Chemistry
5,15-Diaryl-10,20-dihalogeno porphyrins I1 − 6 were obtained by halogenation and hydrolysis of porphyrins synthesized by standard Lindsey protocol in 30%-50% total yields, as shown in Scheme S1 (Vinogradova et al. 2009). The synthesis of 5,15-diaryl-10,20-dibromoporphyrins (II1 − 4) was shown in Scheme S2. An amide condensation reaction occurred among compounds (I1, I2) and amino-acid esters. The compounds (4a-d) were hydrolyzed to give the target compounds (II1 − 4) in 56%-70% total yields. The structures of these porphyrins (I1 − 6, II1 − 4) were characterized by 1H NMR, 13C NMR and HR-MS (Figure. S1-25).
Density functional theory (DFT) studies
In general, a smaller gap of HOMO and LUMO energy level (ΔE) corresponds to a stronger driving force toward the electron-transfer state. Frontier molecular orbitals (FMOs) of the HOMO and LUMO energy (eV) levels for the optimized structures of PS1, I2, I5 and I6 (Fig. 3) were calculated. According to our DFT calculations, ΔE of compounds I2 (0.09637 eV), I5 (0.09675 eV) and I6 (0.09400 eV) were smaller than compound PS1 (0.10184 eV) due to the existence of halogen atoms (Fig. 2). In addition, it was observed that their corresponding ΔE values decreased with the increase of relative atomic mass of halogen atoms of meso-substituents.
Photophysical properties
The UV-vis absorption spectrum of the 5,15-diaryl-10,20-dihalogeno porphyrins (I1 − 6 and II1 − 4) in DMSO were presented in Fig. 4A. All compounds exhibited an intense B band at ~ 425 nm, and four Q bands at around 523 nm, 558 nm, 602 nm and 660 nm. Among these compounds, I6 showed the longest absorption wavelength at 664 nm. The unhalogenated porphyrins PS1 had fluorescence while 5,15-diaryl-10,20-dihalogeno porphyrins (I1 − 6 and II1 − 4) have no fluorescence (Fig. 4B).
Singlet oxygen generation
To evaluate the ability to produce singlet oxygen, 1,3-diphenylisobenzofuran (DPBF) was used as the quencher. The generation rate of singlet oxygen was measured by monitoring the decrease in absorbance at 415 nm with regular intervals of 10 s upon irradiation with 650 nm laser light. As shown in Fig. 5A, decomposition of DPBF did not occur in the absence of singlet oxygen. The representative halogenated porphyrins (I2, I6, II4) were irradiated with 650 nm laser and their absorbance did not decrease with the increase of irradiation time, which indicated that halogenated porphyrins had good photostability. Time-dependent changes of the absorption spectra of various PSs containing DPBF under light irradiation were shown in Fig. 5B, 5C and Figure S26. The singlet oxygen generation rates of all halogeno compounds were summarized in Table 1. The results showed that the ability of singlet oxygen generation of halogenated porphyrins was significantly higher than that of unhalogenated porphyrins PS1and HMME.
Table 1
Singlet oxygen generation rate of compounds I1 − 6, II1 − 4
Compound
|
k×10− 2(s− 1)
|
Compound
|
k×10− 2(s− 1)
|
I1
|
0.27
|
II1
|
0.29
|
I2
|
0.60
|
II2
|
0.43
|
I3
|
0.16
|
II3
|
0.30
|
I4
|
0.64
|
II4
|
0.61
|
I5
|
0.68
|
PS1
|
0.014
|
I6
|
0.64
|
HMME
|
0.026
|
In vitro photosensitizing efficacy
MTT assays were used to test the in vitro cytotoxicity of the target compounds against Eca-109 cells. As shown in Fig. 6A and Table 2, the cell viabilities after the treatment with compounds I3 − 4, and II1 − 4 at 10 µM were higher than 80% under dark condition while other compounds were lower than 80% under the same conditions, which suggested that compounds I3 − 4, and II1 − 4 had negligible dark cytotoxicity. There was little difference between the IC50 values of halogenated porphyrins (I2, I5 and I6) with Cl, Br and I under dark or light irradiation, as shown in Figure S27. The IC50 values of all target compounds were evidently smaller than HMME under light irradiation of 12 J/cm2 (Table 2). It was noteworthy that significant decrease in cell viability of Eca-109 cells after the treatment with compounds I1 − 3, I5 − 6, and II1 − 4 at concentration of 4 µM and a light dose of 12 J/cm2 was observed (Figure S27 and Fig. 6B), which indicated that these compounds had obvious photodynamic activities against Eca-109 cells.
Table 2
IC50 values of I1 − 6, II1 − 4 in Eca-109 cells upon different light dose irradiation
Compound
|
IC50 (µM)
|
0 J/cm2
|
2 J/cm2
|
4 J/cm2
|
8 J/cm2
|
12 J/cm2
|
I1
|
11.1
|
8.1
|
6.2
|
4.6
|
3.5
|
I2
|
23.4
|
7.6
|
4.8
|
3.9
|
3.0
|
I3
|
28.6
|
4.6
|
3.9
|
3.2
|
2.4
|
I4
|
> 30
|
> 30
|
23.3
|
14.0
|
7.7
|
I5
|
22.2
|
20.0
|
5.8
|
3.7
|
2.9
|
I6
|
16.5
|
10.0
|
6.4
|
3.8
|
2.5
|
II1
|
26.6
|
13.8
|
7.2
|
4.1
|
2.2
|
II2
|
26.3
|
4.9
|
2.7
|
1.9
|
0.9
|
II3
|
> 30
|
5.4
|
4.2
|
3.4
|
1.6
|
II4
|
24.4
|
3.9
|
1.9
|
1.6
|
0.4
|
HMME
|
27.9
|
22.4
|
18.6
|
15.0
|
9.2
|
Flow cytometry analysis
To investigate the cell death induced by different PSs, Eca-109 cells after PDT were incubated for 5 h and stained with the V-FITC/PI apoptosis assay kit for flow cytometry analysis. As shown in Fig. 7, no necrotic cells were detected in the control group. After 5 h post-PDT, the proportion of living cells were decreased in all PDT-treatment groups, especially dropped to 48.40% and 45.58% in I3 and II4 groups respectively. II4-PDT mainly induced cell necrosis, while I3-PDT could induce cell necrosis and apoptotic.
Different PSs mediated PDT in vivo
Since compounds I3, II2, II3 and II4 had more obvious photodynamic cytotoxicity than others under the same conditions, their photodynamic efficacy in Eca-109 tumor-bearing BABL/c nude mice were evaluated. The compounds at a dose of 2 mg/kg were injected intravenously into mice, followed by irradiation of 120 J/cm2 light at tumor site for single time. As shown in Fig. 8A, 8B and 8C, almost negligible inhibition of tumor growth was observed from the mice treated with light only. Notably, II2 and II4 showed more prominent anti-tumor PDT effect compared to the control group, light group and HMME group, especially II4.
After 1-day post treatment, the tumor tissue sections were separated then stained with hematoxylin and eosin (H&E) reagent. As shown in Fig. 8D, significant tumor tissue damage in the PDT group could be observed under microscopy compared to the control group. The results of the tumor growth curve and histological examination showed that II4 had the best photodynamic antitumor activity in vivo.