Skip to main content
NCBI home page
ACS Medicinal Chemistry Letters logoLink to ACS Medicinal Chemistry Letters
. 2019 Jul 24;10(8):1110–1114. doi: 10.1021/acsmedchemlett.9b00034

Fluorescent Imidazo[1,5-a]pyridinium Salt for a Potential Cancer Therapy Agent

Fumitoshi Yagishita †,§,*, Jun-ichi Tanigawa †, Chiho Nii †, Atsushi Tabata ‡, Hideaki Nagamune ‡, Hiroki Takanari ∥, Yasushi Imada †, Yasuhiko Kawamura †
PMCID: PMC6693471  PMID: 31417665

Abstract

graphic file with name ml9b00034_0009.jpg

N,N′-Dimethylated imidazo[1,5-a]pyridinium salt having good water solubility and exhibiting fluorescence emission was found to work as not only a bioimaging agent but also a therapeutic agent under UVA-LED irradiation conditions. Because the continuous UVA-LED irradiation to HeLa cells stained by the synthesized salt resulted in the cell death due to the mitochondrial damage, the salt has a potential application as photodynamic therapy agent against tumor cells.

Keywords: Imidazo[1,5-a]pyridinium salt; photodynamic therapy; fluorescence

Photodynamic therapy (PDT) has a tremendous importance in the healthcare field due to noninvasive treatment for various diseases including cancer.1 To achieve this purpose, the photosensitizers were recognized as a key component to generate the reactive oxygen species (ROS) via energy transfer to molecular oxygen. Therefore, many researchers have developed the photosensitizers based on organic dyes2,3 and organic metal complexes including Ir,4,5 Ru,6,7 Cu,8,9 and so on.10,11 However, the O2-dependent mechanism is ineffective for hypoxic tumor cells because of the low production efficiency of ROS. Therefore, the development of a new strategy to overcome the hypoxic condition has drawn considerable attention. For example, Zheng and co-workers reported the Ru complex, which underwent photoinduced dissociation of reactive free radical species upon visible light irradiation.12 Their mechanism has potential for selective inactivation of hypoxic tumor cells, because the generated radical species could photobind and photocleave DNA in anaerobic conditions. On the other hand, mitochondria-targeting photosensitizer was also recognized as an efficient strategy to overcome the hypoxia in photodynamic therapy.13 For example, Thomas and co-workers reported the cancer-mitochondria-targeted photodynamic therapy using a water-soluble cyanine dye.14 Very recently, Cheng and co-workers also reported the mitochondria and plasma membrane dual-targeted photosensitizer for effective photodyanamic therapy using the chimeric peptide.15

We recently reported the synthesis of dimeric imidazo[1,5-a]pyridinium salts having the good fluorescence quantum yield and water solubility suitable for bioimaging.16 Because these salts possess iodide ions as the courter anions, we consider that our salts also could be applied to PDT agent due to the production of ROS through an intersystem crossing process. This time, we found that salt 1, shown in Figure 1, exhibits DNA photocleaving ability upon continuous UVA-LED (Ultraviolet A light emitting diode) irradiation even under anaerobic conditions. In addition, it was found that the salt exhibits photocytotoxicity against HeLa cells due to mitochondrial damage under the continuous UVA-LED irradiation conditions and has potential application as a PDT agent. The therapeutic agents exhibiting photoluminescence have the advantage, because they could visualize and define the affected area during the photodynamic therapy treatment.17

Figure 1.

Figure 1

Open in a new tab

Structure of the salt 1 and proposed processes for PDT under irradiation condition.

Initially, we evaluated the DNA photocleaving ability of salt 1 using supercoiled ΦX174 RF I DNA in TAE (Tris-acetate EDTA buffer). Because salt 1 has its absorption maximum at 353 nm, we selected the UVA-LED (365 nm, 1.1 W) as a light source. The conversion of supercoiled form (Form I) to the nicked circular form (Form II) or the linear form (Form III) was evaluated using an agarose gel electrophoresis. The results are shown in Figure 2. The photoirradiation using UVA-LED did not affect the DNA solution in the absence of salt 1 even after 3 h. On the other hand, salt 1 obviously indicated the DNA cleaving ability to produce the nicked circular DNA (Form II) under the photoirradiation conditions. The band indicating Form II was observed after 10 min (Lane 5), and Form I completely disappeared after 60 min in the presence of salt 1 (Lane 9).

