# Advances in Purpurin 18 Research: On Cancer Therapy
## 4 Photodynamic Therapy  
PDT is an attractive approach to the treatment of many types of cancer and non-cancerous diseases. This approach is based on the administration of photoactive substances, so-called PSs, which themselves (without any stimulation) do not exhibit any biological activity. They are activated by illumination with light of appropriate wavelength upon which a PS transits into an excited state. Then, the excited PS can react with molecular oxygen to form ROS and singlet oxygen. The presence of these particles in a cell leads to strong oxidative stress resulting in cell damage and cell death (Figure 3). However, ROS play a dual role in cancer treatment, since they can trigger multiple cellular cascades leading also to pro-tumorigenis (2017). The ideal PS should meet several criteria, such as high quantum yield of singlet oxygen and absorption in the dark red or near-infrared region of the electromagnetic spectrum (650-800 nm), which ensures deeper penetration of the light into a treated tissue. On the contrary, negligible absorption of the PS in the visible part electromagnetic spectrum (400-600 nm) is desirable due to possible photosensization of the skin (2018). Excitation and emission spectra are very important properties of PSs because human tissue contains many endogenous chromophores that also absorb light. For example, hemoglobin, myoglobin, melanin, and water absorb light in the 600-1000 nm region, which limits the biological window in this range of wavelengths (2019). Therefore, current trend is to develop novel PSs absorbing in the range of the second and third near-infrared (NIR) region of the electromagnetic spectrum (NIR-II: 1000-1350 nm, NIR-III: 1550-1870 nm), which provides the benefit of deeper tissue penetration, enhanced image contrast due to reduced optical scattering, and lower phototoxicity (2019). Related to this, it is another important property of a PS--preferred and ideally selective accumulation of the PS in the target, usually tumor, tissue. Equally important in PDT is none or marginal dark toxicity (without photoactivation) of the PS and its potential metabolites. What makes the use of PDT for cancer treatment more advantageous over conventional surgical methods  
Figure 2: Numbered molecular structure of purpurin 18 (pu18) and positions of frequent modifications. The pu18 molecule is usually modified to improve its photochemical properties, for example, by halogenation (violet dots) or nitration (orange dots), or derivatized by conjugation reactions with peptides or other tumor cell-targeting molecules. The most common approaches are the use of a free carboxyl group (C-173) to form esters or amides (an orange dot) or by the derivatization of a cyclic anhydride to form imides (a blue dot). are its low invasiveness and minimal side effects, which are frequently encountered in chemotherapy and radiotherapy. For this reason, PDT has been a very attractive method in cancer treatment. In PDT, substances originating in natural products are often used as PSs, including the already mentioned pu18. Pu18 in connection to photoactivation has attracted scientific interest already more than thirty years ago when [PERSON] et al. [18] discovered that photoactivated pu18 inhibits the growth of primary human fibroblasts. Thirteen years later, after the reports that photoactivated pu18 induces apoptosis and necrosis also in human cancer cells, namely HL-60 (cells from leukemia) [31; 32], the real boom in pu18 research related to cancer treatment has started. More recently, pu18 has been tested as an effective PS for PDT treatment of commonly incurable triple-negative breast cancer [33]. The phototoxicity of pu18 was determined using a mouse mammary tumor cell line (4T1), which is used as a model of stage IV human breast cancer. 4T1 cells were used in the form of two-dimensional (2D) culture, three-dimensional (3D) spheroids, and, also, in vivo in Balb/c mice with subcutaneously implanted 4T1 cells.