Contents
2 Structural modification of the Ir(Ⅲ)complexes
2.1 Mononuclear Ir(Ⅲ)complex
2.2 Multinuclear Ir(Ⅲ)complex
2.3 Macromolecular⁃Ir(Ⅲ)complex
3 Photophysical properties of Ir(Ⅲ)complexes
3.1 Single⁃photon emission of Ir(Ⅲ)complex
3.2 Two⁃photon emission of Ir(Ⅲ)complex
4 Biotargets for Ir(Ⅲ)complexes
5 Biological applications of Ir(Ⅲ)complexes
5.1 Bioimaging and biosensing
5.2 Antitumor diagnosis and therapy
1 Introduction
Luminescent Ir (Ⅲ) complexes have attracted much attention in the field of luminescent materials because of their tunable excitation/emission wavelength, high quantum yield, large Stokes shift value, long luminescence lifetime and good photostability. Ir (Ⅲ) complexes also show strong cell penetration ability and unique anti-tumor activity, which has attracted wide attention of researchers in the field of biomedicine. Ir (Ⅲ) metal center can form stable complexes with multidentate ligands containing N, O, S, C and other coordination atoms.By modifying and modifying the structure of the ligands, Ir (Ⅲ) complexes have a wide range of applications in organic light-emitting diodes, nonlinear light-emitting materials, biological imaging, photodynamic therapy, sensors, photocatalysis and other fields
[1][2][3][4][5][6]. This paper focuses on the research progress of luminescent Ir (Ⅲ) complexes in the field of anti-tumor.
2 Structural Modification of Iridium (Ⅲ) Complex
Iridium (Ⅲ) complexes are generally six-coordinated with distorted octahedral geometry, and the metal center is often coordinated by bidentate ligands such as C ^ N, N ^ N, O ^ O and tridentate ligands such as N ^ N ^ N, N ^ n ^ C, and many cyclopentadiene derivatives. Iridium (Ⅲ) complexes with various structures can be obtained by structural modification of the main ligand and selection of appropriate auxiliary ligands. In view of the fact that iridium (Ⅲ) complexes show certain luminescent properties in solution, it is necessary to avoid light and carry out the synthesis under the protection of inert gas. When the iridium (Ⅲ) coordination unit is cationic, the synthesis reaction can be completed in a low boiling point solvent at reflux for 4 ~ 6 H, with high yield and convenient purification
[7⇓~9]. However, the preparation of iridium complexes with neutral coordination units generally requires a long reflux reaction (24 ~ 48 H) in a high boiling point solvent, and the coordination reaction is accompanied by redox reactions, resulting in many by-products and cumbersome column chromatography separation of the target products
[10⇓⇓~13]. A large number of Ir (Ⅲ) -containing mononuclear, binuclear and polynuclear complexes and coordination polymers, as well as polymers modified by Ir (Ⅲ) complexes, have been reported in the literature. Due to the limitation of space, only recent typical compounds are selected for description in this paper.
2.1 Mononuclear Ir (Ⅲ) complex
The reported mononuclear Ir (Ⅲ) complexes mainly use cyclopentadiene (Cp*) or phenylpyridine (ppy) derivatives as ligands to construct half-sandwich complexes ( [ (η5⁃Cp*)Ir(L^L)]n+) and cyclometalated complexes ([Ir(N^C)2(L^L)]n+), respectively. The optical properties and biological activities of Ir (Ⅲ) complexes can be adjusted and improved by the rational design of the derivative groups on cyclopentadiene or phenylpyridine or the introduction of auxiliary ligands with specific structures.
Liu et al. Selected triphenylamine-substituted bipyridine as an auxiliary ligand to react with (Cp
*)
2Ir
2Cl
4 to obtain half-sandwich complexes 1.1 – 1.3 (Fig. 1)
[14]. The introduction of triphenylamine group with two-photon absorption (TPA) activity can promote the production of singlet oxygen (
1O
2) and induce apoptosis of Ir (Ⅲ) complexes, and the attachment of methoxy group to the triphenylamine group can also effectively adjust the hydrophilicity and lipophilicity of the complexes, which makes Ir (Ⅲ) complexes useful as photosensitizers for photodynamic therapy (PDT). These complexes can effectively bind BSA and oxidize NADH, and the results of confocal laser microscopy (CLSM) show that the complexes are preferentially targeted to lysosomes.
The mononuclear complex 2 designed by Sadler et al. (Fig. 1) exhibits a broad spectrum of antitumor activity with IC
50 values as low as nanomolar, which is related to its high quantum yield of inducing the production of
1O
2 in tumor cells
[15]. Pyridine, as an auxiliary ligand, can be hydrolyzed and removed, and its hydrolysate can oxidize NADH to form Ir — H, which can catalyze the formation of H
2O
2.
Brabec et al. Reported the effect of the size of the ligand conjugation system on the antitumor activity of complex 3 (Fig. 1), where the activity was strongest when { N, N } was phen and dpq
[16]. The complex showed certain growth inhibitory activity against cisplatin-resistant tumor cells. It has been found that anti-tumor activity is not based on DNA damage, but by preferentially targeting the endoplasmic reticulum and inhibiting protein translation to kill tumor cells. The complexes exhibit good PDT activity, and the IC
50 value can be reduced by two orders of magnitude under visible light.
Mao et al. Prepared mononuclear complex 4 (Fig. 1) by introducing an aldehyde group to phenylpyridine, which can selectively respond to glutamine (Gln) in cells
[17]. The results of density functional theory (DFT) show that Gln interacts with complex 4 through coordination bonds and hydrogen bonds to form a stable complex 5 (Fig. 1). The fluorescence of complex 5 was blue shifted by 82 nm and the luminescence was significantly enhanced compared with that of complex 4. Complex 4 can be used for in situ detection of intracellular Gln.
Metal azacarbene complexes are potential anticancer reagents. Mao et al. Designed and synthesized four mononuclear Ir (Ⅲ) complexes 6.1 ~ 6.4 with stronger cytotoxic activity than cisplatin by using phenylquinoline (or phenylthiophene) and substituted N-heterocyclic carbene as ligands (Fig. 1)
[18]. These complexes can rapidly penetrate the A549 cell membrane and target to the mitochondria. The complex can increase the level of intracellular reactive oxygen species (ROS) and reduce mitochondrial membrane potential, leading to apoptosis. Under the excitation of LED light at 450 nm, the antitumor activity of complex 6.1 against A549 and A549R was increased by 2 orders of magnitude (PI = 500 and 789, respectively), showing obvious PDT activity.
2.2 Polynuclear Ir (Ⅲ) complex
Compared with mononuclear Ir (Ⅲ) complexes, polynuclear Ir (Ⅲ) complexes introduce more ligands and metal centers, and the energy levels and structures of excited States are more complex and diverse, so the photophysical properties of the complexes can be further adjusted and optimized, such as the red shift of the emission wavelength of the complexes, the increase of quantum yield and fluorescence lifetime, and the heteronuclear complexes also show excellent composite properties. At present, the research of polynuclear Ir (Ⅲ) complexes mainly focuses on dinuclear and trinuclear.
