › RGC 21 - Publication in refereed journal › peer-review

1 Citation (Scopus)

Abstract

Photoactivatable systems have received considerable attention in the development of diagnostics and therapeutics due to their noninvasive nature and precise spatiotemporal control. Of particular interest is the 3,6-dithio-1,2,4,5-tetrazine (S,S-tetrazine) unit, which can not only act as a photolabile protecting group for constructing photoactivatable systems but also as a bioorthogonal scaffold that enables the inverse electron-demand Diels–Alder (IEDDA) cycloaddition reaction with strained alkynes. In this study, we designed and synthesised a cyclometallated iridium(III) complex modified with a 3-chloro-6-thio-1,2,4,5-tetrazine moiety (1) for cysteine conjugation. The complex was conjugated with an integrin-targeting peptide c(RGDfC) to afford a tumour-targeting conjugate (1-RGD) for bioimaging and photoactivated therapy. An RGD-free analogue (2) was also prepared for comparison studies. Unlike common iridium(III) complexes, excitation of conjugate 1-RGD and complex 2 resulted in weak emission and negligible singlet oxygen (¹O₂) generation due to the quenching effect of the tetrazine unit. Upon continuous light irradiation, the S,S-tetrazine moiety in conjugate 1-RGD and complex 2 underwent efficient photodissociation, yielding thiocyanate (3) and amide (4) complexes as photoproducts with increased emission intensities and enhanced ¹O₂ generation efficiencies. Interestingly, the IEDDA cycloaddition reaction of the S,S-tetrazine-containing conjugate 1-RGD and complex 2 with (1R,8S,9s)-bicyclo[6.1.0]non-4-yn-9-ylmethanol (BCN-OH) led to significant emission enhancement. Notably, conjugate 1-RGD showed higher cellular uptake and (photo)cytotoxicity (IC_50,dark = 26 μM, IC_50,light = 0.08 μM) in U87-MG cells, which overexpress integrin, compared to MCF-7 (IC_50,dark = 52 μM, IC_50,light = 0.22 μM) and HEK293 cells (IC_50,dark> 50 μM, IC_50,light= 1.3 μM) with lower integrin levels. This work will contribute to the development of photoactivatable transition metal complexes for cancer-targeted imaging and therapy. © 2025 The Author(s). Published by the Royal Society of Chemistry

Original language	English
Pages (from-to)	1148-1155
Journal	RSC Chemical Biology
Volume	6
Issue number	7
Online published	7 May 2025
DOIs	https://doi.org/10.1039/d4cb00316k
Publication status	Published - 1 Jul 2025

Funding

We thank the Hong Kong Research Grants Council (Project No. CityU 11301121, CityU 11317022, CityU 11309423 and C7075-21GF) and the Hong Kong Research Grants Council and National Natural Science Foundation of China (Project No. N_CityU104/21) for financial support. We also thank the funding support from “Laboratory for Synthetic Chemistry and Chemical Biology” under the Health@InnoHK Programme launched by the Innovation and Technology Commission, The Government of Hong Kong SAR, P. R. China. L. H. acknowledges the receipt of a Postgraduate Studentship and Research Tuition Scholarship, both administered by City University of Hong Kong.

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This full text is made available under CC-BY-NC 3.0. https://creativecommons.org/licenses/by-nc/3.0/

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RGC-funded

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This output contributes to the following UN Sustainable Development Goals (SDGs)

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title = "An iridium(iii) 3-chloro-6-thio-1,2,4,5-tetrazine complex for cysteine conjugation, bioimaging and photoactivated therapy",

