2. Advantages and Disadvantages of PDT in Cancer Treatment

While chemotherapeutic drugs exhibit side effects not only on cancer cells but also on normal cells, photosensitizers combined with nanocarriers selectively accumulate in the tumor site and show cytotoxicity only in the area exposed to light irradiation. PDT can be performed instead of surgery in inoperable patients; light irradiation of the surgical site in patients who underwent tumor removal surgery can decrease the risk of cancer recurrence [21]. Combination of PDT with chemotherapy has the advantage of reducing side effects by lowering the dose of anticancer drugs [22].

However, contrary to systemic chemotherapy, local treatment using an optical fiber in PDT is hard to kill the tumor cells present outside the focal area or alter the therapeutic outcomes in patients with advanced-stage cancer [23]. Under light irradiation for PDT, the penetration efficiency of light into the deep tissue is low because of endogenous biomolecules absorbing the light. It is difficult to efficiently excite the photosensitizers located deeper than 1 cm from the tumor surface. Although there have been recent advances in microendoscopic technology or laparoscopic light delivery systems, PDT still has obvious limitations in treating large, deeply hidden tumors [24].

Among photosensitizer classes such as porphyrins, chlorophylls, and dyes, most photosensitizers approved for clinical use are derivatives of the porphyrin moiety [25]. Porphyrins are composed of tetrapyrrole macrocycles connected to each other via methine bridges [26]. Limitations of the first-generation photosensitizers in clinical application to various solid tumors include aggregation in water, low singlet oxygen quantum yield, high doses needed for therapeutic efficacy, low selectivity to the tumor site, skin photosensitivity, and long elimination half-life [27]. The first-generation photosensitizers based on the porphyrin backbone exhibited undesirable hydrophobicity and low penetration depth, hindering their clinical use. The second-generation photosensitizers such as chlorins and phthalocyanines, which are structurally related to tetrapyrrole macrocycles, have been investigated for cancer treatment [27,28]. Their water solubility, pKa value, and stability were improved by introduction of the hydrophilic substituents to pyrrole rings. However, the increase in renal clearance due to improved water solubility tends to decrease bioavailability of the photosensitizer.

In order to be excited in the deeper tissue, the second-generation photosensitizers have been developed to show higher molar extinction coefficient in the near-infrared region than the first-generation photosensitizers [29]. 5-Aminolevulinic acid (5-ALA), a precursor of protoporphyrin IX, has shown good clinical outcomes in cancer treatment [30,31]. Protoporphyrin IX, an intermediate in the heme biosynthesis, exhibits cytotoxicity when excited by light, and loses phototoxicity by binding to iron ions. Administration of excess exogenous ALA promotes production of protoporphyrin IX more efficiently in cancer cells than in normal cells. One of the disadvantages in clinical application of photosensitizers is a risk of skin photosensitivity. Hydrophobic photosensitizers remain nonspecifically accumulated in the skin and eyes after photodynamic treatment [32]. Contrary to other porphyrin-based photosensitizers, protoporphyrin IX produced by systemic administration of 5-ALA is eliminated after 24–48 h with the lower risk of long-term photosensitivity (Table 1).

The ratio between type I and type II reactions of photosensitizers in cancer depends on the concentration of oxygen and substrates in the tumor microenvironment (TME) [33]. When exposed to light of a specific wavelength, photosensitizer molecules are excited to the singlet energy state. While some of the excited photosensitizer molecules return to the ground state by emitting energy in the form of fluorescence, most of the excited molecules transfer to the triplet energy state via intersystem crossing. In type I reaction, the transfer of electrons or hydrogen atoms between photosensitizer molecules excited by light and the surrounding substrates generates radical species, resulting in cytotoxicity through oxidation reaction with intracellular components. In type II reaction, photosensitizer molecules in the triplet state efficiently transfer energy to the surrounding oxygen and generate singlet oxygen [34]. Singlet oxygen induces cytotoxicity via chemical reaction with intracellular components. It is important to consider that the production of cytotoxic ROS in the TME induces oxygen depletion in the tumor tissue, resulting in apoptosis or necrosis in the targeted tissue. After intravenous injection, porphyrin derivatives accumulate in the tumor vasculature, and occlusion by damage of vascular epithelial cells occurs under light irradiation [35]. It induces hypoxia and tumor necrosis. As therapeutic outcomes of PDT are dependent on the preexisting concentration of oxygen at the tumor site, strategies are needed for use of PDT in the treatment of hypoxic tumors [36].

