Chlorin e6 & PDT

Photodynamic action

Over a hundred years ago, scientists found that light and certain chemicals together can cause cell death. In 1903, Herman Von Tappeiner and A. Jesionek used a combination of white light and eosin to treat skin tumors and called it “photodynamic action”. Later on, Von Tappeiner and Jodlbauer proved that oxygen is essential for photosensitization. In 1931, Kautsky and de Bruijn suggested that singlet oxygen is probably involved in these photosensitizing reactions as a reactive intermediate. After many advancements made in the last century, photodynamic therapy was finally approved by the FDA nearly three decades ago, and it is still progressing. 

Three things are essential to perform the PDT: a photosensitizer, a light source, and molecular oxygen. When the photosensitizer in excited through light irradiation, it undergoes photochemical reactions and generates reactive oxygen species (ROS) by interacting with cellular oxygen. ROS mediated cytotoxicity is the cause of cancer cell death in PDT.

Photosensitizers in PDT

Hematoporphyrin and its derivatives are the first reported photosensitizers for PDT. In 1911, Hausmann and Pfeiffer observed light mediated phototoxicity in guinea pigs and white mice injected with hematoprophyrins. In 1924, Policard found that natural porphyrins emit fluorescence and have the propensity to accumulate in the tumor region. These classical photosensitizers like hematoporphyrin derivative and photofrin II are known as first generation photosensitizers. First generation photosensitizers have a complex composition, which is responsible for poor tissue selectivity and low light absorption. Hematoporphyrin derivative and photofrin have their absorbance maxima in the range of 625−635 nm, which results in poor light penetration and cutaneous photosensitive toxicity. In order to alleviate the disadvantages associated with the first generation photosensitizers, a new class of photosensitizers, called second generation photosensitizers, were developed. These photosensitizers are chemically pure, have better photosensitivity, tissue selectivity, and longer wavelength absorption maxima. The majority of the second generation photosensitizers are porphyrin-based photosensitizers, like benzoporphyrins, texaphyrins, and phthalocyanines. Some examples of nonporphyrin photosensitizers are hypericin, methylene blue, toluidine blue, and merocyanine 540. When a second generation photosensitizer is incorporated with targeting moieties, such as antibodies and peptides, or encapsulated into nanocarriers for better accumulation at the tumor site, it is known as a third generation photosensitizer.

Chlorin e6

Tetrapyrroles are known as the pigments of life, because life essential biomolecules like heam, chlorophyll, and bacteriochlorophyll have tetrapyrrole-based structures. A wide range of PDT photosensitizers are tetrapyrroles and have been classified into several classes like porphyrins, chlorins, bacteriochlorins, and phthalocyanines.Chlorins are 18π-electron aromatic compounds, and a number of clinically significant photosensitizers belong to this family. The tetrapyrrole backbone of chlorins has two extra hydrogens in comparison to porphyrins (Figure 1)

and this structural difference causes a significant shift in the absorption bands of these two classes. Light wavelength ranging from 650 to 700 nm is required for the activation of chlorins, which is relatively higher than that of porphyrins. It has been reported that laser light of higher wavelength penetrates deeper into the tissues in comparison to shorter wavelength. Chlorin e6 (Ce6) is a popular photosensitizer of the chlorins family. It is a chlorophyll a derivative with molecular weight of 596.7 Da. Ce6 is formally known as (7S,8S)-3-carboxy-5-(carboxymethyl)-13-ethenyl-18-ethyl-7,8-dihydro-2,8,12,17-tetramethyl21H,23H-porphine-7-propanoic acid. It is a fluorescent molecule having excitation and emission maxima at 400 and 665 nm, respectively. The singlet oxygen quantum yield and triplet lifetime of Ce6 are 0.7 and 760 μs, respectively. Ce6 has been reported to be an effective photosensitizer against a variety of cancer cell lines, including HT29 colon carcinoma, FM3A mouse mammary carcinoma, DL110 leukemia, MH134 hepatoma cells, CBMT mammary cancer cells, and human NHIK3025 cells.

Molecular Mechanism of PDT

The process of PDT starts with the excitation of photosensitizer. After light absorption, PS goes into an excited singlet state from the ground state. The excited PS can return to the ground state by losing its energy in the form of fluorescence emission. Fluorescence emitted by PS is useful in photodetection. Alternatively, the excited PS can change its state from singlet to triplet through intersystem crossing. In the triplet state, the PS interacts with different cellular substrates and produces ROS. Based on the substrate type, the ROS generation process can be categorized as type I and type II mechanisms. In the type I mechanism, interactions between triplet PS and cellular substrates cause the formation of radical ions and free radicals, which interact with oxygen and generate ROS. In the type II mechanism, the triplet PS interacts with molecular oxygen directly and produces singlet oxygen (specific type of ROS)Figure 2 depicts the photochemical mechanism of ROS generation.

An excess amount of ROS in the cell causes oxidative damage to intracellular components like protein, lipid, DNA, and RNA and leads to cell death via apoptosis and necrosis (Figure 3). Additionally, cell death via autophagy and ferroptosis have also been reported. Apart from tumor cells, an activated PS also damages tumor neovasculature. In comparison to normal vasculature, the defective cellular borders in tumor vasculature make it leaky and allow higher accumulation of PS. The series of biochemical reactions during PDT also activates different components of the immune system. A study on squamous cell carcinoma shows increased cytokine levels after PDT. In a clinical study conducted on nine patients suffering from head and neck squamous cell carcinoma, it was found that PDT causes the release of HMGB-1 and increases the levels of IL-6 and NK cells.

Ce6-clinical progress

Photo-based therapy is well reported as a cost-effective alternative to the existing treatment modalities for cancer. In particular, Ce6-based photosensitizers have been shown to be well tolerated with minimal phototoxicity and significant treatment efficacy in several phase I and phase II trials in patients with multiple types of laser-accessible tumors.

LinkChem Ce6 building block series

Reference：

Hak, A.; Ali, M.S.; Sankaranarayanan, S.A.; Shinde, V.R.; Rengan, A.K. Chlorin E6: A Promising Photosensitizer in Photo-Based Cancer Nanomedicine. ACS Appl. Bio Mater. 2023, 6, 349–364.

This article is published by LinkChem Technology Co., Ltd. Welcome to scan the QR code to follow us and explore the mysteries of chemistry together.