Figure 2.

Figure 2

Open in a new tab

0.9% Agarose gel, TAE buffer (40 mmol L–1 of Tris-acetate and 1 mmol L–1 of EDTA (pH = 8.0)). Lanes 1 and 13: DNA marker. Lane 2: DNA sample. Lanes 3, 5, 7, 9, and 11: DNA, TE (Tris-EDTA buffer), and salt 1 in H2O (Irradiation time: 1, 10, 30, 60, and 180 min). Lanes 4, 6, 8, 10, and 12: DNA, TE, and H2O (Irradiation time: 1, 10, 30, 60, and 180 min).

To evaluate the DNA cleaving mechanism by salt 1 under irradiation condtions, we conducted a control experiment using some ROS scavengers. In this study, we used NaN3 for singlet oxygen scavenger and KI, DMSO, and t-BuOH for hydroxyl radical scavengers. As a result, no scavengers could work during the DNA photocleaving experiments as shown in Figure 3. In addition, the formation of Form II was observed even under a nitrogen atmosphere (Figure S1). Therefore, it was suggested that the DNA photocleavage by salt 1 occurred through not only the ROS formation but also the direct electron transfer from the DNA to the electron deficient fluorophore.

Figure 3.

Figure 3

Open in a new tab

0.9% Agarose gel, TAE buffer (40 mmol L–1 of Tris-acetate and 1 mmol L–1 of EDTA (pH = 8.0)). Lanes 1 and 11: DNA marker. Lane 2: DNA sample. Lanes 3 and 5: DNA, TE, and salt 1 in H2O (Irradiation time: 0, 60 min). Lanes 4 and 6: DNA and TE in H2O (Irradiation time: 0, 60 min). Lane 7: DNA, TE, salt 1, and 1.0 × 10–3 mol L–1 of NaN3 (Irradiation time: 60 min). Lane 8: DNA, TE, salt 1, and 1.0 × 10–3 mol L–1 of KI (Irradiation time: 60 min). Lane 9: DNA, TE, salt 1, and 1.0 × 10–3 mol L–1 of DMSO (Irradiation time: 60 min). Lane 10: DNA, TE, salt 1, and 1.0 × 10–3 mol L–1 of t-BuOH (Irradiation time: 60 min).

Next, we examined the continuous UVA-LED irradiation to HeLa cells stained by salt 1 to evaluate the potential of salt 1 for photodynamic cancer therapy. Before the cell staining experiment, we evaluated the cytotoxicity of salt 1 to HeLa cells using CCK-8 assay. The viabilities of HeLa cells incubated for 1 h (for acute toxicity assay) and 24 h (for subacute toxicity assay) with a series of concentrations of salt 1 are summarized in Figure S2. Although the viabilities of the HeLa cells were slightly decreased in the presence of the highest concentration of salt 1 tested in this study (2.5 × 10–4 mol L–1) for 24 h incubation, the significant toxicity of salt 1 to HeLa cells was not observed at lower concentration. Although the actual value of the IC50 for salt 1 was not determined because of its limit of water solubility in the assay condition, it was predicted that the IC50 of salt 1 to HeLa cells is 2.5 × 10–4 mol L–1 or more (Figure S2). We next applied salt 1 to fluorescence bioimaging of HeLa cells. When HeLa cells were incubated with salt 1 at 37 °C for 24 h under the 5% CO2 condition, the cells were labeled with the dot-shaped cyan fluorescent signals under UV irradiation (Figure S3). Because this fluorescence image was similar to that using the methoxy-substituted analogue demonstrated in our previous report,16 we speculated that a part of cyan fluorescent signals indicating salt 1 localized in the endosomes. To evaluate this speculation, we conducted the control experiment using an endocytosis marker. When HeLa cells were incubated with salt 1 and endocytosis marker at 37 °C for 24 h under the 5% CO2 condition, a part of cyan fluorescent signals indicating salt 1 colocalized with the red fluorescent signals indicating endocytosis marker (Figure 4). Therefore, we considered that salt 1 was also incorporated into HeLa cells via the endocytosis process as one of the routes into the cells.

Figure 4.

Figure 4

Open in a new tab

Observed images focused on endosomes in HeLa cells incubated with salt 1 and endocytosis marker: (a) bright field image, (b) cyan fluorescence of salt 1 through a WU filter, (c) red fluorescence of endocytosis marker through a WIY filter, and (d) their merged image. Each of the lower images is the magnified view of the region surrounded by white squares shown in the upper images.