Sun et al. Bridged the Ir(N^N^N)
2 or Ir(N^N^C)
2 by fluorene ring (or fluorenyldiyne), and the molar absorption coefficient of the dinuclear Ir (Ⅲ) complex 7 (Fig. 2) in the visible region was significantly increased
[19]. The valence state of the coordination cation is adjusted by adjusting the end capping group, so that the water solubility of the complex is improved; By extending the conjugated system of the bridging group to adjust the internal charge transfer transition (ILCT) of the ligand, the absorption band of the complex can fall into the PDT window range. The authors compared the effect of structure on the luminescence band and lifetime,
1O
2 yield, and PDT activity of the complexes.
Zhang et al. Bridged Ir(ppy)
2 and Sn(Bu)
3 by terpyridine carboxylic acid to obtain heteronuclear complex 8 (Fig. 2), which exhibited multiphoton luminescence (two-photon/three-photon) phenomenon under light excitation in the near-infrared II region
[20]. In vitro and in vivo experiments showed that complex 8 had significant antibacterial activity against Staphylococcus aureus. The results of two-photon CLSM and stimulated emission depletion fluorescence microscopy showed that complex 8 could target tyrosine proteins in the nuclei of Staphylococcus aureus and cancer cells and cause DNA fragmentation.
Teets et al. Prepared a trinuclear complex 9 (Fig. 2) by self-assembly reaction of highly active Ir(C^N)
2(CNAr
dmp)(PF
6) with a bipyridyl platinum alkyne derivative
[21]. Because the weakly coordinated
is easily replaced by free pyridine groups, the reaction can be carried out at room temperature, and the pure product can be obtained by simple filtration, with a yield of up to 94%. The absorption spectra and cyclic voltammetry show that there is a significant electronic interaction between the Ir (Ⅲ) -containing chromophore and the ground state of the Pt (Ⅱ) -containing chromophore in complex 9, and the emission spectra show that there is a significant energy transfer between the Ir (Ⅲ) and Pt (Ⅱ) centers.
Gupta et al. First attached DPA groups to ppy and bpy molecules, respectively, and then reacted the obtained precursor Ir (Ⅲ) complex [Ir(ppy
*)
2(bpy
*)] with Pt(DMSO)
2Cl
2 to prepare IrPt
2-type trinuclear complexes 10 and 11 (Fig. 2)
[22]. The introduction of the Pt-DPA group has little effect on the luminescent properties of the central [Ir(ppy
*)
2(bpy
*)], but is essential for the binding effect of complex 10 with 5 '-GMP.
Chao et al. Designed and synthesized a mitochondria-targeting heteronuclear complex 12 (Fig. 2) by bridging the Ir (III) coordination unit with the terpyridine Pt (II) coordination unit through the imidazole phenanthroline group
[23]. The mitochondrial accumulation rate of 12 is as high as 76%, and it has strong antitumor activity (IC
50=3.3±0.1μmol/L) against cisplatin-resistant cancer cells (A549R). Complex 12 can damage mtDNA, thus destroying the function of mitochondria, resulting in a decrease in mitochondrial membrane potential and ATP depletion. Complex 12-induced necrosis was characterized by the release of LDH, overexpression of RIP3 protein, and lack of activation of typical apoptotic signaling in caspase 3/7.
Lusby et al. Assembled the [Ir(ppy)
2]
+ into a hexanuclear complex 13 via 1,3,5-tricyanobenzene (Fig. 2)
[24]. Six Ir (Ⅲ) metal centers form an octahedral hole, which can encapsulate four pair anions, and these pair anions can exchange with other anions. The luminescent properties of complex 13 are enhanced compared to mononuclear [Ir(ppy)
2(PhCN)
2]
+. Lo et al. Attached the [Ir(N^C)
2] group to the polyoxysilane to prepare the octanuclear complex 14 (Fig. 2)
[25]. Compared with the mononuclear [Ir(N^C)
2(py⁃im)](PF
6), the octanuclear complex exhibits very low cytotoxic activity whether illuminated or not, and is expected to be used as a developing reagent for living cells.
2.3 Macromolecular iridium (Ⅲ) complex
Maggioni et al. Reacted polyamino acid PhenISA containing 6% phenanthroline repeating units with Ir
2(py)
4Cl
2 to obtain Ir (III) -containing coordination polymer 15 (Fig. 3), and the presence of the polymer backbone greatly improved its biocompatibility
[26]. Compared with simple [Ir(ppy)
2(phen)]
+, the emission wavelength of polymer 15 was blue-shifted, and the fluorescence lifetime and phosphorescence quantum yield were increased. 15 can sensitize the formation of
1O
2, but the Φ
1O
2 is only 1/2 of the [Ir(ppy)
2(phen)]
+. Although the IC
50 of 15 in the absence of light is less than that of [Ir(ppy)
2(phen)]
+, the PDT results show that the amount of apoptosis caused by 15 under Xe lamp irradiation is less than that of [[Ir(ppy)
2(phen)]
+. Therefore, linking the PDT photosensitizer containing Ir (Ⅲ) to the polymer chain is an effective strategy to avoid its dark damage to cells.
Wang et al. Prepared a hyperbranched copolymer 16 (Figure 3) with an oxygen-sensitive Ir (III) complex emitting red phosphorescence as the core and an oxygen-insensitive poly (9,9 ′ -dioctylfluorene) emitting blue light as the conjugated skeleton, and then added poly (styrene co maleic anhydride) (PSMA) to the phosphorescent conjugated polymer by coprecipitation to provide negative charges to ensure the water solubility and biocompatibility of the conjugated polymer
[27]. The hyperbranched Ir (III) conjugated polymer 16 shows excellent O
2 sensing activity because the energy can be effectively transferred from the excited state of Ir (III) to the ground state O
2. The accurate response to O
2 does not depend on the concentration of the probe and is not disturbed by the measurement environment, so it can be used as a hypoxic imaging reagent and a photosensitizer for tumor PDT.
Mao et al. modified polydopamine with β-cyclodextrin and oligomeric amino acids to prepare nanoparticles PDA CD RG, in which Ir (III) complex 17 (Fig. 3) was encapsulated to obtain nanocomposite LysoIr @ PDA CD RGD, which can be applied to photothermal combined chemotherapy targeting tumors, as well as in vivo thermal/photoacoustic imaging and two-photon phosphorescence lifetime imaging
[28].
Tang et al. Encapsulated 18 (Fig. 3) with mPEG and PLGA to make nanocapsules with high biocompatibility. The nanocapsules have good single-photon and two-photon imaging properties, can effectively fluorescently label mouse neural stem cells, and can stably track living mice for up to 21 days
[29].
