Photodynamic therapy of cancer: an update

Abstract

Photodynamic therapy (PDT) is a clinically approved, minimally invasive therapeutic procedure that can exert a selective cytotoxic activity toward malignant cells. The procedure involves administration of a photosensitizing agent followed by irradiation at a wavelength corresponding to an absorbance band of the sensitizer. In the presence of oxygen, a series of events lead to direct tumor cell death, damage to the microvasculature, and induction of a local inflammatory reaction. Clinical studies revealed that PDT can be curative, particularly in early stage tumors. It can prolong survival in patients with inoperable cancers and significantly improve quality of life. Minimal normal tissue toxicity, negligible systemic effects, greatly reduced long-term morbidity, lack of intrinsic or acquired resistance mechanisms, and excellent cosmetic as well as organ function-sparing effects of this treatment make it a valuable therapeutic option for combination treatments. With a number of recent technological improvements, PDT has the potential to become integrated into the mainstream of cancer treatment.

Figure 1. The principles of PDT

A photosensitizer (PS) is administered systemically or topically. After a period of systemic PS distribution it selectively accumulates in the tumor. Irradiation activates the PS and in the presence of molecular oxygen triggers a photochemical reaction that culminates in the production of ¹O₂. Irreparable damage to cellular macromolecules leads to tumor cell death via an apoptotic, necrotic or autophagic mechanism, accompanied by induction of an acute local inflammatory reaction that participates in the removal of dead cells, restoration of normal tissue homeostasis and sometimes in the development of systemic immunity.

Figure 2

Light propagation through the tissues.

Figure 3. Photosensitization processes illustrated by a modified Jablonski diagram

Light exposure takes a photosensitizer molecule from the ground singlet state (S₀) to an excited singlet state (S₁). The molecule in S₁ may undergo intersystem crossing to an excited triplet state (T₁) and then either form radicals via a type 1 reaction or, more likely, transfers its energy to molecular oxygen (³O₂) and form singlet oxygen (¹O₂), which is the major cytotoxic agent involved in PDT.

Figure 4. Three major cell death morphotypes and their immunological profiles

Apoptosis is morphologically characterized by chromatin condensation, cleavage of chromosomal DNA into internucleosomal fragments, cell shrinkage, membrane blebbing and formation of apoptotic bodies without plasma membrane breakdown. Typically apoptotic cells release “find me” and “eat me” signals required for the clearance of the remaining corpses by phagocytic cells. At the biochemical level, apoptosis entails the activation of caspases, a highly conserved family of cysteine-dependent aspartate-specific proteases. Necrosis is morphologically characterized by vacuolization of the cytoplasm, swelling and breakdown of the plasma membrane resulting in an inflammatory reaction due to release of cellular contents and pro-inflammatory molecules. Classically, necrosis is thought to be the result of pathological insults or be caused by a bio-energetic catastrophe, ATP depletion to a level incompatible with cell survival. The biochemistry of necrosis is characterized mostly in negative terms by the absence of caspase activation, cytochrome c release and DNA oligonucleosomal fragmentation. Autophagy is characterized by a massive vacuolization of the cytoplasm. Autophagic cytoplasmic degradation requires the formation of a double-membrane structure called the autophagosome, which sequesters cytoplasmic components as well as organelles and traffics them to the lysosomes. Autophagosome-lysosome fusion results in the degradation of the cytoplasmic components by the lysosomal hydrolazes. In adult organisms, autophagy functions as a self-digestion pathway promoting cell survival in an adverse environment and as a quality control mechanism by removing damaged organelles, toxic metabolites or intracellular pathogens.

Figure 5. PDT-induced effects

Light-mediated excitation of photosensitizer-loaded tumor cells leads to production of reactive oxygen species (ROS) within these cells, leading to cell death (predominantly apoptotic and necrotic). Tumor cell kill is further potentiated by damage to the microvasculature (not shown), which further restricts oxygen and nutrient supply. Tumor cell death is accompanied by activation of the complement cascade, secretion of proinflammatory cytokines, rapid recruitment of neutrophils, macrophages and dendritic cells (DCs). Dying tumor cells and tumor cell debris is phagocytosed by phagocytic cells, including DCs, which migrate to the local lymph nodes and differentiate into professional antigen-presenting cells. Tumor antigen presentation within the lymph nodes is followed by clonal expansion of tumor-sensitized lymphocytes that home to the tumor and eliminate residual tumor cells.

Figure 6. PDT molecular beacons

A peptide linker that is a substrate of a cancer-associated enzyme (e.g. a protease) is conjugated to a photosensitizer (PS) and a singlet oxygen (¹O₂) quencher. Proximity of the PS and quencher ensures inhibition of ¹O₂ generation during irradiation of normal cells. In the presence of an enzyme the substrate sequence is cleaved and the PS and quencher are separated thereby enabling photoactivation of the PS.

Figure 7. The principles of the PCI technology

The photosensitizer (PS) and the therapeutic compound (D) in this example linked to a monoclonal antibody as a targeting moiety are delivered to the target cells. The photosensitizer and the therapeutic compound are both unable to penetrate the plasma membrane and both are thus endocytosed reaching initially the endocytic compartments (endosome). The photsensitizers used in PCI are integrated into the membranes of the endocytic vesicles. Upon light exposure the photosensitizer becomes activated and form singlet oxygen oxidizing membrane constituents resulting in rupture of the endocytic membranes, allowing the therapeutic compound to reach cellular compartments where its therapeutic targets are located (T₁ or T₂ (nucleus)). In the absence of light the therapeutic compound may be degraded in the lysosomes.