We further focused on the use of salt 1 as a PDT agent and conducted the UVA-LED irradiation to HeLa cells stained by salt 1 (Figure 5). Although the photocytotoxicty against HeLa cells was not observed after UVA-LED irradiation for 10 min, the decrease of cell viability was observed after 30 min and 60 min irradiation. On the other hand, in the absence of salt 1, the cell viability was not affected even after 60 min of irradiation. In addition, when HeLa cells were incubated for a further 24 h after the UVA-LED irradiation, the viabilities of the damaged cells did not recover. Especially, significant damage against HeLa cells was observed after the further incubation in the case of the 60 min irradiation against the HeLa cells stained by salt 1. Therefore, it was found that the combination of salt 1 and the UVA-LED irradiation was the trigger for the irreversible cell damage against the HeLa cells.

Figure 5.

Figure 5

Open in a new tab

Cell viabilities of HeLa cells stained by salt 1 (a) after UVA-LED irradiation and (b) after the UVA-LED irradiation followed by the incubation at 37 °C for 24 h in the dark. The statistics were conducted using software R for Mac OS X (version 3.5.1, https://cran.r-project.org/bin/macosx/). The significance of differences between each sample was evaluated by the two-sided Welch’s t-test or two-sided t-test after F-test. ** p < 0.01, * p < 0.05, and “(n.s.)” indicates “not significant”.

To consider the obvious cell damage depending on the UVA-LED irradiation time, the fluorescence image was observed under the UVA-LED irradiation conditions. Interestingly, the fluorescence intensity of dot-shaped cyan signals, which mainly indicated endosomes, was weakened and fluorescent signals were spread to the cytoplasm according to the UVA-LED irradiation (Figure 6). Because some examples on the photoinduced endosomal escape of molecules trapped in endosomes were reported via the generation of ROS using the photosensitizer or fluorescent dye to destabilize the endosomeal membrane, it was expected that diffusion of salt 1 from endosomes into the cytoplasm also occurred in similar fashion.18−20

Figure 6.

Figure 6

Open in a new tab

Photographs of HeLa cells stained by salt 1 (a) before, (b) after 10 min, and (c) after 30 min continuous UVA-LED irradiation; bright field (upper images) and fluorescence image through WU filter (lower images).

To confirm that redistribution of salt 1 from endosomes to cytoplasm was caused by free radicals generated in the cells, we performed live cell imaging of hydroxyl radical and superoxide anion using OxiORANGE and MitoROS 580, respectively. Representative fluorescence images and summarized fluorescence intensity showed that transition of blue fluorescence due to salt 1 started at 10–15 min after the initiation of UV irradiation and completed in 30 min (Figure 7). On the other hand, the fluorescence intensity of OxiORANGE and MitoROS started increasing just after the initiation of UV irradiation, revealing that the generation of ROS preceded the transfer of salt 1 from the endosome to the cytoplasm. From the results, we considered that the ROS destroyed the membrane structure of endosomes to release salt 1 into the cytoplasm. Moreover, the increase in fluorescence of OxiORANGE in the presence of salt 1 was much higher than that in the absence of it (Figure 7c), suggesting that salt 1 might be involved in hydroxyl radical generation in the cells. Although a gradual increase in fluorescence of MitoROS was observed, a significant difference could not be observed, regardless of the presence of salt 1 (Figure 7f).

Figure 7.

Figure 7

Open in a new tab

Fluorescence evaluation of free radical production in HeLa cells. Representative fluorescence images in the absence (a, d) and the presence (b, e) of salt 1. Top pictures are blue fluorescence images obtained through a DAPI filter, middle pictures are red fluorescence acquired through a TRITC filter, and bottom pictures are merged images of blue and red fluorescence. The images before (left) and after (right) 30 min UV irradiation are shown. White bar in each picture indicates 20 μm. Fluorescence intensity with the combination of 1 and OxiORANGE (c) or the combination of 1 and MitoROS (f) is summarized. Endosomal: fluorescence measured at the position where blue fluorescence was first confirmed in endosomes, Cytoplasm: fluorescence intensity measured in cytoplasm, OxiORANGE/MitoROS (+): red fluorescence in the presence of 1, OxiORANGE/MitoROS (−): red fluorescence in the absence of 1.