Black phosphorus (BP) -based nanomaterials have shown great potential in tumor therapy, but their poor stability, low X-ray mass attenuation coefficient, and non-selectivity limit their application in radiotherapy. Chen et al. Prepared a two-dimensional layered nanocomposite 19 (Fig. 3) by compounding an unsaturated coordinated iridium complex with BP nanosheets and attaching a tumor-targeting RGD polypeptide to the Ir (III) complex molecule
[30]. RGD Ir @ BP has high biological stability. The Ir (Ⅲ) complex improves the photoelectric properties and photoinduced carrier dynamics of BP, and Ir @ BP can produce more singlet oxygen after X-ray irradiation. In vivo experiments show that 19 can effectively inhibit the growth of nasopharyngeal carcinoma tumors with little side effects. Based on untargeted metabolomics analysis, 19 can specifically down-regulate tumor proliferation markers of prostaglandin E2 in cancer cells.
Chao et al. Treated the surface of black TiO
2 nanoparticles with Ir (III) complex 20 (fig. 3), and then decorated it with tumor cell membrane CCM to construct a functionalized nanoparticle Ir⁃B⁃TiO
2@CCM
[31]. Ir⁃B⁃TiO
2@CCM is selective for tumor cells versus non-tumor cells and accumulates in the mitochondria of tumor cells, showing hierarchical targeting properties. These nanoparticles can effectively generate heat under NIR II light excitation and catalyze the formation of reactive oxygen species under ultrasonic radiation. With the synergistic effect of light irradiation at 1064 nm and ultrasound irradiation, the nanoparticles were able to act as imaging agents to identify tumor sites with high spatial resolution and as therapeutic agents to completely eradicate cervical cancer tumors in mice.
3 Photophysical Properties of Iridium (Ⅲ) Complex
The photophysical properties of Ir (Ⅲ) complexes are closely related to the structure of the ligand molecule. By modifying the ligand structure,Uch as increasing the dimension of the organic ligand molecule, increasing the polarizable degree of the conjugated structure, increasing the symmetry of the molecule, introducing charges into the molecular system, and the like, can finally influence the energy level and the structure of a 3MLCT excited state of a complex, thereby effectively adjusting the luminescent property. Many mononuclear Ir (Ⅲ) complexes have shown good near-infrared absorption and luminescence properties, and it has also been found that quadrupolar, octupolar molecules and even dendritic molecules or coordination polymers have shown more excellent one- or two-photon luminescence properties.
3.1 Single Photon Luminescence Behavior of Ir (Ⅲ) Complex
Teets et al. Used three nitrogen-containing auxiliary ligands with strong π donors to coordinate with the Ir (III) metal center with the main ligands 1 phenylisoquinoline (PIQ) and 2 (2 pyridyl) benzothiophene (BTP), respectively, to obtain six cyclometalated iridium complexes (Fig. 4), in which the quantum yields (Φ
PL) of complexes 21.1(λ
em=637 nm) and 21.2(λ
em=622 nm) are 0.8 and 0.79, respectively, which are significantly higher than those of commercial red luminescent compounds with excellent properties such as fac⁃Ir(PIQ)
3(Φ
PL=0.45) and Ir(TMQ)
2(acac)(Φ
PL=0.55)
[32][33].
The dinuclear Ir (Ⅲ) complex 22 reported by Shafikov et al. (Fig. 4) exhibits dual emission: NIR phosphorescence (λ
max=714 nm) and green fluorescence (λ
max=537 nm)
[34]. The strong NIR phosphorescence originates from the triplet-bridged ligand (
3LC) with a phosphorescence efficiency of Φ
PL=3.5%. The weak green fluorescence then originates from the lowest excited singlet state (
1LC). The fluorescence generation is caused by the intersystem crossing (ISC) of
1LC to
3LC. The large energy difference (0.57 eV) between the two excited States makes the ISC relatively slow, τ
ISC=2.1 ps. The complex could be taken up into vesicles near the endoplasmic reticulum by HeLa cells, and no enrichment of the complex was detected in the nucleus.
Sun et al. Designed carbazole-bridged quinoline derivatives, which showed strong luminescent properties, and used them as ligands to synthesize mononuclear, binuclear and trinuclear Ir (III) complexes 23.1 ~ 23.5 (Fig. 4)
[35]. The absorption peaks below 400 nm in the absorption spectrum are attributed to the π/π transition of the ligand, and the 400 ~ 700 nm peaks originate from the charge transfer transition (
1CT). The size of the central carbazole derivative π system, the number and orientation of the Ir (Ⅲ) -containing coordination unit can significantly affect the position and intensity of the
1CT absorption peak. The peak emission wavelength of complexes 23.1 ~ 23.3 in CH
2Cl
2 is located at 710 nm, and the NIR emission originates from the excited state of
3MLCT/
3LLCT, while the NIR emission of complexes 23.4 ~ 23.5 originates from the excited state of
3ILCT/
3LMCT. The transient absorption spectra of the triplet States of complexes 23.1 ~ 23.3 and 23.4 ~ 23.5 are also different. These differences are mainly related to the π system of the carbazole derivative which plays a bridging role in the center of the complex molecule, and there is no weak interaction between the Ir (Ⅲ) coordination units.
3.2 Two-Photon Luminescence Behavior of Ir (Ⅲ) Complex
Gao et al. Prepared Ir (Ⅲ) complexes 24.1 – 24.4 (Fig. 4) conveniently and efficiently by using phenanthroline diketone as a ligand to react with Ir
2(ppy)
2Cl
2[36]. The luminescence of the complex originates from the
3MLCT/
3LLCT excited state when R = H, and from the
3LL excited state when R is an electron-withdrawing or electron-donating substituent. The two-photon absorption cross sections (σ
2) of these compounds were measured to be 113 – 168 GM using rhodamine RB as a reference. The quantum yield of the triplet excited state of complex 24.3, which is connected with a strong electron-donating group, is high, and the quantum yield of the induced
1O
2 is also higher, reaching 0.95. The cytotoxic activity of the complex is very weak in the absence of light, and the complex can induce obvious apoptosis in the presence of light. The PI (ratio of phototoxicity to dark toxicity) of two-photon irradiation was significantly higher than that of single-photon irradiation, and the PI of complex 24.3 on A549 cells increased from 179(λ
ex=450 nm) to 253(λ
ex=850 nm).
Chao et al. Reported a two-photon luminescent Ir (III) complex 25 (Fig. 4). The morpholine group attached to the phenylpyridine allows 25 to be effectively targeted and aggregated in lysosomes
[37]. 25 can be used as a NO capture indicator, and the emission intensity of 25 at 587 nm gradually increased to about 350 times with the increase of NO concentration, and the phosphorescence quantum yield increased from 0. 006 to 0. 079. After 25 reacted with NO, the σ
2 increased to 60 GM, and the phosphorescence lifetime also increased from 280.2 ns to 617.63 ns.
Wang et al. Synthesized Ir (Ⅲ) complexes 26.1 and 26.2 with tpy and tpy as ligands, respectively (Fig. 4)
[38]. The maximum σ
2 value of the tpy xpy ligand is 448 GM, and 26.1 has similar two-photon absorption properties to the tpy xpy ligand, but the σ
2 value of 26.2 is only about 5 to 20 GM. Benefiting from the "ping-pong" energy transfer mechanism, 26.1 has a long triplet lifetime (τ = 15 μs) and a
1O
2 quantum yield of 0.98 in CH
3OH, much higher than 0.26 of 26.2 (τ = 65 ns).
