OSCC is one of the most common oral malignant tumors. With approximately 140,000 people dying of oral cancer each year, this condition poses a serious threat to human health. OSCC is located in the face; hence, routine surgery will affect normal pronunciation, chewing, swallowing and other oral functions. Meanwhile, facial loss can lead to negative psychological impact for patients who receive the treatment.[6,7] Photodynamic therapy has superior advantages in the treatment of OSCC due to its non-invasive or minimally invasive characteristics. Photodynamic laser tissue penetration has some limitations, which is only available for superficial parts of the body, such as facial lip tumor, tongue tumor and so on, so 5-ALA-mediated photodynamic therapy may be the best choice for the treatment of OSCC.[9,10] Now 5-ALA-mediated photodynamic therapy has been used in the treatment of actinic keratosis and superficial basal cell carcinoma with beneficial effects. However, through literature review, there are limited studies concerning the use of 5-ALA-mediated photodynamic therapy in the treatment of OSCC, and its specific role and mechanism remain largely unclear. Therefore, the present study aimed to explore the effect and underlying mechanism of 5-ALA-mediated photodynamic therapy on OSCC in vitro.
Photodynamic therapy is a new approach for targeted treatment of cancer, which is ideally suited for solid cancer, benign tumors and some precancerous lesions.[11–13] In clinical treatment, the recommended dose of photosensitizer has no cytotoxicity, but photoactivation with the participation of oxygen can induce cytotoxicity. The second-generation photosensitizer has been used in the clinic, which is not only specific to cancer tissue, but also has a killing effect in the laser irradiated area. Additionally, photosensitizer in normal tissue can disappear within 12 to 24 h, so it causes no harm to normal tissue, which is regarded as the greatest advantage of photodynamic therapy for cancer. 5-ALA is the second-generation photosensitizer in photodynamic therapy. It is synthesized by the action of δ-amino- ƴ -ketovalerate synthetase on succinyl-CoA and glycine, which is the precursor of heme synthesis in vivo. 5-ALA has no photosensitivity, but it can produce strong photosensitizer protoporphyrin IX after a series of transformations in cells. Related clinical studies have confirmed that 5-ALA can greatly increase the content of protoporphyrin IX in tissues to the peak within 3 h, which slowly decreases after 6 h and disappears after 24 h. However, with different sensitivities of 5-ALA to photodynamic process, the amount of 5-ALA absorbed by different types of cancer cells is different; hence, there are different degrees of protoporphyrin IX in the cells. According Wang et al., the fluorescence intensity of 5-ALA is not the same in OSCC cells with different degrees of differentiation. The results of this study showed that the level of protoporphyrin IX in SCC25 cells was determined by 5-ALA concentration and time, which is consistent with previous studies. Our results also found that the best 5-ALA incubation time was not the same as other studies, and the reason may be that cytology had no correlation. Based on aggregated results, 5-ALA-mediated photodynamic therapy is available for the treatment of OSCC. Additionally, compared with the first-generation photosensitizer, protoporphyrin IX has a shorter half-life in the body, so the photodynamic mediated by 5-ALA shows better biosafety, which is able to remarkably shorten treatment time, thus reducing light exposure for patients.
Based on our results, 5-ALA combined with laser irradiation could significantly inhibit the proliferation of SCC25 cells in vitro. When the mass concentration of 5-ALA was 50 mg/L, the cell survival rate was (8.97 ± 2.97) mg/L. Different concentrations of 5-ALA combined with laser irradiation showed remarkable effect on apoptosis, and higher drug concentrations caused more apoptosis. After the intervention of 5-ALA combined with laser irradiation, the proportion of grade I SCC25 cells was significantly higher than that of the blank control group, the proportion of grade IV SCC25 cells was lower than that of the blank control group, and that of grade II and III was slightly lower than that of the the blank control group. A novel study reported that in 5-ALA-mediated photodynamic therapy, Bcl-2, p-Akt, p-mTOR and iNOS were up regulated in OSCC cells, suggesting an activation of anti-apoptosis and cell proliferation pathways. The above results confirmed that 5-ALA-mediated photodynamic therapy offers the potential to regulate the growth of OSCC in vivo and in vitro, and is linked to the regulation of metabolism and proliferation of SCC25 cells. It also indicates that 5-ALA-mediated photodynamic therapy has dual effects and is not controllable, and its clinical implementation needs to be further verified.
In addition, as a minimally invasive or non-invasive therapy, 5-ALA-mediated photodynamic therapy can specifically select cancer cells and effectively shorten treatment time to reduce light exposure. Nevertheless, limitation exists considering that the therapy fails to go deep into the cancer tissue; hence, it is insufficient to kill cancer tissue, and it can cause damage to the normal tissue. Therefore, the latest research is performed to combine fractionated light with iron chelating agent, with an attempt to provide a new insight for OSCC treatment.
BMC Oral Health. 2020;20(258) © 2020 BioMed Central, Ltd.