It was known that some heterocyclic aromatic cations accumulated in the mitochondria due to the negative membrane potential of the mitochondrial inner membrane.21−24 Therefore, we considered that salt 1 released from the endosomes and then accumulated in the mitochondria. Based on this assumption, we conducted the control experiment using a mitochondria marker. When HeLa cells were stained by salt 1 and mitochondrial marker without the UVA-LED irradiation, different localization was observed between the cyan signals indicating salt 1 and the red fluorescence signals indicating the mitochondrial marker (Figure S4a). Conversely, when HeLa cells stained by salt 1 were incubated in the presence of the mitochondrial marker after the 30 min UVA-LED irradiation, cyan and red fluorescence signals partially colocalized (Figure S4b). These results indicated that the decrease of cell viability was caused by the mitochondrial damage. Therefore, we conducted the JC-1 assay for the measurement of mitochondrial membrane potential to evaluate the mitochondrial function after the continuous UVA-LED irradiation. As the positive control, FCCP treatment was also conducted for depolarization of mitochondrial membrane potential (Figure 8c). When the UVA-LED irradiation to HeLa cells was conducted for 0, 30, and 60 min, the changes of red and green fluorescence signals of JC-1 dyes were not observed (Figure 8a). On the other hand, the continuous UVA-LED irradiation elevated the depolarization of mitochondrial membrane potential in HeLa cells stained by salt 1 (Figure 8b). Therefore, it was found that the death of HeLa cells stained by salt 1 was caused by the mitochondrial damage under the irradiation conditions. Because the DNA photocleaving experiments using salt 1 under hypoxic condition indicated the direct electron transfer from the DNA to the electron deficient fluorophore, we also conducted the JC-1 assay under the hypoxic conditions. Unfortunately, the depolarization of mitochondrial membrane potential in HeLa cells stained by salt 1 was not observed (Figure S5). This result indicated that the mitochondrial damage was caused by the ROS generated in the cells. Although we could not completely determine the kind of ROS contributed to the mitochondrial damage in the cells, two processes were expected as follows: (1) Singlet oxygen was produced by the excited salt 1 which acts as a photosensitizer via the intercrossing system, or (2) hydroxyl radical was generated effectively in the presence of salt 1.

Figure 8.

Figure 8

Open in a new tab

Photographs of HeLa cells incubated with the JC-1 dye (a) in the absence and (b) presence of salt 1; fluorescence images through WIY filter (upper), fluorescence images through WU filter (middle), and bright fields (lower).

In summary, we demonstrated the DNA cleaving experiments of salt 1 under the UVA-LED irradiation conditions and evaluated their therapeutic abilities using HeLa cells. Salt 1 exhibited DNA photocleaving ability to form the nicked circular form from the supercoiled form even under the anaerobic conditions. In addition, it was found that salt 1 has a potential application as PDT agent via the mitochondrial damage. Because the use of UVA light source was unfavorable for PDT treatment due to its low light penetration and potential toxicity, the chemical modification of salt 1 is required to endow the broad absorption region or the two-photon absorption character. However, we believe that the synthesized imidazo[1,5-a]pyridinium salt becomes the new molecular design for a PDT agent. In addition, further studies about the application to the drug-delivery system and the use of NIR are currently in progress.

Acknowledgments

We thank the Research Cluster program of Tokushima University (No. 1802001) for partial financial support and NICHIA corp. for supplying LEDs. All the live cell imaging experiments were conducted with technical support of Tokushima Bioimaging Station.

Supporting Information Available

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmedchemlett.9b00034.

  • Details of characterization of compound including 1H NMR spectrum, biological assay protocols, and additional data (PDF)

Author Contributions

The manuscript was written through contributions of all authors./All authors have given approval to the final version of the manuscript.

This work was partially supported by the Research Clusters program of Tokushima University (No. 1802001, No. 1802003, and No. 1903007) and JSPS KAKENHI Grant Number JP19H04443.

The authors declare no competing financial interest.