Ir (Ⅲ) complex 27 reported by Chao et al. (Fig. 4), not only has a high two-photon absorption cross-section (σ
2=101 GM), but also shows a certain three-photon absorption (3PA) effect (σ
3=2.63×10
-7-78cm
6·s
2·photon
-2-2)
[39]. Complex 27 can be used as a three-photon contrast agent for CLSM and lifetime imaging of mitochondria. Tissue model, in vitro tissue and in vivo model were used for comparison. Compared with two-photon excitation (2PE), three-photon excitation (3PE) imaging has higher spatial resolution, deeper tissue penetration, less defocus interference, lower imaging working concentration (50 nmol/L), faster absorption rate (5 min) and lower three-photon excitation power (0. 5 mW).
4 Biological Targets of Iridium (Ⅲ) Complex
Ir (Ⅲ) complexes can be used in bioimaging, biosensing, cancer therapy and other fields, based on their ability to be ingested by cells and to interact with each other.
4.1 Cell membrane
Lipophilic macromolecular cationic Ir (Ⅲ) complexes can act on the cell membrane. The negatively charged cell membrane attracts the coordinating cation, and the lipophilic ligand facilitates the interaction of the complex with the hydrophobic phospholipid bilayer. The attached macromolecular group can prevent the complex from diffusing to the cytoplasm, and the introduction of membrane protein substrates into the ligand is an effective strategy
[40].
4.2 Cytoplasm
Small molecular Ir (Ⅲ) complexes with good water solubility are easier to be located in the cytoplasm, and the introduction of sulfonate, carboxylate, quaternary ammonium salt, or polar long chain or sugar-like group containing polyhydroxyl on the complex molecule can greatly improve its water solubility, showing cell membrane permeability and cytoplasmic specific staining
[41].
Yoshihara et al. Attached arginine peptides of different lengths to Ir (Ⅲ) complexes and synthesized complexes 28.1 and 28.2, respectively (Fig. 5)
[42]. When 28.1, 28.2 and BTQphen were incubated with HeLa cells at 37 ℃ for 2 H, the fluorescence intensities of 28.2 and 28.1 under 500 nm excitation were 5. 5 and 7. 2 times higher than those of BTQphen, respectively, indicating that the introduction of arginine peptide could significantly improve the cellular uptake efficiency of the complexes.
4.3 Organelle
4.3.1 Mitochondria
Mitochondria are intracellular energy factories and the production sites of reactive oxygen species (ROS), which regulate intracellular redox, cell death, anabolism and so on. Mitochondrial fission and fusion control the shape, size and function of mitochondria in cells, and disruption of this process may lead to serious diseases such as Parkinsonism, Alzheimer's disease, metabolic diseases and cancer. There is a transmembrane potential between the inner and outer membranes of mitochondria, and the outer membrane is electronegative. Lipophilic cationic Ir (Ⅲ) complexes are easy to accumulate in mitochondria and interfere with mitochondrial function.
DCA can effectively inhibit mitochondrial pyruvate dehydrogenase kinase PDK1 and restore mitochondrial oxidative phosphorylation (OXPHOS) function in cancer cells. Chao et al. Prepared DCA-Ir (Ⅲ) complex 29 (Fig. 5) by linking DCA to Ir (Ⅲ) complex through a flexible chain, and its σ
2 value at 720 ~ 750 nm was as high as 159 GM
[43]. Co-staining with MTG showed that complex 29 preferentially accumulated in mitochondria with a Pearson coefficient (PCC) of 0.85. Complex 29 selectively changes the metabolic process of cancer cells through good mitochondrial targeting, and shows a certain two-photon PDT effect in hypoxic environment.
Chao et al. Designed [Ir(N^N^N)(C^N)Cl](PF
6) complex 30 with two-photon luminescence properties (σ
2=400 GM) and good photostability (Fig. 5)
[44]. Complex 30 exhibited excellent mitochondrial targeting, with PCC of about 0.89 ~ 0.94 in A549, AGS, DU145, PC ³ 3, SGC ³ 7901, and SHSY5Y cells. Rapamycin was used to accelerate mitochondrial fission and fusion in HeLa cells and C. elegans, and CLSM was used to track mitochondrial dynamics in real time by using complex 30 as a two-photon phosphorescence probe.
Chao et al. Reported an AIE active Ir (III) complex 31 (fig. 5) with a fluorescence quantum yield of about 0.001 in DMSO, but a quantum yield increase of about 40 times in a mixed solvent of H
2O∶DMSO=9∶1
[45]. The σ
2 in the range of 720 ~ 850 nm can reach 214 GM. Inductively coupled plasma mass spectrometry (ICP-MS) and CLSM were used to track the cellular uptake of the complex in real time, and about 80% of it was concentrated in mitochondria. The complexes exhibit good one-photon and two-photon PDT activity, and the activity of the TPA⁃PDT(IC
50=0.35μmol/L,PI=110) is better than that of the OPA⁃PDT(IC
50=0.40μmol/L,PI=75). Chao et al. Also synthesized a series of mitochondria-targeted photosensitizers with triazine derivatives as the main ligands, among which complex 32 (Fig. 5) showed strong phototoxicity to cisplatin-resistant cancer cell A549R, with a PI value of 12.29
[46].
Liu et al. Prepared a series of Ir (Ⅲ) complexes 33.1 ~ 33.3 using dipyriphenazine derivatives as ligands (Fig. 5)
[47]. The complex can significantly increase the accumulation of intracellular ROS, reduce mitochondrial membrane potential, increase the permeability of mitochondrial membrane, impair the normal function of mitochondria, and further inhibit the synthesis of ATP. Complexes 33.1 ~ 33.3 had a different mechanism of cell death induction from cisplatin. Compared with cisplatin inhibiting S phase cell growth, complexes 33.1 ~ 33.3 induced cell cycle arrest at G0/G1 phase. Compared with the control group of 67.4%, the percentage of G0/G1 phase increased by 9.8%, 9.4% and 6.8%, respectively.
4.3.2 Lysosome
Lysosomes are intracellular acidic organelles, and drugs targeting lysosomes can affect the integrity of lysosomes, leading to lysosomal membrane permeabilization and cell death. The pH of the lysosome is significantly lower than that of the cytoplasm, and weakly basic molecules tend to target the lysosome. The Ir (Ⅲ) complex with polypyridine ligand has a protonated N atom, which enhances the water solubility of the complex and promotes the aggregation of the complex in lysosomes.
The Ir (Ⅲ) complex 34 (Fig. 5) with β carboline alkaloid ligand reported by Mao et al. Has high photostability and superior two-photon performance
[48]. The NH site on the indole ring of the ligand can be protonated or deprotonated, which makes the phosphorescence of complex 34 pH-responsive. Complex 34 is capable of specific labeling of lysosomes. The complex can induce autophagy, but not apoptosis. Complex 34 can induce autophagy while tracking the changes of lysosomes, and can be used as a diagnostic reagent to observe the fusion of autophagosomes and lysosomes in real time.
