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 ((1)O(2)) 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 (1)O(2) 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.