Supplementary Material

ml9b00034_si_001.pdf (1.8MB, pdf)

References

  1. Dolmans D. E. J. G. J.; Fukumura D.; Jain R. K. Photodynamic therapy for cancer. Nat. Rev. Cancer 2003, 3, 380–387. 10.1038/nrc1071. [DOI] [PubMed] [Google Scholar]
  2. Bohne C.; Faulhaber K.; Giese B.; Häfner A.; Hofmann A.; Ihmels H.; Köhler A.-K.; Perä S.; Schneider F.; Sheepwash M. A. L. Studies on the mechanism of the photo-induced DNA damage in the presence of acridizinium salts–Involvement of singlet oxygen and an unusual source for hydroxyl radicals. J. Am. Chem. Soc. 2005, 127, 76–85. 10.1021/ja046189m. [DOI] [PubMed] [Google Scholar]
  3. Adarsh N.; Avirah R. R.; Ramaiah D. Tuning photosensitized singlet oxygen generation efficiency of novel aza-BODIPY dyes. Org. Lett. 2010, 12, 5720–5723. 10.1021/ol102562k. [DOI] [PubMed] [Google Scholar]
  4. Nam J. S.; Kang M.-G.; Kang J.; Park S.-Y.; Lee S. J. C.; Kim H.-T.; Seo J. K.; Kwon O.-H.; Lim M. H.; Rhee H.-W.; Kwon T.-H. Endoplasmic reticulum-localized iridium(III) complexes as efficient photodynamic therapy agents via protein modifications. J. Am. Chem. Soc. 2016, 138, 10968–10977. 10.1021/jacs.6b05302. [DOI] [PubMed] [Google Scholar]
  5. Zheng Y.; He L.; Zhang D.-Y.; Tan C.-P.; Ji L.-N.; Mao Z.-W. Mixed-ligand iridium(III) complexes as photodynamic anticancer agents. Dalton Trans 2017, 46, 11395–11407. 10.1039/C7DT02273E. [DOI] [PubMed] [Google Scholar]
  6. Yavin E.; Stemp E. D. A.; Weiner L.; Sagi I.; Arad-Yellin R.; Shanzer A. Direct photo-induced DNA strand scission by a ruthenium bipyridyl complex. J. Inorg. Biochem. 2004, 98, 1750–1756. 10.1016/j.jinorgbio.2004.07.016. [DOI] [PubMed] [Google Scholar]
  7. Ramu V.; Aute S.; Taye N.; Guha R.; Walker M. G.; Mogare D.; Parulekar A.; Thomas J. A.; Chattopadhyay S.; Das A. Photo-induced cytotoxicity and anti-metastatic activity of ruthenium(II)-polypyridyl complexes functionalized with tyrosine or tryptophan. Dalton Trans 2017, 46, 6634–6644. 10.1039/C7DT00670E. [DOI] [PubMed] [Google Scholar]
  8. Patra A. K.; Dhar S.; Nethaji M.; Chakravarty A. R. Visible light-induced nuclease activity of a ternary mono-phenanthroline copper(II) complex containing L-methionine as a photosensitizer. Chem. Commun. 2003, 1562–1563. 10.1039/b303442a. [DOI] [PubMed] [Google Scholar]
  9. Lin R.-K.; Chiu C.-I.; Hsu C.-H.; Lai Y.-J.; Venkatesan P.; Huang P.-H.; Lai P.-S.; Lin C.-C. Photocytotoxic copper(II) complexes with Schiff-base scaffolds for photodynamic therapy. Chem. - Eur. J. 2018, 24, 4111–4120. 10.1002/chem.201705640. [DOI] [PubMed] [Google Scholar]
  10. Roy M.; Saha S.; Patra A. K.; Nethaji M.; Chakravarty A. R. Ternary iron(III) complex showing photocleavage of DNA in the photodynamic therapy window. Inorg. Chem. 2007, 46, 4368–4370. 10.1021/ic062056l. [DOI] [PubMed] [Google Scholar]
  11. Lau J. T. F.; Lo P.-C.; Fong W.-P.; Ng D. K. P. A zinc(II) phthalocyanine conjugated with an oxaliplatin derivative for dual chemo- and photodynamic therapy. J. Med. Chem. 2012, 55, 5446–5454. 10.1021/jm300398q. [DOI] [PubMed] [Google Scholar]
  12. Zheng Y.; Zhou Q.; Lei W.; Hou Y.; Li K.; Chen Y.; Zhang B.; Wang X. DNA photocleavage in anaerobic conditions by a Ru(II) complex: a new mechanism. Chem. Commun. 2015, 51, 428–430. 10.1039/C4CC06552B. [DOI] [PubMed] [Google Scholar]