Chao et al. Synthesized a basic NIR luminescent Ir (III) complex 35 (Fig. 5), which has good cell permeability and low toxicity
[49]. ICP and MS data showed that complex 35 was mainly distributed in lysosomes. The complex can react with peroxynitrite ONOO
- by nucleophilic substitution, and complex 35 can effectively respond to ONOO
- within 200 s, which can be used to detect intracellular ONOO
-. Chao et al. Prepared a stable Ir (III) complex 36 containing a naphthalene ring polypeptide (Fig. 5). 36 was transported into the lysosome by endocytosis. The acidic environment in the lysosome changed the morphology of 36 from a dispersed nanostructure to a nanostructure aligned by π-π interaction, which showed aggregation-induced luminescence (AIE) at very low concentrations
[50]. 36 in lysosomes showed higher photostability under continuous 488 nm laser irradiation. In continuous cell culture, 36 is passed through more than 15 generations in daughter cells, has a lysosomal high labeling rate, and can be used for lysosomal high-resolution long-term follow-up imaging.
4.3.3 Endoplasmic reticulum
ER is an important organelle, and subtle perturbation of its redox signaling pathway may contribute to ER stress-induced apoptosis. Chao et al. Designed a { N, C } ligand with a gradually expanded conjugated system, which was used as the main ligand to mix with Ir _ tpy to obtain a series of ER-targeted Ir (Ⅲ) complexes 37.1 ~ 37.3 (Fig. 5)
[51]. These complexes can accumulate in ER and effectively induce apoptosis after PDT treatment through ER stress mechanism. The quantum yield and cytotoxicity of
1O
2 increase with the extension of { N, C } ligand conjugation system.
4.3.4 Lipid droplet
Lipid droplets are organelles coated by a monolayer of phospholipids for storing cellular fat, which are associated with obesity, Alzheimer's disease, type II diabetes, cardiovascular disease and so on. Because the lipid droplet is a hydrophobic region, most of the Ir (Ⅲ) complexes located in the lipid droplet are lipophilic and electrically neutral. Mao et al. Synthesized Ir (III) complex 38 (Fig. 5), which has two-photon luminescence properties and targets lipid droplets, and used two-photon phosphorescence lifetime imaging to study lipid droplets in living cells: the accumulation process of lipid droplets in cells was monitored, and the complex was used to observe the metabolism process of fat in zebrafish larvae within 7 days
[52].
5 Biological Application of Iridium (Ⅲ) Complex
Luminescent Ir (Ⅲ) complexes have the advantages of good photostability and strong cell penetration ability, and have potential applications in the fields of developing reagents, biosensors, anti-tumor drugs and so on.
5.1 Application of Ir (Ⅲ) Complexes in Bioimaging and Probe Sensing
Intracellular microenvironmental parameters, including hypoxia, pH, polarity, viscosity, and temperature, play a key role in controlling the biophysical or chemical behavior of local molecules. Abnormal changes in these parameters can lead to cell dysfunction, which is a sign of some serious diseases. Some Ir (Ⅲ) phosphorescent complexes can be used as response probes of biological microenvironment parameters for the diagnosis and treatment of cancer, diabetes and other diseases
[53]. He et al. Reported an Ir (III) complex-based dual-mode imaging probe 39 (Fig. 6), which can simultaneously and synergistically sense acidity and hypoxia
[54]. Through the synergistic sensing of acidic pH and hypoxia, a stronger luminescence signal or a longer lifetime can be obtained than that caused by a single factor (acidity or hypoxia). The complex has good optical stability and low phototoxicity, and the synergistic enhancement effect is remarkable when 39 is used for acidic and hypoxic phosphorescence imaging in living monolayer cells and three-dimensional multicellular spheroids. It has important application value in tumor monitoring, image-guided surgery and metastasis diagnosis.
Ir (Ⅲ) complexes with phosphorescent properties have large Stokes shift, long phosphorescent lifetime and strong photobleaching resistance, and their emission wavelength can be adjusted from visible light to near-infrared region by controlling the ligand structure. The excellent luminescent properties of Ir (Ⅲ) complexes make them widely used in cell imaging, signal transduction, biomarkers and other fields. With the development and application of two-photon CLSM technology, Ir (Ⅲ) complexes have been widely used in the field of living tissue and 3D bioimaging. The Ir (Ⅲ) complexes 40.1 and 40.2 (Fig. 6) reported by Chao et al. Have σ
2 values of 115 and 70 GM at 750 nm, respectively, and can image a 3D tumor ball model in a depth of 200 μm under the excitation of near-infrared femtosecond laser, showing good biological tissue penetration
[55].
In addition to phosphorescent probes for proteins, nucleic acids and other biological macromolecules, it is also of great significance to develop biological probes for the detection of reactive oxygen species (ROS), small signaling molecules and metal ions, because their concentration levels, spatial and temporal distribution and other characteristics can not only reflect the physiological and pathological status of cells, but also play an important role in regulating cell functions.
Chao et al. Designed and synthesized dinuclear complex 41 (Fig. 6) by connecting bipyridines through azo bonds to obtain azobpy
[56]. The azo double bond in complex 41 has an electron-withdrawing effect, which can quench the fluorescence of the complex, so that the complex solution can only emit weak phosphorescence. The addition of
and
ions makes the azo double bond structure of complex 41 destroyed, and the phosphorescence of the complex is significantly enhanced, showing a "phosphorescence opening" effect.
can be produced in vivo by thiosulfate sulfurtransferase (TST), and complex 41 can be used as a fluorescent imaging reagent for TST in cells. Chao et al. Further reported a dinuclear complex 42 with good two-photon luminescence properties (Fig. 6), and the two-photon CLSM results showed that the complex targeted to mitochondria and could image SO
2 derivatives in vivo and in vitro
[57]. The authors used two-photon excitation real-time imaging to observe sulfite ions generated in situ in nematodes, and also used the complex to observe the distribution of sulfite ions in the brain and other tissues of mice.
The addition of H
2S to the solution of complex 43 (Fig. 6) reported by Li et al. Can significantly enhance its luminescence intensity, and the response of 43 to H
2S has strong selectivity and sensitivity
[58]. The toxicity of 43 to HeLa cells was negligible, and it was mainly distributed in the mitochondria. The authors successfully used 43 to image the exogenous and endogenous H
2S in the cells, and the complex was also used to image the exogenous H
2S in the legs of mice.
Chao et al. Reported that the mitochondria-targeted antitumor prodrug complex 44.1 (Fig. 6) can undergo dissociation in the presence of H
2O
2 to yield 44.2 (Fig. 6), which has reduced luminescence, as well as methylquinone (QM)
[59]. By tracking the change of the emission spectrum of the complex, quantitative response and real-time imaging analysis of H
2O
2 molecules rich in tumor cells can be carried out. 44.2 As a photosensitizer of two-photon PDT, QM can disrupt the redox homeostasis in mitochondria by scavenging glutathione. Cancer cells are more susceptible to oxidative stress caused by two-photon photodynamic therapy based on the reduction of glutathione.