Effective cytotoxic effects can be obtained by localization of photosensitizers in the intracellular organelles such as mitochondria, nucleus, and lysosomes because of reactivity and short half-life of the ROS generated by light irradiation [37]. In order to allow photosensitizers to accumulate in the target site at sufficient concentrations, modification of photosensitizers has been attempted by using cell-penetrating peptides or conjugation with targeting ligands [38]. Besides, therapeutic efficacy of PDT depends on the wavelength of light, laser power per unit area, and the dosage of photosensitizers [39]. Over the past few decades, extensive attention has been paid to the design and development of various PDT modalities.

3. Advances in Nanocarriers for PDT

To date, numerous nanocarriers such as polymers, micelles, liposomes, dendrimers, and inorganic nanoparticles have been studied for increasing therapeutic efficacy of photosensitizers. It is important to efficiently deliver photosensitizers and the generated singlet oxygen to the target site in the optimum therapeutic range. Pharmacokinetic or pharmacodynamic profiles of nanocarriers should also be checked for clinical use. Multifunctional nanoparticles are also currently being investigated for theranostic purpose or photodynamic/chemo dual therapy. Recent advances in preclinical developments using nanomedicine for PDT are categorized in Table 2.

Natural polysaccharides such as hyaluronic acid (HA), heparin, chitin, chitosan, and fucoidan have been reported as potential photosensitizer carriers owing to their biocompatibility and biodegradability [69,70,71]. HA shell can interact with the core, including chlorin e6 (Ce6) and positively charged CRISPR–Cas9 targeting the phosphatase gene, which constructs a nanocarrier system [52]. The negatively charged HA in the nanoparticles could not only regulate the surface charge of the nanoparticles reducing nonspecific interactions in the physiological environment, but also target the TME via CD44 receptors [72,73]. This multifunctional nanosystem demonstrated high transfection efficiency in B16F10 cells and PDT efficacy under laser irradiation. It can also sensitize the targeted tumors to immunotherapy by promoting the proliferation of cytotoxic CD8⁺ T cells.

Newly developed biocompatible nanocarriers not only encapsulate PDT agents, but also provide additional targeting effects for enhancing anticancer activities. Fucoidan exhibits binding affinity to P-selectin and the targeting effect on P-selectin-positive cancer cells [74,75]. Chung et al. developed a multifunctional nanocomplex carrying photosensitizer verteporfin, which was composed of negatively charged fucoidan, positively charged polyamidoamine (PAMAM) dendrimer, and MnO₂ catalyzing the decomposition of hydrogen peroxide to form oxygen. The nanocarrier can enhance accumulation of verteporfin in the tumor site through both P-selectin targeting and the EPR effect. The dendrimer–fucoidan nanocomplex could specifically target P-selectin-overexpressed breast cancer and tumor-associated vasculature [53]. It also overcame tumor hypoxia using MnO₂ and improved the therapeutic efficacy of PDT for antimetastatic effects. Recent multifunctional nanoplatforms exhibit such characteristics as cancer targeting, improved pharmacokinetics/pharmacodynamics, and high photodynamic efficacy.

In order to overcome drawbacks of chemotherapy, nanocarriers for the combination of chemotherapy and PDT have also been developed. Ce6 was loaded into peroxidase mimic metal–organic nanoparticles and coated with HA [54]. The nanocarrier can react with hydrogen peroxide in the tumor site and form oxygen to prevent hypoxia. It was designed for exhibiting cascade reactions and it could show synergetic chemo–photodynamic therapeutic efficacy.

Attachment of polyethylene glycol to the nanocarrier can provide hydrophilicity, decrease the clearance by the reticuloendothelial system, and extend circulation time. Polyethylene glycol (PEG) conjugated with hydrophobic stearamine carrying doxorubicin and pheophorbide A (PhA) accumulated in the tumor site, and the release of drugs from the nanocarrier with a ROS-sensitive linker was triggered by ROS within cancer cells [55]. The outer PEG layer could be helpful for long circulation of the nanocarrier in blood, and a stable thioketal bond could prevent premature drug leakage. On the other hand, there is growing concern about the immune response induced by PEGylation. Injection of PEGylated multifunctional nanoplatforms or bioconjugates may induce immunogenic reaction in the body [76]. Alternatively, zwitterionic polymers that have a hydrophilic nature and excellent biocompatibility can be used in the nanocarriers [77,78,79,80].