  13. Weinberg S. E.; Chandel N. S. Targeting mitochondria metabolism for cancer therapy. Nat. Chem. Biol. 2015, 11, 9–15. 10.1038/nchembio.1712. [DOI] [PMC free article] [PubMed] [Google Scholar]
  14. Thomas A. P.; Palanikumar L.; Jeena M. T.; Kim K.; Ryu J.-H. Cancer-mitochondria-targeted photodynamic therapy with supramolecular assembly of HA and a water soluble NIR cyanine dye. Chem. Sci. 2017, 8, 8351–8356. 10.1039/C7SC03169F. [DOI] [PMC free article] [PubMed] [Google Scholar]
  15. Cheng H.; Zheng R.-R.; Fan G.-L.; Fan J.-H.; Zhao L.-P.; Jiang X.-Y.; Yang B.; Yu X.-Y.; Li S.-Y.; Zhang X.-Z. Mitochondria and plasma membrane dual-targeted chimeric peptide for single-agent synergistic photodynamic therapy. Biomaterials 2019, 188, 1–11. 10.1016/j.biomaterials.2018.10.005. [DOI] [PubMed] [Google Scholar]
  16. Yagishita F.; Nii C.; Tezuka Y.; Tabata A.; Nagamune H.; Uemura N.; Yoshida Y.; Mino T.; Sakamoto M.; Kawamura Y. Fluorescent N-heteroarenes having large Stokes shift and water solubility suitable for bioimaging. Asian J. Org. Chem. 2018, 7, 1614–1619. 10.1002/ajoc.201800250. [DOI] [Google Scholar]
  17. Lovell J. F.; Liu T. W. B.; Chen J.; Zheng G. Activatable photosensitizers for imaging and therapy. Chem. Rev. 2010, 110, 2839–2857. 10.1021/cr900236h. [DOI] [PubMed] [Google Scholar]
  18. Berg K.; Selbo P. K.; Prasmickaite L.; Tjelle T. E.; Sandvig K.; Moan J.; Gaudernack G.; Fodstad O.; Kjolsrud S.; Anholt H.; Rodal G. H.; Hogest A. Photochemical internalization: a novel technology for delivery of macromolecules into cytosol. Cancer Res. 1999, 59, 1180–1183. [PubMed] [Google Scholar]
  19. Maiolo J. R. III; Ottinger E. A.; Ferrer M. Specific redistribution of cell-penetrating peptides from endosomes to the cytoplasm and nucleus upon lase illumination. J. Am. Chem. Soc. 2004, 126, 15376–15377. 10.1021/ja044867z. [DOI] [PubMed] [Google Scholar]
  20. Matsushita M.; Noguchi H.; Lu Y.-F.; Tomizawa K.; Michiue H.; Li S.-T.; Hirose K.; Bonner-Weir S.; Matsui H. Photo-acceleration of protein release from endosome in the protein transduction system. FEBS Lett. 2004, 572, 221–226. 10.1016/j.febslet.2004.07.033. [DOI] [PubMed] [Google Scholar]
  21. Davey G. P.; Tipton K. F.; Murphy M. P. Uptake and accumulation of 1-methyl-4-phenylpyridinium by rat liver mitochondria measured using an ion-selective electrode. Biochem. J. 1992, 288 (Pt 2), 439–443. 10.1042/bj2880439. [DOI] [PMC free article] [PubMed] [Google Scholar]
  22. Chevalier A.; Zhang Y.; Khdour O. M.; Kaye J. B.; Hecht S. M. Mitochondrial nitroreductase activity enables selective imaging and therapeutic targeting. J. Am. Chem. Soc. 2016, 138, 12009–12012. 10.1021/jacs.6b06229. [DOI] [PubMed] [Google Scholar]
  23. Wang P.; Huang J.; Gu Y. Rational design of a novel mitochondrial-targeted near-infrared fluorescent pH probe for imaging in living cells and in vivo. RSC Adv. 2016, 6, 95708–95714. 10.1039/C6RA22470A. [DOI] [Google Scholar]
  24. Lin B.; Fan L.; Ge J.; Zhang W.; Zhang C.; Dong C.; Shuang S. A naphthalene-based fluorescent probe with a large Stokes shift for mitochondrial pH imaging. Analyst 2018, 143, 5054–5060. 10.1039/C8AN01371C. [DOI] [PubMed] [Google Scholar]

Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

ml9b00034_si_001.pdf (1.8MB, pdf)

Articles from ACS Medicinal Chemistry Letters are provided here courtesy of American Chemical Society

RESOURCES