Pu et al. Designed and synthesized a thiophenol probe 45 based on a red luminescent Ir (III) complex (Figure 6). The [Ir(ppy)
2Phen]
+ on the probe molecule is used as a phosphorescent emission group, the 2,4-dinitrobenzenesulfonyl (DNBS) group is used as a recognition site, and the polyether group introduced to the ligand molecule can greatly improve the water solubility of the complex molecule
[60]. Due to the strong electron-withdrawing ability of DNBS, the probe molecule showed very weak phosphorescence at 600 nm. The addition of thiophenol can break the sulfonamide bond on 45 molecule, and the obvious red phosphorescence at 600 nm can be observed after the departure of DNBS group. The phosphorescent thiophenol probe has a large Stokes shift (180 nm) and phosphorescence lifetime (τ = 5. 265 μs), and the detection limit is LOD=8.68×10
-7mol/L, which can be used for the quantitative detection of thiophenol in real water samples and human serum with high recovery.
Ir (Ⅲ) complex 46 (Fig. 6) designed and synthesized by Pu et al. Can be used for the detection of
and
. The compound has no emission peak at 600 nm. When
and
are added, they can interact with the probe molecule through hydrogen bonding, thus increasing the luminescence intensity of the complex by about 10 times
[61]. Probe 46 has short response time (< 20 s), good stability and low detection limit (
, 1.51×10
-7mol/L,
, 3.95×10
-8mol/L). The author made the probe into a test paper, which can visually detect the
and
in real samples with high accuracy.
Peroxynitrite (ONOO
-) is one of the reactive oxygen/nitrogen species (ROS/RNS) found in the body and plays a crucial role in many physiological and pathological processes. Fan et al. Developed NIR phosphorescent Ir (III) complex 47 (fig. 6), which can detect ONOO
- in a few seconds
[62]. 47 showed excellent selectivity to ONOO
- in the presence of
, ClO
- plasma. The luminescence lifetime of 47 is long, and the background light noise interference can be eliminated by time-resolved spectroscopy. Because of its low cytotoxicity, the authors utilized probe 47 to successfully detect ONOO
- in live cells and in a mouse model of inflammation.
Pikramenou et al. Introduced two boronic acid groups to the phenylpyridine ligand, and prepared Ir (III) complex 48 (Fig. 6), which can recognize both photoluminescence and electroluminescence of monosaccharide molecules
[63]. 48 exhibited orange luminescence at 618 nm, and the luminescence intensity decreased by 70% and 40% when combined with fructose and glucose, respectively. The uptake of 48 in HeLa cells was detected by inductively coupled plasma optical emission spectrometry (ICP-OES), and confocal imaging experiments showed that 48 was suitable for the recognition and analysis of monosaccharides in cells.
5.2 Application of Ir (Ⅲ) complexes in antitumor therapy
Ir (Ⅲ) complexes show certain tumor cell growth inhibitory activity, and are very promising new anticancer reagents to replace platinum-based drugs. Ir (Ⅲ) complexes have the characteristics of high stability, good water solubility, easy synthesis and structural modification, so researchers can improve the anti-tumor activity of Ir (Ⅲ) complexes by fine-tuning their structures. Ir (Ⅲ) complexes exhibit strong phosphorescence effect, long phosphorescence lifetime and oxygen sensitivity, which are favored in the research and development of PDT photosensitizers.
5.2.1 Antitumor activity of Ir (Ⅲ)
Mao et al. Introduced a coumarin group into Ir (III) complex 49 (fig. 7), which showed stronger anti-proliferative activity than cisplatin for a variety of cancer cells 49, and the complex could also overcome the resistance of cancer cells to cisplatin
[64]. Complex 49 exhibited strong phosphorescence emission, and the CLSM results indicated that the complex was able to effectively penetrate into HeLa cells. Real-time monitoring of mitochondrial morphology showed that the complex could induce mitochondrial damage and kill cancer cells. Metformin can relieve cell hypoxia by effectively inhibiting mitochondrial respiration, activating the expression of pAMPKa and inhibiting the expression of HIF-1a. Mao et al. Coupled the Ir (III) coordination unit with metformin to prepare complex 50 (Fig. 7), which can effectively induce apoptosis of tumor cells even under hypoxic conditions
[65]. The cytotoxicity against hypoxic cancer cells A549, HeLa and MCF 73,50 was about 10 times higher than that of Ir (Ⅲ) complexes without metformin group and cisplatin. In addition, 50 showed some antimetastatic and anti-inflammatory activities.
Huang et al. Reported a water-soluble Ir (III) complex 51 with a coumarin group (Fig. 7), and co-staining experiments showed that the complex was mainly localized to lysosomes and mitochondria of tumor cells
[66]. 51 has good photocatalytic oxidation of NADPH and amino acids through a single electron transfer mechanism. Light-induced intracellular redox imbalance and changes in mitochondrial membrane potential lead to necrosis and apoptosis of tumor cells. 51 showed low toxicity (IC
50>100μmol/L) to normal cells in the dark, and very high phototoxicity to the sorafenib-resistant cancer cell line HepG2 SR and the cisplatin-resistant cell line A549R, with PI values greater than 100. 51 also showed high biocompatibility and photocatalytic anticancer effect in zebrafish and mice.
Chao et al. Reported an Ir (III) complex 52 (fig. 7) capable of producing an immunomodulatory response via immunogenic cell death (ICD), which targets the ER, triggering ER stress and inducing ROS production, thereby causing severe intracellular damage
[67]. 52 showed high cytotoxicity to both A549 lung cancer cells as well as cisplatin-resistant A549R cells and induced multiple features of ICD (i.e. Surface exposure of CRT, extracellular release of HMGB1 and ATP). Vaccination of immunocompetent mice with 52-treated dead cells caused an antitumor CD8
+T cell response and depletion of Foxp3
+T cells, which ultimately resulted in long-lasting antitumor immunity through activation of the ICD in lung cancer cells. 52 has the dual effects of ICD and chemotherapy, and is a potential anti-tumor drug.
Guo et al. Prepared a series of half-sandwich Ir (Ⅲ) complexes with phosphinimine derivatives as ligands, and these complexes showed good anticancer activity against A549 cells, which was generally higher than that of cisplatin drugs
[68]. After replacing the cyclohexyl group in the ligand imine with a phenyl group, the resulting complex 53 (Fig. 7) had the highest anticancer activity (IC
50=4.7μmol/L) against A549 cells. Complex 53 can target and destroy the lysosome of A549 cells, resulting in cell death.
Liu et al. Designed and reported three Ir (Ⅲ) complexes 54.1 – 54.3 (Fig. 7), and found that the complexes could effectively inhibit the growth of tumor cells A549, HepG2, SGC 7901, BEL 7402 and normal cells NIH3T3 by detecting the expression of ROS, [Ca
2+], mitochondrial membrane potential, cytochrome C and Bcl 2 protein in cells
[69]. In vivo experiments showed that the growth inhibition rate of A549 cells by complex 54.3 was 76. 34%, and the ROS-mediated mitochondrial and lysosomal dysfunction eventually led to the apoptosis of A549 cells.