abstract = "Photoactivatable systems have received considerable attention in the development of diagnostics and therapeutics due to their noninvasive nature and precise spatiotemporal control. Of particular interest is the 3,6-dithio-1,2,4,5-tetrazine (S,S-tetrazine) unit, which can not only act as a photolabile protecting group for constructing photoactivatable systems but also as a bioorthogonal scaffold that enables the inverse electron-demand Diels–Alder (IEDDA) cycloaddition reaction with strained alkynes. In this study, we designed and synthesised a cyclometallated iridium(III) complex modified with a 3-chloro-6-thio-1,2,4,5-tetrazine moiety (1) for cysteine conjugation. The complex was conjugated with an integrin-targeting peptide c(RGDfC) to afford a tumour-targeting conjugate (1-RGD) for bioimaging and photoactivated therapy. An RGD-free analogue (2) was also prepared for comparison studies. Unlike common iridium(III) complexes, excitation of conjugate 1-RGD and complex 2 resulted in weak emission and negligible singlet oxygen (1O2) generation due to the quenching effect of the tetrazine unit. Upon continuous light irradiation, the S,S-tetrazine moiety in conjugate 1-RGD and complex 2 underwent efficient photodissociation, yielding thiocyanate (3) and amide (4) complexes as photoproducts with increased emission intensities and enhanced 1O2 generation efficiencies. Interestingly, the IEDDA cycloaddition reaction of the S,S-tetrazine-containing conjugate 1-RGD and complex 2 with (1R,8S,9s)-bicyclo[6.1.0]non-4-yn-9-ylmethanol (BCN-OH) led to significant emission enhancement. Notably, conjugate 1-RGD showed higher cellular uptake and (photo)cytotoxicity (IC50,dark = 26 μM, IC50,light = 0.08 μM) in U87-MG cells, which overexpress integrin, compared to MCF-7 (IC50,dark = 52 μM, IC50,light = 0.22 μM) and HEK293 cells (IC50,dark > 50 μM, IC50,light = 1.3 μM) with lower integrin levels. This work will contribute to the development of photoactivatable transition metal complexes for cancer-targeted imaging and therapy. {\textcopyright} 2025 The Author(s). Published by the Royal Society of Chemistry",

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T1 - An iridium(iii) 3-chloro-6-thio-1,2,4,5-tetrazine complex for cysteine conjugation, bioimaging and photoactivated therapy

AU - Huang, Lili

AU - Shum, Justin

AU - Lee, Lawrence Cho-Cheung

AU - Xu, Guang-Xi

AU - Leung, Peter Kam-Keung

AU - Lo, Kenneth Kam-Wing

PY - 2025/7/1

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N2 - Photoactivatable systems have received considerable attention in the development of diagnostics and therapeutics due to their noninvasive nature and precise spatiotemporal control. Of particular interest is the 3,6-dithio-1,2,4,5-tetrazine (S,S-tetrazine) unit, which can not only act as a photolabile protecting group for constructing photoactivatable systems but also as a bioorthogonal scaffold that enables the inverse electron-demand Diels–Alder (IEDDA) cycloaddition reaction with strained alkynes. In this study, we designed and synthesised a cyclometallated iridium(III) complex modified with a 3-chloro-6-thio-1,2,4,5-tetrazine moiety (1) for cysteine conjugation. The complex was conjugated with an integrin-targeting peptide c(RGDfC) to afford a tumour-targeting conjugate (1-RGD) for bioimaging and photoactivated therapy. An RGD-free analogue (2) was also prepared for comparison studies. Unlike common iridium(III) complexes, excitation of conjugate 1-RGD and complex 2 resulted in weak emission and negligible singlet oxygen (1O2) generation due to the quenching effect of the tetrazine unit. Upon continuous light irradiation, the S,S-tetrazine moiety in conjugate 1-RGD and complex 2 underwent efficient photodissociation, yielding thiocyanate (3) and amide (4) complexes as photoproducts with increased emission intensities and enhanced 1O2 generation efficiencies. Interestingly, the IEDDA cycloaddition reaction of the S,S-tetrazine-containing conjugate 1-RGD and complex 2 with (1R,8S,9s)-bicyclo[6.1.0]non-4-yn-9-ylmethanol (BCN-OH) led to significant emission enhancement. Notably, conjugate 1-RGD showed higher cellular uptake and (photo)cytotoxicity (IC50,dark = 26 μM, IC50,light = 0.08 μM) in U87-MG cells, which overexpress integrin, compared to MCF-7 (IC50,dark = 52 μM, IC50,light = 0.22 μM) and HEK293 cells (IC50,dark > 50 μM, IC50,light = 1.3 μM) with lower integrin levels. This work will contribute to the development of photoactivatable transition metal complexes for cancer-targeted imaging and therapy. © 2025 The Author(s). Published by the Royal Society of Chemistry

