# Advances in Purpurin 18 Research: On Cancer Therapy
## 6 Photoacoustic Imaging and Therapy  
Photoacoustic imaging (PAI) is a promising technique for diagnostic and therapeutic purposes in the treatment of cancer due to its high resolution and deep penetration into tissues [69]. In medical applications, photoacoustic imaging is currently in clinical trials for imaging tumors of the breast [70; 71], head and neck [72; 73], thyroid gland [74], colon, and rectum [75]. The principle of PAI is the generation of acoustic ultrasonic waves based on the conversion of energy supplied by pulsed laser light. If a suitable molecule absorbs light energy, this energy is converted into heat, which also leads to an increase in the temperature around the molecule. Based on the thermoelastic expansion caused by the increase in temperature, ultrasonic waves are subsequently generated, recorded, and analyzed to form an image. Naturally occurring chromophores such as oxy-/deoxyhemoglobin, melanin, or exogenous inorganic and organic substances, which have the potential to absorb energy in the near-infrared region of the spectrum, are used as contrast agents in PAI. Endogenous chromophores can be used in a variety of applications, from brain mapping to monitoring angiogenesis and hypoxia, which are one of the main features of tumor presence. However, their main advantage, i.e., endogenous occurrence, is also their main limitation, due to the difficult distinction between the increased signal vs. the background signal (e.g., in tumor imaging). Some PSs, such as indocyanine green [76], and methylene blue [77] have  
Figure 7: Schematic representation of the ultrasound activatable polypeptide drug PTPK. The drug is applied to the bloodstream in a linear form, pu18 is activated to form singlet oxygen, which leads to cleavage of the thiol bond and disconnection of the PEG spacer. The rest of the molecule then consists of hydrophobic nanoparticles, which can more easily penetrate tumor cells. been approved by the US Food and Drug Administration for application in PAI. Besides, due to absorption in long wavelengths, pu18 and its derivatives are also suitable for such application. For example, it could be beneficial in distinguishing very small tumor features (<10 mm) from healthy tissues. PAI probes such as pu18/AHC\(\subset\)L (AHC\(\subset\)L--ammonium hydrogen carbonate encapsulated in liposomes) and CD44v6-pu18/AHC\(\subset\)L with improved photoacoustic properties are nanoparticles composed of a lipid bilayer, in which pu18 and NH\({}_{4}\)HCO\({}_{3}\) aggregates are incorporated [78]. NH\({}_{4}\)HCO\({}_{3}\) is in the hydrophilic core of the particle (pu18/AHC\(\subset\)L). After laser irradiation of the particles, pu18 aggregates are activated to produce heat and photoacoustic signals (Figure 8). The generated heat (42 \({}^{\circ}\)C) further initiates the slow decomposition of NH\({}_{4}\)HCO\({}_{3}\), which leads to the formation of CO\({}_{2}\) bubbles, which significantly contribute to the long-term increase in the photoacoustic signal (8 h). For targeted imaging of bladder tumors, the surface of the nanoparticles was conjugated to an antibody against the surface protein CD44v6 (CD44v6-pu18/AHC\(\subset\)L). This protein has been identified in the colon [79; 80], breast [81], and bladder [82] cancer cells, where it is produced to a greater extent than in healthy tissues. The CD44v6 protein is associated with cell migration, adhesion, and tumor metastasis. By comparing the effect of CD44v6-pu18/AHC\(\subset\)L on tumor and healthy human bladder tissue, a preferential accumulation of CD44v6-pu18/AHC\(\subset\)L was found in tumor tissue with a clear tumor delineation. Very small lesions (<5 mm) were also distinguished, which is a great advantage for early diagnosis of the tumor. The problem of the application of not only PAI drugs is bioavailability, which significantly affects the success of the whole process. An attractive approach of the preparation of new bioavailable drugs is the creation of so-called self-assembly molecules.