Liu et al. Encapsulated complex 55 (fig. 7) with poor cytotoxic activity with liposomes to obtain Ir1LiPo
[70]. The anti-tumor activity of Ir1LiPo was significantly enhanced due to the transmembrane advantage of liposome, and the IC
50 of Ir1LiPo was as low as 2. 6 μmol/L for BEL 7402 tumor cell line. After incubation of Ir1Lipo with EL 7402, the cell cycle distribution was detected by flow cytometry, and the expression of Caspases and Bcl 2 protein was detected by Western Blot. Combined with the increase of intracellular ROS and [Ca
2+] and the decrease of mitochondrial membrane potential, the authors believe that the pathway of apoptosis is through mitochondrial function damage.
Kozie et al. Studied the interaction of Ir (III) complexes 56.1 ~ 56.4 (Fig. 7) containing fluoroquinolone groups with DNA, albumin, and apotransferrin
[71]. These complexes can be enriched to the nucleus, generate free radicals, activate Caspase 3/7 activity, induce cell cycle changes leading to G2/M phase arrest, and induce apoptosis. Although the Ir (Ⅲ) core coordination unit in complexes 56.1 ~ 56.4 can partially intercalate into the minor groove of DNA, and the quinolone group in the ligand molecule also intercalates between the DNA base pairs through a certain π-π interaction, these two interactions are not enough to cause the dissociation of the DNA double helix. The results of fluorescence titration and molecular chimerism simulation showed that complexes 56.1 ~ 56.4 could also bind to tryptophan residues in HSA binding site I (subdomain II A), and could be chimerized into the binding site of apolipoprotein Tf, including binding to nearby tyrosine residues.
Ye et al. Connected the Ir (III) coordination unit with artesunate, which has good anti-tumor activity, and obtained three complexes 57.1 ~ 57.3 (Fig. 7), which can effectively inhibit the growth activity of liver cancer cells
[72]. Imaging results showed that the complex was localized to the mitochondria of HepG2 cells and induced a series of mitochondria-mediated apoptotic events. These complexes can also regulate the cell cycle and inhibit the metastatic cells of HepG2, and are expected to become candidate drugs for the treatment of liver cancer in the future.
Liu et al. Introduced a triphenylamine group into the cyclometalated iridium complex to improve its lipid solubility and antitumor activity, in which the IC
50 value of complex 58 (Fig. 7) against A549 cells was 4.34 μmol/L
[73]. These complexes have dual anticancer mechanisms (metastasis inhibition and lysosome damage): the complexes are transported by serum albumin and electrostatic force quenching mechanism, and can effectively prevent the metastasis of cancer cells, disrupt the G0/G1 phase of the cell cycle, and induce apoptosis; The complex effectively accumulates in lysosomes through an energy-dependent cellular uptake mechanism, inducing lysosomal damage and ultimately cell death.
Liu et al. Introduced the rhodamine group into the half-sandwich iridium complex, and the obtained complex 59 (Fig. 7) had strong anticancer activity against A549, HeLa, and HepG2 cancer cells
[74]. The complex enters the lysosome of the cell through an energy-dependent mechanism, causing lysosomal damage. The anti-metastasis experiment proves that the complex has the ability to prevent the migration of cancer cells. Although complex 59 did not show a greater ability to interfere with the coenzyme NAD
+/NADH pair by transfer hydrogenation, the intracellular ROS content was clearly increased. ROS regulation increases NF κB activity, and the ROS NF κB signaling pathway further induces apoptosis. In addition, cell flow experiments also showed that complex 59 blocked the cell cycle in S phase, but the complex did not cause a significant change in mitochondrial membrane potential.
Liu et al. Introduced tetraphenylethylene group into Ir (III) coordination unit, and the obtained complexes 60.1 ~ 60.4 (Fig. 7) showed a certain selectivity to cancer cells and normal cells, in which the IC
50 value of complex 60.1 to A549 cancer cells could reach 3.56 μmol/L
[75]. Complexes with different structures have different anti-tumor mechanisms: complexes 60.1 and 60.2 can not only catalyze the conversion of NADH to NAD
+(TONs:43.2,11.9), but also induce ROS enrichment. Complexes 60.3 and 60.4 can target lysosomes (PCC is 0.73) and induce lysosomal damage.
5.2.2 Photodynamic therapy
Photodynamic therapy (PDT) is a new approach in the field of anticancer research, and photosensitizer is the core element of PDT. Improving the targeting selectivity and low dark toxicity of photosensitizer is the focus of PDT research. Ir (Ⅲ) complexes are often used as PDT photosensitizers because of their long-lived 3MLCT excited state and strong ability to produce 1O2 under hypoxic conditions.
Natrajan et al. Used complex 61 (Fig. 8)(σ
2=45 GM) for two-photon PDT studies
[76]. In vitro experiments showed that C6 glioma cells were significantly damaged when they were incubated with the complex and irradiated with 740 nm two-photon femtosecond pulse laser, while the cells that were not incubated with the complex in the control experiment did not change. The change of cell morphology is related to the production of
1O
2 induced by the complex.
Gao et al. Designed and synthesized a mitochondria-targeted Ir (III) complex 62 (Fig. 8), which has very low dark toxicity
[77]. After incubation for 48 H, complex 62 showed strong growth inhibitory activity against A549, MCF7 and 22Rv1 tumors under 450 nm light excitation, with a IC
50(A549)=1.35μmol/L,PI=179, which was better than that of the clinically used single-photon PDT photosensitizer 5 ALA. Due to the better tissue penetration of two-photon laser, complex 62 showed stronger phototoxicity under 800 nm laser irradiation, IC
50(A549)=0.96μmol/L,PI=253. The DNA-binding ability of the complex is weak, and its antitumor activity is related to the high
1O
2 quantum yield. Complex 62 can not only be used as a two-photon mitochondrial dye, but also be expected to be a two-photon PDT photosensitizer with excellent performance.
Sun et al. Reported that the dinuclear terpyridine iridium complex 63 (Fig. 8) containing a terminal pyrene group exhibited strong phototoxicity and relatively weak dark toxicity
[78]. The complex can generate ROS in cells, damage the mitochondrial and lysosomal functions of human lung cancer cells A549 and human skin cancer cells A431, and cause cell necrosis in the form of apoptosis.
Lim et al. Synthesized an ER-localized Ir (III) complex 64 (fig. 8) as a PDT reagent
[79]. Complex 64 can exhibit phosphorescent emission under two-photon excitation. Complex 64 has a very strong ability to produce ROS, and can effectively kill cancer cells at low concentrations (≤ 2 μmol/L) and weak light energy (≤1 J·cm
-2). The damage mainly appears in the proteins near the endoplasmic reticulum. It can be found that the complex induces protein cross-linking and protein oxidation in cells by mass spectrometry. Zhao et al. Reported the complex 65 with anthraquinone group (Fig. 8), whose energy gap is between 1.63 and 2.21 eV, which is well matched with the O
2[80]. The highest occupied molecular orbital (HOMO) energy of complex 65 is less than − 5.51 eV, which shows a high
1O
2 quantum yield (Φ
1O
2=0.822) both in solution and in living cells. The complex was targeted to mitochondria with PCC of 0.95. As a PDT photosensitizer, 65 has shown good therapeutic effects not only under normoxic conditions but also under hypoxic conditions.