AB - Photoactivatable systems have received considerable attention in the development of diagnostics and therapeutics due to their noninvasive nature and precise spatiotemporal control. Of particular interest is the 3,6-dithio-1,2,4,5-tetrazine (S,S-tetrazine) unit, which can not only act as a photolabile protecting group for constructing photoactivatable systems but also as a bioorthogonal scaffold that enables the inverse electron-demand Diels–Alder (IEDDA) cycloaddition reaction with strained alkynes. In this study, we designed and synthesised a cyclometallated iridium(III) complex modified with a 3-chloro-6-thio-1,2,4,5-tetrazine moiety (1) for cysteine conjugation. The complex was conjugated with an integrin-targeting peptide c(RGDfC) to afford a tumour-targeting conjugate (1-RGD) for bioimaging and photoactivated therapy. An RGD-free analogue (2) was also prepared for comparison studies. Unlike common iridium(III) complexes, excitation of conjugate 1-RGD and complex 2 resulted in weak emission and negligible singlet oxygen (1O2) generation due to the quenching effect of the tetrazine unit. Upon continuous light irradiation, the S,S-tetrazine moiety in conjugate 1-RGD and complex 2 underwent efficient photodissociation, yielding thiocyanate (3) and amide (4) complexes as photoproducts with increased emission intensities and enhanced 1O2 generation efficiencies. Interestingly, the IEDDA cycloaddition reaction of the S,S-tetrazine-containing conjugate 1-RGD and complex 2 with (1R,8S,9s)-bicyclo[6.1.0]non-4-yn-9-ylmethanol (BCN-OH) led to significant emission enhancement. Notably, conjugate 1-RGD showed higher cellular uptake and (photo)cytotoxicity (IC50,dark = 26 μM, IC50,light = 0.08 μM) in U87-MG cells, which overexpress integrin, compared to MCF-7 (IC50,dark = 52 μM, IC50,light = 0.22 μM) and HEK293 cells (IC50,dark > 50 μM, IC50,light = 1.3 μM) with lower integrin levels. This work will contribute to the development of photoactivatable transition metal complexes for cancer-targeted imaging and therapy. © 2025 The Author(s). Published by the Royal Society of Chemistry

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An iridium(iii) 3-chloro-6-thio-1,2,4,5-tetrazine complex for cysteine conjugation, bioimaging and photoactivated therapy

Abstract

Funding

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GRF: Strategic Design of Photofunctional Transition Metal Complexes as Photoinducers to Trigger Cancer-Selective Immunogenic Cell Death

GRF: Targeting N-terminal Cysteine Residues with Photofunctional Transition Metal Complexes for Site-specific Bioconjugation, Live-cell Imaging, and Phototherapeutic Applications

GRF: Modulation of the Luminescence Properties and Photocytotoxic Activity of Transition Metal Complexes through Dissociative Bioorthogonal Reactions for Diagnostic and Therapeutic Applications

Cite this

An iridium(iii) 3-chloro-6-thio-1,2,4,5-tetrazine complex for cysteine conjugation, bioimaging and photoactivated therapy

Abstract

Funding

Publisher's Copyright Statement

RGC Funding Information

UN SDGs

Access Output

Other Access Options

Permanent Link

Other Files and Links

Fingerprint

Projects

GRF: Strategic Design of Photofunctional Transition Metal Complexes as Photoinducers to Trigger Cancer-Selective Immunogenic Cell Death

GRF: Targeting N-terminal Cysteine Residues with Photofunctional Transition Metal Complexes for Site-specific Bioconjugation, Live-cell Imaging, and Phototherapeutic Applications

GRF: Modulation of the Luminescence Properties and Photocytotoxic Activity of Transition Metal Complexes through Dissociative Bioorthogonal Reactions for Diagnostic and Therapeutic Applications

Cite this