The efficacy of PDT usually depends on the local concentration and diffusion of oxygen. Oxygen-independent photosensitizers are in high demand for cancer therapy due to the hypoxic microenvironment present in solid tumors
[81]. Chao et al. Reported an Ir (Ⅲ) complex 66 (Fig. 8) with an anthraquinone terpyridine derivative as the main ligand, which exhibited significantly enhanced luminescence properties after reduction by reductase under anoxic conditions and high stability under two-photon photoexcitation of λ
ex=730 nm
[82]. After irradiation, the reduced form of complex 66 (red) can induce the production of carbon radicals, leading to a decrease in mitochondrial membrane potential and cell death (IC
50=2.1μmol/L under light excitation, IC
50=58.2μmol/L,PI=27.7 in the dark). After injection of A549 cancer cells into male mice, the body weight of the mice did not change significantly after 2 days of two-photon excitation irradiation (730 nm, 50 mW, 1 kHz, pulse width 35 fs, 5 s/mm), and the tumor tissue in the mice decreased significantly after 14 days. Complex 66 has less toxic and side effects, and has a very good application prospect in the treatment of anaerobic tumors by two-photon PDT.
Ma et al. Coprecipitated the fluorescence-quenched Ir (III) complex 67 (Fig. 8) with polystyrene (PS) grafted and copolymerized with polyethylene glycol (PEG) to prepare water-soluble Ir NPs with greatly enhanced luminescent properties
[83]. IrNPs at a very low concentration (1. 6 μg/mL) can achieve significant PDT efficacy on MCF 7 breast cancer cells under low light intensity (5 mW/cm
2) white light irradiation, and the PDT efficiency is significantly enhanced compared with that of complex 67. The distribution of Ir and NPs in mitochondria was tracked by CLSM, and the nanoparticles could induce ROS in cells, causing mitochondria-mediated apoptosis and cell necrosis.
Ir (III) complex 68 (Fig. 8) reported by Mao et al. Has both mitochondrial and nuclear targeting. The complex can first target mitochondria, induce ROS production under light, damage mitochondria, and then transfer the molecule to the nucleus to further destroy nucleic acids
[84]. DNA photocleavage experiments revealed that 68 induced significant cleavage of DNA superhelix structure under light irradiation. 68 has low dark cytotoxicity to a variety of tumor cell lines, while the IC
50 is significantly reduced under light irradiation, indicating that 68 can be used as a multi-target PDT photosensitizer.
He et al. prepared Ir (III) complex 69 (Fig. 8) by derivatizing benzothienylisoquinoline (btiq) and imidazole-phenanthroline (ipt) as ligands. As a PDT photosensitizer, 69 can induce significant iron death in hypoxic MCF 7 tumor cells under light excitation, which is manifested by the accumulation of lipid peroxidation, mitochondrial contraction, glutathione peroxidase down-regulation, and ferristatin-inhibited cell death
[85]. Mitochondria-targeted (PCC value of 0.84) Ir (III) complex 70 (fig. 8) was obtained by modifying the triphenylphosphine group on ipt. Under light excitation, 70 caused a decrease in mitochondrial membrane potential, inhibition of ATP production, and induction of apoptosis. Due to the dual synergistic effects of iron death and apoptosis, 70 showed superior PDT performance than 69. Complex 70 showed potent PDT activity against 3D multicellular spheroids (MCSs) of refractory tumor cell lines such as triple-negative MDA ³ MB ³ 231 cells, hypoxia-resistant PANC ³ 1 cells, and MCF ³ 7 cells.
6 Conclusion and prospect
Iridium (Ⅲ) complexes developed in recent years have shown unique antitumor activity in vitro or in vivo, and many of them are significantly more active than cisplatin drugs. At present, most researchers choose cyclopentadiene, phenylpyridine and terpyridine as the main ligands, modify them with appropriate groups to improve the water solubility and biocompatibility of iridium (Ⅲ) complexes, and attach targeting groups to the complexes to promote the complexes to enter specific sites (mitochondria/lysosomes/endoplasmic reticulum and other organelles) in tumor cells. In view of the luminescence effect of iridium (Ⅲ) complexes, researchers have modified the structure to improve the luminescence quantum yield and luminescence lifetime of the complexes, and extended the size of the conjugated system of the ligand to enable the complexes to emit in the near infrared, and some complexes also show good two-photon absorption/emission properties, even three-photon emission. Luminescent iridium (Ⅲ) complexes can not only be used as probes and detectors for intracellular ions and signal molecules, but also be applied to live cell imaging, and the distribution of iridium (Ⅲ) complexes in cells can be studied by laser confocal microscopy. Luminescent iridium (Ⅲ) complexes have incomparable advantages in the "visual" study of the anti-tumor mechanism of complexes. It has been reported that iridium (Ⅲ) complexes can induce tumor cell death through apoptosis, necrosis, swelling and other ways. Many iridium (Ⅲ) complexes have shown good PDT activity with high 1O2 quantum yield and PI value, which are expected to become PDT photosensitizers for clinical application.
Although great progress has been made in the study of iridium (Ⅲ) complexes with antitumor activity,However, there are still many problems to be solved: (1) Tracking the intake and metabolism of "luminescent" iridium (Ⅲ) complexes in the whole process: a comprehensive and complete understanding of the existence and role of iridium (Ⅲ) complexes in the complex process from entry to metabolic exit will provide more theoretical and practical basis for the clinical application of iridium (III) complexes; (2) Explore and clarify the interaction with biological target molecules: It has been reported that iridium (Ⅲ) complexes can interact with DNA, BSA, GSH and other biological macromolecules, as well as endogenous or exogenous , , H2S and other signaling molecules, and can also induce the production of ROS. In the future, we need to explore more possible target molecules, study the interaction mode between iridium (III) complexes and them by various means, and study what functional defects will be caused by this interaction, so as to finally clarify the target and anti-tumor mechanism of iridium (III) complexes. (3) Synergistic optimization of luminescence and antitumor properties of complexes: The photosensitizer based on iridium (Ⅲ) complexes for two-photon PDT is a hot research topic at present, but the molecular design rule of "synergistic" optimization of two-photon luminescence and antitumor properties of complexes is not yet clear. It is a great challenge to develop iridium (Ⅲ) complexes with large two-photon absorption cross section, good water solubility and biocompatibility, long excited state lifetime, high 1O2 quantum yield, strong phototoxicity, low light dose and good stability. Nevertheless, luminescent iridium (Ⅲ) complexes are very promising as a new generation of cancer diagnostic and therapeutic drugs.
Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), Lianyungang Petrel Project, Opening Foundation of Jiangsu Key Laboratory of Functional Control of Advanced Materials(jsklfctam201808)
Jiangsu Provincial Graduate Scientific Research and Innovation Plan(KYCX20_2950)
PDF
